Momentum Diffusion Flame Characteristics and the Effects of Water Spray

Abstract

For water spray suppression of gas well blow-out fire applications, reasonably large scale (7 MW) subsonic methane diffusion flames have been investigated near the Froude number limit. Characteristics of this limit include constant flame height, increasing lift-off height, and decreasing radiative fraction. Flame blow-off has been observed with pipe sizes up to 30 mm diameter. Flame and lift-off heights, centerline temperatures, and incident radiative flux to nearby targets have been measured with and without water spray suppressant. Using a simple participating cylindrical flame model an effective gray absorption coefficient approaching 0.2 m⁻¹ was determined at the blow-off limit. Away from the limit, this number rises rapidly as does flame temperature decrease with decreasing jet velocity, i.e., luminosity increases as temperature goes down. The derived shape of the functional dependence of decreasing radiative fraction with increasing jet Froude number in the limit is consistent with small scale literature experiments. The effect of adding water spray to the gas flow below the base of a lifted flame is to shift or raise the flame above its normal position, and to lower peak flame temperature and radiation levels despite increased emissivity due to the radiatively active steam. Extinguishment near blowoff is though to be due to the former effect, i.e., a shift in flame position.

Keywords:

• blow-off
• blowout fires
• diffusion flames
• entrainment
• lift-off
• extinguishment
• radiative fraction
• suppression
• water sprays