Fuel Particle Heat Transfer Part 2: Radiation and Convection during Spreading Laboratory Fires

Abstract

ABSTRACT The effects of radiation and convection in determining the heating that leads to ignition of fuel particles were explored using experiments with spreading laboratory fires and a numerical fuel particle heating model. As a follow-on to “Fuel Particle Heat Transfer, Part 1” (this issue), two sizes of square wood fuel particles (1 mm and 12 mm) were instrumented with fine-wire thermocouples to measure particle surface temperatures and adjacent gas temperatures. Radiation heat flux from the approaching fire was measured at the fuel bed particle location. Seven laboratory fires with varying fuel beds, spread rates, and wind speeds were used to measure the conditions experienced by both particles. Experimental results demonstrated that intermittent flame contact ahead of the advancing flame front was principally responsible for heating fuel particles to ignition even with peak irradiance up to 44 kW/m2. A two-dimensional numerical fuel particle heat transfer model was employed to separate the influences of the measured irradiance (radiation) and gas temperature (convection) on particle heating as the flame front approached. Results demonstrated that radiation heating was insufficient for ignition of both particle sizes during the spreading laboratory fires. The low convective heat transfer of the 12 mm particle did not significantly cool the particle, and the brief duration of irradiance was insufficient for ignition. The greater 1 mm particle convective heat transfer offset radiant heating by convectively cooling with ambient air, but rapidly heated the particle during intermittent impingement of hot gases from the flaming front leading to particle pre-ignition.