A study of the structure of a turbulent line fire subjected to cross-flow using large eddy simulations

Abstract

The general objective of this project is to provide a basic understanding of the transition between the different flame regimes observed in fires with cross-flow and/or fires along inclined surfaces. We consider here a simplified configuration corresponding to a methane-air, buoyancy-driven, turbulent line flame stabilized on top of a horizontal floor surface and subjected to different air cross-flow velocities. At high values of the cross-flow velocity, the flame features a horizontal shape and develops as a boundary layer flame in the vicinity of the floor surface; the flow downstream of the flame is attached to the floor surface and air entrainment into the flame is one-sided. In contrast, at low values of the cross-flow velocity, the flame features a tilted vertical shape and develops as a pool-like flame; the flow downstream of the flame separates from the floor surface and air entrainment into the flame is two-sided. In the present study, we analyze the transition from an attached flame to a lifted flame using wall-resolved large eddy simulations (LES). Simulations are performed with an LES solver developed by FM Global and called FireFOAM. The simulated line burner is 50-cm wide and 5-cm long; the flame power is 50 kW; and the air cross-flow velocities range between 0.75 and 3 m/s. The LES simulations provide a detailed description of the different contributions to flow kinetic energy in the horizontal and vertical directions and thereby provide unique insights into the competing effects of the external momentum of the horizontal cross-flow and the internal momentum of the vertical buoyant motions produced by the combustion heat release and the resulting unstable thermal stratification. A new criterion is proposed to measure the relative strength of external/cross-flow-driven versus internal/buoyancy-driven motions and to thereby predict the transition from an attached to a lifted flame regime.