Extinction Dynamics of a Co-flow Diffusion Flame by Very Small Water Droplets Injected into the Air Stream

Abstract

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Computations are performed to examine the effectiveness of mono-disperse water droplets in extinguishing a co-flow, propane diffusion flame by injecting the droplets into the air stream. The calculations show that the droplets entrained into the reaction kernel at the flame base are crucial for extinction. The reaction kernel detaches from the burner rim and blows-off when the droplet concentration is increased to a critical value (extinction concentration). At the critical value, the maximum chain-branching reaction (H2+O=OH+H) rate in the reaction kernel was found to be reduced by a factor of 5 in our computations. A large decrease in the reaction rate indicates that the maximum heat generations rate and Damkholer number are too low to sustain the flame, and cause the flame blow-off. Large drops are more effective than small drops, and the extinction concentration of water increases from 10.5% to 15% by mass as the size is reduced from 32 to 4 micro m. As the droplet size is increased further (>32 micro m), the trend will reverse as the evaporation rates get too small despite increased penetration of flame core by the drops. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S ACRONYM(S) 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) SPONSOR / MONITOR'S REPORT NUMBER(S) 28-07-2008Memorandum Report

Effects of Fine Water Mist on a Confined Blast

“…One dimensional, unsteady, r-component of the conservation equations [22,23] for the gas phase are given by continuity,…”

“…The droplet phase equations describe mass, momentum and energy balances for each droplet injected [22,23]. The Lagrangian approach is used to obtain the evolution of droplet position, r pn , velocity, u pn , temperature, T pn , and diameter, d pn , with time.…”

Suppression Dynamics of a Co-flow Diffusion Flame with High Expansion Aqueous Foam

“…Propane represents the essential features of a longer hydrocarbon chain (e.g., heptane) because of the presence of middle and end carbon atoms. In our earlier works [11][12][13], we have successfully modeled the cup burner flame using the CFD software package Fluent, and demonstrated excellent agreement with experimental data for extinction by nitrogen gas and water mist. We have used this cup burner flame model as the basis for the gas phase sub-model in the present work.…”

“…h 0 i and M i are the enthalpy and molecular weight of specie i, respectively. The last term on the right-hand side of Equation (13) represents the latent heat absorption (per unit volume and unit time) due to the evaporation of water contained in the foam. The energy, E, is related to temperature by…”

“…The gas phase equations can be recovered by setting foam volume fraction, a f , to zero in Equations ( 1)- (17). The resulting gas phase equations are identical to the gas phase equations described by Ananth and Mowrey [11][12][13] for the specified single-step chemistry [18]. Equations ( 1)-( 17) contain specie and mixture properties and fluxes, which are described elsewhere by Ananth and Mowrey [12,13] and Ananth et al [24,25] for the gas phase.…”