ABLATION BY PULSED PLASMA JETS

The ability of pulsed plasma jet igniters to evaporate liquid fuel when cold-starting alcohol- and diesel-fused automobile engines is investigated using solid samples of wax (which is thermally similar to alcohol), polymethylmethacrylate and glass. The known properties of the plasma jet imply a potential rate of heat transfer from the jet to an adjacent surface of the order of 10(8) W m-2. The computed flux thresholds for evaporation of wax and polymethylmethacrylate are of the order of 10(7)-10(8) W m-2 and in fact a mass evaporation of 10(-9)-10(-8) kg per pulse is measured. The threshold for glass is in excess of 10(8) W m-2 and no evaporation is observed. The mass of wax evaporated is in good agreement with calculations made on the basis of a self-limiting mechanism in which an insulating layer of vapour is established at the surface through an equilibrium between evaporation of material into the layer and convection of plasma into the layer.