Effect of unheated length on heat transfer in a vertically solid burning

Abstract

Upward flame almost always exists during the development of fires, which is one of the most rapid and hazardous fire. In the actual fire scenario, whether inherent or designed, there is usually an unheated length prior to the flame. But this is contrary to the classical theoretical assumptions, and there is no experimental study on the effect of unheated length on burning. This paper performed a detailed experimental investigation to give the effect of varied unheated length upon the vertically oriented Polymethyl methacrylate (PMMA) burning with a long duration, under natural convection accompanied with a low velocity concurrent forced airflow. Utilizing the gas-phase temperature profile and temperature gradient, the paraments of heat transfer were obtained to illustrate the effect of unheated length on the three stages of solid burning. In the initial stage of burning, as there is viscous drag along the unheated segment, the forced airflow velocity at the end of the segment decreases. So, with the longer unheated section, the buoyancy induced velocity superposed by the terminal forced velocity in the fuel section decreases, which decreases the oxidizing flow velocity near the fuel surface, further leading to the larger flame standoff distance and lower burning rate. While with increasing burning time, the increasing turbulence intensity and the maximum wind speed induced by natural convection can gradually counteract the forced airflow speed, so the effect of unheated length on heat transfer is diminished gradually. The findings of this paper verify the previous theoretical research on heat transfer and provide a deep understanding of the unheated segment effect in theory and numerical simulation of solid burning in natural convection.