Limitations of the assumptions in the classic de Ris formula (1) for opposed-flow flame spread rate over thick fuels in the thermal regime are explored through numerical experiments in which the assumptions used in the original theory are eliminated one by one from the mathematical model. While the assumptions of infinite-rate chemistry, mass flux liearization, and neglect of radiation are found to be reasonable in the thermal regime, assumptions of a slug flow, a surface-hugging flame, and constant properties are responsible for the predictions of spread rate being in error by as much as a factor of 10.
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