International Journal of Advanced Engineering Research and Studies E-ISSN

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1 Research Paper ANALYSIS OF TWISTED TAPE WITH STRAIGHT WINGLETS TO IMPROVE THE THERMO-HYDRAULIC PERFORMANCE OF TUBE IN TUBE HEAT EXCHANGER Mr.S.D.Patil 1, Prof. A.M. Patil 2, Prof. Gutam S. Kamble 3 Address for Correspondence 1 Mechanical Engg.Dept P.V.P.I.T,Budhgaon, Dist.-Sangli (India) 2 Vice-Principal, Mechanical Engg.Dept P.V.P.I.T,Budhgaon, Dist.-Sangli (India) 3 Associate Professor, TKITE,Warnanager,Dist.-Kolhapur ABSTRACT Experimental investigation of heat transfer and friction factor characteristics in a double pipe heat exchanger fitted with Straight delta winglet and typical twisted tape elements were studied. The inner and outer diameters of the inner tube are 20.5 and 26 mm, respectively and cold and hot water were used as working fluids in shell side and tube side. The twisted tapes were made of the Aluminum strip with thickness of 2.01 mm and the length of 1500 mm. They were inserted in the test tube section in two different cases: (1) Straight delta winglets twisted tape at different twisted ratios (y/w=3.5, 4.5 and 5.5) and depth of cut ratios (d/w=0.1, 0.2 and 0.3), and (2) typical twisted tape with twist ratios (y/w=3.5, 4.5 and 5.5). The results, obtained from the tube with straight delta twisted tape insert, were compared with those typical twisted tape. The results show that the heat transfer coefficient increased with decrease in twist ratio (y/w). Whereas the increase in the depth of cut ratio (d/w) would improve both the heat transfer coefficient and friction factor. The results from each case were correlated for Nusselt number and friction factor. Subsequently, the predicted Nusselt number and friction factor from the correlations were plotted to compare with the experimental data. It was found that Nusselt number was within ±20% and ±15% for friction factor KEYWORDS Passive Methods, Tape inserts, Heat transfer enhancement, Straight delta winglet, Friction factor, typical twisted tape, Twist ratio, swirl flow. INTRODUCTION Design of heat exchanger demands critical attentions to different modes of heat transfer, pressure drop, sizing, long term performance estimation as well as economic aspects. Most of the designs are based on compactness of the unit, defined as the ratio of the heat transfer surface area to the heat exchanger volume. Generally, a heat exchanger with a surface area density > 700 m2/m3 is referred to as a compact heat exchanger. As the heat exchanger gets older, the resistance to heat transfer rate increases due to fouling or scaling. This is particularly true in heat exchangers used in marine as well as chemical industries. Also in some industries there is a need to increase heat transfer rate in the existing heat exchangers. Therefore to achieve / maintain the desired heat transfer in an existing heat exchanger, several methods have been investigated. These methods are classified as Passive techniques Active techniques Passive techniques do not require any direct input of external power; rather they use it from the system itself which ultimately leads to an increase in pressure drop. They generally use surface or geometrical modifications to the flow channel by incorporating inserts or additional devices. They promote higher heat transfer coefficients by disturbing or altering the existing flow behavior. In active techniques, external power is used to facilitate the desired flow modification and the concomitant improvement in the rate of heat transfer. Augmentation of heat transfer by this method can be achieved by stirrers, surface vibration, fluid pulsation etc. LITERATURE SURVEY:- An extensive literature survey of all types of heat transfer augmentation techniques with inserts has been discussed by Bergles (1985). In the present study delta winglets twisted tapes have been selected as an insert for heat transfer augmentation. Hence, important literature pertaining to these inserts alone is discussed in this paper. The work on twisted tape first started with tape inserts in circular ducts. Later, it has been extended to noncircular ducts as well. Numerous geometrical variations are possible in twisted tape inserts to enhance heat transfer and to reduce associated pressure drop. Eiamsa-ard et al, 2006 used the cold and hot water as working fluids in a double pipe heat exchanger fitted with regularly spaced twisted tape elements. Results show that the increase in the free space ratio would improve both the heat transfer coefficient and friction factor. Eiamsa et al, 2007 revealed that the lowest value of regularly spacing twisted tape gives the heat transfer lower than full length twisted tape around (5-15 %) while it can be decreased the pressure drop around 90%. Sivashanmugam and Nagarjan, 2007 proved that the heat transfer coefficient enhancement through a circular tube fitted with right and left helical screw inserts is higher than that for straight helical twist inserts of equal and unequal length for a given twist ratio. Chang et al, 2007 used serrated twisted tape with different twist ratios to enhance the heat transfer inside tube by ( ) times the heat transfer level in the tube fitted with smooth twisted tape. Promvong 2008 indicated that the presence of wire coils together with twisted tapes inside circular tube leads to a double increase in heat transfer over the use of wire coil twisted tape alone especially at smaller twist and coil pitch ratio under the same conditions. Thianpong et al, 2009 revealed that both heat transfer coefficient and friction factor in the dimpled tube fitted with the twisted tape, are higher than those in the dimple tube acting alone and plain tube and increases as the pitch ratio and twist ratio decrease. Jaisankar et al, 2009 found that the minimum twist ratio gives higher percentage of enhancement performance of twisted tape solar water heater collector compared to the plain one. Rahimi et al, 2009 proved experimentally and theoretically that the Nusselt number and performance of the jagged insert are higher than other modified twisted tape inserts by increasing of

2 (31%and 22%), respectively. Murugesan et al, 2009 observed a significant increase in heat transfer coefficient and friction factor inside tube fitted with full length trapezoidal cut twisted tape. Sharma et al, 2009 showed considerable enhancement of convective heat transfer with AL2O3 nano fluids compared to flow with water in a circular tube fitted with twisted tape inserts. Seemawute and Eiamsaard, 2010 showed that the heat transfer rates in the tube fitted with the peripherally cut twisted tape with alternate axis, normal peripherally-cut twisted tape, and typical tape are respectively enhanced up to (184%,120% and 57%) of heat in the plain tube where the testing fluid is the water. Syam sunder and Sharma, 2010 observed that the heat transfer coefficient of Al2 O3 nano fluid is (33.5 %) times higher compared to flow of water in a tube equipped with twisted tape inserts with twist ratio of five. Jian Guo, at,al 2011 found-a center-cleared twisted tape aiming at achieving good thermohydraulic performance. A comparative study between this type and the short-width twisted tape was performed numerically in laminar tubular flows. The computation results demonstrated that the, for tubes with center-cleared twisted tapes, the heat transfer can be even enhanced in the cases with a suitable central clearance ratio. The thermal performance factor of the tube with center-cleared twisted tape can be enhanced by 7-20% as compared with the tube with conventional twisted tape. WORKING PRINCIPLE:-Twisted tape inserts generate swirls, which direct fluid in the centre towards the pipe wall and fluid near the pipe wall towards centre. This phenomenon is the result of fluid flowing in a curvilinear path, which promote mixing and increase in heat transfer coefficient. EXPERIMENTAL SETUP The schematic of the experimental setup designed and fabricated for the present study is shown in Fig. 1 Fig. 1: Experimental setup Fig. 2: Sectional view of test section Fig.3-Typical Twisted tape Fig.4-Straight delta winglet Twisted tape It is a double pipe heat exchanger consisting of a calming section, test section, rotameters, water tank for supplying cold water & a constant temperature bath (500 litre capacity) for supplying hot water with in-built heater, pump & the control system. The test section was a Plain copper tube with dimensions of 1600mm length, Inner tube-20.5mm ID, and 26mm OD; Outer MS pipe-52mm ID, and 60 mm OD. The outer pipe was well insulated using 200mm diameter glass wool to reduce heat losses to the atmosphere. Two calibrated rotameters, with the flow range 250 to 2550 LPH, were used to measure the flow of cold & hot water. The water, at room temperature was drawn from tank by using pump. Similarly a rotameter provided to control the flow rate of hot water from the inlet hot water tank. Cold water flow rate was kept constant at 1450LPH. Two pressure tapings- One just before the test section and the other just after the test section are attached to the U-tube manometer for pressure drop measurement. Carbon tetrachloride (CCl 4 ) is used as the manometric fluid. Four RTDs measure the inlet & outlet temperature of hot water & cold water (T3,T4,T5 &T6) through a multipoint digital temperature indicator and simultaneously record surface temperature (T1,T2,T3,T4,T5,T6,T7 &T8) by using other eight RTDs which is touched on inner tube. EXPERIMENTAL PROCEDURE following procedure was adapted for experimental work. All the rotameters & RTDs were calibrated. For Nusselt Number and heat transfer coefficient calculation: Heater was put on to heat the water to 48 C in a constant temperature water tank of capacity 500 litres. The tank provided with a centrifugal pump & a bypass valve for recirculation of hot water to the tank & to the experimental setup. Hot water at about 48 C was allowed to pass through the tube side of heat exchanger at desired flow rate. Then Cold water allowed to passed through the annulus side of heat exchanger in counter current direction at a 1450LPH (mc= Kg/sec). The water inlet and outlet for both hot water & cold water and surface temperatures are were recorded only after temperature of both the fluids attains a constant value. The procedure was repeated for different Hot water flow rates ranging from Kg/sec. For friction factor determination: Then, Pressure drop was measured for each flow rate with the help of manometer at room temperature.

3 The U-tube manometer used carbon tetrachloride (CCl 4 ) as the manometric liquid. Air bubbles were removed from the manometer so that the liquid levels in both the limbs were equal when the flow stopped. Water at room temperature was allowed to flow through the annulus pipe of the heat exchanger. The manometer reading was noted. RESULTS AND DISCUSSION:- EXPERIMENTAL CALCULATION FOR REYNOLDS NUMBER Sample calculation- decreases the Nusselt number increases for a given Reynolds number and reaching a maximum for the twist ratio of 3.5 due to fact that as the twist ratio decrease, the intensity of swirl generated increases with the maximum intensity for the twist ratio 3.5 One can also observe that the Nusselt number for Straight delta winglet twisted tapes is almost close to Typical twisted tape inserts but more than that for Plain tube for a given twist ratio. This may be due to reason that spiral movement of fluid during course of flow through twisted tape will enhance the heat transfer by virtue of efficient mixing in the radial direction. 1. Discharge of water (Q) = 2. Mass flow rate m (Kg/s) = Density (ρ) Discharge (Q) 3. Velocity of Hot water (v) = = ) 4. Reynolds Number (R e ) = 5. Prandlt Number P r = 6. Heat balance Equation is given by, (m C p ) inner (Change in temp. of inner fluid) = (m C p ) outer (Change in temp. of outer fluid) (m C p ) (T hi - T ho ) = (m C p ) (T co - T ci ) Q h = Q c 7. % of Error = It was less than 4% 8. Rate of heat flow from inner fluid to surface = Rate of change of internal energy of Hot fluid q = m C p ( T hi - T ho ) 9. Rate of Heat transfer by the convection in hot fluid = Rate of change of internal energy of Hot fluid q= h A (T s -T f ) Where A= T s = average Surface temperature. T s = T f =Hot fluid bulk mean temp. 10.Heat transfer coefficient h Inner = w/m 2 k Fig.5 - Nusselt Number Vs Reynolds Number for Plain tube, Typical Twisted Tape and Straight delta Winglet for all the cases B) Performance Evaluation Analysis:- Bergles [15] have suggested several criteria for the performance evaluation of heat transfer enhancement devices. The performance of the heat transfer enhancement device evaluated on the basis of constant mass flow rate. R1=ha/ho where ha= Heat transfer coefficient for tube with Inserts, ho= Heat transfer coefficient for plain tube. In fig. 6 a plot between performance evaluation criteria R1 Vs. Reynolds no. for straight delta winglet twisted tape was shown. Maximum value of R1 is observed for S-DWT, y/w=3.5 insert (d/w=0.3). From this we can conclude that this was the best arrangement out of all arrangements tested for this experiment. 11. Nusselt Number (N u ) = PRESSURE DROP AND FRICTION FACTOR CALCULATION- Area = m 2, v = m/s ) g h N/m 2 Friction factor f= A) Effect of straight delta winglets :- Fig.5 shows variation of Nusselt number with Reynolds number for Straight delta winglet twisted tapes (S-DWT) and typical twisted tape at different twist ratio (y/w=3.5,4.5 &5.5). Nusselt number for the tube fitted with Straight delta winglet twisted tapes is higher than that for typical twisted tape for a given Reynolds number attributing to heat transfer enhancement due to swirl flow. As the Reynolds number increases the Nusselt number increases due to increased convection. Also, as the twist ratio Fig.6. Performance Evaluation Criteria (R1) Vs Reynolds Number for Typical twisted tape and straight delta winglet twisted tape for all the cases. C)Effect of straight delta winglets on Friction factor:- Fig.7 shows the variation of friction factor vs Reynolds number for the tube fitted with Straight delta winglets with y/w=3.5,4.5 & 5.5. The friction factor for the tube fitted with Straight delta winglets inserts was higher than that for Typical twisted tape and decreases with Reynolds number for a given

4 twist ratio. However, the friction factor increases with decrease in twist ratio for a given Reynolds number and reaching maximum for the twist ratio 3.5. Fig.-7 Friction Factor Vs Reynolds Number for Plain tube, Typical Twisted Tape and Straight Delta Winglets for all the cases. CONCLUSION The present work was regarding the study on passive methods for augmentation techniques. In this study the effect of twisted tape inserts has been investigated over the range of Reynolds number ( ) covering turbulent flow. In this Paper the main conclusion based on the work carried out are presented followed by the overall assessment. Use of Straight delta winglet (S-DWT) and typical twisted tapes (TT) can significantly intensify the heat transfer enhancement. Some typical examples for use of twisted tape inserts are water tube boilers, condensers, evaporators, radiators, heat exchanger in chemical & textile industries etc. use of twisted tape inserts increase the swirl flow. The swirls formed inside the tube result in thinning and disturbing the thermal boundary layer formed over the surface during fluid flow and hence increasing the turbulence and serve ultimately to bring enhancement of heat transfer between the fluid and its neighboring surface. 1. With decrease in twist ratio, heat transfer coefficient increases but at the same time pressure drop also increases. 2. For same twist ratio, Straight delta winglets (S-DWT) shows greater heat transfer coefficient & friction factor than the value we got form inserts Typical Twisted Tape (TT) because of increased degree of turbulence created. 3. New proposed correlations for the Nusselt number and friction factor based on the present experimental data are given for practical uses. The agreement between the results obtained from the experimental data and those obtained from the proposed correlations is reasonable Nomenclature A Heat transfer surface area, m 2 C p Specific heat of fluid, KJ/Kg. o C d i Inside diameter of the test tube, m DR Depth of wing cut ratio. f friction factor = P/((L/D)(qU 2 /2)) h Heat transfer coefficient, Wm 2 K I Current, A k Thermal conductivity of fluid, W/m o C L Length of the test section, m m Mass flow rate, kg /s N u Nusselt number = hd i/k P Pressure of flow in stationary tube, Pa P Pressure drop, Pa P r Prandtl number = µc p/k Q Heat transfer rate, W R e Reynolds number = t Thickness of the test tube, m T Temperature, o C T s Average surface Temperature, o C T f Bulk mean temperature, o C v velocity of fluid, m/s Q h Volumetric flow rate of hot water, m3/ s Q c Volumetric flow rate of cold water, m3/ s w tape width, m y/w Twisted tape pitch, mm Greek symbols ρ Fluid density, kg m 3 d Twisted tape thickness, m µ Fluid dynamic viscosity,kg/m.s Subscripts b Bulk c Convection i Inlet o Outlet p Plain pp Pumping power s Surface tu Turbulator W Water Abbreviations TT Typical twisted tape S-DWT Straight delta-winglet twisted tape REFERENCES 1. S. Eiamsa-ard, C. Thianpong, P. Promvonge, Experimental investigation of heat transfer and flow friction in a circular tube fitted with regularly spaced twisted tape elements, Int. Commun. Heat Mass Transfer 33 (2006) S. Eiamsa-ard, C. Thianpong, P. Eiamsa-ard, Turbulent heat transfer enhancement by counter/co-swirling flow in a tube fitted with twin twisted tapes, Experimental Thermal and Fluid Science 34 (2010) S. Eiamsa-ard, P. Seemawute, K. Wongcharee, Influences of peripherally-cut twisted tape insert on heat transfer and thermal performance characteristics in laminar and turbulent tube flows, Experimental Thermal and Fluid Science 34 (2010) S. Eiamsa-ard, P. Promvonge, Performance assessment in a heat exchanger tube with alternate clockwise and counter-clockwise twisted-tape inserts, International Journal of Heat and Mass Transfer 53 (2010) Khwanchit Wongcharee, Smith Eiamsa-ard, Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis, International Communications in Heat and Mass Transfer 38 (2011) Jian Guo, Aiwu Fan, Xiaoyu Zhang, Wei Liu, A numerical study on heat transfer and friction factor characteristics of laminar flow in a circular tube fitted with center-cleared twisted tape, International Journal of Thermal Sciences 50 (2011) S.W. Chang, T.L. Yang, J.S. Liou, Heat transfer and pressure drop in tube with broken twisted tape insert, Experimental Thermal and Fluid Science 32 (2007) P.K.Nagarajan and P.Sivashanmugam, Heat Transfer Enhancement Studies in a Circular Tube Fitted with Right-Left Helical Inserts with Spacer, World Academy of Science, Engineering and Technology A.W. Date, U.N. Gaitonde, Development of correlations for predicting characteristics of laminar flow in a tube fitted with regularly spaced twisted-tape elements, Exp. Therm. Fluid Sci. 3 (1990)

5 10. A. Dewan, P. Mahanta, K. Sumithra Raju, P. Suresh Kumar, Review of passive heat transfer augmentation techniques, J. Power Energy 218 (2004) R.L. Webb, Performance evaluation criteria for use of enhanced heat transfer surfaces in heat exchanger design, Int. J. Heat Mass Transfer 24 (1981) J.P. Holman, Heat Transfer, ninth ed. McGraweHill, New York, N. Sahiti, F. Durst, A. Dewan, Strategy for selection of elements for heat transfer enhancement, International Journal of Heat and Mass Transfer 49 (2006) R.M.Manglik. Heat TransferEnhancement, Thermal- Fluids and Thermal Processing Laboratory.Chapter No- 14, Bergles, A.E., Blumenkrantz, A.R. Performance evaluation criteria for enhanced heat transfer surfaces. 5 th International Heat Conference, Tokyo (1974), Vol. 2, pp