Crisis Phenomena and Heat-Transfer Enhancement during Boling and Evaporation in Horizontal Liquid Films (Review)
А. Н. Павленко, В. И. Жуков, D. A. Shvetsov
Abstract
A brief review is presented of studies in the field of heat-transfer enhancement and for increasing the critical heat fluxes (CHFs) during boiling and evaporation in thin horizontal liquid layers. The effect of the liquid layer height on heat-transfer efficiency at various heat fluxes has been analyzed. Decreasing the film thickness enhances heat transfer at low heat fluxes, while heat-transfer augmentation occurs at high heat fluxes in films whose height is greater than the capillary constant. The heat-transfer coefficient (HTC) first rises with a layer height, and then decreases to pool boiling values. The structures formed in thin liquid layers are examined in a wide range of layer heights and pressures. The mechanisms of formation of various structures and the effect on heat-transfer enhancement depending on process conditions were also discussed. The existence of regimes with heat-transfer enhancement during evaporation of a liquid layer at reduced pressures has been demonstrated. The heat-transfer coefficient in these regimes is higher than during nucleate boiling in a layer of the same height at a higher pressure. This is caused by the effect of the structures formed in these regimes. It has been found that CHF rises with an increase in the layer thickness to pool boiling values. The heat-transfer coefficients during nucleate boiling in thin films on capillary-porous surfaces are approximately three to five times higher than on a smooth surface. It has been demonstrated that there is an optimal film surface at low pressures, which provides higher heat-transfer coefficients than those on a smooth surface. Higher heat conductivity coatings of the same shape considerably increase CHFs in the entire pressure range for liquid layer thicknesses of the order of the capillary constant. It was found that, with a characteristic spacing between the coating fins, which is equal to the capillary constant of the liquid, the highest heat-transfer coefficients are attained in the experiments.