A numerical analysis of ignition to steady downward flame spread over a thin solid fuel

Kuo Kuang Wu, Chiun-Hsun Chen*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

11 Scopus citations


A numerical analysis using an unsteady combustion model is presented to study the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid-fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal runaway, occurs as soon as a flammable mixture is formed and the gas-phase temperature, heated by the solid fuel, becomes high enough. During the induction period, the reactivity and temperature in the gas phase are mutually supportive. The thermal runaway consists of a burning premixed flame as the flow moves with the flame front. This is followed by a transition from a premixed flame into a diffusion flame. The flame front extends along and toward the upstream virgin fuel-as the diffusion flame is formed. Finally, steady flame spread takes place as burnout appears. The ignition delay time is found to be controlled mainly by the time required to form the flammable mixture and is almost independent of the gravity level and the ambient oxygen index. The ignition delay time increases nearly linearly with an increase in solid-fuel thickness within the range of 0.005cm ≤ τ̄ ≤ 0.02cm and is proportional to (Q̄max) -1.11 within 2W/cm2 ≤ Q̄max ≤ 8W/cm2. The steady downward flame-spread rate decreases with increases in the gravity level or fuel thickness and with decreases in the ambient oxygen index but is independent of the incident peak heat flux. The blowoff limit is around 6.7 ḡe and the extinction limit is found to be Yo∞ = 0.131.

Original languageEnglish
Pages (from-to)933-964
Number of pages32
JournalCombustion science and technology
Issue number5
StatePublished - 1 May 2003


  • Downward flame
  • Fuel thickness
  • Gravity
  • Ignition
  • Incident peak heat flux
  • Oxygen index

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