Pool fire dynamics: Principles, models and recent advances

Pool fire is generally described as a diffusion combustion process that occurs above a horizontal fuel surface (composed of gaseous or volatile condensed fuel) with low (similar to zero) initial momentum. Fundamentally, this type of diffusion combustion can be represented by basic forms ranging from a small laminar candle flame, to a turbulent medium-scale sofa fire, and up a storage tank fire, or even a massive forest fire. Pool fire research thus not only has fundamental scientific significance for the study of classical diffusion combustion, but also plays an important role in practical fire safety engineering. Therefore, pool fire is recognized as one of the canonical configurations in both the combustion and fire science communities. Pool fire research involves a rich, multilateral, and bidirectional coupling of fluid mechanics with scalar transport, combustion, and heat transfer. Because of the unabated large-scale disasters that can occur and the numerous and complex 'unknowns' involved in pool fires, several new questions have been raised with accompanying solutions and old questions have been revisited, particularly in recent decades. Significant developments have occurred from a variety of different perspectives in terms of pool fire dynamics, and thus the scientific progress made must be summarized in a systematic manner. This paper provides a comprehensive review of the basic fundamentals of pool fires, including the scale effect, the wind effect, pressure and gravity effects, and multi-pool fire dynamics, with particular focus on recent advances in this century. As the fundamentals of pool fires, the theoretical progress made with regard to burning rates, air entrainment, flame pulsation, the morphological characteristics of flames, radiation, and the dimensional modelling are reviewed first, followed by new insights into the fluid mechanics involved, radiative heat transfer and combustion modeling. With regard to the scale effect, recent experimental and theoretical advances in internal thermal transport and fluid motions within the liquid-phase fuel, lip height effects, and heat transfer blockage are summarized systematically. Furthermore, new understandings of aspects including heat feedback and the burning rate, flame tilt, flame length and instability, flame sag and base drag, and soot and radiation behavior under wind, pressure and gravity effects are reviewed. The growing research into the onset and the merging dynamics of multiple pool fires in the last decade is described in the last section, this research will be helpful in the mitigation of threatening outdoor massive (group) fires. This review provides a state-of-the-art survey of the knowledge gained through decades of research into this topic, and concludes by discussing the challenges and prospects with regard to the complex coupling effects of heat transfer, with the fluid and combustion mechanics of pool fires in future work.