Investigation of heat transfer by evaporation in capillary grooves with a porous coating

Abstract

536.24The results of experimental investigations of heat pipe evaporators with axial grooves of various profiles are presented. A method for enhancing heat transfer by liquid evaporation from capillary grooves of evaporators is proposed. It is realized by applying to the capillary groove surface a thin (25-100 μm) porous coating containing pores up to nanosize. The use of such a coating has made it possible to increase the heat transfer coefficients by evaporation by a factor of 1.5-2.Introduction. It is known that the heat transfer coefficients by evaporation in thin liquid films are much higher than the coefficients by evaporation and boiling in a large volume. To form and maintain thin liquid films, capillary coatings can be used. This is realized to the greatest extent in heat pipes, where the capillary structure provides not only enhancement of evaporation and a uniform distribution of the liquid over the evaporation surface, but also the transportation of the liquid to the evaporation zone. At present, capillary structures in the form of capillary grooves are often used. Such structures have the most extensive applications in heat pipes for systems providing the temperature regimes of spacecrafts. Capillary grooves in a heat pipe are obtained by relatively simple methods, providing thereby repetition of geometrical characteristics, a high mechanical reliability, and good capillary and hydraulic parameters. The properties of these structures and heat pipes using them have been studied fairly well, and perfeting them consists of optimizing the shapes and sizes of capillary grooves depending on the purpose and concrete operating conditions. At the same time, along with the undoubted advantages, such structures also create certain limitations in liquid evaporation and boiling connected with the specificity of the heat transfer in a capillary groove. The next stage of perfection of the heat transfer parameters is intensification of the evaporation transfer processes by applying an additional thin (25-100 μm) porous coating to the surface of grooves. In [1-3], it was shown theoretically and experimentally that the application of a porous coating to the groove surface has a positive effect on the heat transfer coefficient. This is due to the specificity of the processes during liquid evaporation from a capillary groove.Heat Transfer by Evaporation in Capillary Grooves. At the present time, the most widely used evaporation model is that in which practically the entire process of evaporative heat transfer occurs in the bounded zone of the lateral surface of the groove rib in the zone of the liquid meniscus (Fig. 1). According to this model, the liquid meniscus in the capillary groove near the rib surface has a prolate form and is arbitrarily divided into three zones: 1 (zone of the main meniscus), 2 (transition zone of the thin film), and 3 (zone of the equilibrium thin film).It has been shown [1, 4-6] that in zone 2 of the above model the most intensive evaporation of the liquid occurs and practically the whole of ...

Comparative study on the heat transfer performance of micro-grooved anodized thermosyphon with R134a, R600a and R717 for low-temperature applications

“…The anodization process forms a thin porous coating that enhances the evaporation and boiling heat transfer. Further, the anodization process enhances the porosity, the generation of additional active nucleation sites which promotes the nucleate boiling, liquid absorbing capability and the increase in surface area results in an enhancement in evaporation heat transfer [47]. The variation in the condenser heat transfer coefficient of non-anodized and anodized grooved thermosyphon with an increase in heat input was calculated using Eq.…”