Dropmall interaction criteria and their applications in diesel spray modeling

Drop/wall interaction studies are reviewed with emphasis on diesel spray modeling applications. The wall temperature and drop Weber number have major influences on the outcome of drop/wall interactions. It is known that the drop will rebound with a near-spherical shape once the wall temperature exceeds the Leidenfrost temperature due to a vapor blanket that forms between the liquid drop and the wall. The Leidenfrost temperature increases significantly as the ambient pressure increases. The in-cylinder pressure already exceeds the critical pressure of the fuel drop at the time of injection such that the critical temperature of a fuel drop is a better indication of its wall-wetting possibility. It is unlikely that the surface temperature will exceed either the Leidenfrost temperature or the critical temperature under diesel engine conditions. Thus, the liquid drop will at least be in partial contact with the surface and a fuel film is likely to form. However, the amount of wall film will be less than that observed from single drop/wall impingement experiments due to the interaction of the flattened liquid drops that generates splashing droplets from the wall. On the other hand, nucleate boiling may not occur due to the high in-cylinder pressure that prevents bubbles from nucleating and escaping from the liquid despite the high piston surface temperature. Various splashing criteria are compared and a critical splashing limit is proposed that relates the drop Weber number and Reynolds number to the surface roughness and the existing film thickness. Due to the interactions of fuel drops at the impact site that result in more splashing droplets and less fuel film, previously developed drop/wall interaction models that do not emphasize the formation of wall films can still perform reasonably well in diesel spray modeling.