Thermal structure of flame spread in partially premixed atmospheres and effects of fuel Lewis number

The experimental study on the flame spread in partially premixed atmospheres was presented. Two samples of different filter thickness and density were used. Gaseous fuels of hydrogen, methane, and propane were added in the opposed air, focusing on the fuel Lewis number (Le). In the partially premixed atmospheres, the pyrolysis zone and the region of high temperature are expanded. Since the flame temperature is increased, the heat transfer rate to the preheat zone of the filter is promoted, which is confirmed by the de Ris's formula of the thermally thin model. The heat transfer due to the heat conduction from the flame is 60-80% of total heat transfer. By comparing the results of the preheated air, the corresponding temperature rise caused by fuel addition is evaluated. For all fuels, the linear increase of temperature rise with the fuel concentration (Cf) is confirmed. However, at the fixed fuel concentration, the temperature rise depends on the fuel type. Generally, there are two factors directly related with thermal structure of combustion field such as the flame temperature. One is the heat of combustion (H), and the other is the fuel Lewis number, because the opposed flow is the lean mixture. Since Le is proportional to the inverse of the fuel diffusivity, the temperature rise could be corrected by 1/Le. Resultantly, for all fuels, the temperature rise collapses to a single curve with the correlation of (Cf ∗ H/Le). Thus, the Lewis number is a very important and useful parameter to discuss the thermal structure and the flame spread rate in partially premixed atmospheres.