Slope effect on laboratory fire spread: contribution of radiation and convection to fuel bed preheating

Two series of 16 and 18 laboratory fire experiments were conducted to explore the respective roles of radiation and convection heat transfer in slope effect on fire spread. The first series attempts to measure fuel temperature and gas temperature simultaneously and at the same location using an infrared camera and thermocouples respectively. The second series measures the incident radiant heat flux as would be received by a small fuel bed volume ahead of the fire line. These measurements are used to compute a fuel bed heat balance for each slope angle (0°, 10°, 20° and 30°). Overall, radiative heating is found to be the heat transfer mechanism that dominates in the slope effect between 0° and 20°, but close to the fire line (<10 cm), the flux due to convective heating is also significant, reaching one-third of the net heat flux at a 20° slope angle. When the slope angle increases from 20° to 30°, the rate of spread rises by a factor of 2.5 due to a marked increase in convective heating, while radiative heating no longer increases. Far from the fire line, cooling by convection is found to be substantial except at the 30° slope angle.