Thermal performance of a cubic cavity with a solar control coating deposited to a vertical semitransparent wall

摘要

We present a theoretical and experimental study of combined heat transfer in a cubic cavity containing non-participating air. The cubic cavity has a vertical semitransparent wall (glazing) with a solar control coating (SCC); an opaque black isothermal wall forms its opposite side. The top, bottom and side walls are opaque, gray and adiabatic. In the theoretical study, the 3-D steady state conservation equations for the mass, momentum and energy, along with the coupled radiation and conduction equations, were solved numerically by the finite volume method. The conduction for the semitransparent wall and the radiative energy flux were coupled through their boundary conditions at the convection model. Also, the semitransparent wall with SCC exchanges heat by convection and radiation to the exterior of the cavity. In the experimental study, the solar absorptance of the SCC was simulated experimentally using a thin film electrical resistance located on the glazing surface. Infrared imaging thermography was used to measure the temperature of the exterior surface temperature of the glazing. The interior air temperatures of the cavity were measured using thermocouples. The measured exterior surface temperatures of the glazing were introduced into the theoretical model as a boundary condition and the temperatures of the air at the interior of the cavity were compared with the theoretical ones predicted from the computational code for Ra = 2.3 × 10~6. Their average difference was 1.86%. Through these results, detailed descriptions of the air flow and temperature profiles in the cubic cavity are presented. The influence of radiative process on the overall heat transfer in the cavity is given particular attention, thus distinguishing the convective and radiative heat transfer in the cavity was shown separately. A parametric study was carried out for SCC absorptances of 0.08, 0.50 and 0.64 and exterior temperatures of 15 ℃, 25 ℃ and 30 ℃. It was found that for an exterior temperature of 25 ℃, the radiative heat flux increases as the absorptance of the SCC increases from 0.08 to 0.64, but the solar heat gain coefficient (SHGC) decreases from 0.94 to 0.52. A new correlation for the Nusselt number as a function of the SCC absorptance is introduced as Nu = 0.9525α + 10.985 for an ambient temperature of 25 ℃.