The Influence of Upstream Prevaporization on Flame Extinction of One-Dimensional Dilute Sprays

Abstract

The influence of upstream prevaporization on flame extinction of one-dimensional dilute sprays is analyzed by using large activation-energy asymptotics. The heat loss mechanism is composed of the external heat loss represented by the bulk heat conduction form the spray to a tube wall, and the internal heat loss associated with the droplet gasification process. The enhancement of upstream prevaporization is achieved by varying the wall temperature, and thereby the spray experiences simultaneously the external heat gain and loss in upstream. Results show that the enhancement of upstream prevaporization weakens the partially prevaporized burning of both lean and rich sprays, but has no influence on the completely prevaporized sprays. On the basis of a critical droplet radius, it is found that the extinction curve of a spray having smaller droplets is composed of a upper branch representing the partially prevaporized burning, and a lower branch showing the completely prevaporized burning; and that the flame extinction occurs at the completely prevaporized burning which is originally identified to be unstable in the absence of the enhancement of upstream prevaporization. Conversely, for a spray having larger droplets, the flame extinction is totally controlled by the partially pre-vaporized burning, and the S-shaped curve shows the influence of the internal heat loss from liquid droplets. The critical droplet radius and the minimum wall temperature are parametrically analyzed and discussed for both lean and rich sprays.