Heat Loss Analysis of Flamelets in Near-Limit Spread Over Solid Fuel Surfaces

Abstract

The profile of a spreading flamelet is analyzed by examining the heat losses to surrounding surfaces. The study addresses the reasons why flamelets have shapes ranging from round hemispherical “caps” to flat “coin-like” discs. A parabolic shape profile is used for the thin flame sheet, which provides both flame length and flame curvature. A third parameter specifies the height of the flame from the surface beneath it. Radiation and conduction heat losses from the flame sheet are calculated for various flame shapes. Overall heat losses as well as heat losses to the surface beneath the flamelet are examined. Some of the heat “losses” are misnamed because they produce the necessary surface decomposition for subsequent gaseous flame fuel vapors. Strictly, then, “losses” do not contribute appreciably to the maintenance of the flame. Physical arguments are made to explain observed flame spread behavior and flame shapes in response to prevailing flow and environmental conditions.