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Heat Transfer Research

ISSN for PRINT: 1064-2285

Institutional price:	$2485.00
Issues per year:	8

Best Paper Award Selection - Editorial Board Site

2008, Volume39

90 pages

DOI: 10.1615/HeatTransRes.v39.i2

Issue price - $372.00

Modeling and Simulations of Fine Water Spray in Buoyant Turbulent Diffusion Flame

A. Yu. Snegirev
Laboratory for Applied Mathematics and Mechanics, St. Petersburg State Polytechnic University, 29 Politekhnicheskaya Str., St. Petersburg, 195251, Russia

A. L. Lipjainen
Laboratory for Applied Mathematics and Mechanics, St. Petersburg State Polytechnic University, 29 Politekhnicheskaya Str., St. Petersburg, 195251, Russia

ABSTRACT

The overall aims of this work are better understanding of fire suppression mechanisms by fine water spray (water mist) and search for optimum water spray characteristics to mitigate turbulent diffusion flames more rapidly, with less amount of water released. The paper presents the mathematical model of a water spray issuing from the nozzle, and numerical investigation of the interaction of the sprays with the stand-alone buoyant turbulent diffusion flame. The gas phase is modeled by solving the Navier-Stokes equations for the multicom-ponent reacting mixture. URANS simulations of 15-kW methane flame are presented. To model water spray, a Lagrangian approach is used with the groups of droplets tracked upon its movement in the gas mixture. In the model, droplet-gas heat transfer, droplet evaporation, and droplet dispersion due to turbulence are accounted for. To describe initial droplet-size distribution, Rosin-Rammler distribution is assumed. In modeling spray-flame interaction, approximate probabilistic approach is used to allow for partial extinction of the flame-lets due to excessive concentration of water vapor in the reactants. Simulations have been performed for coarse (initial median droplet diameters of 630 μm) and fine (80 μm) polydisperse evaporating sprays. Numerical simulations demonstrate transition between the mechanisms of fire suppression when the droplet size changes. It is characteristic of coarse spray to directly attack the fuel source (the burning surface), while fine spray more effectively mitigates the gas flame. Simulations demonstrated that in the fire scenario considered, fine water spray results in faster flame extinguishment with smaller water flow rate.