Managing the thermal conditions under the hood of passenger cars is of a crucial importance. The underhood typically consists of two main heat sources: radiator and engine that develop high temperature zones, which are required to be carried away to protect the compartment from the damage.
Moreover, the modern vehicles employing tightly packed engine and associated components make the situation even more complex. The thermal protection of engine compartment is also dependent on the vehicle aerodynamics.
One of the vital challenges in thermal management is the prevention of “thermal soak” or “hot-stop”, which results when a vehicle is driven at high load followed by shutting off the engine. In this condition, the underhood cooling relies only on natural convective heat transfer.
Additionally, steady state heat loads are also required to be calculated to avoid excessive building up of temperature under the hood. These steady state heat loads are the conductive and convective heat liberated from the engine block, air flow past the radiator that carries away the heat from the flowing coolant and the heat generated from auxiliary components such as turbo, starter motor and pump and heater core.
A conventional approach is to perform experimental tests by developing prototypes and testing them in wind tunnels. However, the process is extremely time consuming and costly, affecting the manufacturing cycle and time-to-market schedules.
The use of computational fluid dynamics (CFD) proves to be a cost-effective tool to perform preliminary design assessment and prediction of the thermal environment, considering the heat loads mentioned above. However, developing a CFD model of the engine compartment requires comprehensive design information and physical fluid flow conditions.
Additionally, it is required to set up the boundary conditions and a fluid domain with adequate mesh to capture the flow physics and heat transfer phenomena more accurately. The aerodynamic analysis through CFD simulations can provide the details about the air flow and the subsequent convective heat transfer rates. Moreover the use of fluid-solid interaction models, the effect of thermal soak can be simulated.
While the results obtained through simulation techniques are not always accurate, they are certainly useful in determining the requirement of air vents, sizing the front grille and optimizing the compartment design for equal temperature distribution and better heat transfer possibility through the air flow.
It is also possible to identify components that lead to heat concentrating regions and damage the thermal environment. Predicting these regions provide design engineers to place these components optimally in the compartment or alter their design, without conducting experimental trials. The finalized design based on CFD simulations can then be utilized to develop a prototype and conduct physical experiments for validation. This process significantly reduces the number of experimental iterations required, reducing the cost and subsequently shortening the development cycle.
Image Source: http://www.ansys.com/Industries/Automotive/Application+Highlights/Underhood
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