Effect of moisture content on thermal decomposition and autoignition of wood under power-law thermal radiation

• Published in: Applied thermal engineering 2020-10, Vol.179, p.115651, Article 115651
• Main Authors: Gong, Junhui, Cao, Jialei, Zhai, Chunjie, Wang, Zhirong
• Format: Article
• Language: English
• Subjects: Auto-ignition; Beech; Beech wood; Cracks; Evaporation; FireFOAM; Heat conductivity; Heat flux; Heat transfer; Ignition temperature; Mathematical analysis; Mathematical models; Moisture content; Numerical prediction; Power law; Pyrolysis; Radiation; Spontaneous combustion; Surface temperature; Thermal decomposition; Thermal radiation; Time-varying heat flux; Water vapor
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2020.115651

•Effect of moisture content (MC) on pyrolysis and autoignition of wood is studied.•Power-law thermal radiation and MC from 0% to 38% are examined.•Analytical and numerical models are developed to predict experimental results.•No substantial discrepancy is found when MC > 25% due to the cracks of char layer.•Measured critical temperature, 395 °C, can be used as ignition criterion. Pyrolysis and autoignition of beech wood, with moisture content (MC) from 0% to 38%, exposed to power-law thermal radiation is investigated. An experimental facility capable of irradiating time-varying heat flux was utilized to conduct bench-scale tests. An analytical model was proposed to estimate the ignition time, surface and in-depth temperatures. Meanwhile, a transient numerical solver, FireFOAM, was employed to simulate the experimental measurements. The results show that surface and in-depth temperatures rise more quickly with larger heat flux and lower MC. Although the analytical model overestimates surface temperature slightly due to the ignoring of pyrolysis, both the analytical and numerical predictions are at acceptable levels. In-depth temperatures cannot be accurately predicted due to the recondensation of the migrated water vapor beneath the evaporation layer. No substantial discrepancy is found between the 25% and 38% MC surface temperatures since significant cracks emerge on surface upon heating. The average measured ignition temperature is 395 °C which is used as the ignition criterion. Ignition time increases with lower heat flux and larger MC when MC  25% due to the crack effect. The predicted ignition times match the experimental results relatively well despite some minor deviations.