Influences of material properties on thermal design of impinging flame jets

This paper presented a study of the development of an area-averaged heat flux model for the thermal design of impingement premixed gas-fired round flame jets in terms of the flame and surface conditions. The influencing parameters concerning flame condition include Reynolds number and equivalence ratio of the air–fuel jet, and nozzle-to-surface distance. In terms of the impinging plate surface condition, the two influencing parameters are thermal conductivity and surface emissivity of the plate material, which affect directly the thermal conduction and radiation, respectively. Experimental studies were performed to measure the heat flux and temperature distributions of the impingement plates fabricated with materials of different thermal conductivities and surface emissivities. The premixed air–butane flame was impinging vertically upwards upon the plate’s flat bottom surface essentially at laminar flow conditions. In order to examine the influence of thermal conductivity on thermal conduction, there were three materials: brass (k = 61 W/mK), bronze (k = 26 W/mK) and stainless steel (k = 14.9 W/mK), used to fabricate the plate in the present study. Similarly, influence of surface emissivity on thermal radiation was examined by fabricating the brass impingement plate with surface emissivities of 0.1, 0.38 and 0.98, respectively. Based on the response surface methodology, one model was developed by taking into consideration the Reynolds number, equivalence ratio, nozzle-to-surface distance and material’s thermal conductivity. Another model considering the Reynolds number, equivalence ratio, nozzle-to-surface distance and material’s surface emissivity was also developed. The predicted area-averaged heat flux of an impingement plate has been found to agree closely with that of the experimentally measured data at a 95% confidence level. It is found that the influence on thermal characteristics of an impingement plate will be more significant due to its thermal conductivity rather than surface emissivity.