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Lecture 16 - Enhancement of Heat Transfer compact Heat Exchangers
WEBVTT Kind: captions Language: en
00:00:14.880 --> 00:00:22.520 Hello everyone. So, we are again back to the course Heat Exchangers Fundamentals and Design 00:00:22.520 --> 00:00:29.360 analysis. Now if you recall ah we have given some introduction and after that we have tried 00:00:29.360 --> 00:00:37.270 to have a ah have an overview of different type of heat exchangers we are having, and 00:00:37.270 --> 00:00:44.160 I you should appreciate that this is a very wide variety. And in that variety we have 00:00:44.160 --> 00:00:51.140 got different classification of heat exchangers and one of the classification is based on 00:00:51.140 --> 00:00:57.390 the compactness of the heat exchangers. So, heat exchangers can be ah divided into 00:00:57.390 --> 00:01:04.680 two groups compact heat exchangers and non compact heat exchangers based on surface area 00:01:04.680 --> 00:01:10.829 per unit volume of the heat exchanger. Some details I will ah or any of these factors 00:01:10.829 --> 00:01:16.360 we will try to provide as the course proceeds, but this is one of the important classification 00:01:16.360 --> 00:01:27.149 of heat exchanger and then then ah we have also seen that the compactness or the ah threshold 00:01:27.149 --> 00:01:34.220 of compactness based on some magnitude of ah amount of surface area per unit volume 00:01:34.220 --> 00:01:41.399 of the heat exchanger, that varies whether depending on whether our fluid is a gas or 00:01:41.399 --> 00:01:48.229 a liquid. What we will do? We will ah explain some principles 00:01:48.229 --> 00:01:55.969 of ah of mentation heat transfer and then some typical ah method of getting compact 00:01:55.969 --> 00:02:04.959 heat exchanger ah and design and analysis, that is getting some sort of ah compactness 00:02:04.959 --> 00:02:12.160 in the heat exchanger with the use of extended surfaces or fields that we will try to describe 00:02:12.160 --> 00:02:16.540 in few lectures to come. So, today is the beginning this lecture is 00:02:16.540 --> 00:02:25.260 the beginning of that. At the beginning we like to have an overview of ah what is what 00:02:25.260 --> 00:02:29.980 are the different methods of augmentation of heat transfer. 00:02:29.980 --> 00:02:38.110 So, before that let us see what are the desirable features of a good heat exchanger. The there 00:02:38.110 --> 00:02:45.060 are number of points, I do not claim that the the points I have mentioned here are ah 00:02:45.060 --> 00:02:52.240 kind of ah flow. So, there could be some other attributes or some other quality, but definitely 00:02:52.240 --> 00:03:01.040 I have ah jot it down the most important features most important ah features we we expect from 00:03:01.040 --> 00:03:05.210 a good heat exchanger design like high heat duty. 00:03:05.210 --> 00:03:10.800 Heat exchanger should have a high heat duty of course, it should be qualified that for 00:03:10.800 --> 00:03:18.000 a given volume ah or given envelop say of a heat exchanger, it should have the highest 00:03:18.000 --> 00:03:27.760 heat heat duty ah as far as ah possible as far as practically by the ah technology available. 00:03:27.760 --> 00:03:34.320 Then low temperature difference. Basically we are transferring heat between two streams, 00:03:34.320 --> 00:03:41.230 and the potential for this transfer process is the temperature difference of the two streams. 00:03:41.230 --> 00:03:46.190 So, it is obvious that more the temperature difference more will be the heat transfer, 00:03:46.190 --> 00:03:52.000 but can we have very large heat transfer with a small temperature difference. So, that is 00:03:52.000 --> 00:03:56.750 the goal or that is the desirable feature of the heat exchanger. 00:03:56.750 --> 00:04:05.500 Then compact size and less weight. Compact size and less weight it is I mean always it 00:04:05.500 --> 00:04:13.880 is some ah goal of any engineering design, ah because you see ah because you see that 00:04:13.880 --> 00:04:21.269 we have got restriction of this, we have got cost of material, we have also the restriction 00:04:21.269 --> 00:04:27.020 for weight. Particularly if a system is a mobile system we have got different implication 00:04:27.020 --> 00:04:34.800 if we go overweight ah. So, for all these things compactness is one point; apart apart 00:04:34.800 --> 00:04:39.200 from that what we want to have? We want to have less pressure drop. 00:04:39.200 --> 00:04:44.830 Generally what happens pressure drop depends on many parameters, one of the ah most important 00:04:44.830 --> 00:04:51.640 parameter is the fluid velocity, but along with this if the fluid within the heat exchanger 00:04:51.640 --> 00:04:57.600 has to pass a very long pathway, then there will be high pressure drop. So, we have to 00:04:57.600 --> 00:05:04.030 also see that the pressure drop should be low and up to these I have given different 00:05:04.030 --> 00:05:10.280 color because at the beginning or somewhere in the course we are going on mentioning, 00:05:10.280 --> 00:05:16.460 that we you will look into the thermal hydraulic aspects of heat exchanger may not be the other 00:05:16.460 --> 00:05:22.260 design aspects, other ah manufacturability aspect. So, these four points are very important 00:05:22.260 --> 00:05:28.340 for us. But on totality for an exchange for an engineer 00:05:28.340 --> 00:05:34.240 who is designing heat exchanger or ah working on heat exchanger, see other points are also 00:05:34.240 --> 00:05:40.070 important these points could be less prone to fouling ease of cleaning and maintenance, 00:05:40.070 --> 00:05:47.270 that is the serviceability of the heat exchanger and over and above for any engineering system 00:05:47.270 --> 00:05:55.310 for any manmade system cost is a prime factor. So, whatever we design our economy should 00:05:55.310 --> 00:05:59.389 allow that. So, we should have the lowest cost of the heat exchanger. 00:05:59.389 --> 00:06:05.760 So, four points the for ah um point at ah mentioned at the top, though those are very 00:06:05.760 --> 00:06:11.120 important as far as the thermal hydraulics of heat exchanger is concerned. So, we have 00:06:11.120 --> 00:06:17.580 to have high duty high heat duty, we we ah desire to have a low temperature difference 00:06:17.580 --> 00:06:23.699 between the two streams we; obviously, wants to have our heat exchanger to be small and 00:06:23.699 --> 00:06:28.830 less weight light weight and there should be less pressure drop. 00:06:28.830 --> 00:06:38.820 So, with these four points let us proceed. So, how we can have it? So, basically we can 00:06:38.820 --> 00:06:45.760 have this four points we have told out of that at least the first three points; that 00:06:45.760 --> 00:06:55.050 means, high heat duty, ah low temperature difference and ah lesser lesser ah volume 00:06:55.050 --> 00:07:01.920 or weigh of the heat exchanger that we can have with a very good ah or very effective 00:07:01.920 --> 00:07:09.610 way of heat transfer. And we can also argue that if our heat transfer ah process is very 00:07:09.610 --> 00:07:15.380 effective ah very good very efficient, then also we land up with a design where there 00:07:15.380 --> 00:07:22.570 will not be excessive pressure drop. So, this comes that whatever normal heat heat transfer 00:07:22.570 --> 00:07:28.860 is there, we will try to enhance the rate of heat transfer or augment the rate of heat 00:07:28.860 --> 00:07:33.979 transfer. This is very important particularly for the heat exchanger industry, and also 00:07:33.979 --> 00:07:39.020 for other places like electronic component cooling heat sink design etcetera. 00:07:39.020 --> 00:07:45.610 So, augmentation of heat transfer or enhancement of heat transfer is a very special topic and 00:07:45.610 --> 00:07:54.060 very useful branch of heat transfer in general. Now in heat exchanger normally there are three 00:07:54.060 --> 00:08:00.449 modes of heat transfer conduction conviction and radiation, but as we are transferring 00:08:00.449 --> 00:08:08.240 heat in most of the cases from a moving fluid to another fluid stream conviction is the 00:08:08.240 --> 00:08:14.850 main mechanism of heat transfer and if we can improve the improve convictive heat transfer, 00:08:14.850 --> 00:08:16.990 then we have improved the heat transfer process . 00:08:16.990 --> 00:08:24.889 So, first I have written augmentation of heat transfer primarily give saving of material 00:08:24.889 --> 00:08:32.990 and energy whenever we are talking about advance heat exchanger compact heat exchanger; obviously, 00:08:32.990 --> 00:08:38.829 one of the main goal is to reduce the size of the heat exchanger. So, the basic energy 00:08:38.829 --> 00:08:48.330 balance equation the basic energy balance equation what we can get ah ah the basic energy 00:08:48.330 --> 00:08:59.490 balance equation what we can get ah is that let me highlight it. So, this is the basic 00:08:59.490 --> 00:09:02.620 energy balance equation for convictive heat transfer. 00:09:02.620 --> 00:09:13.320 Q dot is equal to hA T w minus Tf. I have shown it ah with the help of a hm ah ah with 00:09:13.320 --> 00:09:17.530 the help of an ah illustration, which is very common to you this is some sort of a solid 00:09:17.530 --> 00:09:24.260 surface on which ah there is a flow of liquid ah sorry flow of a fluid and the solid surface 00:09:24.260 --> 00:09:32.090 is kept at a surface temperature let us say that is equal to ah T T valve and the fluid 00:09:32.090 --> 00:09:38.540 is flowing over ah the surface with a velocity T infinity. So, on the solid surface there 00:09:38.540 --> 00:09:45.100 will be a velocity gradient ah due to formation of boundary layer, there will be a temperature 00:09:45.100 --> 00:09:48.660 gradient due to the formation of thermal boundary area. 00:09:48.660 --> 00:09:54.070 And heat transfer we depend on the difference, that T infinity that is the temptrue of the 00:09:54.070 --> 00:10:00.800 fluid outside the thermal boundary layer and T w is the temperature of the fluid ah T w 00:10:00.800 --> 00:10:09.420 is the temperature of the fluid on the surface. So, Q will be ha T w minus T f. So, you see 00:10:09.420 --> 00:10:19.660 in this equation we want to maximize let us say we want to maximize Q, our T w and T s 00:10:19.660 --> 00:10:27.190 are fixed, then we have got this quantities hA h is the transfer coefficient and A is 00:10:27.190 --> 00:10:31.730 the surface area. So, when we are going to augmentation of heat 00:10:31.730 --> 00:10:37.920 transfer particularly we are having single phase fluid and the mode of heat transfer 00:10:37.920 --> 00:10:45.500 is convection, convection from the ah between the wall and the ah fluid stream then we have 00:10:45.500 --> 00:10:53.870 to concentrate on h A; either separately or as a combination . So, augmentation technique 00:10:53.870 --> 00:11:06.650 aim at modifying h A by improving h or A ah if we go to the next slide ah well ah. 00:11:06.650 --> 00:11:19.410 So, before that let me tell you h A let let me go back to our ah previous slide and sorry 00:11:19.410 --> 00:12:31.740 sorry let me go back to the previous slide ok ah sorry for this small ah problem. So, 00:12:31.740 --> 00:12:38.250 ah what we see that here we are having h A A is the area and h is the heat transfer coefficient. 00:12:38.250 --> 00:12:46.880 So, h depends on ah various thing that depends on the ah course on the fluid property fluid 00:12:46.880 --> 00:12:52.390 velocity and the surface geometry. Particularly the features which available 00:12:52.390 --> 00:12:58.649 on the surface. So, there are many ways of increasing or ah sorry there are many ways 00:12:58.649 --> 00:13:08.910 of enhancing h, and there are ways of ah enhancing a area or A also. There are some ways by which 00:13:08.910 --> 00:13:16.050 the combination can be improved; that means, both h and A can be improved. So, you will 00:13:16.050 --> 00:13:26.040 see some of them ah. So, techniques of heat transfer augmentation ah ah a class of technique, 00:13:26.040 --> 00:13:31.800 that those are known as passive techniques. As the name suggest in this technique we do 00:13:31.800 --> 00:13:39.160 not need any power. ah We have some design modification and with the design modification, 00:13:39.160 --> 00:13:45.020 we can have the ah heat transfer augmentation. Like we can have treated surface, um there 00:13:45.020 --> 00:13:49.720 is some sort of a ah surface treatment. So, that there are microscale features on the 00:13:49.720 --> 00:13:55.800 surface ah ah in case of ah phase change heat transfer there could be the chemical features 00:13:55.800 --> 00:14:01.779 of the surface that could also be different. Surface can be rough end ah there could be 00:14:01.779 --> 00:14:08.930 extended surface. While rough surface ah or the ah treated surface gives feature of very 00:14:08.930 --> 00:14:17.290 small scale, extended surfaces or fields which are as as they are known very ah popularly. 00:14:17.290 --> 00:14:25.600 So, these are large features present on the surface and that increases the surface area. 00:14:25.600 --> 00:14:34.300 Then inserts and swirl devices ah which can be ah put in the ah put e e inside the flow 00:14:34.300 --> 00:14:40.010 passage, then additives for liquids and gases and then rib roughened surface ok. 00:14:40.010 --> 00:14:45.640 So, some of the then let us see we will have a look and we will have some appreciation 00:14:45.640 --> 00:14:54.580 of some of this devices. Ah Before that let us see ah this small illustration, 00:14:54.580 --> 00:14:59.580 here we have a corrugated surface which is an I mean I have shown an exaggerated view. 00:14:59.580 --> 00:15:54.660 So, in this we have got this is the direction sorry again we ah went forward. So, in this 00:15:54.660 --> 00:15:59.980 ah the features I have shown in an exaggerated manner. So, basically there is not much change 00:15:59.980 --> 00:16:06.050 on the surface area, but due to the placement of this feature, there will be change in the 00:16:06.050 --> 00:16:12.700 velocity field very near the ah wall and we will have some increase in h. 00:16:12.700 --> 00:16:18.779 Ah . So, this is where we are increasing h without increasing the area here I have shown 00:16:18.779 --> 00:16:24.630 one extended surface or field. So, basically it is increase in the surface area and here 00:16:24.630 --> 00:16:31.480 there are offset strip fin ah. So, it is like this that small small fins are ah placed in 00:16:31.480 --> 00:16:36.710 the staggered manner on the flow field or in the path of the flow. So, on the field 00:16:36.710 --> 00:16:41.150 there will be boundary layer development, but as the fins are small the boundary layer 00:16:41.150 --> 00:16:48.459 cannot develop infinitely, and cannot reduce the ah rate of heat transfer that side of 00:16:48.459 --> 00:16:53.940 the field there will be vertex formation. So, boundary layer will get some sort of an 00:16:53.940 --> 00:16:59.990 obstacle for further growth heat transfer coefficient will start increasing, and at 00:16:59.990 --> 00:17:05.930 the same time the small fields they are giving an increase in area. I would ah like to show 00:17:05.930 --> 00:17:13.409 you some other example as we proceed. Here ah we have shown techniques for heat 00:17:13.409 --> 00:17:21.120 transfer ah augmentation, ah basically passive augmentation. The a first one you see this 00:17:21.120 --> 00:17:26.030 is a tube, but the tube surface has been given some sort of a shape so, that the boundary 00:17:26.030 --> 00:17:34.620 layer is disturbed and gives more heat transfer. ah In ah the second one that is b ah or um 00:17:34.620 --> 00:17:42.330 b. So, there are some sort of micro field over the ah um um over the tube surface. In 00:17:42.330 --> 00:17:50.130 ah c there are fin inside the tube different kind of fins small small fins inside the tube, 00:17:50.130 --> 00:17:56.000 and these fins as they are small they are not only they are not only ah and ah particularly 00:17:56.000 --> 00:18:01.929 when they are inside. So, they are not only increasing the surface they are also ah giving 00:18:01.929 --> 00:18:07.560 a rise in the heat transfer. D there is again ah on the tube geometry there 00:18:07.560 --> 00:18:14.340 are some variation so, that will give some enhancement in ah heat transfer. ah Below 00:18:14.340 --> 00:18:22.520 what we can see ah below what we can see that here ah we have got some sort of a tape insert 00:18:22.520 --> 00:18:28.450 here in this diagram, we have got a tape insert inside the tube. So, the heat transfer will 00:18:28.450 --> 00:18:34.480 increase here ah the last one here we have got some sort of wire insert inside the tube 00:18:34.480 --> 00:18:37.820 which will give good mixing and better heat 00:18:37.820 --> 00:18:46.150 And then active techniques; active techniques means that we have to spend some energy more 00:18:46.150 --> 00:18:52.190 than what, we are spending for the fluid flow additionally we have to spend certain amount 00:18:52.190 --> 00:18:59.160 of energy. So, there could be mechanical aids like scraping or agitation, ah in case of 00:18:59.160 --> 00:19:05.820 many chemical reactors where heat exchanger is also integrated with the reactor we have 00:19:05.820 --> 00:19:10.600 got this agitators. Ah For viscous fluid we have got ah scrap 00:19:10.600 --> 00:19:15.280 surface heat exchange as I have mentioned in some of the lectures earlier, then there 00:19:15.280 --> 00:19:22.280 could be surface vibration. So, all these thing two goals are there, either we are trying 00:19:22.280 --> 00:19:30.200 to disturb the ah boundary layer, which gives the main resistance to heat transfer and which 00:19:30.200 --> 00:19:36.420 falls on the wall. So, either we are trying to disturb that or we are trying to give a 00:19:36.420 --> 00:19:41.520 good mixing to the bulk of the fluid. So, by passive technique also we try to do that, 00:19:41.520 --> 00:19:46.880 and by active technique also we try to do that. By passive technique we try to do another 00:19:46.880 --> 00:19:51.540 thing more that we sometimes try to provide more surface area . 00:19:51.540 --> 00:19:58.820 So, surface vibration is one thing, as surface vibration can give ah good amount of mixing 00:19:58.820 --> 00:20:05.260 then similarly fluid pulsation can give good amount of mixing then electrostatic field 00:20:05.260 --> 00:20:11.820 this is very unique. So, if there is a dielectric ah fluid, then we can provide some sort of 00:20:11.820 --> 00:20:19.720 a electro electrostatic field and with the help of electrostatic field we can have ah 00:20:19.720 --> 00:20:29.300 we can have ah ah a ah good ah kind of transport of the particles and that will give higher 00:20:29.300 --> 00:20:34.500 heat transfer. Again these are particles which are micro 00:20:34.500 --> 00:20:42.220 scale particle fluid ah I mean ah the molecules of the fluid, when they had a movement due 00:20:42.220 --> 00:20:48.500 to the electrostatic field so obviously, at the microscopic level they produce some sort 00:20:48.500 --> 00:20:53.870 of agitation. When we are providing some sort of mixer insert etcetera, that is providing 00:20:53.870 --> 00:20:59.170 agitation at the macroscopic level, but this is at the microscopic level. 00:20:59.170 --> 00:21:09.570 Now, many cases we go for hybrid technique and compound enhancement. ah Like ah ah two 00:21:09.570 --> 00:21:14.352 techniques are combined together, let us say two passive techniques can be can be can be 00:21:14.352 --> 00:21:20.330 combined one passive and one active technique can be combined and two active technique can 00:21:20.330 --> 00:21:27.009 be combined. Let me give one example and with this example we can probably end our discussion 00:21:27.009 --> 00:21:37.900 So, here this is kind of a think of a fin 00:21:37.900 --> 00:21:45.390 tube ah ah. So, heat exchanger. So, first what one can think that tubes are used for 00:21:45.390 --> 00:21:51.450 circulating some sort of a liquid and this liquid has to be either cooled or heated. 00:21:51.450 --> 00:21:59.440 So, what we can provided that on on the on on the outer surface of the tubes we can circulate 00:21:59.440 --> 00:22:05.410 some amount of gas. But gas side heat transfer is small. So, what 00:22:05.410 --> 00:22:15.280 we can do to improve the gas side heat transfer, we can provide ah extended surface ah or fin 00:22:15.280 --> 00:22:20.380 which is shown by this trick. So, now, you can understand the arrangement. These are 00:22:20.380 --> 00:22:27.830 showing the tube cross section through which your some liquid is passing; surrounding the 00:22:27.830 --> 00:22:35.179 tube we have got plate thin plate which will act as an extended surface or fin, this will 00:22:35.179 --> 00:22:41.830 give augmentation of heat transfer and this is a passive augmentation of heat transfer. 00:22:41.830 --> 00:22:50.240 So, this is the basic design and on this basic design what I have done here we have provided 00:22:50.240 --> 00:22:58.880 rectangular inlet ah for alternatives. So, when the fluid flow will take place. So, there 00:22:58.880 --> 00:23:05.850 some sort of a vortex will be created behind this v inlet. So, fluid flow is ah taking 00:23:05.850 --> 00:23:12.750 from this side and behind this v inlet, we will have some sort of a vortex which will 00:23:12.750 --> 00:23:18.590 give higher heat transfer due to the change in the heat transfer coefficient. 00:23:18.590 --> 00:23:25.150 So, one passive technique is increasing the surface area, the another passive technique 00:23:25.150 --> 00:23:30.780 is increasing the heat transfer coefficient. Two different passive technique we have used 00:23:30.780 --> 00:23:40.380 and with that we are getting a much larger heat transfer. So, ah . So, this is one way 00:23:40.380 --> 00:23:47.530 how we can combine two ah different augmentation techniques together. Let me give some sort 00:23:47.530 --> 00:23:55.940 of ah concluding note for ah whatever we have summary and concluding note for whatever we 00:23:55.940 --> 00:24:03.660 have discussed. Though we are calling passive technique does not meet any kind of hm auxiliary 00:24:03.660 --> 00:24:11.000 power, but most of the passive technique we have increase in pumping power. We have not 00:24:11.000 --> 00:24:18.750 to provide any kind of auxiliary device auxiliary arrangement for power supply, new power supply 00:24:18.750 --> 00:24:26.130 obviously, but the ah fluid pumping power that generally increases. So, this is one 00:24:26.130 --> 00:24:32.559 point people have to be careful when we are selecting a particular augmentation technique 00:24:32.559 --> 00:24:37.000 that is one thing. Second thing generally augmentation techniques 00:24:37.000 --> 00:24:44.720 when you adopt augmentation technique. So, you have increase in initial cost, increase 00:24:44.720 --> 00:24:52.970 in weight and volume of the volume of the heat exchanger or heat transfer device. So, 00:24:52.970 --> 00:25:02.540 this is also one point which needs to be considered. Here again I like to mention one thing that, 00:25:02.540 --> 00:25:10.380 first what I have ah informed that either we have to use auxiliary power or if we are 00:25:10.380 --> 00:25:17.320 using passive technique, then what we have to do then what we have to do that we have 00:25:17.320 --> 00:25:26.710 to go for excess of pressure drop that pressure drop penalty we have to admit. 00:25:26.710 --> 00:25:34.799 So, if we are admitting excess pressure drop then what is happening? We are again spending 00:25:34.799 --> 00:25:44.240 some more amount of power ah. So, here some reference from thermodynamics may be drawn, 00:25:44.240 --> 00:25:51.030 that we want to enhance the rate of heat transfer for that we have to spend some amount of power. 00:25:51.030 --> 00:25:59.419 ah When we are trying to have more heat transfer we are giving with a low grade energy, but 00:25:59.419 --> 00:26:04.650 when we are spending pumping power we are dealing with a high grade energy. 00:26:04.650 --> 00:26:11.600 So, this is like this, ah it is like this that we have to be very careful our heat transfer 00:26:11.600 --> 00:26:18.110 should be much more or from the energy point of view, magnitude of energy point of view 00:26:18.110 --> 00:26:25.450 our ah ah ah rate of heat transfer enhancement should be much more compared to the amount 00:26:25.450 --> 00:26:32.000 of pressure ah work we are spending for the changing design. So, this is the one point 00:26:32.000 --> 00:26:41.710 you have to be very careful . Performance evaluation of augmented surfaces. 00:26:41.710 --> 00:26:49.690 So, first thing what we can see that reduction of size keeping delta tm and Q constant. So, 00:26:49.690 --> 00:26:56.460 this could be one way that suppose we are keeping Q and delta tm constant that is the 00:26:56.460 --> 00:27:03.850 total amount of heat transfer and the hm and the ah ah the ah mean temperature difference 00:27:03.850 --> 00:27:06.470 between the two fluid stream we are keeping constant. 00:27:06.470 --> 00:27:12.960 So, whether we can reduce the size of the heat exchanger by ah the augmentation techniques 00:27:12.960 --> 00:27:19.470 adopted. So, this could be one. Then then the second one could be that reduction of 00:27:19.470 --> 00:27:26.549 ah delta tm keeping Q and size constant for heat exchanger. So, let us say that earlier 00:27:26.549 --> 00:27:32.100 my process I have designed with a large temperature difference between the two fluid streams now 00:27:32.100 --> 00:27:39.799 what I will do, I will have the same amount of heat transfer I will have the same area 00:27:39.799 --> 00:27:48.950 ah ah rather same volume of the heat exchanger ah, but I will be ah I have gone for ah augmentation 00:27:48.950 --> 00:27:53.650 technique. So, I will have lesser amount of delta T m. 00:27:53.650 --> 00:28:01.320 So, this this could be also one goal and it may be possible not in all cases, that by 00:28:01.320 --> 00:28:08.690 all these process as I have made my heat exchanger very small. As I have made my transfer processes 00:28:08.690 --> 00:28:14.700 very efficient that I may achieve this with a lesser pressure drop. So, this may happen 00:28:14.700 --> 00:28:21.360 this may not happen. So, it may be possible to reduce delta p keeping q constant; however, 00:28:21.360 --> 00:28:26.450 compact heat exchanger generally aim at a reduction in size. So, this is one thing we 00:28:26.450 --> 00:28:31.970 have to ah remember, that whenever we are talking about compact heat exchanger generally 00:28:31.970 --> 00:28:36.820 we mean that there is a reduction in size 00:28:36.820 --> 00:28:43.920 Ah Let us quickly see that augmentation of heat transfer during phase change boiling 00:28:43.920 --> 00:28:50.400 and condensation. So, this is initially I have discussed ah what we should do for single 00:28:50.400 --> 00:28:57.750 phase flow, but when there is a phase change ah the the principles are ah completely different. 00:28:57.750 --> 00:29:03.700 In case of boiling generally what we try to do that some sort of a degree of super heat 00:29:03.700 --> 00:29:12.030 is needed between the solid surface and the fluid for um ah for initiate nucleation. So, 00:29:12.030 --> 00:29:18.600 our surface design should be such that this delta super fit is reduced; that means, basically 00:29:18.600 --> 00:29:25.039 we are we are reducing the effective temperature difference between the surface and the fluid. 00:29:25.039 --> 00:29:30.200 The generation of more nucleation site, we need to have more evaporations ah or more 00:29:30.200 --> 00:29:37.770 ah vapor formation. So, from much more many more places there should be formation of vapor. 00:29:37.770 --> 00:29:43.730 Then increase in the range of nucleate boiling ah actually ah I will touch up on boiling, 00:29:43.730 --> 00:29:51.170 but this is as we are discussing ah augmentation. So, this point I want to tell, that nucleate 00:29:51.170 --> 00:29:57.570 boiling is a very high rate of heat transfer and safe rate of safe ah method of heat transfer 00:29:57.570 --> 00:30:03.440 during boiling. So, we want to increase its range ah temperature 00:30:03.440 --> 00:30:09.360 range basically and we want to increase the magnitude of critical heat flus. Critical 00:30:09.360 --> 00:30:15.480 heat flux is again one heat flux probably most of the you know it, because ah as the 00:30:15.480 --> 00:30:21.210 prerequisite of this course probably all of you have attended heat transfer course and 00:30:21.210 --> 00:30:28.200 there you are familiar with different regimes are ah of boiling. So, maximum heat flux that 00:30:28.200 --> 00:30:34.250 we can allow ah in many industrial equipment is the critical heat flux, then if there is 00:30:34.250 --> 00:30:40.020 any excessive vapor formation. So, there should be immediate removal of vapor . 00:30:40.020 --> 00:30:46.120 Condensation in condensation if we can have ah drop wise condensation that is good and 00:30:46.120 --> 00:30:50.870 quick removal of condensate filed if we are having film wise condensation. So, these are 00:30:50.870 --> 00:30:56.081 the method of augmentation in boiling and condensation, probably we will revisit and 00:30:56.081 --> 00:31:02.800 ah see closely some of them when we will ah ah learn about the phase change heat exchangers. 00:31:02.800 --> 00:31:10.620 . So, with this I like to come to an ah end come to an end of this lecture and let us 00:31:10.620 --> 00:31:15.220 see some augmentation technique in detail in our next lecture. 00:31:15.220 --> 00:31:15.830 Thank you.
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