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Convective Ignition

Recent research conducted at the Missoula Fire Lab has found that the amount of radiant heat in wildland fires is not sufficient to ignite fine fuel particles such as needles and grasses. These fine fuels are highly efficient at convective heat transfer, so any amount of airflow can easily offset the radiant heat generated by the fire. As a consequence, fine wildland fuels do not ignite until bathed by the flame. As radiant heating has been assumed to drive ignition in wildland fires as it does in structural fires, ignition by convective heating is not well understood.

Experiments are underway to determine if and how ignition due to convective heating is different than that from radiative heating. We built an apparatus using two electrical heaters to heat air up to 1200°C (2200°F). So far, wood cylinders and disks of different diameters and thicknesses have been tested. A simple model has been shown to predict the ignition times of these simple fuels with reasonable accuracy. Several differences between convective and radiative heating have been noted. For example, the convective heating and ignition process is far more sensitive to fuel size and shape than radiative heating. Another major difference between these modes of ignition is the large surface temperature gradient that forms due to convective heating that is largely missing under radiant heating.

Understanding the ignition process due to convective heating will allow for better prediction of the transition from surface to crown fire and crown fire spread, two aspects of wildland fire behavior that are largely misunderstood.

Photo: Bubbles and blisters on live fuel
Modified: Apr 03, 2018

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