Stability of vapor film in subcooled film boiling on a sphere

摘要

A linear stability analysis of vapor film in subcooled film biling on a sphere has been presented. The analysis considers the effects of liquid intertia, vapor viscotiy and compressibility, and periodic heat conduction in the vapor f ilm and surrounding liquid. The disturbance is assumed to be a standing wave type. An alebraic equation that relates the amplification factor of disturgance to the vapor film thickness distribution around the sphere is derived. Assuming a uniform vapor film thickness around the sphere, numerical solutions are obtained for different wave numbers of disturbance k. The critical vapor film tickness delta _(cr), below which the vapor film becomes unstable, is larger for smaller k. The theoretical predictions of delta _(cr) are compared with available experimental data of the average vapor film thickness at the minimum-heat-flux point delta _(MHF) that were obtained from quenching experiments of small spheres in water. Generally, delta _(MHF) decreases as the liquid subcooling increases (i.e., as the natural disturbance on the vapor film decreases). At a high liquid subcooling, delta _(MHF) agrees well with delta _(cr) for k chemical bounds 1, which suggests that a uniform disturbance of k chemical bounds 0 is not included in the natural disturbance.