Возможности повышения критических тепловых потоков при кипении на поверхностях с пористыми покрытиями (обзор)

Abstract

A review of recent experimental investigations into burnout at porous surfaces under natural convection conditions (during pool boiling) is presented. Despite a generally positive progress that has been made in the technical solutions aimed at increasing critical heat fluxes, it is still difficult to point out any prevailing method for shaping heat-transfer surfaces. As regards boiling on macroscale heat-transfer surfaces (which is exactly the one of practical interest), the use of nanoparticle coatings has no advantages in comparison with other ways of heat transfer enhancement. In a number of studies, the gain from using nanoporous coatings applied on the base surface (in terms of increasing the critical heat flux) was hardly more than 10%. At the same time, the use of a deforming cutting technology made it possible to increase the surface area up to 8.3 times and the critical heat flux values up to 4.1 times. Attempts to interpret the obtained experimental data theoretically encounter certain difficulties stemming from the fact that burnout depends on the porous layer texture and on its key geometrical features. It follows from the analytical part of the reviewed publications that the mechanism governing the enhancement of boiling heat transfer on porous surfaces has not been fully understood. Attempts to obtain a universal correlation for calculating critical heat fluxes will hardly meet with success because the mechanisms governing burnout are in all likelihood different for different types of porous structures. On the other hand, studies carried out on structured surfaces with a geometrically regular morphology may be expected to yield useful results for revealing the boiling mechanisms. The article presents an approximate analysis of burnout for a microfinned surface with fins having the shape of microcolumns with a square cross-section constant over the column height. Central to the analysis is an approach linking the burnout with irreversible growth of dry spots area on the heated surface. It is shown that with the applied burnout model it becomes possible in principle to explain — both in qualitative and quantitative manner — the effect the heated surface microstructure has on the onset of burnout during the boiling of liquids under natural convection (pool boiling) conditions.