Transport phenomena within the liquid phase of a laboratory-scale circular methanol pool fire

Abstract

The effects of altering the lower thermal boundary condition of a methanol pool from −5°C to 50°C was investigated within a 90mm diameter and 12mm deep quartz burner under steady state burning condition in a quiescent air environment. Both the burning rate and the flame height were observed to increase by 15% with increasing bottom temperature over this range of bottom boundary conditions. The temperature and velocity within the liquid were measured by a single thermocouple traversed through the pool and PIV, respectively, in order to better understand the transport of mass and energy in the liquid. Temperature measurements revealed a distinct two-layer vertical thermal structure with the upper layer of the pool being almost uniform and near the boiling temperature of the fuel, while the lower layer experienced an increasing temperature gradient as the bottom boundary temperature was lowered. The thickness of the thermally uniform layer increased as the bottom temperature was increased. The measured fluid velocity showed a complementary two-layer structure with the upper layer being dominated by a pair of counter-rotating vortices that kept this portion of the liquid well mixed and transferred heat from the hot pool wall to the pool center, while the flow in the lower layer was uniformly low in value and vertical. A model was presented to aid in understanding the energy transfer within the liquid phase. In the lower layer, the Peclet Number was in the order of unity and required that the energy transfer throughout the liquid phase to be modeled as a combination of conduction and convection. Using this physical model, the change in burning rate over the full 55°C change in bottom temperature was predicted within 2%, thereby supporting the proposed mechanism for energy transfer into the pool’s depth.