Abstract
Numerical simulation and benchmarking the total energetic performance of vehicles, requires specific values regarding the behavior of heat transfer. This is particularly of interest if energetic simulation models are employed, to evaluate the performance of automotive heating, ventilation and air conditioning systems. Measurements of the heat transfer through enclosure components have to account thermal bridge effects to adjacent components. Within this paper we present a simple methodology for the evaluation of characteristic values of the heat transfer through an arbitrary automobile enclosure component. It is based on state of the art measurement methods and enhanced for determining additional coefficients of the heat transfer processes. In a stationary vehicle a defined and constant heat load is generated until the heat flow through the vehicle's envelope has reached steady state. Simultaneously, the mean inner and outer air temperatures as well as the surface temperatures of the investigated component are determined. Hence, the total heat transfer over the vehicle's envelope can be evaluated by the measured data. Afterwards, the tested component is provided with a defined external insulation and the overall thermal transmittance of the total vehicle envelope is determined again. On the basis of the difference between the heat transfer values with and without insulation the coefficients for internal and external convective heat transfer and the coefficient for conductive heat transfer can be estimated as integral quantities.
In addition, the effect on the convective heat losses at the vehicle outer envelope is examined by the incident flow at the vehicle in a wind tunnel. Therefore, the results of the thermal conductivity resistance can be validated with regard to its independency of the incident flow characteristics. For this paper we examined the front and rear door, the windscreen, the trunk lid and the vehicle roof and ground of a test vehicle.
Using this method, it is possible to divide the entire vehicle envelope in a variable quantity of components and to investigate them with a limited number of measurements in terms of their convective and conductive properties. The obtained results can be used for evaluating further thermal insulation measures for future automobile model generations and as inputs to energy simulations.
KEYWORDS – Heat Transfer, Heat Conductivity, Measurement, Energy Efficiency, Automobile