A Simplified Numerical Analysis for the Performance Evaluation of Intercooler

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1 , pp A Simplified Numerical Analysis for the Performance Evaluation of Intercooler Vashahi Foad, Myungjae Lee, Jintaek Kim, Byung Joon Baek* School of Mechanical System Engineering, Chonbuk National University, Jeonju, , Korea Abstract. The effects of design parameters such as the tank shape, tube size, internal turbulator and external fin on the temperature and pressure drop must be investigated in order to ensure the optimum intercooler performance. A simplified CFD analysis applying porous region instead of Louver/strip fins with complex geometries has been performed. Full scale 3-D model has been simulated using the commercial software STAR-CCM +7.0 in order to visualize the flow pattern and investigate the pressure drop and effect of parameters such as heat transfer coefficient while the boundary conditions was imported from the 1-D performance prediction program using well-known equations. Keywords: Intercooler, Charge air, Louver fin, Offset strip fin, ε-ntu, Optimization. 1 Introduction Heat exchangers are used in wide range and variety of applications in automotive industries in multifarious size and efficiency. Using turbo and super chargers in vehicles reasoned to develop an idea to decrease the outlet charged air from these devices to a lower temperature. Intercoolers are devices which provide an indirect airto-air or air-to-water heat transfer to reduce the charged air temperature. This causes an increase in the air density and will help to improve the internal combustion and the engine efficiency. To reduce the size of these heat exchangers, most common solution is to use the fins which based on desirable pressure drop or decrease in temperature a variety of designs can be found. Comparisons between different types of intercoolers were studied by many researchers. Louvered fin heat exchangers studied by A Sahnoun and R.L.Webb [1]. They proposed a correlation for the heat transfer and friction factor for a specific type. Fin spacing and height effects was investigated by Bilirgen, H. et al. [2], they proved that an increase in the fin height will result in a significant increase in the heat transfer and pressure drop because of an increase in the surface area. CFD approaches were done by several scientists. The effects of geometrical parameters were studied by P. Gunnasegaran, et. al [3]. They compared different louver fins with different attack angles and visualized the flow path around the louver fins. They found that increasing the louver angle will result in increase in the heat transfer and pressure drop. Porous region approaches instead of fin geometry were considered as an important issue since designing, defining and solving fin geometries are time ISSN: ASTL Copyright 2013 SERSC

2 consuming for CFD software. Meanwhile using porous region instead of fin geometry was investigated by Jon Woodcock and Anthony Baxendale [4], and Kim S, Yoo J, Jang S.[5] Their results confirmed that using porous region are totally in agreement with the experimental results and it s applicable. In the present study, a CFD simulation applying the porous region using commercial software STAR CCM+7.0 was performed in order to investigate the intercooler performance in detail to be able to optimize the performance of the intercooler. 2 Analysis model The intercooler model is consisting of three basic parts as the inlet tank, the core, and the outlet tank. Fig.1 (a) shows the actual model of the intercooler which has been investigated in this study. The core consists of headers and 10 tubes with the specification of mm and the total surface area of m 2 while the inlet and outlet tanks diameters are measured at 75mm. The Louver and offset type fins are attached on external and internal sides of the tubes, respectively. Schematic of the louver and offset fins applied in intercooler has been shown in Fig.1 (b) and Fig.1 (c). Fig.1 Actual intercooler model (a) A Schematic of Louver (b) and strip fin (c) 3 CFD analysis and boundary conditions A CFD analysis for the thermal performance of the 3-D the intercooler was performed by using the commercial software STAR-CCM+7.0. Simulation is performed defining the porous region instead of fin geometry inside the tubes. Porous region is defined based on: P L = P v U + P i U 2 (1) While the P v and P i are the viscous and inertial terms, respectively and U represents the superficial velocity, defined by experiments. The effective thermal conductivity in porous region is based on the bed porosity ε=0.935 (which is the fraction of the volume of voids over the total volume) as an experimental value as follows: k eff = εk fluid + (1 ε)k solid (2) And the solid thermal conductivity is being set to W/m-K. The outside fin replaced by the heat transfer coefficient and the ambient temperature obtained from the 1-D prediction code. The 1-D code prediction is based on ε-ntu method and using Copyright 2013 SERSC 35

3 detailed and actual fin and model geometries. It is cable of predicting the ambient heat transfer coefficient and other design parameters. The momentum, energy equations and the standard k-ε turbulence model with the high y+ wall treatment and segregated flow is applied. The k-ε turbulence model is a two-equation model in which the transport equations are solved for the turbulent kinetic energy k and its dissipation rate ε. Following equations defines the turbulence modeling; Kinetic eddy viscosity: Turbulence kinetic energy: ν T = C µ k 2 ε (3) Dissipation rate: k t + U j k = τ ij U i ε + ν + ν T σ k k (4) ε + U ε ε t j = C x ε1 j τ U i k ij C ε2 ε 2 k + ν + ν T σ ε ε (5) The polyhedral mesh generation with 4 prism layers applied. Fig.2, demonstrates the mesh generation while total number of about 565,000 cells achieved and mesh resulted in topologically valid with no negative cells. The inlet mass flow rate and temperature at charged air defined as kg/s and 165 C respectively. The simulation considered as steady state and the residuals observed convergence after 2000 iterations. Finally, we investigated the effect of design parameters on the thermal performance. 4 Results and discussion Temperature distribution along the tube length appeared in Fig.3. The temperature drop is drastic at the first half of the tubes by about 90 C and tends to gradually decrease to the exit temperature at the second half of the tube length. The minimum entrance temperature belongs to the tube number five which is located in the center while the maximum entrance temperature happens in the topmost tubes (number 1). Fig.4 depicts the mass flow rate and pressure distribution in the tubes from top to bottom. It s been observed that the mass flow rate is not equally divided into the tubes and the minimum mass flow rate belongs to the tube number four and five due to the 36 Copyright 2013 SERSC

4 Fig.2. Mesh generation with 4 prism layers Fig.3. Temperature distribution along tubes Fig.4. Pressure and mass flow rate distribution in tubes. Fig. 5. Recirculation area next to entrance of tubes recirculation areas which prevents the flow to divide equally in the tubes because of the inlet tank design. This also makes the flow guide to the top of the tank and reasoned the maximum mass flow rate at tube number one. The recirculation areas can be seen in Fig.5, as the velocity contour is drawn. The maximum pressure drop happens in tube number one as well, due to the higher mass flow rate. This phenomenon also results in non-uniform temperature distribution along the tubes. 5 Conclusion An actual intercooler is modeled, simplified and simulated in the CFD commercial software STAR CCM+ 7.0 applying porous region. The boundary conditions were imported from the 1-D code prediction program based on ε-ntu method. The temperature distribution along the tube direction is drawn and the mass flow rate and pressure distribution were also investigated. Flow pattern through the inlet tank are observed and two recirculation areas have been found which impacts the flow uniformity. The effect of mass flow rates on pressure drop was also investigated. It`s been found that an increase in mass flow rate will result in an increase in pressure drop. References 1. Sahnoun AA, Webb RL. Prediction of Heat Transfer and Friction for the Louver Fin Geometry. J. Heat Transfer. 1992;114(4): doi: / Harun Bilirgen, Stephen Dunbar, Edward K. Levy, Numerical modeling of finned heat exchangers, Applied Thermal Engineering, Volume 61, Issue 2, 3 November 2013, Pages , ISSN , 3. P. Gunnasegaran, N. H. Shuaib, and M. F. Abdul Jalal, The Effect of Geometrical Parameters on Heat Transfer Characteristics of Compact Heat Exchanger with Louvered Fins, ISRN Thermodynamics, vol. 2012, Article ID , 10 pages, doi: /2012/ Copyright 2013 SERSC 37

5 4. Woodcock, J., Baxendale, A., and Fish, G., "An Evaluation of the Use of CFD for Investigating the Performance of Intercooler Assemblies," SAE Technical Paper , 1997, doi: / Kim S, Yoo J, Jang S. Thermal Optimization of a Circular-Sectored Finned Tube Using a Porous Medium Approach. J. Heat Transfer. 2002; 124(6): doi: / Copyright 2013 SERSC