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Lecture 3 - HOT WIRE ANEMOMETRY — Transcript

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0:00[Music]
0:20you
0:25so welcome back we were discussing about
0:28the non-invasive measurement technique
0:30and in that line we have discussed about
0:32the pitot tube
0:33so next invasive measurement technique
0:35is hot weather imagery now this
0:37technique is one of the widely used
0:39technique to understand the turbulence
0:41in single phase flow and the technique
0:44has also been used to analyze the
0:46multi-phase flow and this is the
0:48technique you can say that whatever the
0:49understanding of the turbulence
0:51experiments we have and initially which
0:53we have developed is all because of this
0:55technique isn't this technical piece of
0:57major rule to understand the turbulence
0:59and why because this technique have a
1:02very high response time okay both
1:04spatial resolution as well as the
1:06temporal illusion of this technique is
1:07very high so we try to understand first
1:10the basics of this technique okay then
1:13what is the measurement principle and
1:15then we will try to understand that how
1:17the same technique can be used for the
1:19military's flow what will be the
1:20limitation and overall advantage and
1:22drawback of this technique so this will
1:24start about this technique in this class
1:27and as I said that the name itself is
1:30hot wire anemometer so you can
1:32understand it means that there is some
1:34wire is being used which is hot at
1:36certain condition and that hot condition
1:39is being used to measure the velocity
1:41and that's why the name is called hot
1:43wire anemometer II so the typically the
1:46setup or the kind of a probe which is
1:49being used to in hot wire anemometer II
1:51it looks something like this
1:52okay it looks something like this so it
1:55has actually four parts one is the probe
1:58body which is this okay which is
2:00actually support to the probe and the
2:01backside of this the electronics is
2:03being there which is used for the
2:04measurements we discuss about those
2:07electronics then there is a branch
2:09actually which is being used as a
2:10support then there is some other support
2:13here which you see from this side and
2:15then we use a small wire this is the
2:17wire because of that the name is hot
2:19wire mama tree and that wire is being
2:21used for the measurement how we will say
2:23about that when we discuss about that so
2:25this is the active length of the wire
2:27and this is the diameter of the wire and
2:29these are the support which actually
2:31hold the wire so this technique is being
2:33used and the basic major main principle
2:36of this technique is
2:37that it is being connected with this
2:39wire is actually one leg of the V T
2:41Stonebridge now we will discuss about
2:42the circuit also so what happened we
2:45know that in the Wheatstone bridge say
2:48this is a typical Vita Stonebridge this
2:58is the typical say PT Stonebridge if I
3:05give a current voltage then what will
3:07happen at volt difference then the
3:09current will be actually divided based
3:11on their resistance ratio say R 3 R 4 so
3:14it will be divided based on their
3:16resistance ratio and you can find it out
3:18that how much current will be flowing
3:20here okay when each kind of circuit or
3:23each line now if you change the
3:25resistance of one then what will happen
3:27the overall current distribution of old
3:29distribution will change and that world
3:32distribution will change will be
3:33proportional to the change in the
3:34resistance and that is the basic
3:37principle which we use in the hot wire
3:39anemometer II so what we do we give
3:41certain voltage potential initially so
3:44each because of the voltage potential
3:45some current will pass so each
3:47resistance or each wire will be
3:50maintained at a particular temperature
3:51now if suppose one of the wire I connect
3:54with the flute if we are exposed to the
3:56flute so this is my flute length this is
3:57the wire then what will happen that that
4:00because the fluid will come it will
4:03change the temperature because of the
4:05heat transfer now the change in the
4:07temperature will be recorded as a change
4:09in the resistance and that change in the
4:11resistance will sure cause change in the
4:13current and because of that the change
4:15in the current will be proportional if
4:17you do that the calculation will sue the
4:19numerical you see the numerical part it
4:21will be proportional to the velocity so
4:23you can measure the velocity of the
4:24fluid and that is what the principle is
4:27that in this place in hot wire
4:29rheumatory a hot wire is used the
4:31diameter of the hot wire is in the range
4:34of micrometer this is diameter is
4:38currently this is the range of 5
4:40micrometer okay only it was much bigger
4:42but in the current state of art of hwa
4:45the wire thickness is very low and it
4:47comes in the range of 5 micrometer okay
4:49and the
4:50is around 1 to 5 mm depending upon the
4:55application and different type of the
4:56probes so one length varies in the range
4:59of 1 to 5 mm diameter varies in the
5:01range of say 5 micrometer this is being
5:04exposed this is wire is being exposed to
5:07the flow field or being inserted in the
5:09flow field you know what will happen the
5:11flow field actually the velocity can be
5:13cysts by the convective heat transfer
5:15measurement so because of that that
5:18while is being there as I said that
5:20there is some temperature will be given
5:23because of that particular current or
5:25curve kilovolt which has been applied Y
5:27will will be maintained at a certain
5:29temperature now it will expose that wire
5:32to the fluid then what will happen
5:34because of the fluid some cumulative
5:35heat transfer will take place and that
5:37conductive heat transfer will change the
5:39temperature of the Novaya and that's why
5:42the it will change the temperature
5:43resistance of the wire and the current
5:45so what you are actually solving is a
5:48very simple equation which you will have
5:50might have done in your transport
5:51phenomena course in which what we do a
5:542d cylinder is being suspended in the
5:56fluid now why we are taking to the
5:58cylinder because the diameter of the
6:00probe is very very small so if the
6:02diameter of the probe is very small you
6:04can always assume it to be a theta
6:05symmetric and you can assume that the
6:07temperature is completely in kind of
6:09symmetric here so it's a 2d cylinder
6:11something like this which is suspended
6:12in the flue and you find a convective
6:15heat transfer and that convective heat
6:16transfer coefficient you correlate with
6:18the your resistance change so that is
6:21the basic principle of the hot wire MoMA
6:23tree by which we record the signal so
6:27what as I said already said but what is
6:30the formula the basic principle working
6:32principle of the hot wire Mohammed tree
6:33is that hard wire in the military uses a
6:36fine wire as I said on the order of
6:39several micrometers in the order of
6:41actually 5 micrometer with the current
6:43state of the art which are maintained at
6:46a particular temperature it is heated at
6:48a particular temperature which is above
6:50din the ambient temperature okay then
6:53what we do the electrical resistance of
6:56the conductor actually is changes which
6:58the temperature and hence the resistance
7:01of the wire is also changed and once you
7:03put that wire in a flowing fluid what
7:05will happen because of the convective
7:07heat transfer the fluid will try to cool
7:09the temperature of the wire or cool the
7:12wire and because of that there will be
7:14change in the resistance and resistance
7:15will be reduced and because the
7:17resistance will be reduced as the
7:19current distribution will change so you
7:21can do that and by monitoring that
7:23registration change of heated wire and
7:25taking the temperature of the
7:27surroundings load if you measure the
7:29surrounding fluid temperature we measure
7:30that how much resistance change has been
7:32taking place you can calculate the
7:34velocity of the flute okay so that is
7:38the basic principle which is being used
7:40in hot wire anemometer okay to
7:42understand that so what we do actually
7:44again I will come back to the same that
7:46we make a feed stone bridge circuit so I
7:50put some of the distance here I put
7:53again some resistance here in this then
7:57again I put some resistance this okay
8:03now I connect it to say certain voltage
8:10okay and I will make it a switch
8:15arrangement and say I'm making an
8:19amplifier okay and this and then I'm
8:23giving a voltage difference of voltage
8:28we introduced it and say that this u so
8:34this is a typical circuit okay and this
8:37place you can if you want you can ground
8:38it okay so what will happen say this is
8:41my r1 this is my R - this is my R wire
8:45and this is my r3 okay now in the wheat
8:50in the hot wire with me tree what we do
8:52each stand just resistance and I will
8:54write make this resistance something
8:56like this that this and we have extended
8:59it and expose this resistance to the
9:02flute okay and this becomes my RW okay
9:09so what we are going to do we know that
9:11if there
9:12be current with us with the VT
9:13Stonebridge principle that it is going
9:15to be the ratio R 1 upon R 2 is going to
9:19be equal to R W upon R 3 these are going
9:24to equal and that will be at a ratio
9:26which will be called at the resistance
9:28ratio R and generally typically we
9:30balance a resistance ratio of around 1.5
9:34okay so that is the way this
9:37distribution will be there this R will
9:38be here okay and the current will be
9:40passed through this if you give a
9:42voltage difference a particular current
9:44will be passing now because the
9:46particular volt difference we are
9:47already giving the each resistance will
9:50be maintained at a certain temperature
9:52okay so let's assume that the
9:55temperature is TW here now if I expose
9:58this to a fluid which is moving say in
10:00any direction in this direction if
10:02suppose your fluid is moving and I
10:04expose this wire to the fluid what will
10:07happen if suppose the flute temperature
10:08is TF then because of this flute the
10:12resistance this wire temperature will
10:14cool down okay because of the heat
10:16transfer the moment the wire temperature
10:18will be cooled down this ratio will be
10:20disturbed and then what will happen your
10:23voltage will change your current will
10:25change okay so what you try to do you
10:28maintain that you measure the change in
10:30the current and that change in the
10:32current is actually being converted in
10:34terms of the velocity measurement okay
10:36by using Kings law I hope some of you
10:40might be knowing this in the electrical
10:42engineering courses and even in heat
10:44transfer courses you might have heard
10:47about that so by using the King's law we
10:49convert the this change in the
10:51resistance in terms of the velocity okay
10:55so that is the basic principle of hot
10:58wire mama tree we use sum or difference
11:00we witch
11:01some switches we'll discuss this this
11:03circuit again and wish you some
11:05amplifier again once again so that is
11:12the basic principle of the measurement
11:14and based on that we calculate that
11:16change in the current or change in the
11:18resistance in terms of the velocity now
11:20you can do both we have both the luxury
11:22and we'll
11:23just that what is the type of this so as
11:26I said that there's different type
11:28depending upon whatever you are doing
11:30whether you are maintaining the change
11:32maintaining the current change or you
11:34are seeing that resistance change you
11:36can divide the hot wire anemometer II in
11:38different parts or different types now
11:41the type has been divided based on
11:43number of wires and also based on the
11:47wave mode of operation so we discussed
11:49the mode of operation before that I will
11:51discuss that number of wires that have
11:52been kind of divided the one first one
11:56is called single wire as I have
11:57discussed and the photograph I have
11:58already shown you this is a single wire
12:01anemometer so what it does it does that
12:04the single wire anemometer is being
12:06placed in the flow okay so suppose this
12:09is the flow this the fluid will be past
12:12it in this way okay so that say this is
12:17the solute what will happen now the
12:20fluid will pass cylinder has been
12:22exposed through the flute okay and it
12:24will be like a past so immersed body now
12:28this cylinder is being exposed because
12:30of the flute temperature of loop
12:32velocity and convective heat transfer
12:33the temperature of this this cylinder or
12:36this wire will be reduced and you can
12:38convert that change in the resistance in
12:41terms of the velocity okay so you can
12:43measure the velocity but what happened
12:45with the single wire that it can nature
12:47only the one direction of velocity or if
12:50there is a little longitudinal velocity
12:51it can major but if you have a two
12:54dimensional velocity it means
12:55longitudinal as well as transverse
12:57velocity is there it cannot major that
12:59so in that case we use a x wire type of
13:02hot wire anemometer so what it does the
13:04same thing the fluid is being flowing
13:09with it in this way with a particular
13:11velocity what will happen both the wire
13:13will get cooled down
13:14now both the wires connected to
13:15individual VT stonebridge the current
13:18will kind of the temperature will be
13:20modified if their temperature is
13:22modified what is going to happen the
13:24resistance is going to be modified and
13:26the current will also be changed so
13:28again whatever you are measuring you
13:30want to measure the change in the
13:31resistance such
13:32in the current you can calculate the
13:34velocity and you can calculate the both
13:36the velocity both directional velocity
13:38in this how you can calculate that we
13:40will discuss again in the mathematical
13:41part of it once we will discuss the
13:43mathematical part now if suppose you
13:45have all the three dimensional velocity
13:46okay then we use the triplet wire okay
13:50or triple split so this is the name will
13:52being used some people say triple wire
13:53some people say triple split wire there
13:56is nowadays a three wires are there
13:58again each wire is an individual BT
14:00stone bridge it will major that what is
14:03the change in the temperature because of
14:05the fluid flow and the change in the
14:07temperature will be recorded in terms of
14:09the change in the resistance or in terms
14:11of the change in the current so you can
14:13again measure the velocity so what
14:15happened that as I said that with the
14:17single wire you can allow to measure the
14:19early longitudinal velocity and velocity
14:21fluctuation for sure okay that is the
14:24major advantage of this you can measure
14:25the velocity as well as the velocity
14:27fluctuation this allows you XY I allow
14:30you to measure the transverse velocity
14:32and velocity fluctuation that will be
14:34always been there the three wire will be
14:36beYOU not this the sign and value of the
14:39two component of the velocity and they
14:41can major simultaneously both the
14:43components of the velocity and
14:44definitely they are going to provide the
14:46fluctuation - okay so depending upon
14:50what kind of flow you have you have one
14:52dimensional flow you have longitudinal
14:54flow transverse flow or to dial two
14:56component of the velocity whatever you
14:58want to measure you can use the number
14:59of wires and you can measure that okay
15:02so that is one way to divide the types
15:04of hot wire anemometer second way which
15:07is critical actually on the mode of
15:09operation so as I discuss in the circuit
15:13that what you will happen that you are
15:15going to see the change in the
15:16resistance now based on that and then
15:19that change in the resistance can be
15:21recorded in terms of the change in the
15:23current now based on the mode of
15:25operation how you want to operate the
15:27auto by Rama tree is actually being
15:29divided mainly in two parts that one is
15:31called constant current hot wire
15:33anemometer and second one is called
15:35constant temperature hot wire anemometer
15:37now as the name suggests that in the
15:39constant current you are going to keep
15:41the currents
15:43and in the constant temperature you are
15:45going to keep the temperature same now
15:47temperature same means resistance same
15:49so you can also say that is a constant
15:51temperature some people also say
15:53constant resistance so both are same
15:58okay so you can make the constant
16:01resistance or you can make the constant
16:03temperature so that is the way it has
16:05been divided each technique have their
16:08own advantage or each mode have their
16:10own advantage and disadvantage however
16:11mostly constant temperature hot wire
16:14anemometer is being used why we
16:15discussed so now what is the constant
16:18current thought while no meter as I said
16:20that constant current Hardware witta
16:21means you will maintain the current
16:23constant and we will change the
16:25resistance okay so that is the way what
16:28is called constant current constant
16:30temperature means you are going to keep
16:32the resistance same by watching some
16:34servo amplifier okay or feedback
16:35controller and you will keep on changing
16:38the current okay so that that the
16:41overall resistance you are going to keep
16:43on same okay so that is the basic way it
16:46has been defined the constant current
16:47and constant temperature we will try to
16:49understand that so what is the constant
16:51temperature in constant temperature mode
16:53what we do the current through the are
16:55just wire is adjusted to maintain the
16:58constant temperature so how the
16:59temperature will be given if I maintain
17:02that current constant okay then what
17:04will happen that sorry if you want to
17:06maintain the current pressure constant
17:07what you need to do you have to vary the
17:09current so suppose this is the
17:11Wheatstone bridge again I will make the
17:13same
17:20so this is my say Wheatstone bridge
17:22circuit okay and say this and I'm going
17:26with this this I put a switch okay then
17:31I am coming here I went for a servo type
17:34amplifier and then I'm given a voltage
17:36difference now what will happen
17:38initially say r1 r2 are W and r3 okay
17:44they are maintained and this is
17:45maintained at temperature TW
17:47now if you expose it to the flute so if
17:49I expose it to a flute then what will
17:51happen that the temperature of this wire
17:54resistance will change and because of
17:55that the RW will also change now the
17:58moment RW will change the ratio of the
18:00resistance will change and you need to
18:02supply the extra current okay to
18:04maintain the same temperature how
18:06because if you want to maintain a same
18:07temperature in the same current should
18:09be supplied in this length if the
18:11current will be the same the overall
18:12temperature will also be the same so
18:14what you need to do now it has been
18:15cooler down resistance has reduced so
18:18you have to increase the current so that
18:20your power to this wire remains same now
18:22to improve that current what you will do
18:24you will use those sulfur type amplifier
18:26feedback amplifier and that amplifier
18:29will actually pass the current if it
18:30will suppose cool down where it is in
18:32there so what will happen this which
18:34will be not disconnected and the current
18:36will be stopped and you will see that
18:38that the temperature of the wire has
18:40changed okay so that is the main circuit
18:42is being used and you use the amplifier
18:44if you want to use the constant current
18:47amplifier then what you have to do you
18:49have to keep the current similar now to
18:51keep the current similar what you need
18:53to do you have to maintain this ratio
18:55same somehow so R 1 upon R 2 should
18:58remains same as RW upon R 3 so what you
19:02will do you will put the extra
19:04cholesterol resistance here somewhere
19:05and that resistance you will be
19:07manipulating in such a way that your
19:09ratio remains same this ratio remains
19:11same so the circuit is almost same well
19:13you maintain instead of varying the kind
19:15of current by using the server type
19:17amplifier you are now wearing the
19:20resistance so that the resistance ratios
19:22remain same so that is the basic which
19:25we use in constant temperature and
19:27constant current okay so that is what we
19:30have said that what will happen that
19:32because
19:32the flow the temperature will change and
19:34the temperature need to be maintained
19:36and that will be maintained by supplying
19:39the extra current because the power will
19:40remain the same it is equal to I square
19:42P is equal to I square R so if the
19:45change in the temperature is causing
19:46change in the resistance if I increase
19:48that current what will happen your
19:50temperature will again be increased okay
19:52because I square R value will remain
19:53same so that is the way we do it now we
19:57used to do that current manipulation we
19:59use a feedback circuit so this is the
20:00feedback circuit we kind of see that how
20:02the current is being reduced based on
20:04that we amplify the current okay and
20:07that is the way it is being done so what
20:10is there that it should be clear that
20:12the current required maintaining the
20:15wire at a constant temperature is going
20:17to be proportional to the convective
20:19heat loss okay so more the heat loss
20:22more the current you will require so
20:23that is going to be the proportional and
20:25that proportionality will actually be
20:27can be related with the velocity and
20:30because heat loss through the convective
20:32heat loss is a function of velocity we
20:34all know that and so the velocity can be
20:37measured or can be correlated with the
20:39increase in the current requirement okay
20:42so that is the principle which we use
20:44for the constant temperature hot value
20:46meter now in constant current hot wire
20:48anemometer as I said that what we do we
20:51maintain the constant current mode okay
20:53so constant current means your constant
20:55he to use there so what you are going to
20:57do you are going to fit an electrical
20:59current flow through the wire and that
21:02is what will happen that we have to kind
21:04of pass approximately same current
21:07through the wire if you will pass the
21:09approximately same current through the
21:10wire which is being exposed as I said
21:12that I square R value will remain same
21:14and then you will have maintaining the
21:17constant current now though I square R
21:20value will not be the same but you will
21:22maintaining the constant current okay so
21:23that's what is the objective that you
21:25have to flow the constant current
21:26through the wire now that is been
21:29possible to attain that the equilibrium
21:31temperature will change okay and that
21:34equilibrium temperature between the
21:36internal heat generation due to the
21:38electrical resistance okay and the
21:40convective of wire convective heat
21:43of the wire okay or convective heat
21:46transfer of the wire to the moving fluid
21:48will be actually get balanced okay so
21:51they will be equilibrium so that way
21:53what you can do that the wire
21:55temperature change okay must be adjusted
21:58with the convective loss okay until the
22:01equilibrium of the temperature is
22:02achieved so what will happen because we
22:04are keeping the temperature current
22:06frame that the temperature is going to
22:08be changed now how much temperature of
22:10the wire will change that will depend on
22:12how much heat loss has been taken place
22:14from wire to the flute okay so that will
22:18be the temperature change will be
22:20recorded and that convict temperature
22:22change will be actually the function of
22:24velocity because again the convective
22:26heat transfer so the temperature change
22:28on the wire will be actually the
22:31function of the velocity of the fluid
22:32because how much temperature change you
22:34are going to see that how much
22:35convective losses are taking place from
22:38the wire okay so in that way you can
22:41again correlate the temperature change
22:43recorded to the velocity of the fluid
22:46and the velocity of the fluid can be
22:48measured okay so that is the constant
22:50current heat or a hot wire anemometer is
22:52being used and the circuit what we do we
22:55put some additional resistance and we
22:56keep on changing that resistance in such
22:58a way that your current flow should
23:00remain same okay so that is the way that
23:03is the principle of the constant current
23:05hot wire anemometer now obvious question
23:08will be which one I should use whether I
23:10should use the constant current or
23:11should I use the constant temperature so
23:13now you can compare so what is the basic
23:16principle of basic difference in this in
23:18the constant temperature hot wire
23:20anemometer it is used in the same way
23:23that is being calibrated now as I
23:25already said earlier in introduction
23:26class that most of the measurement
23:28technique is not a direct measurement
23:30you are not directly measuring the
23:32velocity per se you are measuring some
23:34other quantity and that quantity is
23:36being calibrated in terms of the
23:38velocity or it's kind of being
23:39recalculated in terms of the velocity so
23:42in such measurement which is indirect
23:44measurement definitely you need to
23:46calibration and calibration is very very
23:48critical and the accuracy of your
23:50measurement actually depend on the
23:52accuracy of the calibration and I will
23:54keep on repeating this thing because
23:56most of the technique we will discuss
23:57will be actually doing the indirect
23:59measurement so the calibration becomes a
24:01very very integral part of all the
24:04measurement techniques so in this also
24:06you do the calibration and while doing
24:08the calibration the standard procedure
24:10is to maintain the temperature of the
24:13wire same okay so it means the major
24:16benefit of the hot wire anemometer which
24:19is operated on the constant temperature
24:20mode that the way you do the calibration
24:23and the way you do the measurement is
24:25remain same okay so that is the major
24:28advantage of this because it it has the
24:30accuracy okay then in constant current
24:33what happened that as I said that then
24:35the constant current also the
24:36calibration is performed by maintaining
24:38the constant temperature and then that
24:41is being converted in terms of the
24:43constant current so what you have to do
24:45you have to again do a conversion and
24:46then that conversion will be again being
24:49converted in terms of the velocity so
24:52one more extra layer of the conversion
24:54is coming and that's why we would like
24:56to prefer the constant temperature
24:58because you are doing the calibration
25:00and measurement exactly in the same way
25:02while in constant current you do the
25:04calibration by using the constant
25:06temperature approach we will do the
25:07measurement by using constant
25:09temperature mode so that makes a
25:11difference and that me kind of hamper a
25:13little bit of your sensitivity of the
25:15probe so that is the major thing then
25:18the major disadvantage of the constant
25:20current hot wire anemometer is that wire
25:23burn out now what does the wire burn out
25:26means because we are actually allowing
25:28to change the temperature now if you
25:30allow to change the temperature in the
25:32constant current mode what will happen
25:35if the temperature increases beyond a
25:37certain point the wire will burn out or
25:39if the temperature reduces we long a
25:42certain point it means the resistance
25:43will be very very low so the power the
25:46current flow can be increased
25:48momentarily before you get into control
25:50into it it can burn the element itself
25:53which can Brown burn your wire so that
25:55that risk is very very high in case of
25:58the
25:59constant-current hot-wired new media
26:01while in constant temperature hot wire
26:03anemometer the wire temperature always
26:05remain constant so wire burnout is very
26:09very low again because of some
26:10fluctuation some instability or some
26:14kind of error performance or kind of
26:17your not good performance of your
26:19controller it may kind of burn the wire
26:23can burn but most of the time it's the
26:26probability of burning wire in case of
26:28the constant temperature hot wire
26:30anemometer is very very low
26:32okay and that gives a major boost again
26:34because the cost of the sort wanama
26:36meter if you buy from any kind of
26:39commercial vendor like then take and all
26:41the cost is very costly it's not it's
26:43not very easy very cheap technique like
26:45a patate q it will be costly technique
26:47so definitely you don't want a burn out
26:49of the wire and because of that again
26:52constant temperature hot wire no meteor
26:54is being preferred okay and again as i
26:56said that life of the sensor in the
26:59constant temperature hot wire anemometer
27:00is higher not only because of the
27:02burnout possibility is low but also
27:04because the wire is always maintained at
27:07a particular constant temperature so it
27:09need not to go from the thermal shock or
27:12thermal cycle in the constant current
27:14what will happen the temperature of wire
27:16will keep on changing so sometimes it
27:18will reduce sometimes it will increase
27:20so the tire is always having a thermal
27:22cycle of the from the wire or during in
27:24the wire that sometimes it is being
27:26gated heated sometimes the temperature
27:28is going down so that will again what it
27:30will do it will reduce the life of your
27:33wire and that is the region that most of
27:35the places constant temperature hot wire
27:38no meter are used but again it doesn't
27:40mean that you cannot use constant
27:42current you can do that constant current
27:44also but the constant temperature is
27:46being preferred because the life of the
27:48constant temperature wires are much
27:51higher compared to the constant current
27:53probes or constant current wires okay
27:55and then the calibration is always an
27:57edge so this is the major advantage that
28:00why the constant temperature hot wire no
28:02meteors are used now this hot wire no
28:05meters though we can buy from a
28:07commercial vendor but ideally speaking
28:09you can also prepare
28:10the hot wire anemometer at your place
28:12and is very simple to make you just need
28:14a wire made of tungsten or platinum so
28:16both of this kind of most of this wires
28:19used in the hot wire anemometer is
28:21either made of congestion or made of
28:22platinum some of sometimes they are
28:25mixed materials also so you need just a
28:27tungsten wire or platinum wire you have
28:29to maintain a large L by D ratio as I
28:32said that the length is in the order of
28:351 to 5 mm and Daiya is in the order of 5
28:41micrometer so if you see that L by D
28:43ratio is going in the range of thousand
28:46if I take 5 mm and 5 micron so in the
28:49range of thousand we keep a very large
28:52Jewish L by D ratio now why we keep a
28:55very large L by D ratio is important
28:57because if you remember the picture and
28:59I will take you back little bit that if
29:02you remember this picture whichever I
29:03have shown that how it will look like
29:06they will be a branch or support which
29:08will be held this support so I will tell
29:10it as a branch ok this is actually the
29:13support which is holding the wire and
29:17then this is the wire which is being
29:20used for all the measurement so what
29:22will happen if your wire diameter will
29:24be high then conduction losses from the
29:27wire to this support will be higher okay
29:30are you getting my point so let me
29:32explain again suppose this is the
29:34support what I am talking about and this
29:36is a branch branch in between that there
29:44is a wire knife suppose the wire
29:47thickness is very very small then what
29:49will happen because this Wireless is
29:51being heated there will be conduction
29:53losses from this wire to dis supports
29:55also okay now if the wire thickness is
29:58very very small the contact area will be
30:00very very small and in that case you can
30:02say that the losses because of this
30:05conduction is negligible compared to the
30:08loss because of the fluid motion which
30:09is the fluid motion is being caused by
30:11convective heat transfer loss but if
30:14your wire thickness is very big suppose
30:16now you have increased the wire
30:17thickness something like this this is
30:19now your current wire thickness then
30:21what will happen
30:22your contact area is very big so the
30:25convective with conductive losses will
30:26also be higher and you cannot neglect
30:28that will not be in a position to
30:30neglect that so your calculation will be
30:32much difficult we will see the
30:34mathematics part we see that how you do
30:36the measurement but your calibration
30:38will be very very typical and we'll show
30:40you that why it will be typical you have
30:41to take additional term into the account
30:43that will be the conductive loss to the
30:46support or to the crunch okay so that is
30:49the way it has been there so that's why
30:51the L by D ratio should be higher okay
30:54why the L need to be higher because we
30:56know that the resistivity of the wire
30:59depends on the length and the area both
31:01so if you increase the area resistivity
31:03will increase if you increase the length
31:04resistivity you will also increase you
31:06need certain resistance so that there
31:08will be a particular temperature will be
31:09maintained either it will be burned out
31:11even at a small current past I square R
31:13valance if this is R is there you know
31:15the resistance will be very low it will
31:16burn out immediately so you need certain
31:19length so that you can have maintain
31:22certain resistivity so that's why you
31:24are not doing that with increasing
31:25diameter but you are doing that by
31:27increasing the length of this wire so
31:29that is the reason that why the L by D
31:31ratio should be very high in such a kind
31:34of a probe to measure the conductive
31:37losses to the support okay
31:39then again the smaller diameter will
31:42increase your response time why because
31:44if the diameter is small if you suspend
31:47it in the fluid the fluid the
31:49temperature of this world will suddenly
31:51get uniform very fast it will get
31:52uniform because there is no radial
31:54variation and the whole length is being
31:56dipped in the flute so the length wise
31:58the temperature will be the same and
32:00there is no diameter there is no radial
32:02temperature gradient so it will maintain
32:04it will show the temperature difference
32:05very fast if you suppose have a very big
32:08wire in this way then what will happen
32:10if you if you suspend in the fluid it
32:12will take some time before the
32:14temperature becomes uniform and constant
32:16everywhere okay so we take some time so
32:18what will happen till it will not take
32:20that time what you will see you will
32:22keep on seeing the change in the
32:23temperature and change in the resistance
32:25whatever the mode you operate whether
32:27the constant temperature or the constant
32:29current in both the places you will see
32:30the problem so what will happen it will
32:33reduce this temporal response
32:35why because you have to wait till it is
32:38not coming to a constant temperature so
32:40the smaller diameter actually increases
32:42the temperature response and the
32:43temperature in spawns of this technique
32:45is very high you will discuss that is
32:47very very high because you are using a
32:49very thin wire further thing it means
32:52maximizes your spatial resolution now
32:55hot wire manometry like a pitot tube
32:58also gives a point measurement so what
33:01does it mean because there is only one
33:02wire and which is being exposed so this
33:06is my hot wire in 1 meter okay
33:09sorry so this is being exposed to the
33:12flu no this is being it's both this is
33:16the blown branch but this is being
33:19exposed to the flu so what will happen
33:21it will major the velocity at this
33:22location only it cannot mean give you
33:24the velocity distribution or radial
33:26velocity distribution it cannot give you
33:28the velocity everywhere in all the flow
33:30field suppose if you put inside the
33:32column it can give you the velocity at a
33:34particular location only it cannot give
33:35you the velocity at all the locations so
33:37what will the possible they if you want
33:40to measure the velocity at all the
33:41location you have to keep the wire at
33:43all the possible locations if initially
33:44put it here then you put at this
33:46position then you put at this position
33:48this position this position now what is
33:50the specialization how close you can
33:52measure the velocity is smaller the
33:54thickness of the wire a smaller will be
33:57that distance okay and maximum will be
33:59the spatial resolution say if I use a
34:01wire of thickness 1 centimeter the next
34:04point will be minimum after the 1
34:07centimeters so suppose if I take the
34:09wire of 1 centimeter of diameter okay
34:13the next point will mean even if I keep
34:15it just next to it it will be like this
34:17if it will be like this this will be 1
34:20centimeter so centre to centre distance
34:21also remain 1 centimeters so what will
34:23happen your spatial resolution will be
34:26limited to 1 centimeter
34:27okay clear so the smaller wire thickness
34:31actually maximizes your spatial
34:33resolution it minimizes your noise okay
34:36it reduces your noise and improves your
34:38signal-to-noise ratio okay it also
34:41includes reduces and this removes
34:43critical the intrusive nature of the
34:46probe because you are now ready
34:47in the diameter so the effect will be
34:50much lower compared to the bigger wire
34:53effect so suppose if you are putting a 1
34:55to 5 micrometer your diameter wire there
34:59will be some change in the probe or
35:00velocity field I am NOT saying there
35:01will be no change there will be some
35:03change but that change will be much
35:05lower compared to if you intrude a probe
35:07which is of 1 centimeter in diameter
35:09okay so that is the major region and how
35:12the wire dimension should be taken place
35:14and why it has been taken place in this
35:16place and what should be the material
35:17definitely material should be very
35:19conductive and it should have certain
35:21resistance to V and the temperature so
35:23thermal resistance as well as the
35:25electrical resistance all the materials
35:27should be very good and it should be
35:28conductive very this would be very very
35:30conductive so that's why in the
35:32conduction and platinum are generally
35:34used for the wire material now coming
35:37back to the measurement principle
35:39whatever we have discussed now we can
35:40see that how exactly the things happen
35:43so what I have said till now that
35:45suppose there is a hot wire no medium is
35:48there is a wire and that wire is being
35:53suspended in the flute and we maintain
35:57that this wire is maintained at a
35:59temperature TW resistance is our W say
36:02area is yes that is the wire
36:05okay so TW will write it anyway so now
36:08if you spend in the fluid what will
36:10happen there will be heat transfer
36:11taking place because wire is maintained
36:12at a certain temperature
36:13fluid is maintained at a certain
36:15temperature so because of that
36:17temperature gradient there will be some
36:18heat loss now what will be the heat loss
36:20or total heat loss say if I write it in
36:22terms of the Q so that total heat loss I
36:25will say the Q T will be what it will be
36:27because of Q natural convection I will
36:32write it as natural convection that will
36:36also take place plus Q force convection
36:47Plus Q of our radiation radiation to the
36:59surroundings plus Q see conduction and
37:04we'll say that conduction to support or
37:12branch
37:15so that is the way the heat transfer
37:17losses will take place so what will
37:20happen the fluid will be there it will
37:22be taking the heat through the natural
37:23convection it can take the heat from the
37:25forced convection they can be radiation
37:27lossless okay and there will be some
37:29conduction losses now as I said that if
37:32the diameter of this wire is very very
37:34small the conduction losses to the
37:36branch can be neglected anyway first
37:38let's see that how to mathematically
37:39write it so for natural convection what
37:42we know we know that there are several
37:44equations available we can use that for
37:46the first convection the equation is
37:48generally we use Newton's law of cooling
37:49so H the heat transfer coefficient into
37:52a s okay into TW minus TS that will be
37:59the natural convection Hatton's heat
38:01transfer coefficient
38:09siient es is surface area area of wire
38:19TW is wire temperature richer and TF is
38:28loot temperature low temperature okay
38:36now the radiation we know that Stefan
38:39Boltzmann equation we can use it will be
38:40a s star Sigma star epsilon mr. t w-4
38:47minus T a or t f4 okay so we can do that
38:54emissivity okay it's the
38:55stefan-boltzmann constant Sigma surface
38:57area TW and TS and then the conduction
39:00losses can be written QC can be written
39:03as minus K into es okay into DT upon DX
39:11that'll be the conduction losses okay
39:13clip so we can have this equation and
39:17ideally if you want the total heat loss
39:19will be because of this now we know that
39:21if the wire temperature is very very
39:23small okay or is relatively smaller not
39:25very very small then I can neglect with
39:29kind of QR value and it will not
39:31generate much error in my measurements
39:33okay and it will simplify my
39:35calculations so I can neglect the QR
39:37value if the temperature difference is
39:39very very low it's not very high
39:41similarly if your wire is very thin okay
39:44you can say that the convective losses
39:46conductive losses to the branch or to
39:49the support can also be neglected if the
39:53velocity is there if the fluid is moving
39:54with the velocity and sufficient
39:56velocity the natural convection part can
39:58also be neglected so what is going to
40:00happen the heat loss which is going to
40:02take place from this wire will be purely
40:05because of the convective heat transfer
40:06okay so this Q T will be equal to
40:10actually q FC and that will be equal to
40:13H into S into TW minus TS that will be
40:20your total heat loss
40:22okay which will be taking place from the
40:24wire now we know that how the wire
40:27temperature will change our resistance
40:29will change with the this heat loss for
40:32that we know that equation we know the
40:34correlation between the wire resistance
40:37and the temperature and that is being
40:39given say our W or of reference say a
40:44reference temperature or reference
40:45resistance which was there earlier one
40:48plus or constant alpha it will be T of W
40:53T of flute okay minus T of reference
41:00okay so that is the way it has been
41:03maintained that the way it will be there
41:07okay so in this way it will be
41:11maintained so our W will be what it is
41:13the wire resistance RF is reference
41:21resistance TS is the surface temperature
41:29of the wire
41:37better if we write the same term as a TW
41:40instead of CS let's not kind of simplify
41:43this equation so we can say TW and T
41:46reference is the reference temperature
41:51temperature at reference in the distance
41:59okay so that is the way we know that how
42:03the RW is changing okay now we can use
42:06that equation QT is H a TW minus T of
42:15Pluto this is a of surface yes now this
42:19is the convective heat transfer now we
42:21know that from the Kings law that heat
42:24heat efficient heat transfer coefficient
42:26is also a function of velocity and how
42:29it has been related to the velocity H is
42:32being related to a plus B into Z where V
42:36is the velocity raised to the power C
42:40and generally this value of C is equal
42:43to 0.5 so you can say that it will be a
42:46plus B into V raised to the power 0.5 H
42:50what we can do we can replace the H
42:53value here in the Qt by this equation so
42:56your equation will be modified and your
42:58equation will be instead of H you will
43:00get a plus B V raised to the power 0.5
43:05into Ayers into TW minus TS okay and
43:10that will be equal to Qt ya now this QT
43:16the heat transfer will be take place how
43:19much heat transfer will be take place it
43:21will be equivalent to how much power
43:23supplied to the wire so that is the heat
43:27transfer which will be taking place or
43:28the kind of overall heat transfer will
43:30be there so whatever the voltage drop or
43:32the power will be given to the wire that
43:35will be equal to the convective heat
43:36transfer loss once the equilibrium will
43:38be achieved at this condition okay so it
43:40means QT the power to the resistance
43:42applied to the wire will be exactly what
43:45you can write it in terms of the I
43:48I square RW which is nothing but the
43:51current okay you got my point that the
43:55electrical power input will be equal
43:58okay to the convective losses so
44:01electrical power input to the wire will
44:03be equal to the convective losses based
44:04on this planet Wheelock temperature so
44:07you can write it I square RW will be
44:09equal to H a TW minus TF as I have
44:13converted in terms of the Kings law so
44:15you will get it it in this value okay so
44:18you can write it here I square R W will
44:24be equal to a plus V into V raised to
44:28the power 0.5 into S into TW minus T of
44:34that is the way we can write RW again
44:37you can convert in terms of the T and T
44:41F so in terms of the reference so I
44:43square you can say will be equal to a
44:47plus B raised to the power 0.5 okay
44:52in two years that will be in two years
44:58TW minus TS and this RW can be written
45:02as our reference 1 plus alpha it will be
45:08T s or TW minus T so that is the I
45:14square value now it means what it says
45:17it says that your current change is
45:19directly proportional to change of the
45:21fluid temperature and that flue
45:23temperature is measured with the
45:25velocity so if you are measuring the
45:27current change if you are measuring the
45:29flute temperature what you can find you
45:31can easily find that what is your
45:33velocity because you will get that our
45:35value also okay that how your resistance
45:37value will change with this formula
45:39whatever I have done so if you know your
45:41reference temperature you know that how
45:43much temperature you have changed you
45:44reference temperature also you know you
45:46can calculate the velocity so what you
45:48need you need flu temperature you need I
45:50square if you know that you can
45:52calculate this value if you don't know
45:54that what you can do you can convert it
45:56in in terms of the constant temperature
45:58if you maintain the campus
45:59constant ok you can find that how much
46:02current value need to be accounted so
46:04either you can keep this i-square
46:05constant or you can keep this
46:08temperature difference constant okay so
46:10the temperature constant will be there
46:11this would be constant temperature and
46:13you can measure the correlation you can
46:15measure the velocity and the equation
46:17will be simplified and you will get that
46:19what will be the velocity for say
46:21constant temperature if we do the
46:23anemometer finally this you can write I
46:26in terms of the v square so this will be
46:28I square will be v square upon R square
46:30it will come this will be cancelled out
46:32and it will be simply as a Kings law
46:35into ministry four point five here the V
46:40is volt so I will not confuse it I will
46:42write it as e e square V is both okay so
46:49what is the volts applied so is equal to
46:52V upon R okay so you do that or not be
46:56upon us a because sorry I'm just doing
46:58this upon R where is the voltage so you
47:03can major the voltage difference and
47:05that voltage difference can be measured
47:07in terms of the velocity and in most of
47:09the constant temperature anemometer okay
47:12this is constant temperature is mo meter
47:14constant temperature anemometer we can
47:25reduce it into the this form in square
47:27is equal to a + b v2 v is to the power
47:300.5 0.5 we can find it out the a and b
47:33value and we can see that how this e v
47:36and z is being correlated and what you
47:38need to do that you have to do the
47:40calibration okay if you do the
47:42calibration you will find that how the e
47:44NV is correlated in the constant
47:47temperature in meteor you do the
47:48experiments and maintain the temperature
47:50constant by changing the voltage it
47:52means you have to change the kind of
47:54voltage or current in any way whatever
47:56you say you measure the whole difference
47:58at the outlet and you will find that how
48:00the velocity will be what will be the
48:02velocity so what we do we do the
48:04calibration and in the calibration what
48:06we do in the calibration part the
48:12suspend the probe in a domain say this
48:15is the flute fleet okay or and I suspend
48:19the wire here I suspend the wire so this
48:23is the probe I suspended it here
48:25somewhere here in this way the branch
48:29okay I suspend the wire and I major that
48:33velocity now how I measure the velocity
48:35what I do I put a patate you and that is
48:38the way the calibration is being done in
48:40hot wire no media so I put up it or tube
48:42and that we taught you means being
48:44connected to a man o meter you measure
48:52the H we know that velocity in this pit
48:54or tube is going to be under root 2 into
48:56G H or you can say rupee my P into Rho P
49:00minus Rho and P I have just converted in
49:02terms of the manometer ad so it will
49:03come into under to Rho gh ok so that is
49:06the way you can calculate the velocity
49:08and you can record that volt that what
49:11is the whole difference so you put a
49:12circuit your voltmeter is there you can
49:15see that what is the Volt is recorded so
49:17what you can do you can put a plot
49:20between a square and V raise to the
49:23power 0.5 if you pull that a square
49:26whatever the formula of the applet is a
49:28plus B into Z raised to power 0.5 if I
49:32pot plot a graph between Eastburn 3005
49:34what I will get I will get a straight
49:36line which will have a intercept the
49:39value of intercept will be a and the
49:41slope of line will be B so you will get
49:44this calibration parameter you will get
49:46the value of a you will get the value of
49:48B and then what you can do you can
49:50calculate that what will be your
49:51velocity okay
49:53now in the real experiments here what
49:54you don't want knowing you are knowing
49:56that at this velocity what will be the
49:57whole difference in the real experiments
50:00you will be measuring the whole
50:01difference and you will be calculating
50:02the velocity so you will get that what
50:05is the velocity of the flute and not
50:07only the velocity now what will happen
50:09with the change in the current if there
50:11is a turbulent flow and that is the
50:13region white is being used so fast
50:14because the temporal response of the
50:17electrical signals are very very high it
50:19can go up to 1 megahertz it means you
50:21can acquire at tens power minus six
50:24seconds did
50:25equation time delta T so what will
50:27happen if you do that you will see that
50:29you can find this slope you can acquire
50:33the data at a very high speed and if the
50:35flow is turbulent
50:36if the fluctuations level is very high
50:37you can acquire at a very high frequency
50:39and you can not only get the velocity
50:41but you can also get the fluctuations
50:43that how the fluctuation is taking place
50:45so with East fluctuation the temperature
50:47the kind of will try to be modified you
50:50will keep the temperature same current
50:52will be modified a volt will be modified
50:54and you will see the volts luxation
50:55reading and this voltage in reading it
50:58it in this way with the time and this
51:00volt fluctuation or reverses T you can
51:02get this can be calibrated with the
51:04velocity that how the velocity will be
51:05there you can calculate the mean with
51:07the time average mean value you can find
51:09it out what will be the fluctuating
51:11component of the velocity in this case
51:13okay so you can do all this with the hot
51:16wire no meter and that's why this
51:18technique is very very famous and being
51:20widely used okay now I would like to
51:23also do the comparison between hot wire
51:25anemometer and pitot tube because we are
51:27calibrating the hot pan medium with the
51:29pitot tube you should not confuse that
51:31it is only as good as P tot Q so what is
51:35the comparison so the major competition
51:37major advantage that the pitot tube
51:39because the response time is very low
51:41the manometer will take time before it
51:43will get to stabilize it can measure
51:45only the time averaged mean velocity
51:47while hot wire anemometer the frequency
51:50response is very high so what you can
51:53measure you can measure the fluctuation
51:55velocity you can major the real
51:56real-time fluctuation velocity you can
51:58measure the mean velocity for sure okay
52:01the advantage of the pitot tube and why
52:03it is being used also is that there is
52:05no calibration required you are using P
52:07tot you have to calibrate hardware you
52:09meet you
52:09so no calibration is required definitely
52:13the cost is much much lower than
52:14whatever you are using in the hot 100
52:16meter so the cost factor is also coming
52:19into the picture it's very cheap
52:20compared to the hot 100 meter okay
52:22the major thing is that the hot wire
52:25anemometer calibration is very very
52:27tickly
52:27very very typical critical and it is not
52:30a very easy job it is a time consuming
52:32job you have to do the calibration for
52:35sufficient long
52:36I mean the denim only you will get that
52:38and as I said accuracy of your hot wire
52:41anemometer measurement will depend on
52:43the accuracy of your calibration curve
52:44so need to prepare the calibration curve
52:47properly and the calibration in Hwa is
52:50performed using the pitot tube to obtain
52:52the correlation coefficients okay to
52:55measure the speed at the outlet voltage
52:57so you can find that correlation only by
53:00using the pitot tube so that is the way
53:02so how do I know meter the major
53:04advantage is your frequency response
53:06time is very very high it means it's a
53:08temporal it's very high ideally speaking
53:11you can achieve very high spatial
53:13resolution if you use an array of hot
53:16wire anemometer probe okay or you keep
53:19the probes at several locations you can
53:20ideally speaking can use achieve a very
53:22high spatial resolution too and it can
53:25give you not only the mean velocity it
53:27can also give you the fluctuating
53:28velocity and real time fluctuation
53:30velocities so that is the major
53:32advantage of the hot wire anemometer
53:34over the conventional pitot tube then
53:37the same approach because it was being
53:40used very widely and in 1942 248 this
53:43development has been done for the single
53:44phase flow and then many people have
53:46used this for the single phase flow and
53:48again I am Telling that most of our
53:50understanding on the turbulence is
53:51generated with the data obtained for the
53:53hot 100 meter now the same concept in
53:57late 1970 okay we use before people have
54:01tried to use for the multi-phase flow so
54:03in the multi phase flow all they were
54:04whatever I have said is remain same what
54:07you can do you can use single wire you
54:08can use X wire you can use triple split
54:10wire depending upon what kind of a
54:12velocity coefficient you have definitely
54:15the single wire has a very limited use
54:16because most of the multi-phase flow is
54:18having at least two dimensional velocity
54:20it's not three okay even if you assume
54:23the theta directional symmetry you will
54:25have the three dimensional velocity or
54:27at least two dimensional velocity so you
54:29need to use either triplet or ax fire
54:31most of the time this triplet
54:33triple split wire is being used in the
54:35multi-phase flow then there is
54:37additional complexity whatever the
54:38equation I have solved it is solved for
54:40only one phase because each phase we
54:42have not accounted that heat capacity of
54:45that phase now because water and
54:48have a different heat capacity values
54:50what is going to happen that the your
54:53heat transfer will be different in case
54:55of the water or a temperature difference
54:57will be different in case of the water
54:59is touching the wire and in case of air
55:02is touching the wire if suppose you are
55:03operating a constant temperature so if
55:06you are using a constant temperature
55:07anemometer then the current requirement
55:10to maintain the constant temperature was
55:12the air comes into the contact and once
55:14the water comes into the contact will be
55:16different even if they are moving with
55:17the same velocity so what you need to do
55:19you have to do the double labor
55:21calibration one the calibration will be
55:23only with the water to see that what
55:25will be the Volta
55:26once the calibration will be only with
55:28the air and you have to see the what's
55:29the Volt level and you will able to
55:31measure the velocity of both the phases
55:33only and only if the Volt level readings
55:36are different for both the phases like
55:38it has been done by Davis in 1972 they
55:41have done the experiment with air and
55:43water and they have found that with the
55:45air and water with the hot pan of
55:46amateur the Volt labelled readings are
55:48different air gives a certain volt of
55:50this range and whenever it water comes
55:52the water volt rating is this so now you
55:54have the reading volt reading for each
55:56volt reading for each page you have to
55:58do the calibration you will find that
56:00velocity even if you want you can also
56:02find the volume fraction in the same
56:03paper they have tried to show that that
56:05each curve we will show that how much is
56:08the contact time it will show that how
56:10much fraction of the time that phase was
56:12staying at that time you are measuring
56:14that funk fraction the area under the
56:16curve with the time that will also give
56:18you that what will be the fraction of
56:20that phase okay
56:21but whatever we are interested in mostly
56:22in the hot wire anemometer is the
56:25velocity measurement that you can easily
56:26do here okay the only thing is it will
56:29be typical it will not be that easy why
56:32because you will have the multiple
56:33components here okay now the second
56:37thing which is important is that how to
56:39major the velocity or two dimensional
56:41velocity particularly so what you can do
56:44if suppose I use a X wire I will just so
56:48one example
56:49Triplette will be the same you can try
56:51to do that so suppose if I am using the
56:53at wire okay and there is a velocity in
56:55this direction okay let's do it with the
56:59one wire which is being inclined or
57:01now this is say velocity this is the you
57:05are going to get it it in this way okay
57:07now this say is you and this is being
57:11converted incog of you this okay and the
57:15other thing is converted in terms of the
57:17you this okay so this is the way it has
57:22been converted this is you effective you
57:24are doing it in this way that this is
57:26say you effective this is your U of X
57:29direction this is U of Y direction okay
57:32and this is your angle which it is
57:34making with the wire okay this is your
57:36theta say this is Theta this will be 90
57:39plus theta okay 90 degree so what will
57:42happen you effective you can convert in
57:45terms of this so you effective will be
57:47what you say this will be equal to u X
57:51okay this will be cos 45 plus uy into
58:00cos 90 plus 45 90 plus theta sorry this
58:07will be theta this will be treated cos
58:09theta plus 90 plus theta okay
58:11now this will be what you X minus uy cos
58:16theta so if I know the angle between
58:19these two is what is the angle of this
58:21slope then I can find it out that what
58:23will be the you effective okay most of
58:26the time this angle is kept 45 degree
58:29okay so this angle is most of the time
58:31disc a 45 degree okay not centigrade 45
58:34degree so what will happen the COS theta
58:37will be cos 45 so you can say UX minus
58:39uy cos 45 and cos 45 value is 1 upon
58:44under root 2 so you can say UX minus uy
58:471 upon under root 2 okay for the probe
58:51which is coming at it in this direction
58:52it means the probe is it placed it it in
58:55this direction and the fluid is coming
58:56from so probe is in this direction and
58:58fluid is coming from this side okay now
59:01if the probe is in the opposite
59:02direction okay if the probe is in the
59:04opposite direction so in this way and
59:06the fluid is again coming it it in this
59:09way in this direction what will happen
59:10again
59:11can do that okay that say this is the
59:13flow direction is coming this is 45 and
59:16this is the unaided direction so what
59:18will happen now it will be theta so it
59:21will be say this is your u effective
59:23this is your u X this is your u Y so
59:27what will happen this will be 45 it will
59:30be cos u effective u X will be equal to
59:32what au effective will be UX cost 45 uy
59:35cos 45 and you will get that values and
59:38that value will be equal to that what is
59:42your value so this will be u X cos 45
59:44plus uy cos fortifies and this will be u
59:47effective will be equal to your UX plus
59:51uy 1 upon under root 2 okay I hope you
59:54got that point that in one case what we
59:57have done we have seen that because this
59:59is the way this wire is there so if the
1:00:01flue flow is coming at it in this way
1:00:03this will be the angle so that angle
1:00:04will be 4 it will be 90 degrees for the
1:00:07UX and then the effective will be more
1:00:09than that theta in this case once the
1:00:11wire will be on this side and the fluid
1:00:13is coming it it in this way it will be
1:00:14been in the teacher so that value will
1:00:16be within the theta so it will be 45
1:00:18degree here okay if the wire angle is 45
1:00:20this will be also at u theta will be at
1:00:2245 degree so it will be u X cos 45 u Y
1:00:26sine cos 45 and you will get that UX
1:00:29plus UI 1 upon under root 2 so you get
1:00:33the you effective values we know that
1:00:35for each wire is say I will say even a
1:00:38square will be a 1 plus B 1 into V
1:00:42raised to the power 0.5 okay and V is
1:00:44nothing but is the effective or you
1:00:46effective now you effective can be
1:00:48written for depending upon eetu will be
1:00:51e 2 plus b 2 into u effective 0.5 this
1:00:57also that consuming you effective okay
1:01:000.5 now this u effective can be written
1:01:03based on that which wire we are talking
1:01:05about the upward wire or downward side
1:01:07wire it means plus 35 or minus 45 wires
1:01:09we can have the values we can have you
1:01:12place the you effective from this place
1:01:13say this will go here let's say this is
1:01:15going here we will replace that we will
1:01:18do the calibration one for the dis wire
1:01:20and enter for this wire you will get the
1:01:22constant so you
1:01:23now to calibration curve that's why it
1:01:24will becomes typical interpolate we have
1:01:27to do it three times okay so you can do
1:01:29that and you can have different wires to
1:01:31measure all the direction velocity in
1:01:33the basic principle again remains same
1:01:35okay so now concluding this part of hot
1:01:38wire anemometer what I will say that
1:01:40advantage of the hot wire anemometer it
1:01:43has the major advantage there is a very
1:01:45good frequency response it means the
1:01:48temporal response is very very high but
1:01:51that is the major advantage it can
1:01:53measure the velocity and the velocity
1:01:55fluctuation over a wide range of
1:01:57distribution not in a small range so not
1:01:59very worried about that what is the
1:02:01velocity you are worried about the lower
1:02:03velocity we will see why you can use it
1:02:05for temperature measurement also
1:02:07actually because you need the reference
1:02:08temperature you can calculate the
1:02:10turbulent quantities because you are
1:02:12using the fluctuation velocity you can
1:02:13calculate the turbulence in turbulent
1:02:15quantities like what is dissipation rate
1:02:17etc all the turbulence quantity you can
1:02:19calculate okay it will scan measurement
1:02:22the containing the continuous turbulence
1:02:24flow and it can be used even for the
1:02:27bubbles like in the patottie of the
1:02:28problem as we discussed that if you use
1:02:30for the bubble flow bubble can burst and
1:02:32then the local pressure difference will
1:02:34be very high so in case of multi-phase
1:02:35flow the pitot tube applications are
1:02:37lower compared to multi-phase flow
1:02:40because if you have a distributed bubble
1:02:42you can simply use this okay the way I
1:02:45have shown the example but it doesn't
1:02:47mean that it has all the advantage it
1:02:49has several drawbacks and that's why the
1:02:52use is limited and advanced techniques
1:02:53more advanced techniques are needed and
1:02:55being invented the major it want
1:02:58disadvantages its intrusive in nature so
1:03:01even if you are minimizing the size of
1:03:04the wire we are not able to minimize the
1:03:06size of the probe so sign up the probe
1:03:08or support of tranche is actually little
1:03:11bit bigger and that actually create the
1:03:14invasive nature a lot you cannot
1:03:16minimize that actually and that's why
1:03:18the point the flow can change at the
1:03:20point of measurement itself okay then
1:03:23the second problem is you are using a
1:03:24wire and if you are using it in a fluid
1:03:26which is contaminated or having
1:03:28deposition can this tendency or it has
1:03:31been already contaminated with
1:03:32subsolid or something some chemicals
1:03:34what will happen the deposition will
1:03:36take place on the wire the moment you
1:03:37will expose it to a surface the moment
1:03:39the deposition will take place what will
1:03:41happen your resistance will change your
1:03:43overall calibration will also get
1:03:45modified so that is the major problem
1:03:48that in case of impure environment when
1:03:50deposition tendency is there you can not
1:03:52use it the probe can break in the solid
1:03:55environment if you put it in the solid
1:03:56it will immediately break can also be
1:03:59break due to the burnout which will be
1:04:01because of very low temperature with
1:04:03very low velocity or very high
1:04:04temperature why the low velocity because
1:04:06the low velocity will not able to remove
1:04:08the heat properly so that the burnout is
1:04:10possible you can use array of multiple
1:04:13detector use multiple enemy no meter is
1:04:15used hot wire in one meter to find the
1:04:17radial distribution of the velocity so
1:04:19overall the cost if you want to find the
1:04:20radial distribution will be very high
1:04:22the calibration is very valid with a
1:04:24certain velocity range if you go outside
1:04:26of that velocity range you will not able
1:04:28to use that you have to actually
1:04:29calibrate it again okay in the flow of
1:04:31speed is too low assuming the force
1:04:34convection is not valid you cannot use
1:04:36the equations which we have developed if
1:04:38there is a natural convection is also
1:04:40taking place if the velocity is too low
1:04:41you have being clued that and then the
1:04:43process will not be very simple okay the
1:04:45only the can this assumption of force
1:04:48convection will not be true okay and the
1:04:50major disadvantage is this that if you
1:04:53have a reverse flow somewhere in between
1:04:55then your hot wire the meter will still
1:04:58measure the velocity it is not going to
1:05:00tell you anything about the direction so
1:05:02you will get the speed but you will not
1:05:04get the direction why because if suppose
1:05:05the probe is being suspended whether the
1:05:08fluid is flowing from this side or it is
1:05:10coming from this side somewhere the heat
1:05:12loss will be the same so it will not
1:05:13able to sense that whether it is fluid
1:05:15is moving from top to the bottom or
1:05:17bottom to the top so it will not able to
1:05:20sense that so reverse flow condition if
1:05:22you are using that you will only get the
1:05:23speed not the velocity will not get the
1:05:25direction and that is the major
1:05:27limitation in the multi phase flow
1:05:28because of the bubbles or discrete phase
1:05:31presence the flow can reverse at any
1:05:33moment okay so that is the major
1:05:35drawback and that's why the more
1:05:37advanced technique is being invented and
1:05:38which will be discussed later thank you
1:05:42[Music]
1:05:59you

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