Breakdown of a falling wave liquid film during nonstationary heat release

The results of an experimental investigation of the dynamics of development of heat transfer and crisis phenomena in falling wave nitrogen films during nonstationary heat release from the surface of a thin-wall heater are given. It is shown that at small values of the heat flux density in crisis regimes under the conditions of stepwise increase in loading, the breakdown of a laminar-wave liquid film occurs with the appearance of metastable regular structures with liquid jets and large-scale unwetted zones between them. Experimental data are obtained on the times of expectation of liquid boiling up, development of regular structures, and drying out of heat-releasing surface depending on the heat flux within the range of Reynolds numbers Re-in = 90-1690. From the analysis of experimental data it follows that in calculating the time of expectation of liquid boiling up in the case of stepwise heat release in the investigated range of heat flux values it is necessary to taken into account the influence of the convective component of heat transfer and the development of intensive evaporation from the free surface of the wave film. The charts of transient regimes for various Reynolds numbers are constructed; they isolate the region with a basically different character of the breakdown of a film at high heat fluxes with rapid expulsion of a liquid in the form of droplets and complete drying of virtually the entire surface.