Combustion of a Fuel Droplet Surrounded by Oxidizer Droplets

The interaction between a burning fuel droplet and satellite oxidizer droplets is studied analytically. The effects of droplet spacing and droplet size ratio on the flame configuration of a burning fuel droplet with a satellite oxidizer droplet are analyzed in a high-temperature oxidizing environment by using the bispherical coordinate system. Three combustion modes including normal combustion, conjugate combustion, and composite combustion are identified at appropriate droplet size ratio and droplet spacing. The burning rate of the fuel droplet is found to be greater than that of an isolated burning fuel droplet, and to increase with the decreasing distance between two droplets. This result has shown a positive effect on the interaction between fuel and oxidizer droplets, in contrast to that of two interacting fuel droplets where the burning rate decreases with decreasing droplet spacing. The combustion configuration of a fuel droplet surrounded by six satellite oxidizer droplets symmetrically is also examined by the method of images. The flame that encloses the fuel droplet is found to be “compressed” and distorted to a nonspherical shape due not only to the group effect among oxidizer droplets but also to the interaction of bipropellant droplets. The results indicate that the burning rate of a fuel droplet increases and the flame size decreases significantly as a result of an increased supply of oxidizer vapor provided by the surrounding oxidizer droplets. Therefore properly optimized bipropellant combustion is potentially able to achieve a desired combustion performance with a much smaller combustor than a conventional spray burner.