Spray Formation by Homogeneous Flash Boiling

Injection systems are integrated in a variety of applications including fuel injection systems, painting and coating processes, medical inhalers and more. In some applications, and particularly in fuel injection systems, the size and size distribution of the droplets are extremely important properties. For example, in jet engines and internal combustion engines the droplet’s diameter affects the combustion performances; smaller diameter usually means better combustion efficiency and reduced air pollution. Additionally, since even small amount of large droplets increase the pollutants emission and reduce the efficiency; the size distribution is also important. There are various injecting methods to produce a fine spray that contains small and uniform droplets; in most of them there is a use of the fluid energy (plane or swirl atomizers), an added gas energy (air blast and air assist atomizers) or mechanical energy (rotary or slinger atomizers) to shatter the liquid jet into fine spray. One of the more effective methods is to inject the fluid under suitable conditions to attain flash boiling atomization; i.e. injecting a pure liquid or a mixture under initial conditions that ensure an evaporation of the liquid or at least one of the components in a mixture during the rapid depressurization caused by the expansion in the orifice. In a binary mixture for example, a high vapor pressure propellant might be dissolved into a low vapor pressure solvent in order to produce rapid evaporation of the propellant during the depressurization through the orifice; the rapid evaporation of the propellant disintegrates the solvent into the desired fine spray.
The majority of the studies in this field deal with the most common case in which the bubbles are formed in preferred sites on the orifice walls or near contaminants (heterogeneous nucleation). Studies dealing with homogeneous nucleation, in which the bubbles are formed in the entire bulk, are quite rare although finer spays are anticipated since the typical bubble formation rate in the case of homogeneous regime is much higher. The current study deals with homogeneous flash boiling through a plane orifice. The spray properties of pure liquid and binary mixture were characterized experimentally, under variety injecting conditions (injecting temperature and pressure) and a wide range of propellant mass concentration. Also, the border line between the two regimes was examined. Additionally, since the majority of the studies deal with the heterogeneous case, there are neither suitable correlations nor analytical estimations for the typical droplets’ diameter under homogeneous nucleation regime. In the current work an analytical model has been developed to estimate the characteristic droplet diameter in the spray for flash boiling atomization. One significant conclusion is that under homogeneous flash boiling, remarkably small and uniform droplets are achieved; therefore by using this relatively simple method extremely fine sprays are obtained comparing to the other methods.