SPECIFIC FEATURES OF BOILING OF REFRIGERANT FILMS FALLING DOWN THE SURFACES WITH SEMI-CLOSED SURFACE CAVITIES

The paper presents the results of an experimental study of heat transfer at the film flow of a mixture of refrigerants R114/R21 on vertical cylinders with a differently microstructured outer surface. The film Reynolds number varied in the range 300-1500. The density of heat flux step-by-step increased from zero to values corresponding to the boiling crisis. It is shown that the heat transfer coefficients at nucleate boiling regime on surfaces with a microstructure with a smaller height and fin pitch exceed values for surfaces with larger characteristics. Surfaces with artificially created closed micropores at boiling regime have an advantage in heat transfer in comparison with microstructured surfaces without such pores.