The effects of emissivity on the performance of steel in furnace tests

The role that surface emissivity plays in the standard furnace test (BS476) is considered for steel sections. Steel samples coated with either a low-emissivity paint or a high-emissivity paint were subjected to furnace tests and cone calorimeter tests in order to quantify the degree to which emissivity affects performance. The cone calorimeter experiments were designed primarily to determine the emissivity of the coatings and to compare with the manufacturer's estimates. However, a welcome additional benefit of this analysis was an estimate of the average convection heat transfer coefficient h for horizontal test specimens in the cone calorimeter. Our measurements suggest that h has been significantly underestimated (in some cases by at least 50%) in the literature to date. Most studies appear to assume a value for h that is close to the value for free convection for a hot plate with hot surface uppermost (something in the region 10–15 W m−2 K−1). Our results suggest that a figure closer to 28 W m−2 K−1 is more appropriate. The furnace tests showed that emissivity has a low-order effect on performance and so we are able to conclude that convective heat transfer is dominant in these situations.