Thermal Analysis of Engine Bay in Star-CCM+. Method Development and Correlation with Experimental Data.

Abstract

The use of computational fluid dynamics have a central role today in the automotive industry, but still a lot of decision making is based on knowledge achieved with prototype testing. With the industry driven towards shorter development cycles, the need of more accurate virtual computational methods grow. When the time available for testing is shortend, more and more of the technical ground for decision making need to be entrusted to simulations. In this thesis a method is developed and evaluated for investigating the thermal balance in the engine bay of a car with focus on the components in close proximity of the exhaust system. A CFD simulation is setup with a finite volume model of a car in a wind tunnel with a fully resolved engine bay, including the internal exhaust gas flow from the exhaust manifold downstream to the first sections of the exhaust pipe. The solids containing the exhaust gases are also included with volume meshes, along with several other solid components included for temperature correlation. In the following simulations the fluid flow and heat transfer is calculated using the steady state Reynolds-Averaged Navier-Stokes equations, the SST k −! turbulence model and the effect of surface-to-surface radiation included. The method also include dual stream modeling and porous media treatment of the front heat exchangers and modeling of the cooling fan with a moving reference frame. The results show temperatures for several solid components located slightly further away from the exhaust system within a 12% deviation from the test data. This concludes the overall temperature in the engine bay is well captured. The internal exhaust gas temperatures are shown to be predicted within a margin of 6%. However, with the high temperatures of the exhaust gases, this 6% error means around 50 K difference. This deviation can only partially explain the results for the component closest to the exhaust system, the manifold heat shield, which give an error of as much as 40%. This, along with other significant deviations from the test data closer to the exhaust system indicate these components need to be more thoroughly modeled to achieve a full representation of the thermal balance in the engine bay.