Experimental study on the combustion characteristics of square fire

Abstract

In this paper, the combustion characteristics of square fire are studied experimentally, with emphasis on the changes in flame structure during the flow evolution. A series of fire tests with different fuel types, heat release rates, and fire scales are performed. The time-averaged temperature and velocity profiles and the flame length are measured and discussed. Experimental results show that as the square fire progresses downstream from the flame base, its cross-sectional shape undergoes a square-to-round transition. In this process, the self-induced axis-switching phenomenon intensifies the square fire's turbulence, manifested as more significant axial and radial temperature fluctuations than the round fire. This enhanced turbulent mixing promotes flame burning, resulting in a greater axial temperature and velocity of the square fire than the round fire. At different fire scales, the critical height at which a square fire completes the axis-switching first increases and then slows down with increasing heat release rate, which can be well correlated by the dimensionless heat release rate normalized by the side length. Experimental data verify that Quintiere and Grove's classic model can well normalize the flame lengths of square fires at different scales. A square fire's three-dimensional flow and enhanced turbulent mixing cause a higher constant representing the near-field entrainment strength than a round fire.