Experimental study of convection and radiation interaction in a headlight model using pressure variation

An experimental study is presented on the coupled heat transfer of natural convection and specular reflected radiation in an automotive headlight. To simplify the geometry, an automotive reflector with a standard H7 bulb is arranged inside a cylindrical enclosure, representing housing and lens of the headlight. To vary the coupling effects between the convective and radiative heat transfer, the pressure inside the enclosure is reduced and convection is limited. Since the Rayleigh number scales according to Ra ∼ p2 with pressure, the variation of Ra extends over six-orders of magnitude. In the experiments, the temperature distribution at the lens is investigated in dependence of the Ra number, the aspect ratio and the emission coefficient. It is found that for low emission coefficients of the reflector, the influence of the radiation heat flux exceeds convection effects. On the other hand, for high emission coefficients of the reflector, both effects are of the same order of magnitude. The interaction significantly affects the temperature and location of the hot spot on the lens.