Heat Transfer in Liquid Film Falling Down Vertical Cylinder with Single-Layer and Gradient Two-Layer Mesh Coatings

The paper presents the results of a study of heat transfer in laminar-wave liquid films falling down the outer surface of a vertical cylinder. As intensifiers of heat transfer during boiling, single-layer micromesh coatings with different geometric characteristics and a two-layer gradient mesh coating were used. The working liquid was mixture of refrigerants R114-R21 with the initial concentration of the low-boiling component (R114) equal to 12%. The film Reynolds number at the entrance to the test section varied from 400 to 1300; the heat flux density varied in the range of 0-6 W/cm(2). The results of measuring the heat transfer coefficients in the regimes of evaporation and nucleate boiling of the film are presented. A comparison is made of the results obtained on the single-layer and two-layer mesh coatings, as well as with previously obtained data for a combined coating (created by the method of deformational cutting in combination with a mesh coating). It has been shown that, in comparison with the smooth surface, the heat transfer coefficient during boiling of the falling film can be increased up to 2 times with the single-layer mesh coating and up to 1.7 times with the two-layer gradient mesh coating.