Review on Thermal Performance of cross Flow Heat Exchanger Using Non-Circular Shape of Tubes

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1 Review on Thermal Performance of cross Flow Heat Exchanger Using Non-Circular Shape of Tubes 1 Snehal A. Powar, 2 Ashish R. Wankhade, 3 Neelam Gohel 1 snehal @gmail.com, 2 wankhade_ashish@ymail.com, 3 nsgohel@sinhgad.edu Abstract: The shape of tubes affects the thermal and hydraulic performance of heat exchanger. Circular tubes are widely used in heat exchanger, circular shape causes severe separation of boundary layer and large wake region formation due to which produce high pressure drop observed in the shell side fluid. This study provides a literature review on different shape of tubes such as oval, cam and wing shape tubes used in cross flow heat exchanger. The friction factor for oval shape tubes are less compare to circular tubes, which helps to decrease the wake region, pressure drop and hence required less pumping power. Cam, wing shape tubes study shows increase in heat transfer rate as the angle of attack increases. Keywords:Cam shape of tube, drag coefficient, friction factor,nusselt number,oval shape of tube, pressure drop, Wing shape of tube I. INTRODUCTION Heat exchangers have wide range of applications in the area of thermal power plant, automobile, waste heat recovery system, air conditioning and refrigeration system. The design of heat exchanger which should give high thermal performance and economical is preferable. The design of cross flow heat exchanger using non circular shape tube provides high heat transfer rate, compact design, require less pumping power hence reduces the cost. To manufacture the cross flow heat exchanger different shape of tubes are used. Circular tubes are widely used due to ease of manufacturing process, it causes severe separation flow and large wake region produces with high pressure drop. To overcome the problem of flow separation and wake region one of the method is changing the shape of tube. In recent years, lot of study is going on the change in shape of tubes. Study is carried out on different shapes such as oval, flat, elliptical, cam, wing, hexagonal. A. Nouri-Borujerdi [1, 2], studied the heat transfer characteristics and pressure distribution of an isothermal cam shaped tube in cross flow. The angle of attack is varied in the range of 0 < α <180 with Reynolds number <Re eq < The result show that the heat transfer from a cam shaped tube is maximum at α = 90 and minimum at α =30. Cam shape tube give larger value of St/Cd except at α = 90 and 120 relative to the circular tube. Ala Hasan[3] studied the thermal-hydraulic performance of oval tubes in a cross-flow of air. It is seen from result that Nusselt number Nu D for oval tubes are close to that for circular tube for Re D < 4000 and for higher Reynolds number, Nu D is lower than that of circular tube and it decreases with increase in the axis ratio R. The drag coefficient C d is also measured; it shows better combined thermal-hydraulic performance of oval tubes compare to circular tube. AlaHasan [3] performs the investigation on circular and oval tube in an evaporative cooled heat exchanger under similar operating condition. Result found that average mass transfer Colburn factor (j m ) for oval tube is 89% that of circular tube and the average friction factor (f) for oval tube is 46% that of circular tube. This shows that oval tube have better combined thermal-hydraulic performance than circular tube. Mesbah G. Khan [5],carried out the experiment for characterization of cross-flow cooling of air via an inline elliptical tube array. For experimental analysis, range of Reynolds number for air side is < Re a < and for water side < is considered. Result shows that Nusselt number and hence the heat transfer rate increases with increase in Reynolds number in a power law fashion. Nu-Re correlation is found as Nu a = 0.26Re a 0.66 Sayed Ahmed E[8] study the cross flow air-cooling process via water-cooled wing-shaped tubes in staggered arrangement at different angles of attack. In this study both experimental and CFD analysis is carried out for different angle of attack. For water Re w = 500 and for air Re a =1800 to 9700.Results were obtained that wing shape tube bundle in heat exchanger gives best result for heat transfer coefficient, effectiveness, efficiency at zero angle of attack and as the value of Re a increases,nu a increases whereasst a decreases. The highest value and lowest value of Nu a andst a occurred at θ= 45 o, 135 o, 225 o, 315 o and at θ= 0, 180 respectively. From experimental results they develop new correlation for Nu a and St a number in terms of Re a, Pr and angle of attack and compare with previous work which shows thatnu a increased by 24% compare to circular tube 24

2 whereas by 76% compare to elliptic tube at zero angle of attack. The objective of this paper is to present a review on the work done on different shape of tubes in cross flow heat exchanger. Tubes used for studies are oval, cam and wing shape tubes. II. CROSS FLOW ACROSS THE CIRCULAR CYLINDER Analysis of any heat exchanger both internal and external flow should be considered, the internal flow through the tubes and external flow over the tubes.when a fluid flow over a stationary solid body, body experiences two types of forces, drag and lift.the drag force is the net force exerted by a fluid on a body in the direction of flow due to combined effects of wall shear and pressure forces. The components of the pressure and wall shear forces in the normal direction to flow tend to move the body in that direction is called lift force. The drag force F D depends on the density ρ of the fluid, the upstream velocity V and size, shape and orientation of the body. The drag characteristics of a body is represented by the dimensionless drag coefficient C D, defined as Figure1. Boundary Layer Separation and Wake Region [6] C D = F D 1 (1) 2 ρav2 Where A is the frontal area, the area projected on a plane normal to the direction of flow. The drag coefficient is primarily a function of the shape of tube, Reynolds number and surface roughness. The total drag coefficient is given by C D = C D,friction + C D,pressure (2) The wall shear stress is produces due to friction between fluid and tube wall, this is also called as skin friction drag, C D, friction. The friction drag is proportional to the surface area. Therefore larger surface area experiences a larger friction drag. The pressure drag is also called as form drag because it is strongly depend on the form or shape of the body, C D. The pressure drag is proportional to the frontal area and the difference between the pressure acting on the front and back of the immersed body. When a flow is separated from the body, it creates a separator region due to low pressure behind the body, where recirculation and backflow of fluid occurs. As the separated region increases, pressure drag is increases. The separated region ends when the two separated flow streams reattach. The region of flow trailing the body where the effects of the body on velocity are felt is called wake as shown in fig (1). The wake region keeps on growing behind the body until the fluid in the wake region regain its velocity. Figure 2. Variation of Nu θ along the circumference of a circular cylinder [6] The phenomena, affect the drag force also affect the heat transfer and this effect comes into Nusselt number. The variation in local Nusselt number Nu θ around the periphery of a cylinder is shown in fig.(2). For all values of Nu θ is high at θ = 0 and it decreases as the value of θ increase, it is minimum at θ = 80, which is separation point in laminar flow. At θ = 90 sudden increase in value of Nu θ because of transition from laminar to turbulent flow. Because of thickening of the boundary layer it reaches to second minimum at θ =140, which is separation point in turbulent flow. [6] As the formation of wake region is large and also boundary layer separates in circular tube, so to reduce the formation of wake region and separation of flow, changing the shape of tube is one method. As we changes the shape of tube from circular to oval, elliptical, cam or wing shape, it help to reduce formation of wake region and separation of boundary layer. It helps to decrease the drag force on the tube. III. OVAL SHAPE TUBES IN CROSS FLOW HEAT EXCHANGER Study the thermal-hydraulic performance of oval shape tube compare to circular tube, experimentation is carried 25

3 out in two evaporative cooled heat exchangers, in which one is having ovalshape of tube and other is having circular shape of tube under similar operating conditions.the heat exchanger cross-sectional area is mm 3. The oval shaped tubes are manufacture from circular copper tube. The circular tube of 18 mm OD is heated then press and formed oval shape of tube having major axis of 25.3mm and minor axis of 8.2mm. In an evaporative cooled heat exchanger, a thin layer of water is formed on the surface of tube by spraying water. The heat is transferred from hot water flowing inside the tube, to a thin layer of water on the surface of tube then to cold airpass over the tubes. Heat transfer from a thin layer of water to cooled air contains both latent and sensible heat. Latent heat is formed due to evaporation of sprayed water whereas sensible heat is formed due to temperature difference between sprayed water and cooled air. [3] θ = t h1 t h2 (t h1 t wb 1 )D (3) It is concluded from the graphfig (4), that θ is lower for oval tubes, on average it is 79% of that of circular tube, because circular tubes have large frontal area and the higher turbulence is induced on its backside which helps to increase the rate of heat and mass transfer. Combined Thermal-Hydraulic Performance Pressure drop in air flow over the oval and circular tube bank is measured to calculate friction factor. The friction factor is calculated as f = p 0.5ρv x 2 (4) Where p is pressure drop across one tube row.the mass transfer Colburn factor j m is calculated using equation (5)whereSh is Sherwood number, convective mass transfer coefficient given by S = m l D and Sc is Schmidt number, given byθ D. j m = S ReSc 1/3 (5) Figure 3. Distribution of Circular and Oval Tubes In Test Section [3] Thermal Performance of Circular and Oval Shape Tubes To analyzed the thermal performance of tubes heat flux is selected as thermal performance parameter θ which is defined as heat transfer per surface area given as follows: Fig (5) show the graph of f vs. Re, it show that the friction factor (f) for oval shape tubes have lower value compare circular shape of tubes. Lower friction factor requires less energy for air movement across the tube to achieve required heat transfer rate and as the friction factor f, for oval tube is 46 % that of circular tube, the use of oval tubes save the energy.in similar fig (5) graph of j m vs Re and j m /f vs Re as shown to analyzed the combined thermal-hydraulic performance of oval tubes compare to circular tube. The graph show that the value of mass transfer Colburn factor j m for oval tube is 89% that of circular tube and the ratio of j m /f for oval tube is times that of circular tube. Figure 4. Thermal Performance Parameter θ vs. Air Velocity[3] Figure 5. Mass Transfer Colburn Factor j m and Friction Factor f For Circular and Oval Tubes[3] IV. CAM SHAPE OF TUBE IN CROSS FLOW HEAT EXCHANGER The experiments are conducted on cam shape of tube to study the pressure distribution and drag coefficient over the surface of tube. The tubes are made up of copper 26

4 plate having thickness of 0.3 mm and a length of 120 mm. The three tubes have same diameters D = 22 mm and d = 12 mm with different distance between the centers, l = 11 mm, 29 mm, 66mm. To measure the pressure distribution over the tube surface, 20 holes are drilled with 18 of intervals at the centre. The angle of attack is measured between flow direction and the axis of tube in clockwise direction. [2] Figure 8.Pressure drag coefficient of a cam shaped and circular tubes vs. angle of attack [2] Figure 6. Schematic of Cam Shaped Tube and Thermocouple Location In Cross Flow [2] The pressure coefficient C p is calculated as: C p = P i P 1 2 ρu 2 (6) The pressure drag coefficient C D is defined as: C D = 20 1 C p,i cos ψ i S i /D eq (7) Where, P Pressure,U - Velocity, ψ i - hole angle, S i - streamline coordinate From experimental data graphs are plotted, Fig. (7) show the pressure distribution over the surface of cam shape tube at α = 0, U = 15m/s for different Re eq and corresponding l/d eq. Positive and negative values from X-axis indicates (S/D eq ) the measured distance along upper and lower part of the cylinder respectively. For l/d eq = 0.4, 0.8, 1.1, flow separates at S/Deq = ±0.75, ±0.85, ±0.5and reattach ats/deq = 1.3, 0.8. This pressure distribution decreases the drag produce around the surface of tube. As shown in Fig. (8) at different values of Reynolds number, curves are repeated after every 150 degree. Maximum value of C D is at α = 90 and 270 and minimum value of C D is appear at α = 30, 180, 330. Atα = 180, the laminar boundary layer is formed over the most part of the tube surface and separation is delayed which help to reduce the extent of the wake region and the magnitude of the form drag. Figure 9. The average Nusselt number ratio of the camshaped tube to an equivalent circular tube [2] Fig. (9)show The maximum value of Nu cam /Nu cir >1 at α = 90 and 270, corresponding values are 1.05 and 1.8 for various Reynolds number. The Minimum value of Nu cam /Nu cir 1 at α = 180 is 0.87, this is because large wake region form below critical Reynolds number over a large region of cam shape tube. V. WING SHAPE TUBES IN CROSS FLOW HEAT EXCHANGER Figure 7. Pressure Coefficient For Three Different l/d eq and Reynolds Number [2] The experiments were conducted in open wind tunnel. The test section of size mm 3 is placed in the middle of the wind tunnel. The wingshaped tube dimensions are shown in fig.(10). Tubes are fixed in such way that angle of attack can be adjusted by turning the tubes. The air was passed over the tubes bundle and water was flowing inside the tubes. Air was heated with the help of 4kW electric heater up totemperature of56.5 ± 1.5 DBT and 26 ± 1.5 WBT. A 134A refrigeration system used to maintain inlet temperatureof water at 10.8 ± 1.5. T-type of 27

5 thermocouples was used to measure the temperature and air side pressure drop measured by an electric micromanometer. [8] Figure 10. Dimensions of wing shape tube in mm [8] Q w = m w C pw T we T wi = m w C pw T w (9) Q a = m a C af T ai T ae = m a C af T a (10) The average waterside and airside heat transfer Q was taken for analysis as, Q = Q a + Q w 2 (11) Heat transfer through condensation and radiation is neglected. Therefore, the overall heat transfer rate Q = Q convecti on = a A so T ln (12) Where, A so is the total outer surface area for the tubes and T ln is the logarithmic mean temperature difference calculated as follows: T ln = T ai T ae ln (T ai T s ) (T ae T s ) (13) Air side average heat transfer coefficient is calculated as: a = Q A so T ln (14) The Reynolds Number Re a is given by: Re a = ρ af V ai D eq μ af (15) Figure 11. Schematic Drawing of Experimental Setup [8] Table 1. Drawing Legend Sr. Description Sr. Description No. No. 1 Electrical Heater 9 Water Control valves 2 Honey Comp 10 Water pump 3 Pitot-Static Tube 11 Electrical micromanometer 4 Pressure Taps 12 Water tank 5 Test Section 13 Evaporator 6 Gate hp Compressor 7 Fan 15 Expansion valve 8 Electrical motor Experimental data was collected after min, the steady state is reached. The water flow rate was measured at the beginning of test run. The mean air velocity was calculated by eq.(8). For analysis, Reynolds number of water (Re w =500) kept constant and air side Reynolds number is varied from Re a = V ai = 2g( ρ w ρ a f ) dyn (8) The Nusselt Number Nu a is given by: Nu a = ad eq k af (16) Where,D eq is outer equivalent diameter of the tube and k af is the air thermal conductivity, W/mK. As shown in fig (12), Re a plotted on X-axis and Nu a on Y-axis for different angle of attack. Graph show that Nusselt number increases as Reynolds number and angle of attack increases. Increase in Nusselt number helps increase in the turbulent intensity and also to enhance the convective heat transfer. Figure 12.Nu a vs. Re a For Different Angle of Attack [8] Heat transfer on water side and air side are calculated by equation (9) and (10) respectively. 28

6 Figure 13.Nu a vs. angle of attack (θ 1, 2, 3 ) at different Re a [8] Fig.13 show that, Nu a versus angle of attack (θ 1,2,3 ) at different Reynolds number are plotted. The highest values of Nu a are occurred at θ 1,2,3 =45,135,225,315 whereas lowest values of Nu a are occurred at θ 1,2,3 =0,180. Based on experimental results, correlation between Nu a, Re a, Pr and at different angle of attack is obtained as follows: Nu a = a. Re a b. Pr θ θ 90 (17) Where a, b, c are constant, mention in table, this correlation is applicable for1800 Re a Table.2. Constants for Proposed Correlation for Nusselt Number [8] Angle A B C 0 θ 1,2, θ 1,2, θ 1,2, θ 1,2, Figure14. Comparison of Present Nu a vs Re a Results with Different Tube Shapes at0 Angle of Attack [8] The result of new correlation between Nu a - Re a at different angle of attack, obtained from wing shaped tube is compared with other shape of tubes such as circular and elliptical at zero angle of attack fig.(14). The study shows that Nu a increases, by 24% compare to circular tubes and by 76% compared to elliptical tubes bundle. SUMMARY The thermal hydraulic performance of oval shape tube shows better performance than circular tube. Friction factor for oval shape tube is decreases compare to circular tube which helps to reduce wake region and hence it reduces the drag force, pressure drop. Study of cam shape tube show the reduction in pressure drag coefficient and increase in heat transfer coefficient. In case of wing shape tube, Nusselt number for air side increases with increase in Reynolds number and angle of attack. The heat transfer enhancement and better thermal-hydraulic performance is achieved by changing the shape of tube. It also helps to reduces large wake region and separation of flow, which reduces the drag force. The heat exchanger with different shape of tube requires less pumping power, which helps to save energy. The size of heat exchanger is reduced which gives the compact and less costly heat exchanger. NOMENCLATURE Alphabet Upper Case C D Pressure drag coefficient C p Pressure coefficient D eq Equivalent circular diameter, m Q Heat transfer rate, W S Streamline coordinate U Velocity, m/s Greek Letters α angle of attack, thermal diffusivity θ thermal performance parameter ρdensity μdynamicviscocity Δdifference Dimensionless number f Friction factor, j m mass transfer Colburn factor Nu Nusselt number Re Reynolds number Sc Schmidt number Sh Sherwood number Subscripts a Air eq equivalent i Inlet k thermal conductivity m mass flow rate o Outlet w water freestream REFERENCES [1]. A.Nouri-Borujerdi,A.M. Lavasani, "Experimental study of forced convection heat transfer from a cam shaped tubes in cross 29

7 flows, International Journal of Heat and Mass Transfer, 50 (2007) [2]. A.Nouri-Borujerdi, A.M. Lavasani, Pressure loss and heat transfer characterization of a cam shaped cylinder at different orientation, Journal of Heat Transfer, Vol. 130/ (Dec. 2008). [3]. AlaHasan, Kai Siren, Performance investigation of plain circular and oval tube evaporatively cooled heat exchanger, Applied Thermal Engineering 24 (2004) [4]. G. P. Merker, H. Hanke, Heat transfer and pressure drop on the shell-side of tubes-banks having oval-shaped tubes, Int. J. Heat Transfer, Vol. 29, No. 12,pp [5]. Mesbah G. Khan, Amir Fartaj, David S.-K. Ting, An experimental characterization of cross-flow cooling of air via an in-line elliptical tube array, International Journal of Heat and Fluid Flow 25 (2004) [6]. S. Tiwari, D. Maurya, G. Biswas, V. Eswaran, Heat transfer enhancement in cross-flow heat exchangers using oval tubes and multiple delta winglets,international Journal of Heat and mass transfer,46(2003) [7]. S. A. Nada, H. El-Batsh, M. Moawed, Heat transfer and fluid flow around semi-circular tube in cross flow at different orientations, Heat Mass Transfer (2007) 43: [8] Sayed Ahmed E. Sayed Ahmed, Emad Z. Ibrahiem, Osama M. Mesalhy, Mohamed A. Abdelatief, Study of cross flow air-cooling process via water-cooled wing-shaped tubes in staggered arrangement at different angles of attack, Part 2: heat transfer characteristic and thermal performance criteria, World academy of science, Engineering and Technology, Vol: [9]. Cengel,Y. A. and Ghajar, A. J., Heat and Mass Transfer, 4 th ed., Tata McGraw Hill, pp [10]. Shah, R. S. and Sekulic, D. P., Fundamentals of Heat Exchanger Design, John Wiley and sons, pp 1-3,