Observation of boiling heat transfer and crisis phenomena in falling water film at transient heating

This paper deals with the experimental study on transient heat transfer at boiling and development of crisis phenomena in the falling film of subcooled liquid at stepwise heat generation. Experimental data with a high temporal and spatial resolution were obtained with the use of high-speed infrared thermometry and high-speed video recording synchronized simultaneously. It is shown that at vapor bubble appearance and condensation the convective streams are formed there, which lead to a decrease in temperature in the area behind the active nucleation site and local increase in heat transfer intensity. At the same time on the free surface of liquid film the so-called “temperature waves” propagating along the liquid flow are formed. The boiling curves were obtained on the basis of experimental data, and comparison with the known models for description of heat transfer at pool boiling of saturated liquid was performed. It is shown that mechanism of appearance and development of crisis phenomena depended significantly on the Reynolds numbers. Crisis development at high Reynolds numbers relates to dry spot formation at quasi steady-state boiling and their following expansion by the mechanism of longitudinal heat conductivity in the heating wall. The wave development on the free surface at low Reynolds number (Re = 601), studied in current research, leads to the breakdown of the film without nucleate boiling due to the action of thermocapillary forces. The critical heat fluxes under these conditions are significantly lower than the critical values obtained for high Reynolds numbers at boiling in the falling films of subcooled water.