Enhancement in the Performance of Heat Exchanger by Inserting Twisted Tape Turbulators

ISSN 2395-1621 Enhancement in the Performance of Heat Exchanger by Inserting Twisted Tape Turbulators #1 D.S. Nakate, #2 S.V. Channapattana, #3 Ravi.H.C 1 ameetnakate@gmail.com 2 svchanna@yahoo.co.in. 3 ravi.honnebagera@gmail.com #1 PG Student, Department of Mechanical Engineering, Dr. D Y Patil School of Engineering Academy, ambi, savitribai phule pune university, MH, India. #2 Professor, Department of Mechanical Engineering, Rajarshi Shahu College of Engineering Pune savitribai phule pune university, MH, India. #3 Assistant Professor, Department of Mechanical Engineering, Dr. D Y Patil School of Engineering Academy ambi, savitribai phule pune university, MH, India. ABSTRACT Enhancing heat transfer surfaces are used in many engineering applications such as heat exchanger, air conditioning, chemical reactor and refrigeration systems, hence many techniques have been investigated on enhancement of heat transfer rate and decrease the size and cost of the involving equipment especially in heat exchangers. One of the most important techniques used are passive heat transfer technique. These techniques when adopted in Heat exchanger proved that the overall thermal performance was improved significantly. This experimental works can be taken by researchers on Augmentation Technique such as Twisted Tape. So, Researchers tried to increase the effective surface area Contact with fluid to increases the heat transfer rate in the heat exchanger. We tried to enhance the Heat transfer rate with the help of Twisted Tape insert with Turbulator s and find out the effect of Turbulator s on Flow of Fluid. In case of twisted tape with modified geometry, more turbulence is created during the swirl of fluid and gives higher heat transfer rate compared to plain twisted tape and modified twisted tape. The result shows that for modified twisted tape geometry, the heat transfer rate is higher with reasonable friction factor for both laminar and turbulent flow. ARTICLE INFO Article History Received :18 th November 2015 Received in revised form : 19 th November 2015 Accepted : 21 st November, 2015 Published online : 22 nd November 2015 Keywords Augmentation, friction factor, Heat Exchanger, Tube, Turbulators, Twisted Tape, Vortex I. INTRODUCTION Heat exchangers are devices that can be used to transfer heat from a fluid stream (liquid or gas) to another fluid at different temperatures. Heat Exchanger is a device in which the exchange of energy takes place between two fluids at different temperature. A heat exchanger utilizes the fact that, where ever there is a temperature difference, flow of energy occurs. So, that heat will flow from higher temperature heat reservoir to the lower temperature heat reservoir. The flowing fluids provide the necessary temperature difference and thus force the energy to flow between them. Heat exchangers are used in different processes ranging from conversion, utilization & recovery of thermal energy in various industrial, commercial & domestic applications. These include power production, process, chemical and food industries, electronics, environmental engineering, and waste heat recovery, manufacturing industries and air conditioning, refrigeration and space applications. Examples of heat exchangers that can be found in all homes are heating radiators, the coils on your Refrigerator and room air conditioner and the hot water tank. The development of high performance thermal systems has stimulated interest in methods to improve heat transfer. The study of improved heat transfer is referred to as heat transfer enhancement, augmentation or intensification. The performance of

conventional heat exchanger can be substantially improved by a number of enhancement techniques. A great deal of research effort has been devoted to developing apparatus and performing experiments to define the conditions under which an enhancement technique will improve heat transfer. Heat transfer enhancement technology has been widely applied to heat exchanger applications in refrigeration, automobile, process industries etc. The goal of enhanced heat transfer is to encourage or accommodate high heat fluxes.this result in reduction of heat exchanger size, which generally leads to less capital cost. Another advantage is the reduction of temperature driving force, which reduces the entropy generation and increases the second law efficiency. In addition, the heat transfer enhancement enables heat exchangers to operate at smaller velocity, but still achieve the same or even higher heat transfer coefficient. This means that a reduction of pressure drop, corresponding to less operating cost, may be achieved. All these advantages have made heat transfer enhancement technology attractive in heat exchanger applications. For shell and tube heat exchangers, the tube insert technology is one of the most common heat transfer enhancement technologies, particularly for the retrofit situation. With tube insert technology, additional exchangers can often be avoided and thus significant cost saving becomes possible. Furthermore as a heat exchanger becomes older, the resistance to heat transfer increases owing to fouling or scaling. These problems are more common for heat exchangers used in chemical industries and marine applications. In this case the heat transfer rate can be improved by introducing a disturbance in the fluid flow by different enhancement technologies (breaking the viscous and thermal boundary layer). In this projects, a review of heat transfer enhancement tool i.e. Inserting Twisted Tape Turbulators is done, for laminar and turbulent flow. Since it is most commonly used enhancement tool. Inner pipe OD=25mm Outer pipe ID =53mm Outer pipe OD =61mm Material of construction= Copper Heat transfer length= 2.43m Pressure tapping to pressure tapping length = 2.825m Water at room temperature was allowed to flow through the inner pipe while hot water (set point 60 C) flowed through the annulus side in the counter current direction. III. TYPES OF INSERTS USED For experimentation, three types of twisted tape inserts made from stainless steel strips of thickness 1.80 mm were used. 1. Reduced Width Twisted Tape(RWTT): Twisted tapes of width 16mm, thickness 1.80 mm were used in the inner pipe of ID 22mm as shown in fig 2. Fig 2. Reduced Width Twisted Tape (RWTT) 2. Baffled Reduced Width Twisted Tape (BRWTT1): Baffles in the shape of rectangular strips of size 16mm 10mm 1.80mm were attached in such a way that they were projecting at right angles on each side to the surface of twisted tape. A constant distance of 20 cm was kept in between two consecutive strips as shown in fig 3. Fig 1.Schematic diagram of Experimental Setup II. EXPERIMENTAL SETUP THE EXPERIMENTAL STUDY IS DONE IN A DOUBLE PIPE HEAT EXCHANGER HAVING THE SPECIFICATIONS AS LISTED BELOW:- Specifications of Heat Exchanger: Inner pipe ID = 22mm

Fig 3. Baffled Reduced Width Twisted Tape (BRWTT1) 3. Baffled Reduced Width Twisted Tape with holes (BRWTT2): In these twisted tapes, holes of diameter 6 mm were drilled at midpoint of two consecutive strips of BRWTT1 as shown in fig 4. manometer at room temperature. 3. Preparation of Wilson chart: Where Rd is the dirt resistance (1) all the resistance, excepts the first term on the RHS of equation (1) are constant for this set of experiments. For Re > 1000, seider tate equation for smooth tube is of the form : there fore equation can be written as (2) Where k is constant And it is found to be from the Wilson chart ( as the intercept on the y-axis K =6.613 After confirmation of validity of experimental values of friction factor and heat transfer coefficient in smooth tube with standard equations, friction factor and heat transfer studies with inserts were conducted. Standard equations:- I. Friction factor (fo) calculations: II. For Re 2100 f = (3) For Re 2100 f = (4) Heat transfer calculations i) Laminar Flow: For Re < 2100 Nu= f(gz) (5) Where Gz = For Gz<100, Hausen Equation is used. (6) Fig 4 Baffled Reduced Width Twisted Tape with holes (BRWTT2) 4. The present work deals with finding the friction factor and the heat transfer coefficient for the various types of twisted tapes with twist ratios (y w =3.69, 4.39, 5.25) and comparing those results with that of smooth tube and finally finding the heat transfer enhancement in comparison to a smooth tube on constant flow rate basis (R 1 ) as well as constant pumping basis (R 3 ) IV. EXPERIMENTAL PROCEDURE III. (8) IV. For Gz>100, Seider Tate equation is used (7) Transition Zone: For 2100<Re<10000, Hausen equation is used. ( ) ( ) Turbulent Zone: 1. Twist Ratio(y) of the twisted tapes were calculated. Twist Ratio, y w = H/W Where H = Linear distance of the tape for 180 rotation, W = Width of twisted tape. 2. For friction factor determination: Pressure drop is measured for each flow rate with the help of (9) Viscosity correction Factor is assumed to be equal to 1 for all calculations as this value for water in present case will be very close to 1 & the data for wall temperatures is not measured.

Sample calculation: 1. PRESSURE DROP & FRICTION FACTOR CALULATIONS: For BRWTT1 having y w =3.69 m=0.2090 Kg/sec Experimental friction factor Area A= = =3.8 10-04 m 2 m/sec Fig.6. Temperature in different RTDs = (1603-1000) = 4916 n/m 2 fa = = =63.29-3 For viscosity calculation: LMTD = Δ ΔT1 = T4 - T1 = (49.1-38.5) =10.5 ΔT2 = T3 T2 = (51.0-41.1) ) =9.9 Δ Δ Δ Q1 = mc Cpc (T2 - T1) =0.2090 4187 (41.1-38.5) = 2275W Q2 = mh Cph (T3 - T4) =0.2715 4187 (51.0-49.1) = 2160W Heat balance error = Qavg = (Q1+Q2)/2 = (2275+2160)/2 =2218 W Heat transfer Area, Ai = = 0.022 2.43 =0.1860 m 2 ( ) =1289 w/ =18089 hi can be calculated using Eq.1 Fig 5. Viscosity vs. Temperature 4 10-11 T 4-9 10-9 T 3 +9 10-7 T 2-5 10-5 T+0.0017 (10) Theoretical friction factor calculation for smooth tube: (11) Ha = 8721 W/m 2 C Theoretical Calculation for smooth tube = 17218 Fo = 0.046 = 0.046 = 6.54 10-3 (12) For Prandtl Number calculation: HEAT TRANSFER COEFFICIENT CALCULATION: For BRWTT1 having yw=3.69 mc= 0.209 kg/sec (750lph) & mh=0.2715kg/sec T1 = 38.5 T2 =41.1 T3 =51 T4 =49.1 Fig 6. Prandtl Number vs. Temperature (13) 3 10-7 T 4-8 10-5 T 3 +0.0072 10-7 T 2-0.387T+11.995

ho (hi for smooth): ho = ho=2601 w/ R 1 = =8721/2601=3.35 Pr (at T=Tavg) =3.695 To calculate fa at Re =18089, use correlation for fa vs.re for BRWTT1 having Yw=3.69 Fa = 0.9409 For equal pumping power, =0.06226 (14) A xo = = =3.8 10-04 m 2 A xa = = =3.48 10-04 m 2 Re o = 39341 ho at Reo(Equivalent Reynolds number in the smooth tube for same pumping power) 1) ho = Fig.7. Friction factor vs. Reynolds number for Smooth tube, RWTT, BRWTT1 & BRWTT2 Fig.8. shows the variation of fa/fo with Reynolds number for RRWTT, BRWTT1 &BRWTT2. 1. fa/fo increases with decrease in twist ratio due to increase in swirl flow created with decreasing twist ratio. 2. fa/fo is found to be maximum for BRWTT2 (8.86-14.44) followed by BRWTT1 (7.79-11.23) followed by RWTT (3.23-5.96) because degree of turbulence decreases in the same order. Fig.9. shows the correlations for friction factor for different twisted tapes for RRWTT, BRWTT1 & BRWTT2. These correlations were used while calculating performance evaluation criteria3. 2) ho = ho = 4843 W/m 2 C R 3 = = 1.80 V.RESULTS & DISCUSSION Fig.7. shows the variation of friction factor (fa) with Reynolds Number for Smooth tube, Reduced width twisted tape (RWTT), Baffled Reduced width twisted tape (BRWTT1), Baffled Reduced width twisted tape with holes (BRWTT2) for different twist ratios (yw=3.69,yw=4.39, yw=5.25). As the twist ratio decreases, a higher degree of swirl is created which leads to higher pressure drop & hence higher friction factor. In case of BRWTT1 & BRWTT2, a much higher friction factor is observed because of increase in degree of turbulence created by the respective tapes. Fig. 8. fa/fo vs. Reynolds Number for RRWTT, BRWTT1 & BRWTT2 Fig.9. Correlations for variation of Friction factor with Reynolds Number HEAT TRANSFER COEFFICIENT RESULTS: Fig.10. shows the variation of heat transfer coefficient (ha) with Reynolds Number for Smooth tube, Reduced width twisted tape (RWTT), Baffled Reduced width twisted tape (BRWTT1), Baffled Reduced width twisted tape with holes (BRWTT2) for different twist ratios (yw=3.69, yw=4.39, yw=5.25). As the twist ratio decreases, a higher degree of swirl is created which increases turbulence & hence the heat transfer coefficient increases as the twist ratio decreases. In case of BRWTT1 & BRWTT2, a much higher heat transfer coefficient is observed because of increase in degree of secondary flow created which disturbs the entire thermal boundary layer & hence the heat transfer coefficient increases as the twist ratio decreases.

Fig.10. Heat transfer coefficient vs. Reynolds Number for Smooth tube, RWTT, BRWTT1 & BRWTT2 Plot for Performance evaluation criteria, R1 (based on constant flow rate) vs. Reynolds number for different tapes is shown in Fig.11. Fig.11. Performance evaluation criteria, R1 vs. Reynolds Number for RWTT, BRWTT1, BRWTT2 Plot for Performance evaluation criteria, R3 (based on constant pressure drop) vs. Reynolds number for different tapes is shown in Fig.12. For obtaining R3, correlation between friction factor & Reynolds Number for respective tape is used. Fig.11. Performance evaluation criteria, R3 vs. Reynolds Number for RWTT, BRWTT1, BRWTT2 VI.CONCLUSION Experimental investigations have been carried out to examine the effect of RWTT, BRWTT 1, BRWTT 2 With different twist ratio within the pipe. Depending on the analysis of the result the following conclusion can be carried out. I. For same twist ratio, Baffled reduced width twisted tape with holes & Baffled reduced width twisted tape shows higher heat transfer coefficient & friction factor increase because of higher degree of turbulence created. II. III. For same twist ratio, Baffled reduced width twisted tape with holes & Baffled reduced width twisted tape gives higher heat transfer coefficient than the reduced width twisted tapes. The correlations derived from friction factor values have R2 (Correlation coefficient) values very close to 1. So, the correlations can be used for finding friction factor values for respective designs in the given range of Reynolds number. IV. On the basis of performance evaluation criterion R3, twisted tapes-brwtt 1 & BRWTT 2 with y w = 3.9 were found to be the augmentations. Further studies can be done using this study as base. Some of the possibilities are mentioned below: I. Experimental work can be done at low Reynolds number using viscous liquids, as the tapes have shown comparatively better results at low Reynolds number II. III. istance between two consecutive baffles & holes can be varied to see their effect on heat transfer & friction factor. eometry of baffles: Circular baffles instead of rectangular baffles can be used. Baffles can be kept at an angle to flow of liquid instead of putting them perpendicular to flow of liquid. Size of baffles can be varied. The proposed designs can be used for cooling of liquids NOMENCLATURE Ai Heat transfer area, m 2 A xa Cross- section area of tube with twisted tape, m 2 A xo Cross-section area of tube, Cp Specific heat of fluid, J/Kg.K d i ID of inside tube, m d o OD of inside tube, m f Fanning friction factor, Dimensionless Friction factor fa for the tube with inserts, Dimensionless

f 0 Theoretical friction factor for smooth tube,dimensionless g acceleration due to gravity, m/s 2 Gz Graetz Number, Dimensionless h Heat transfer coefficient, W/m 2 C h a Heat transfer coefficient for tube with inserts,w/m 2 C h o Heat transfer coefficient for smooth tube, W/m 2 C h i(exp) Experimental Heat transfer coefficient, W/m 2 C h i(thero) Theoretical Heat transfer coefficient, W/m 2 C H Pitch of twisted tape for 180 rotation, L heat exchanger length, m LMTD Log mean temperature difference, C m Mass flow rate, kg/sec Nu Nusselt Number, Dimensionless Pr Prandtl number, dimensionless Q Heat transfer rate, W Re Reynolds Number, Dimensionless R 1 Performance evaluation criteria based on constant flow rate, Dimensionless R3 Performance evaluation criteria based on constant pumping power, Dimensionless Ui Overall heat transfer coefficient based on inside surface area, W/m 2 C v flow velocity, m/s 2 W Width of twisted tape, m w Width ratio (w/d i ) Dimensionless Twist ratio of twisted tape (H/W), Dimensionless y w Greek letters h Height difference in manometer, m P Pressure difference across heat exchanger, N/m 2 µ Viscosity of the fluid, N s/m 2 µ b Viscosity of fluid at bulk temperature, N s/m 2 µ w Viscosity of fluid at wall temperature, N s/m 2 δ Density of the fluid, kg/m 3 ACKNOWLEDGMENT I wish to express my sincere gratitude to Dr. V. 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