On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires - Part Ⅰ: Spread rate and soot production

摘要

In microgravity, the extended time scales associated with the absence of buoyancy lead to peculiar flame features that are likely to affect the risk associated with flame spread in the case of a spacecraft fire. Investigating a non-buoyant flame spreading over the polyethylene coating of an electrical wire in an opposed laminar flow, recent studies especially evidenced and quantified the major role of soot in the radiative heat transfer, which affects local quenching at the flame tip as well as heat feedback from the flame to the coating. Consequently, the control of soot production in such a flame needs to be explored. In the present paper, the role of basic flow features, i.e. oxygen content, flow velocity, and ambient pressure, is documented. Conducted in parabolic flights, a set of 142 experiments spreads over 91 flow conditions, with oxygen content ranging from 18% to 21%, flow velocity kept between 100 mm s(-1) and 200 mm s(-1), and pressure ranging from 51 kPa to 142 kPa. The implementation of the Broadband Modulated Absorption/Emission (B-MAE) technique allows the fields of soot temperature and volume fraction to be measured within the spreading flames. The flame spread rate is thus shown to be an increasing function of oxygen content, but is independent of flow velocity and pressure. Concomitantly, both oxygen content and flow velocity affect soot production residence time, while pressure has a marginal impact on it. Maximum soot volume fraction is a function of all three parameters. Complementing these results with a scaling analysis, soot production rate is third-order in pressure, and very sensitive to oxygen content. An increase in flow velocity promotes two competitive processes with respect to maximum soot volume fraction, i.e. a reduction in residence time and an increase in flame temperature. A numerical model supports the experimental finding that the latter phenomenon prevails, hence that maximum soot volume fraction increases with flow velocity. These conclusions will serve as basis for upcoming quenching and radiative heat feedback analysis. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.