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1 SPECIAL ISSUE (ICRAME-2015) International Conference on Recent Advances in Mechanical Engineering In collaboration with International Journal of Engineering and Management Research (IJEMR) Page Number: Numerical Analysis of Multi-Stream Cross-Counter Flow Heat Exchanger using Computational Fluid Dynamics Arvind A. Dev 1, P. M. Ardhapurkar 2 1 UG Student, 2 Professor, Mechanical Engineering Department, S. S. G. M. College of Engineering, Shegaon (M.S), INDIA ABSTRACT Heat exchangers are widely used in many industries such as diary, food, beverage and pharmaceutical industries. The performance of heat exchanger depends on the relative flow arrangements of fluids in addition to various operating parameters. Multi-stream heat exchangers are important for the applications which demand higher effectiveness. In the present work, simulation of the cross-counter flow type of heat exchanger is carried out. It is found that the crosscounter flow arrangement is better than either purely cross or counter arrangement in the case of multi-stream heat exchanger. Further, the paper also studies and suggests the best proportion of the cold fluid from cross and counter direction to get the maximum heat transfer performance. Keywords Heat exchanger; cross-counter flow; effectiveness; Computational Fluid Dynamics (CFD). I. INTRODUCTION There are many types of heat exchangers which are used based on the applications. Computational fluid dynamics (CFD) is used as a tool to model any heat exchanger. It has been widely reported by many researchers that a CFD code can be useful to predict the heat transfer. Heat exchanger can be classified on the basis of heat exchange process, geometry, number of fluids, flow arrangements etc. Depending on the number of fluids, these can be referred as two-stream heat exchanger or multi-stream heat exchanger. There are numerous experimental and numerical studies related to design and performance analysis of conventional two-stream heat exchanger. These studies mainly include different aspects of heat exchanger such as flow arrangement, effect of geometry parameters, heat transfer enhancement, etc. H. Van Der Vyver et al. (2003) studied a tube-in-tube counter flow heat exchanger. They analysed and compared experimental results with the CFD results. Further, friction factor is also studied using analytical correlation, CFD and experimentation. Vikas Kumar et al. (2003) considered a cross flow air to air, tubular heat exchanger and observed the relation between heat transfer and Dean s number, keeping the flow rate constant in the outer region. It is assumed that heat loss occurred through convection from the top surface of the exchanger only, whereas all radiation losses are neglected in their study. M. Sneha Priya and G. Jamuna Rani (2012) carried out two-dimensional numerical study on cross-flow heat exchanger. They analyzed effects of the mass flow rate and tube diameter on the heat transfer rate. The software - GAMBIT is used for modelling and meshing of the heat exchanger. They also studied the effect of the tube material on the performance of heat exchanger. Patel et al. (2012) performed thermal analysis of tubular heat exchanger using CFD technique. They studied the effect of different tube materials on rate of heat transfer. In literature, few studies are devoted on multi-stream heat exchanger. O. García-Valladares (2004) carried out numerical simulation of triple concentric tube heat exchanger. Thermal and hydraulic performance of the heat exchanger is carried out under both the steady and the transient conditions. They also compared the performance of heat exchanger with that of parallel and counter flow heat exchanger arrangement. The effectiveness of the multi-stream heat exchanger is crucial to determine its performance. However, literature review suggests that there are only few studies on the performance of the multi-stream heat exchanger. Additionally, the analytical solutions for these heat exchangers are complex; therefore, it is of more significance to study such heat exchanger with the help of numerical techniques. Further, the reported studies on heat exchanger are either related to cross flow or counter flow arrangements of heat exchanger. 40 Copyright Vandana Publications. All Rights Reserved.

2 In the present work, the multi-stream heat exchanger selected is cross-counter flow type. The objective is to evaluate the performance of the heat exchanger in comparison with the counter flow and cross flow arrangement. II. CFD METHODOLOGY In the present work, heat exchanger geometry is created in Ansys TM workbench. ICEM-CFD is used to generate the mesh. The results are obtained in CFX solver after specifying the boundary conditions and the solver parameters. The details of heat exchanger geometry and mesh quality are presented in the following sections. A. Heat exchanger modeling The heat exchanger investigated is a cross-counter flow type multi-stream heat exchanger. Fig. 1, shows the geometry of the present cross-counter flow heat exchanger studied. There is one hot stream flowing through the annulus area formed due to two concentric tubes and two cold streams flowing through the heat exchanger. The cold fluid flows through the inner tube in counter direction to the hot fluid as well as in cross direction to the hot fluid over the surface of the outer tube. Table I shows the specifications of the heat exchanger. B. Grid Generation The unstructured type mesh is generated using ICEM- CFD. The prism mesh is created nearby the surface of the tube to study the boundary layer phenomenon properly. Fig. 2 shows the meshing of cross-counter flow heat exchanger. The whole geometry is symmetrical about an axis; therefore, only half symmetrical geometry is taken for mesh analysis. This reduces the cell count leading to the reduced computational time. In order to obtain mesh independent solution, the mesh density is increased in steps, systematically, from 8 cells/mm 3 to 12 cells/mm 3. Further, increase in cell density up to16 cells/mm 3, does not show any significant improvement in the results. In the present work, the results are obtained with cell density as 14 cells/mm 3. The mesh quality corresponding to this cell density is 0.3. Table II shows the number of nodes and elements created for the simulation of the heat exchanger. Fig. 1.Geometry of the cross-counter flow arrangement of heat exchanger Table I. PARAMETERS OF CROSS-COUNTER FLOW MULTI-STREAM HEAT EXCHANGER Parameter Dimension (mm) Inner tube inner diameter, din 10.5 Outer tube inner diameter, Din 23 Thickness of tube, t 1 Cross flow rectangular opening 31x500 Length of heat exchanger, L 500 Fig. 2. Grid of cross-counter flow arrangement for CFD simulation TABLE II. MESH STATISTICS Domain Nodes Elements Counter cold fluid Cross fluid Hot fluid Inner tube solid Outer tube solid All Domains C. Solver parameters and boundary conditions Table III shows the boundary conditions and solver parameters. Symmetry is applied to the plane about which the geometry is symmetrical. Various cases have been modeled with different mass flow rate conditions for cold fluid in counter and cross flow direction. However, the total flow rate of the cold water is kept constant at 0.25 Kg/s. 41 Copyright Vandana Publications. All Rights Reserved. III. VALIDATION OF SIMULATION PROGRAM Validation of simulation program is achieved in two parts. In the first part of validation, the simulation results are compared against the experimental data which

3 is available in the literature [1] for the counter flow arrangement. In the later part, analytical results are obtained for counter flow and cross flow arrangement. These analytical results are compared against the simulation results for counter flow and cross flow arrangement. TABLE III. SOLVER PARAMETERS AND BOUNDARY CONDITION FOR CROSS-COUNTER FLOW HEAT EXCHANGER Parameters Values Domain Fluid, solid Fluid Water Solid Copper Mass flow rate of hot fluid (kg/s) 0.35 Hot fluid inlet temperature (K) 363 Total mass flow rate of cold fluid (kg/s) 0.25 Cold fluid inlet temperature (K) 283 Relative pressure at outlet (Pa) 0 Heat transfer model Thermal energy Turbulence model Shear stress transport (SST) Solver scheme Upwind Convergence criteria (energy equation) Convergence criteria (Mass and momentum equation) A. Simulation results for counter flow arrangement In order to validate the simulation results against the available experimental results [1], initially, simple tube-in tube heat exchanger is simulated. The specifications of the counter flow heat exchanger are taken from the literature, for which experimental results are available. Table IV shows the dimensions and boundary conditions of the heat exchanger used for this simulation. TABLE IV. PARAMETERS OF FLOW DOMAIN AND BOUNDARY CONDITIONS FOR COUNTER FLOW HEAT EXCHANGER Solid Copper Cold fluid velocity at inlet (m/s) 1, 2.8, 4.9, 7 Hot fluid velocity at inlet (m/s) 0.5, 0.8, 1.4, 2 Cold fluid Temperature at inlet (K) 283 Hot fluid Temperature at inlet (K) 355 Fig. 3, shows the comparison between experimental and simulation results. It is noted that the simulation results using CFD matches well with that of the experimental data. Nusselt number Simulation Results Experimental Results [1] Reynolds number Fig. 3. Comparison of experimental and simulation results of a tube-in-tube counter flow heat exchanger. B. Validation with analytical results of purely counter flow heat exchanger. The individual counter flow heat exchanger is simulated in CFD and the results are compared with the analytical results. The cold fluid flows through the inner area and the hot fluid flows through the annulus area. The dimensions of the flow domain are kept same as that of the proposed multi-stream heat exchanger but limited to the counter flow arrangement. Fig. 4 shows the grid for the purely counter flow heat exchanger. Parameters Values Inner tube s inside diameter (mm) 8 Inner tube s outer diameter (mm) 10 Tube thickness (mm) 1 Inside diameter of outer tube (mm) 16 Domain Fluid, solid Fluid Water Fig. 4. Grid of purely counter flow heat exchanger arrangement 42 Copyright Vandana Publications. All Rights Reserved.

4 The temperature at the outlet of the counter flow heat exchanger for both the hot and the cold fluid are predicted using ε NTU method. The forced convective heat transfer coefficients are calculated using Dittus-Boelter correlation as given in Eq. (1). symmetrical geometry which reduces computational time and provides option for dense meshing Nu = Re Pr (1) where Nu is Nusselt Number, Re is Reynolds Number, Pr is Prandtl Number. In ε NTU method, effectiveness, ε for counter flow heat exchanger is defined in Eq. (2). 1 exp[ NTU (1 R)] ε = 1 R exp[ NTU(1 R)] (2) Fig. 5. CFD modeling of a purely cross flow heat exchanger where R= C min / C max (3) C min and max C are minimum and maximum heat capacity rates, respectively. NTU is number of transfer units for the heat exchanger which is given in Eq. (4). U NTU = C (4) min Table V compares simulation results against theoretical predictions. It is noted that the predicted temperatures of the hot and the cold fluid at the outlet using simulation are close to those obtained analytically. TABLE V. VALIDATION AND SIMULATION RESULTS FOR PURELY COUNTER FLOW HEAT EXCHANGER Predicted temperature, K Condition Simulation Analytical Inlet Cold fluid Outlet inlet Hot fluid outlet C. Validation with analytical results of purely cross flow heat exchanger Purely cross flow is simulated and the results are compared with the analytical results. Fig. 5 and Fig. 6 show the model and the grid of the purely cross flow arrangement, respectively. Grid is generated for half Fig. 6. Meshing of purely cross flow heat exchanger The outlet temperatures of the hot and the cold fluid are predicted using Churchill s correlation and ε NTU method. Churchill s correlation for cross flow is given in Eq. (5). Nu. (5) cyl hd 0.62 Re Pr Re = = k Pr where h is convective heat transfer coefficient, k is thermal conductivity of the fluid. The effectiveness of heat exchanger for cross flow arrangement when both the fluids are unmixed, is expressed in Eq. (6). Table VI shows the outlet temperatures predicted using empirical correlation and simulation method. It is clear that the outlet temperatures for the cold and the hot fluids obtained using simulation are close to those obtained analytically NTU 0.78 ε = 1 exp( [exp( RNTU ) 1]) R (6) 43 Copyright Vandana Publications. All Rights Reserved.

5 TABLE VI. COMPARISON OF ANALYTICAL AND CFD RESULTS FOR PURELY CROSS FLOW HEAT EXCHANGER Fluid Cold/Cross Fluid Hot Fluid Location Temperature, K Simulation Analytical inlet outlet inlet outlet Fig. 8. Temperature contours along the length of heat exchanger vertically. IV. RESULTS AND DISCUSSION The present arrangement comprises of two cold streams. One cold stream is in counter direction to the hot fluid through the inner area and another is in cross direction to the hot fluid over the outer tube. The total mass flow of the both the cold streams is 0.25 kg/s. Various simulations are carried out for varying mass flow of cooling fluid through counter and cross direction to the hot fluid. It is assumed that both the cooling fluids are mixed after heat exchange. The cold water flows over the outer tube and also through the inner tube. Fig. 7 shows the stream lines for the cold fluid flow over the outer tube surface. The present arrangement comprises of two co Fig. 9. Temperature contours on counter cold fluid outlet plane In the cross-counter arrangement pressure drop characteristics of the inner cold fluid are studied using CFD and compared with the analytical pressure drop calculations. The dimensionless number that incorporates the pressure drop through the heat exchanger is the friction factor, f. It is calculated using Eq. (7). Fig. 7. Cross flow over the outer tube The change in temperature of the three fluids can be seen from temperature contours obtained on the section plane of three fluids. Fig. 8 and Fig. 9 show the temperature contours in the domain. 2d h p f = 2 ρlv (7) where p is pressure drop in flow length L; v and ρ are velocity and density of flow, respectively. Analytically, friction factor, f is calculated by ( 1.82 log Re 1.64) 2 f = 10 (8) Fig.10 shows the variation in friction factor for inner cold fluid with percentage of cross fluid. It is seen that the pressure drop of inner cold fluid increases with the increase in the percentage of the cross fluid flow. The variation in friction factor calculated using CFD and analytical method shows similar trend. 44 Copyright Vandana Publications. All Rights Reserved.

6 Friction factor CFD FRICTION FACTOR ANALYTICAL FRICTION FACTOR Fraction of total cold fluid in cross direction (%) Fig. 10. Variation of friction factor with percentage of cross fluid flow Table VII shows the variation in mass flow rate of cold fluid in cross direction to the hot fluid. The mass flow rate of the cold fluid flowing in counter and cross flow direction is varied; however, total mass flow rate of the cold fluid in both, cross and counter flow directions is kept constant at 0.25 kg/s. It is noted that, the effectiveness of the heat exchanger depends on the fraction of cold fluid flow in cross and counter flow direction to the hot fluid. In the present case, effectiveness of the heat exchanger is calculated for various mass flow rates of the cold fluid in cross direction. Fig. 11 shows the variation of effectiveness with percentage of cross fluid flow. It is assumed that the two cooling fluids, i.e., cold fluid in counter direction to the hot fluid and cold fluid in cross direction to the hot fluid, after heat exchange get mixed with each other and reach to a common temperature. The mixing temperature is determined by applying energy balance equation to both the fluids. The effectiveness corresponding to 0 % cross fluid refers to the purely counter flow and that corresponding to 100 % cross fluid refers to the purely cross flow arrangement of the heat exchanger. Effectiveness Fraction of total cold fluid in cross direction (%) Fig.11. Variation of effectiveness of cross-counter flow heat exchanger with percentage of cross fluid flow It is clear from Fig. 11 that the effectiveness of purely counter flow heat exchanger arrangement is more than purely cross flow arrangement. It is also seen that the multi-stream heat exchanger arrangement is more effective as compared to either purely counter or purely cross flow type of heat exchanger. As the cooling fluid through the cross direction increases, effectiveness first increases and then decreases after a point. The effectiveness corresponding to the 25 % cross fluid flow is found to be maximum. It implies that out of total cooling fluid available, 25 % cold fluid flows in cross direction to the hot fluid and 75 % in the counter direction to the hot fluid. The variation of outlet temperature of two cooling fluid from cross and counter flow direction with the percentage of cross fluid flow is shown in Fig. 12. It is observed from Figures 11 and 12 that the cross-counter arrangement has maximum effectiveness when the outlet temperature of both cooling fluids are same. TABLE VII. VARIATION OF COLD FLUID FROM CROSS DIRECTION TO HOT FLUID (MASS FLOW RATE OF HOT FLUID = 0.35 kg/s) Mass flow rate, kg/s Case Cross cold Counter cold fluid fluid Copyright Vandana Publications. All Rights Reserved.

7 Hot fluid outlettemperature (K) cross fluid outlet Maximum effectiven TABLE VIII. VARIATION OF HEAT TRANSFER RATE WITH ARRANGEMENT OF HEAT EXCHANGER Arrangement Heat transfer rate, W Purely Cross flow 4545 Purely Counter flow 7721 Counter-Cross flow Fig. 12. Variation of outlet fluid temperature with percentage of cross fluid flow The outlet temperature of the hot fluid varies with the percentage of cross fluid flow. It is seen that the outlet temperature of the hot fluid is minimum when the temperature of the two cooling fluids are same for which the effectiveness is maximum. Fig. 13 shows the variation of hot fluid temperature with respect to the fraction of total cold fluid flow in cross direction. It is observed that the temperature of hot fluid at the outlet is minimum where the effectiveness of the heat exchanger is maximum. The heat transfer rates obtained for different flow arrangements are compared in Table VIII. The heat transfer rate is more in the case of cross-counter flow heat exchanger. It is found that the present arrangement is 22.6 % more efficient than the purely counter flow heat exchanger and % more efficient than the purely cross flow heat exchanger. Hot fluid outlet temperature (K) Fraction of total cold fluid in cross direction (%) Fraction of total cold fluid in cross direction (%) Fig. 13. Variation of hot fluid outlet temperature with percentage of cross fluid flow V. CONCLUSION In the present work, multi-stream heat exchanger is investigated using numerical simulation technique. The detail analysis of the different flow arrangements shows that the cross-counter flow arrangement gives better performance of the heat exchanger. The model developed can be an excellent tool In the present work, multi-stream heat exchanger is investigated using numerical simulation technique. The detail analysis of the different flow arrangements shows that the cross-counter flow arrangement gives better performance of the heat exchanger. The model developed can be an excellent tool to optimize the efficiency of concentric tube-in-tube heat exchangers and consequently the energy consumption will be reduced. REFERENCES [1] Hilde Van Der Vyver, Jaco Dirker and Josua P. Meyer, Validation of a CFD model of a three dimensional tube-in-tube heat exchanger Third International Conference on CFD in the Minerals and Process Industries CSIRO, Melbourne, Australia, Dec [2] Vikas kumar, D. Gangacharyulu, Parlapalli M S Rao, R. S. Barve. CFD analysis of cross flow air to air tube type heat exchanger The Phoenics Journal of computational fluid dynamics and its applications, Vol. 16 and 17, , UK. [3] M.Sneha Priya, G. Jamuna Rani 2012 Periodic flow simulation and heat transfer analysis using computational fluid dynamics International Journal of Engineering Research and Applications (IJERA) ISSN: Vol. 2, Issue 3. [4] O. García-Valladares (2004), Numerical simulation of triple concentric-tube heat exchangers International Journal of Thermal Sciences 43 (10), pp [5] Paresh Patel, Amitesh Paul (2012), Thermal analysis of tubular heat exchanger by using ansys International journal of engineering research and technology (IJERT) Vol.1 issue 8, ISSN: Copyright Vandana Publications. All Rights Reserved.

8 [6] Abhijeet P. Shah, Suresh M. Sawant (2011), Performance analysis of different types of heat exchanger 5 th international conference on Advances in Mechanical Engineering (ICAME-2011) SVNIT, Surat, Gujrat, India. [7] D. Bhanuchandrarao, M. Ashok Chakravarthy, Dr. Y. Krishna, Dr. V.V. Subba Rao, T.Hari Krishna 2013 CFD analysis and performance of parallel and counter flow in concentric tube heat exchangers International Journal of Engineering Research and Technology (IJERT) Vol. 2 (11). [8] Rustum, I.M. and Soliman, H.M. (1990) Numerical analysis of laminar mixed convection in horizontally internally finned tubes, Int. Journal of heat and mass transfer, vol. 33, pp [9] L. Cabezas-gomez, H.A. Navarro, S.M. Godoy, J.M. Saiz-Jabardo A new cross flow thermally efficient heat exchanger flow arrangement 5 th European thermalsciences conference, The Netherlands. [10] G.A. Quadir, Saqab S. Jarallah, N.J. Salman Ahmed, Irfan Anjum Badruddin (2013), Experimental investigation of the performance of a triple concentric pipe heat exchanger International Journal of Heat and Mass Transfer 62, Copyright Vandana Publications. All Rights Reserved.