The effect of wall emissivity on radiator heat output

Abstract

The variation in the heat output of panel radiators obtained by altering the emissivity of the wall behind them has been examined. This work was conducted through experiments and Computational Fluid Dynamics (CFD). The results indicate that the presence of a high emissivity (black, such as the usual painted or wallpapered) surface to the wall increases the mass flow rate and air velocity behind the heat source compared to a reflective material. This is due to the radiation heat transfer to the wall creating an additional convecting surface behind the radiator. The results imply that the heat transfer rate can be increased by 20% through the use of a black instead of a reflective wall. The work concentrated on the air-gap behind the radiator, so these results will not be directly applicable to a normal radiator. An extrapolation indicates that the output of single bank (plate) radiator will be increased by 10% and a double radiator by 5%. Wall surface temperature results indicate that a reflective wall does indeed decrease the heat loss through the wall. The trend shown in the data obtained from the CFD analysis agreed well with the experimental results. The flow and temperature plots obtained from the CFD work help to explain the heat exchange and fluid flow processes that take place between the radiator and the wall. This understanding should lead the engineer to a better consideration of radiator placement and design.