The Dynamics of Buoyant Fires and their Suppression with Chemically Enhanced Water Mist

Abstract

This thesis presents experimental results based on the Sydney Fire Burner that produces 10 - 40 kW CNG turbulent buoyant diffusion flames, focusing on revealing complex fire phenomena, specifically buoyant fires, and the effects of enhanced fuel-turbulence levels and their suppression through chemically enhanced water mist. The novelty of the burner employed is the ability to control fuel-turbulence purely through the inclusion of a perforated plate that is recessed inside the burner.

The burner is characterised through means of LDV, used to obtain exit plane boundary conditions of velocity for the different fuel flow rates, and fuel-turbulence levels. DSLR images and LDV show that the changes observed in flame structure due to enhanced turbulence are achieved purely through the inclusion of the perforated plate and not through the change in HRR. High-speed (250 Hz) simultaneous imaging of OH-PLIF and Mie scattering of soot is applied to all flame cases, where the measurements are used to temporally resolve the evolution of flame structure with increasing fuel-turbulence levels. Classical flame pinch-off instabilities are observed where local quenching creates separate flame pockets. The flames begin to transition from a varicose to sinuous type instability with increasing levels of fuel-turbulence, thereby shifting the mechanism of oxidiser transport into the flame.

The flame co-flow is then seeded with fine water mist droplets (d32 = 50um) at varying concentrations of alkali metal salts (KHCO3 and NaHCO3) to observe the effects of enhanced suppression through water mist. An unprecedented characterisation of the boundary conditions is measured through PDPA, obtaining droplet size distributions, velocity and flux. Average flame extinction delay times are measured for varying flow rates of water mist and concentrations of inhibitors to produce flame stability maps, comparing suppression efficacy, giving the following inhibitor rankings: KHCO3 > NaHCO3 > H2O.