Decay of the Falling Wavy Liquids Films at Nonstationary Heat Release

Abstract

This paper deals with investigation results on boiling up and crisis phenomena for nonstationary heat release in falling liquid films. According to the experimental results, in the studied range of irrigation degree alteration (Rein = 50–1300), parameters, characterizing decay of the falling liquid film with stepped heat release (distribution of time of boiling incipience along the liquid film, velocities of movable boundaries in the boiling-up and drying fronts), depend complexly on the Reynolds number, wave characteristics and heat flux density. Experiments were carried out with the use liquid nitrogen. Step-wise heat release was supplied on the vertical plane constantan foil of the 25-mkm thickness and 40-mm length. When loading impulses of high intensity, film decay is determined by dynamic characteristics of propagation of the self-maintained fronts of evaporation and the complex shape of structures, formed during its development. The effect of heat flux density on the time of boiling-up expectation and structures of evaporation fronts is shown for different Reynolds numbers. The experimental data obtained on the average propagation velocity of the self-maintained front of evaporation are compared with the simulation model results.

Investigation of transient processes at liquid boiling under nonstationary heat generation conditions

“…The effects of nonstationary back pressure in liquid, providing a significant decrease in the efficient temperature head and rise in the density of vapor, generated at the front interface, play the determining role for variability of propagation velocity of the head and lateral points of the front. It was shown in [83] that consideration of the rapidly growing disturbances determining the two-dimensional structure and branching of the front lead to rapidly increasing average propagation velocity of the front at high temperature differences, compared to the base solution (the system of Eqs. (1), (2)) for the undisturbed one-dimensional front.…”

“…Meanwhile, the rapid liquid boiling leads to rapid liquid discharge in the form of drops and to complete drying out of the entire heat-releasing surface in a narrow time interval. According to analysis of digital records [80,82,83], propagation of boiling fronts in the liquid film falling over a plane surface demonstrates some features. Vapor cavities of a complex shape propagate rapidly over the film at boiling-up.…”

“…Recent experimental investigations carried out in [80][81][82][83] for liquid boiling in nitrogen films falling on extended surfaces under stepwise heat generation have shown that in the area of high heat loading, the drying crisis is also determined by the dynamics of propagation of fast self-sustained evaporation fronts. Meanwhile, the rapid liquid boiling leads to rapid liquid discharge in the form of drops and to complete drying out of the entire heat-releasing surface in a narrow time interval.…”