Heat Exchange under the Evaporation and Boiling of a Film Showering a Bundle of Horizontal Finned Tubes
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Exchange under the Evaporation and Boiling of a Film Showering a Bundle of Horizontal Finned Tubes I. I. Gogonin* Kutateladze Institute of Thermophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia *e-mail: [email protected] Received March 18, 2019; revised October 24, 2019; accepted November 12, 2019
Abstract—An algorithm for calculating heat exchange in the course of evaporation and boiling of a film that showers a bundle of finned tubes is described and a comparison of calculated and experimental data is performed. The calculated parameters are in quite satisfactory agreement with the experimental data. Keywords: finned tube bundle, heat exchange under evaporation and boiling, heat-exchange enhancement DOI: 10.1134/S0040579520030033
INTRODUCTION Film-type heat exchangers are successfully used in many industrial branches. The finning of tubes of the bundle is one of the main ways to intensify heat exchange. If it is taken into account that the finning leads to a significant stabilization of the film flow and a multifold intensification of heat-exchange processes, such heat exchangers are especially attractive. However, the number of journal publications devoted to studies concerning heat exchange in the course of evaporation and boiling of a film that showers a bundle of horizontal finned tubes is extremely scarce. Heat and mass transfer in this case is described by empirical equations. In this paper, an attempt is made to describe heat-exchange processes occurring under evaporation and boiling by equations obtained based on physical studies concerning the hydrodynamics of film-type flows on the surface of finned tubes. The processes of heat exchange under boiling in the film is always accompanied by heat exchange owing to evaporation, and wall cooling depends on both processes. HYDRODYNAMICS OF A FILM-FLOW SHOWERING A BUNDLE OF FINNED TUBES The authors of [1] have found that the amount of liquid retained between fins significantly changes depending on the geometrical parameters of finning. Figure 1 shows a photo image where one can distinctly see the flooding of the interfinal cavity by the liquid in the lower part of the finned cylinder. Figure 2 shows a physical model of liquid flow along a finned tube. It is assumed that, in the course of the liquid flow in the interfinal space, the forces of surface tension significantly exceed the forces of grav-
ity. This assumption is confirmed by visual observations and video analysis. In the course of the showering of finned tubes with ethyl alcohol, the emitted microbubbles have made it possible to see quite distinctly the trajectory of the liquid along the lateral surface of the fin. A scheme of this motion is shown in Fig. 2. The results of systematic measurements of the height of a liquid layer retained between the fins performed in [1] are shown in Fig. 3. The height of capillary liquid retention has been measured using a cathetometer. Figure 3 also shows the results of experiments obtained for finned cylinders with a rectangular and
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