Transient processes in falling films of liquid under conditions of unsteady-state heat release

Results are given of experimental investigation of the dynamics of development of heat transfer and critical phenomena in falling wavy films of nitrogen under conditions of unsteady-state heat release from the surface of a thin-walled heater. The experiments involve the investigation of transient modes with formation of dry spots and development of dryout in the case of stepwise and quasi-steady laws of heat release. It is demonstrated that the disintegration of laminar-wavy film of liquid in the case of low heat flux in critical modes under conditions of stepwise increase involves the emergence of metastable regular structures with jets of liquid and large-scale non-wetted zones between them. The generalization of experimental data is used to demonstrate that the value of characteristic transverse dimension between jets of liquid is more than twice the respective values obtained under conditions of steady-state heat release with boundary condition $T_{\text{heater}} \sim \text{const}$. In the case of high heat flux, the vigorous boiling of liquid leads to a rapid ejection of liquid in the form of droplets and to complete drying (almost simultaneously) of the entire heat-emitting surface. Experimental data are obtained on the times of expectation of boiling of liquid, of development of regular structures, and of drying of the heat-emitting surface as functions of heat flux in the range of variation of Reynolds number $\text{Re}_{\text{in}} = 90$–$1690$. It follows from analysis of experimental data that, in calculating the time of expectation of boiling of liquid in the case of stepwise heat release in the investigated range of variation of heat flux, one must take into account the development of intense evaporation from the free surface of laminar-wavy film.