Heat transfer and bubble formation on horizontal copper tubes with different diameters and roughness structures

Abstract

Heat transfer in flooded evaporators of the refrigeration, air conditioning or process industries is mainly enhanced by modifying the surface structure of evaporator tubes in the micro and/or macro range. To quantify the effect of such modifications, however, the influence of the basic roughness structure on the heated surface has to be separated. Starting from recent publications, experimental results of heat transfer and bubble formation from horizontal copper tubes with different outer diameters (8 or 25 mm) and roughness structures to various boiling liquids are analyzed in this paper to improve our knowledge of the specific events connected with the formation of bubbles at active nucleation sites and their effect on local heat transfer. It is shown that a single, standardized roughness parameter like the (integral) mean roughness height P a is not sufficient to explain the effect of the heating surface structure on nucleate boiling heat transfer. Instead, detailed information on characteristic roughness parameters of the heated surfaces is necessary for the analysis, making it possible to define the size and form of cavities included in the roughness structure and their positions on the surface. An analysis that aims in this direction is given in a separate contribution to this special issue by A. Luke, who prepared the surfaces and provided the basic data on the set of standardized roughness parameters, the probability distributions of which are used in this paper.