INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD 382 481, 8-10 DECEMBER, 2011 1 Comparison of Heat transfer Enhancement in Tube in Tube heat exchanger using Different Turbulent Generator. A. Mehta Kushal K. and B. Lakdawala A M. A & B. Mechanical Engg. Department, Institute of Technology, Nirma University, Ahmedabad Abstract--In the present work experimental analysis for heat transfer in Tube In Tube heat exchanger with and without inserts and delta winglet is carried out. The heat transfer augmentation in the form of Nusselt number variation is established for two different length of insert and delta winglet. Thermal performance and pressure drop for 20 cm insert, 50 cm insert and delta winglet for Reynolds numbers range of 1400 to 6500 at annulus side have been studied. Results shows that the Nusselt number for long inserts is increase by 53% and the short inserts is increase by 27% as compared with the plain tube in tube heat exchanger. Keywords-- Delta Winglet; Heat Transfer Enhancement; Tube In Tube Heat Exchanger. H I INTRODUCTION eat transfer enhancements in heat exchangers is gaining industrial importance because it gives one the opportunity to reduce the heat transfer surface area required for a given application and thus reduce the heat exchanger size and cost, increase the heat duty of the exchanger for fixed surface area, reduce logarithmic mean temperature difference (LMTD) for fixed heat duty and surface area, and reduce pumping power for fixed heat duty and surface area. The automotive and refrigeration industries routinely use enhanced surfaces in their heat exchangers. Also, the process industry is aggressively working to incorporate enhanced heat transfer surfaces in their heat exchangers. Heat transfer enhancement in heat exchangers has been the subject of many experimental and analytical investigations. These techniques can be categorized as `active' or `passive'. The active techniques require external power, such as surface vibration, fluid vibration, injection, suction, and electric or acoustic fields. Passive techniques employ special surface geometries for enhancement, such as extended surfaces, rough surfaces, displacement enhancement devices, swirl flow devices, obstruction devices, and treated surfaces. Different methods used to transfer the heat either through increasing the heat input, by increasing the velocity and by using the winglet in the flow path of the fluid. The last method used i.e. the passive way to increase the heat transfer by using the delta winglet is found to be most effective method to increase the heat transfer without any external input. In parallel flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter flow heat exchangers the fluids enter the exchanger from opposite ends. Fig.1 Overall Heat Transfer vs. Reynolds Number In the Fig.1 trial 1-4, representation parallel flow, 5-8 representation Countercurrent, 9-10 representation cross flow. It is evident from the Fig. 1 that in all case overall heat transfer coefficient increase with cold flow Reynolds number, but it is also seen that for all cold flow Reynolds numbers the heat transfer coefficient of counter flow arrangement is always higher than other two. This is the main motivation behind selecting the counter flow arrangement. II Experimental Setup The schematic diagram of the experimental setup used is as shown in Fig.2 As the motivation of the experiments was to study the effect of turbulant creator as well as the effect of Reynolds number on the heat transfer of heat exchanger, the tube in tube heat exchanger was selected to avoid the complexity of the problem. Moreover, various apparatus, as shown in Fig.2 such as, Rotameter, Pump, Hot water tank, Cold water tank, Thermocouples, and the Control valve were implemented in the experimental setup as per the requirement of the experiments.
2 INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 Fig.2 Line Diagram of Tube in Tube heat exchanger Fig.3. shows hot (insulated) water tank and cold water tank filled with water. The left side of the tank is attached with the heater with variac and the entire tank is insulated. Water in the insulated tank is heated up to 80 C temperature with the help of heater. While the tank in the right side is filled with water at the atmospheric temperature. As the water in the insulated tank is reached to 80 C both the water Fig.3 Experimental Setup Mass flow rate of hot fluid (LPM) TABLE I MATRIX OF EXPERIMENTS PERFORMED Mass flow rate of cold fluid (LPM) Temperature variation of Tube side fluid 4 6, 8 & 10 40 to 80 6 4, 8 & 10 40 to 80 8 4, 6 & 10 40 to 80 10 4, 6 & 8 40 to 80 pump are operated in order to supply the hot and cold fluid through the test rig. During the experiment the hot fluid is passed through the tube side and the cold fluid is passed through the shell side. The mass flow rate at which the water is passing through the respective channel i.e. tube side and shell side can be varied with the help of rotameter. Mass flow rate on both the side of test rig i.e. Tube side and shell side is kept fixed with the help of the rotameter as shown in the Fig.3 Total twelve thermocouples are provided in the experimental setup as shown in the Fig.3. Out of this twelve thermocouple one is fixed in the hot water insulated tank located in the left side below the test rig i.e abbreviated as T1. At the inlet of the tube side one thermocouple is provided abbreviated as T2. Similarly one thermocouple is also provided at the output of the tube side flow T3. Rest thermo-couples are kept in the test rig. Starting from the right side of the test rig each thermocouple is fixed as constant distance on the shell side. they are abbreviated from T4 to T12 res. Number of experiments are performed by varying the mass flow rate in both the channels and by varying the inlet temperature of the water at the tube side. The matrix of the experiments performed is shown below in the Tab.2 As shown in the above Tab.2 flow both the rotameter is set to the specific value. Also we can see that the value for the hot fluid rotameter does not match with the value as that in cold fluid rotameter. i.e. the value for both the rotameter is never kept same. This is due to the fact that specific heat at constant pressure at a specific temperature remains constant. After measuring the readings from the various thermocouple the results are calibrated to and the various parameters of the flow inside the tube and the shell side. Inner Tube: As the hot fluid water is made to pass from the tube side and the cold water is made to passed from annulus side, the heat transfer takes place between the hot water and the cold water. There are mainly three resistance which obstruct the heat transfer between hot and cold fluid, which are 1) Convective resistance between hot fluid and inner surface of tube 2) Conduction resistance offered by the tube wall 3) Convection resistance between cold fluid and outer surface of tube To insure maximum heat transfer from tube wall (i.e less conduction resistance), copper as tube material was selected which is shown in Fig4. Outer Tube: The outer tube was insulated in order to keep the heat loss from the shell side as low as possible. In order to provide sufficient strength and rigidity the outer tube was made of the stainless steel. Fig4. shows the annulus side tube with insulated glass wool. Delta Winglets: Delta winglet plays an important role in the enhancement of tube in tube heat exchanger. It was made of the alluminium foil. Purpose of the delta winglet is to obstruct the flow of the water flowing from the shell side. As a result the flow path of the water is disturbed and hence turbulence is created. For preparing delta winglet long strips of alluminium was cut from the alluminium sheet. Size of this alluminium sheet was kept 100cm long and 4cm in width. After the strip is cut, triangles of 2cm base and 1cm height were marked over the entire length of the strip keeping a distance of 2cm between two triangles. After marking the strip was fold from the centre line of the strip along its entire length. Cut was made on the two side of the triangle as per the marking and than the long strip was unfold in order to erect the triangle (delta winglet) from
INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD 382 481, 8-10 DECEMBER, 2011 3 TABLE II SPECIFICATION OF EXPERIMENTAL SETUP PART NAME SPECIFICATION NO. INNER TUBE MATERIAL OUTER TUBE ID : 25MM, OD : 28MM, MATERIAL : COPPER. ID : 62.5MM, OD : 65MM, MATERIAL : SS STEEL Thermocouple TYPE : K AND TEMPERATURE RANGE : -5 C T0 550 C ROTAMETER MASS FLOW RATE : 0 TO 10 LIT/MIN, LIST COUNT 1ML, Mercury Manometers MATERIAL : GLASS LEAST COUNT 1CM 1 Heater Variac CAPACITY: 3 KW 1 With Thermostatic Switch PUMP CAPACITY : 1/2 HP AND TYPE : FHP 2 DELTA WINGLET MATERIAL : ALLUMINIUM - 1 1 12 2 Fig.6 DeltaWinglet Long Inserts: Long inserts are also made from the alluminium material. For this long strip of alluminium material was cut from the alluminium strip. The length of the strip was so adjusted so as to make the inserts of 50 cm long and the width was kept of 1cm. The same strip was wound around the rod of diameter 9.5 cm by keeping the distance of 1cm between two wounds. In this way the inserts of 50 cm long, 1 cm of diameter and 1cm pitch was generated. The same strip was tied on the peripheral surface of the tube as shown in Fig7. along the entire length of the tube. Angle of 45 degree is kept between two consecutive inserts. Short Inserts: Short inserts are also made from the alluminium foil. The size of this insert is 20 cm long, 1cm in diameter and 1cm pitch between the consecutive wound. Making of this insert is same as that of the long strip. Here, the length of insert is kept 20 cm. This inserts also were fitted in the same way as that done in long inserts as shown in Fig7. Fig:4 Inner tube. the base. Thus at every 2cm distance such triangles was erect. The same strip with delta winglet was wounded around the inner tube as shown in Fig6. Fig.7 Long Insert Fig:5 Outer tube Fig.8 Short Insert
4 INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 III EXPERIMENTAL PROCEDURES CROSS SECTIONAL AREA OF THE INNER TUBE IS GIVEN BY: for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. The larger the LMTD, the more heat is transferred. The use of the LMTD arises straight forwardly from the analysis of a heat exchanger with constant flow rate and fluid thermal properties. Similarly, Cross sectional area of the shell side is given by: Equivalent diameter of the shell : Rotameter gives the reading in Liter per minute (LPM) which is converted first into Liter per second by dividing the readings of rotameter by 60 and than converted into m3/s by dividing Liter per second by 1000 as shown below: Similarly Cp and Ch value with respect to the temperature is calculated as shown below: Value of specific density (), Prandtl number (Pr) and Thermal conductivity ( W/mK) is taken from the table of thermo-physical properties of water: When the mass flow rate is divided by the cross sectional area of the channel through which the flow is passing it gives velocity of flow: For finding whether the flow is laminar flow or turbulant flow, Reynolds number for the hot fluid is calculated as shown below: The conductive heat transfer for the tube side flow (hot side) is given by: The conductive heat transfer for the shell side flow (cold side) is given by: Overall heat transfer coefficient Uo is determined from the known parameters as shown in the equation below: Similarly; Reynolds number for the shell side fluid is calculated as shown below: Using the equation heat transfer h in the shell side is calculated as shown below: Friction factor for the tube flow is given by: Similarly; the Prandtl number is given by: Nusselt number for the flow of the fluid is also given by; Internal heat transfer coefficient is given by: Tavg is the mean temperature of the different temperature reading obtained by the thermo couple fitted at equal distance over the entire length of the shell: Log mean temperature difference is abbreviated as LMTD. It is defined as the log mean temperature difference (also known by its acronym LMTD) is used to determine the temperature driving force IV RESULT AND DISSCUSION The experiments have been carried out using different flow rates to change the Reynolds number. On tube side as well as on annulus side, the flow rate was varied from 4 LPM to 10 LPM. The analysis was performed using 12 different combinations of flow rates (i.e. Reynolds number). Similar analysis was performed using different turbulent generator that are short insert (20 cm long), long insert (50 cm long), and delta winglet (pitch 2 cm, height 1cm). The variation of Nusselt number with Reynolds number is presented in this chapter. Fig.9 shows the effect of Reynolds number on the Nusselt number.i.e. overall heat transfer coefficient, for the plain tube, short inserts, long inserts and delta winglet. There is monotonic increase in the Nusselt number with the increase in Reynolds number, also the Nusselt number increases in the ascending order for the short insert, long insert and delta winglet respectively when compared with the plain tube. It is also seen that the increase in the Nusselt number for lower Reynolds number is less as compared to the increase in the Nusselt number for the higher Reynolds number. When Fig.9 (a), (b), (c) and (d) is compared with each other it is seen that the percentage increase in Nusselt number is more for tube flow of 6 LPM as compared to the flow of 4 LPM, 8 LPM and 10 LPM. Also the percentage increase in the Nusselt number is more for the higher Reynolds however for tube side flow of 8 LPM, there
INSTITUTE OF TECHNOLOGY, NIRMA UNIVERSITY, AHMEDABAD 382 481, 8-10 DECEMBER, 2011 5 is slight decrease in the Nusselt number at Reynolds number 6423 as compared to the increase in Nusselt number at Reynolds number 3854 as shown in Fig.9 (a). respectively for short insert, long insert and delta winglet. The average increase in the Nusselt number for short insert is 9%, for long insert is 11% and for the delta winglet is 17%. Fig.9 Nusselt number vs Reynolds number As shown in Fig.9 (a) the percentage increase in the Nusselt number at Reynolds number 3854 for short, long inserts and delta winglet is 17%, 95% and 83% respectively. For the Reynolds number 5138 percentage increase is 12%, 51% and 55% respectively and for Reynolds number 6423 percentage increase is 27%, 71% and 62%. respectively for short insert, long insert and delta winglet. The average increase in the Nusselt number for short insert is 19%, for long insert is 75% and for the delta winglet is 66%. As shown in Fig.9 (b) the percentage increase in the Nusselt number at Reynolds number 2569 for short, long inserts and delta winglet is 7%, 20% and 33% respectively. For the Reynolds number 5138 percentage increase is 32%, 79% and 92% respectively and for Reynolds number 6423 percentage increase is 70%, 120% and 130%. respectively for short insert, long insert and delta winglet. The average increase in the Nusselt number for short insert is 36%, for long insert is 75% and for the delta winglet is 85%. As shown in Fig.9 (c) the percentage increase in the Nusselt number at Reynolds number 2569 for short, long inserts and delta winglet is 11%, 16% and 22% respectively. For the Reynolds number 3854 percentage increase is 22%, 54% and 63% res. and for Reynolds number 6423 percentage increase is 18%, 27% and 48%. res. for short insert, long insert and delta winglet. The average increase in the Nusselt number for short insert is 17%, for long insert is 32% and for the delta winglet is 44%. As shown in Fig.9 (d) the percentage increase in the Nusselt number at Reynolds number 2569 for short, long inserts and delta winglet is 0.17%, 0.59% and 5% respectively. For the Reynolds number 3854 percentage increase is 20%, 19% and 26% respectively and for Reynolds number 5138 percentage increase is 7%, 13% and 21%. Fig.10 Nusselt number vs Prandtl number Fig.10 shows the effect of Prandtl number on the Nusselt number with and without turbulent generator. From the Fig.10 it is clear that the as the Prandtl number increases the Nusselt number goes on decreasing.i.e. in the other way as the inlet temperature of the hot water increases the Nusselt number goes on increasing. Their is monotonic increase in the Nusselt number with the increase in the temperature.i.e. with the decrease in the Prandtl number. Also the increase in the Nusselt number at higher Prandtl number is less as compared with increase in the Nusselt number at lower Prandtl number. For all Fig.10 (a), (b), (c) and (d) the increase in the Nusselt number value is in descending order for Plain tube, short insert, long insert and delta winglet. However, as shown in Fig.10 (a) the Nusselt number for delta winglet at Prandtl number 2.2 and 2.7 decreases as compared to that with the long insert and Fig.10 (c) the Nusselt number for delta winglet at Prandtl number 2.53 decreases as compared to that with the long insert. As shown in Fig.10 (a) the average percentage increase in Nusselt number for short insert is 39%, for long insert is 99% and for the delta winglet is 105% as compared to the plain tube. For Fig.10 (b) the average percentage increase for short insert is 15%, for the long insert is 46%, for delta winglet 83% as compared to the plain tube. In Fig.10 (c) the average percentage increase for short insert is 28%, for long insert is 87% and for the delta winglet is 36% as compared to plain tube. In Fig.10 (d) the average percentage increase for short insert is 39%, for the long insert 52% and for the delta winglet is 93% as compared to plain tube.
6 INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN TECHNOLOGY, NUiCONE 2011 V. CONCLUSION Fig.11 Nusselt number vs Prandtl number As shown in Fig.11. (e) the average percentage increase for short insert is 33%, for long insert is 95% and for the delta winglet is 120% as compared to the plain tube. For Fig.11 (f) the average percentage increase for short insert is 53%, for the long insert is 110% and for delta winglet 122% as compared to the plain tube. In Fig.11 (g) the average percentage increase for short insert is 19%, for long insert is 28% and for the delta winglet is 37% as compared to plain tube. In Fig.11 (h) the average percentage increase for short insert is 22%, for the long insert 6% and for the delta winglet is 68% as compared to plain tube. As shown in Fig.12 (i) the average percentage increase for short insert is 49%, for long insert is 67% and for the delta winglet is 82% as compared to the plain tube. For Fig.12 (j) the average percentage increase for short insert is 18%, for the long insert is 42% and for delta winglet 4% as compared to the plain tube. In Fig.12 (k) the average percentage increase for short insert is 19%, for long insert is 2% and for the delta winglet is 3% as compared to plain tube as shown in table4.15. In Fig.12 (l) the average percentage increase for short insert is 0.8%, for the long insert 1% and for the delta winglet is 2% as compared to plain tube. When the flow at tube side (mh) is 4 LPM and annulus side (mc) flow is 6 LPM percentage increase in the Nusselt number for short insert is 32%,for long insert is 99% and for delta winglet is 109% as compared to plain tube. For mh = 4 LPM and mc = 8 LPM percentage increase in the Nusselt number for short insert is 15%, for long insert is 46% and delta winglet is 83% as compared to plain tube. For mh = 4 LPM and mc = 10 LPM percentage increase in the Nusselt number for short insert is 28%, for long insert is 87% and for delta winglet is 36% as compared to plain tube. For mh = 6 LPM and mc = 4 LPM percentage increase in the Nusselt number for short insert is 39%, for long insert is 52% and for delta winglet is 93% as compared to plain tube. For mh = 6 LPM and mc = 8 LPM percentage increase in the Nusselt number for short insert is 33%, for long insert is 95% and for delta winglet is 121% as compared to plain tube. For mh = 6 LPM and mc = 10 LPM percentage increase in the Nusselt number for short insert is 53%, for long insert is 110% and for delta winglet is 122% as compared to plain tube. For mh = 8 LPM and mc = 4 LPM percentage increase in the Nusselt number for short insert is 19%, for long insert is 28% and for delta winglet is 37% as compared to plain tube. For mh = 8 LPM and mc = 6 LPM percentage increase in the Nusselt number for short insert is 22%, for long insert is 6% and for delta winglet is 68% as compared to plain tube. For mh = 8 LPM and mc = 10 LPM percentage increase in the Nusselt number for short insert is 49%, for long insert is 67% and for delta winglet is 82% as compared to plain tube. For mh = 10 LPM and mc = 4 LPM percentage increase in the Nusselt number for short insert is 18%, for long insert is 42% and for delta winglet is 0.80% as compared to plain tube. For mh = 10 LPM and mc = 6 LPM percentage increase in the Nusselt number for short insert is 19%, for long insert is 2% and for delta winglet is 2% as compared to plain tube. For mh = 10 LPM and mc = 8 LPM percentage increase in the Nusselt number for short insert is 0.80%, for long insert is 1% and for delta winglet is 2% as compared to plain tube. VI. REFERENCES [1] C.B. Allison, B.B. Dally, Effect of a delta-winglet vortex pair on the performance of a tube fin heat exchanger, International Journal of Heat and Mass Transfer 50 (2007) 5065-5072. [2] Chi-Chuan Wang a, Jerry Lo b, Yur-Tsai Lin b, Chung-Szu Wei, Flow visualization of annular and delta winglet vortex generators in fin and tube heat exchanger application_, International Journal of Heat and Mass Transfer 45 (2002) 3803-3815. [3] K. Torii, K.M. Kwak, K. Nishino, Heat transfer enhancement accompanying pressure-loss reduction with winglet-type vortex generators for fin-tube heat exchangers, International Journal of Heat and Mass Transfer 45 (2002) 3795-3801. [4] K.M. Kwak, K. Torii, K. Nishino, Simultaneous heat transfer enhancement and pressure loss reduction for finned-tube bundles with the first or two transverse rows of built-in winglets_, Experimental Thermal and Fluid Science 29 (2005) 625-632. [5] Kenan Yakut a, Bayram Sahin a, Cafer Celik b, Nihal Alemdaroglu a, Aslihan Kurnuc, _Effects of tapes with double-sided delta-winglets on heat and vortex characteristics, Applied Energy 80 (2005) 77-95. [6] Liting Tian, Yaling He, Yubing Tao, Wenquan Tao, _A comparative study on the air-side performance of wavy _n-and-tube heat exchanger with punched delta winglets in staggered and in-line arrangements, International Journal of Thermal Sciences 48 (2009) 1765_1776. Fig.12 Nusselt number vs Prandtl number