CFD analysis of triple concentric tube heat exchanger

Available online at www.ganpatuniversity.ac.in University Journal of Research ISSN (Online) 0000 0000, ISSN (Print) 0000 0000 CFD analysis of triple concentric tube heat exchanger Patel Dharmik A a, V. D. Dhiman b, Jignesh J. Patel c, Ravi Engineer d a PG Student, MGITER, Valsad-396001, Gujarat, INDIA b Associate Professor Govt. Engg. College, Bharuch-392001, Gujarat,INDIA c Assistant Professor MGITER, Navsari-396 450, Gujarat, INDIA, d Professor, GEC Valsad-396001, Gujarat, INDIA Abstract Triple concentric tube heat exchanger consists of three tubes of different diameters connected concentrically. Triple concentric tube heat exchanger performs better than double concentric tube heat exchanger. Most of the previous studies used two fluids for different arrangement. Cold fluids flow from inner tube and outer annulus and hot fluid from inner annulus. Different parameters were found which affect performance of triple concentric tube heat exchanger. The present work involves CFD analysis of triple concentric tube heat exchanger by using previous research s mathematical model, experimental model and correlation. The comparison is carried out between CFD result and literature result. Effectiveness of heat exchanger increases with increased inner tube diameter. Effectiveness of heat exchanger increases with increased inner annulus diameter at the beginning and decreases when flow becomes laminar. Keywords: Triple concentric tube heat exchanger; CFD analysis; Parametric study 30

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 31 1. Introduction Heat exchanger is a device which transfers thermal energy from hot fluid to cold fluid with maximum rate and minimum investment and running cost. Concentric tube plays an important role in food industry. The most common concentric tube type is double pipe heat exchanger. Figure 1. Triple concentric tube heat exchanger Introducing an intermediate pipe to double pipe heat exchanger which performs better than double pipe heat exchanger and it called triple concentric pipe heat exchanger shown in Figure 1. The chief function of third pipe is to improve the heat transfer rate through an additional flow passage and larger transfer per unit exchanger length. Nomenclature A heat transfer area, ( m 2 ) d diameter, (m) h coefficient of heat transfer, (W/m 2 k ) k thermal conductivity, (W/m k) L length, (mm) m mass flow rate, (Kg/s) Q heat flow rate, (W/s) Re Reynolds number T temperature,(k) University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

32 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 U overall heat transfer coefficient, (W/m 2 k) Subscript C1 cold fluid in the central tube C2 cold fluid in the outer tube H hot fluid in intermediate tube Greek symbol ρ density, (kg/m 3 ) Ɛ effectiveness In triple concentric tube heat exchanger thermal fluid (hot fluid) contacts with heat transfer fluid (hot fluid) from both sides, therefore, it provides larger heat transfer area and increases heat transfer rate. Generally, there are two types of fluid used which are cold and hot. In this arrangement, cold fluids flow from inner tube and outer annulus and hot fluid from inner annulus as shown in Figure 1. The technical and economic advantages that are ensured by the use of triple concentric tube heat exchanger in comparison to the double pipe heat exchanger are represented by the higher heat transfer area per unit length and higher velocity due to presence of annular space. Replacing the double pipe heat exchanger with triple pipe heat exchanger application of triple concentric pipe heat exchanger is mainly in food industries such as pasteurization, sterilization, drying, cooling, freezing, evaporation, refrigeration etc. Theoretical analysis was carried out which concludes that relative size of triple pipe heat exchanger influence the performance of heat exchanger (Unal et al., 1998) (Unal et al., 2001). Mathematical formulation, equation for effectiveness (Unal et al., 2003), and correlation for partial coefficient was developed (Sinziana et al., 2012). Detailed review on thermal design of triple pipe heat exchanger was carried out (Zuritz, 1990). More generic way of calculating heat transfer coefficient and mean temperature difference method was developed (Batmaz et al., 2003). 32

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 33 Performance analysis was carried out which concludes that N-H-C (normal hot cold fluid) perform well than C-H-N (cold- hot- normal fluid) arrangement (Quadir et al., 2014) (Quadir et al., 2013). Heat transfer rating and performance analysis with fin also investigated (Rajsekar et al.). Numerical model was developed to analyse the behaviour of triple pipe heat exchanger (Garcia-valladares, 2004). Triple pipe heat exchanger was also investigated as milk sterilizer which performs better than double pipe heat exchanger (Sahoo. et al., 2000) (Satyanarayana et al., 1995) (Nema et al., 2006). PCM material was used in triple pipe heat exchanger to investigate the performance (Sopiana et al., 2013) (Basal et al., 2013) (Long Jian-you, 2008)). Main problem related with the double pipe heat exchanger is size of heat exchanger. The objective is to increase the heat transfer area by adding intermediate tube in double pipe heat exchanger in order to reduce the size and the cost and to increase the heat transfer rate. Relative sizes of tubes are the important parameters which affect performance of the heat exchanger. 2. CFD analysis of triple concentric tube heat exchanger 2.1 Material and method Due to the thermal properties that was determined easier and also because of its availability and safety, water has been used as fluid for all three tubes. All three tubes were made from copper because of its high thermal conductivity than aluminium and steel. Hot water flows from inner annulus and cold water flows from inner tube and outer annulus. Table 1 represents the input parameters used for CFD analysis. Table1. Input parameters for CFD analysis Input parameters Symbols Values Diameter of central pipe, mm d in1 12 Diameter of intermediate pipe, mm d in2 26 University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

34 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 Diameter of outer pipe, mm d in3 40 Thickness of each pipe, mm T 1 Length, mm L 1193 2.2 Geometry of triple concentric tube heat exchanger In triple concentric tube heat exchanger three tubes are concentric. Therefore, axis symmetrical 2-D geometry modelling was generated which is presented in Figure 2. Figure 2. Geometry of triple concentric tube heat exchanger 2.3 Meshing of triple concentric tube heat exchanger Quadrant type meshing was selected. For selecting map size Grid Independence Test was carried out for five different map sizes (0.25, 0.20, 0.15, 0.1, and 0.09). 34

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 35 Figure 3. (a) Outlet temperatures of three fluids (b) effectiveness for different map size Structured type mesh was selected because it gave better accuracy for 2-D problem. Temperature of hot fluid, inner cold fluid and outer cold fluid were nearly same values for same mass flow rate condition (Figure 3 (a)). Effectiveness also decreased with number of map size (Figure 3 (b)). So quadrilateral mesh type with 0.25 map size was selected which gave better accuracy with less number of element and nodes as shown in Figure 4. Figure 4. Meshing with 0.25 map size University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

36 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 2.4 Solver FLUENT was used as solver. 2.4.1 Boundary conditions Table 2 represents the boundary conditions were used for CFD analysis. Table 2. Boundary conditions Inlet Velocity inlet Outlet Axis Wall Pressure outlet Axis Wall Interfaces Interface 3. Results and discussions After CFD modelling validation, the parametric study was carried out for different mass flow rates of hot fluid, inner cold and outer cold fluid. 3.1 Validation Eight different mass flow rate conditions were used for validation of CFD model. Outlet temperatures of three fluids were used for validation as shown in Table 3 and Figure 5. Table 3. Outlet temperatures of hot, inner cold and outer fluid for experiment and CFD for different mass flow rates Inner cold water Hot water Outer cold water V T C1i T C1 T C1e V T H1i T He T He V T C2i T C2e T C2e (Exp.) (CFD) (Exp.) (CFD) (Exp.) (CFD) 1 100 14.2 23.6 21.8 150 55.5 42.5 44.3 100 14.2 24 22.01 2 100 14.2 25.1 23.7 250 55.5 46.5 47.0 100 14.2 25.3 22.57 36

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 37 3 100 14.2 27.1 26.1 450 55.5 49.6 49.1 100 14.2 27.4 24.13 4 140 13 21 18.4 150 55.5 39.2 43.7 210 13 19.1 18.08 5 210 13 19.2 17.7 150 55.5 38.6 43.4 140 13 21.5 19.79 6 310 13 18.4 16.9 150 55.5 36.8 43.1 140 13 20.6 19.76 7 200 10.8 21.8 23.0 490 55.5 45.7 47.8 470 10.8 16.1 14.16 8 300 10.8 19.1 19.2 490 55.5 44.7 47.6 470 10.8 16.5 14.55 The data of column 3, 7 and 11 of Table 3 were taken from references Sinziana et al. (2012) for the comparison of experimental results with CFD results. Figure 5. outlet temperatures of (a) inner cold fluid (b) hot fluid (c) outer cold fluid for experiment and CFD 3.2 Contours Contours of temperature and pressure are shown in the following Figure 6: University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

38 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 Figure 6. Contour of (a) temperature distribution (b) pressure distribution 3.3 Parametric study of triple concentric tube heat exchanger After validation, analysis was carried out for different inner tube diameters and inner annulus diameters. Heat transfer rate, heat transfer coefficient, overall heat transfer coefficient and effectiveness were calculated by using sizing and rating theory of triple pipe heat exchanger (Matawala V. K. et al., 2014). 3.3.1 Different diameters of inner tube Effect of different inner tube diameters on performance of triple concentric tube heat exchanger was observed for five different inner tube diameters (8 mm, 10 mm, 12 mm, 14 mm and 16 mm). 38

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 39 Figure 7. (a) Heat transfer rate (b) Heat transfer coefficients of hot fluid, inner cold fluid and outer cold fluid for different inner tube diameters Heat transfer rates of hot fluid, inner cold fluid and outer cold fluid were increased with the increase in inner tube diameter due to increase in temperature differences of three fluids (Figure 7 (a)). Heat transfer coefficient of hot fluid was increased and heat transfer coefficient of inner cold fluid was also decreased with the increase in inner tube diameter. Heat transfer coefficient of outer cold fluid was remained constant (Figure 7 (b)). Figure 8 (a) Overall Heat transfer coefficients (b) Effectiveness for different inner University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

40 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 tube diameters Overall heat transfer coefficient based on outer diameter of inner tube was decreased whereas overall heat transfer coefficient based on inner diameter of inner annulus was increased with the increase in diameter of inner tube (Figure 8(a)). Effectiveness of triple concentric tube heat exchanger was increased with the increase in inner tube diameter (Figure 8(b)). 3.3.2 Different diameters of inner annulus Effect of different inner annulus diameter on performance of triple concentric tube heat exchanger was observed for five different inner annulus diameters (22 mm, 24 mm, 26 mm, 28 mm and 30 mm). Figure 9 (a) Heat transfer rates (b) Heat transfer coefficients of hot fluid, inner cold fluid and outer cold fluid for different inner annulus diameters Heat transfer rates of hot fluid and inner cold fluid were increased up to 26 mm after that it was decreased due to decrease in temperature difference and flow became laminar. Heat transfer rate of outer cold fluid increased with the increase in diameter of inner annulus (Figure 9 (a)). Heat transfer coefficient of hot fluid decreased whereas heat transfer coefficient of outer cold fluid increased with the increase in inner annulus diameter. Heat transfer coefficient of inner cold fluid was remained constant (Figure 9 40

D. Patel et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 41 (b)). Figure 10. (a) Overall Heat transfer coefficients (b) Effectiveness for different inner annulus diameters Overall heat transfer coefficient based on outer diameter of inner tube decreased whereas overall heat transfer coefficient based on inner diameter of inner annulus increased with the increase in diameter of inner annulus (Figure 10 (a)). Effectiveness of triple concentric tube heat exchanger increased up to 26 mm and after that it was decreased with the increase in inner annulus diameter due to flow become laminar (Figure 10 (b)). 4.Conclusions For triple concentric tube heat exchanger parameters which affect the performance should be relative in sizes or radius of inner tube, inner annulus and outer annulus, mass flow rate, material of tube. With the increase in inner tube diameter heat transfer rates of hot fluid, inner cold fluid and outer cold fluid increased, heat transfer coefficient of hot fluid was increased and heat transfer coefficient of inner cold fluid was decreased. Heat transfer coefficient of outer cold fluid remains constant. University Journal of Research, 1(1), 2015 published by Ganpat University. All rights reserved.

42 Patel D. et al./university Journal of Research Vol. 01, Issue 01 (2015) ISSN: 0000 0000 Overall heat transfer coefficient based on inner diameter of inner annulus and effectiveness was increased with the increase in inner tube diameter. With the increase in inner annulus diameter heat transfer rate of hot fluid and inner cold fluid increased up to 26 mm after that it decreased due to decrease in temperature difference and flow became laminar. Heat transfer coefficient of hot fluid decreased whereas heat transfer coefficient of outer cold fluid was increased. Overall heat transfer coefficient based on inner diameter of inner annulus was also increased. Effectiveness of triple concentric tube heat exchanger was increased up to 26 mm and after that it was decreased. 5. Acknowledgments I would like to thanks Asst. Prof. Prakas R. Patel, Asst. Prof. Tejendra Patel and Asst. Prof. Hiren Patel for their guidance. 6. References Basal B., Unal A., (2013). Numerical evaluation of triple concentric-tube latent heat thermal energy storage. Solar Energy, 92, 196 205. Batmaz E., Sndeep., (2003). Overall heat transfer coefficients and axial temperature distribution of fluids in a triple tube heat exchanger, Food Science, 1, 1-34. Garcia-valladares, O., (2004). Numerical simulation of triple concentric pipe heat exchangers, International Journal of Thermal sciences. 43, 979-991. Long J., (2008). Numerical and experimental investigation for heat transfer in triplex concentric tube with phase change material for thermal energy storage. Solar Energy, 82, 42

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