Heat transfer and critical heat flux at evaporation and boiling in refrigerant mixture films falling down the tube with structured surfaces

The results of an experimental study of heat transfer in the liquid films flowing down a vertical cylinder with different types of surface structuring are presented. The methods of heat transfer enhancement and critical heat flux increase using horizontal ribbing, rhomb-shaped notches and mesh surface coating at the wave-laminar film flow were investigated. The mixture of refrigerants R114 and R21 was used as a working liquid; initial concentration of volatile component R114 in the mixture was varied from 0% to 19%. It is found out that a change in the character of the wave film flow on the surface with horizontal ribbing at film Reynolds numbers Re > 400 leads to heat transfer intensification at evaporation compared with the smooth and rhomb-shaped surfaces. At evaporation and development of nucleate boiling, the heat transfer coefficient on the surface with mesh coating is higher than for the smooth and textured surfaces. At nucleate boiling there is no effect of surface texture on heat transfer. The influence of the initial composition of a binary mixture of refrigerants with close saturation temperatures on the heat transfer coefficient at evaporation and boiling is insignificant. The development of crisis phenomena over the studied structured surfaces is determined by the regularities of dry spot formation, specific to evaporation of the wave liquid film.