Explosive Homogeneous nucleation boiling in ethanol

Abstract

The characteristics of homogeneous boiling in ethanol subjected to rapid heating with a linearly increasing boundary temperature condition have been studied by applying a macroscopic model based on analytical solution of 1-D semi-infinite heat conduction in association with molecular theory of homogeneous nucleation boiling. In this model, a finite control volume or cluster at the liquid boundary is considered where energy is stored due to external heating while some energy is consumed because of bubble nucleation and subsequent growth. The corresponding energy balance equation in the liquid cluster is obtained by considering these two competing parallel processes. A particular state has been defined as the boiling explosion condition when massive scale vaporization causes the liquid sensible energy to decrease i.e. the instantaneous rate of boiling heat consumption at a moment exceeds the external energy deposition within the liquid cluster. Two different length scales for the size of liquid cluster has been adopted- (i) size of a critical vapor embryo and (ii) thermal penetration depth. For ethanol heating with initial and boundary conditions identical to those reported in the literature, the present study shows that the model can predict the timing of the boiling explosion condition with reasonable agreement if the size of liquid cluster is considered to be that of a critical vapor embryo at maximum liquid superheat. Then for different boundary heating rates, boiling explosion time and temperature for ethanol are obtained. The obtained times of boiling explosion vary almost in inverse order of heating rate, while boiling explosion temperatures vary about 90C over the entire range of heating rates. These results have been compared with the theoretical explosion condition obtained from molecular dynamics and pseudo-atomic model study as reported in the literature. It has been found that, the time and temperature at which explosive boiling in ethanol occurs; vary with the degree of liquid subcooling. Therefore, these parameters are not sufficient to describe the explosive boiling phenomena as a unique state regardless of heating method and parameter. To find a unique criterion for explosive boiling, an energy approach is taken in this study. Using this approach, for a given initial temperature, it has been found that, for a given liquid initial temperature (250C), when cumulative energy density reaches 3.28108 - 3.44108 J / m3 for ethanol, explosive boiling will occur irrespective of liquid boundary heating conditions. Therefore, a unique criterion is found in the form of cumulative energy density near the boundary for the occurrence of explosive boiling in the liquid.