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Coupled Heat and Mass Transfer on Horizontal Tubes Feeded by Falling Droplets

R. H. Wassenaar
Laboratory of Refrigeration, Delft University of Technology, Delft, The Netherlands

C. H. M. Machielsen
Laboratory of Refrigeration, Delft University of Technology, Delft, The Netherlands

P. D. Iedema
FDO Consultants BV, Hengelo, The Netherlands


Heat pumps for ordinary heating or cooling purposes can be divided in two kinds: compression and absorption heat pumps. The last ones are not driven by mechanical but by thermal energy. The compressor is substituted by a mixture circuit that transports the vapour from evaporator to condensor. The vapour produced in the condensor at a low temperature (e.g. 0°C) and at a low pressure is absorbed in the absorber by the mixture, and the heat of absorption is released at a moderate temperature level (e.g. 50°C). From the absorber the mixture is pumped to the desorber (also called generator), here high temperature heat is supplied (e.g. 120°C) driving the vapour out at a high pressure to the condensor. From the desorber the mixture flows through an expansion valve back to the absorber. With these absorption cycles [1] considerable energy savings are possible. Gas fired heat pumps for domestic heating can reach COP's above 1.3. With absorption heat transformers in industrial applications waste heat at a moderate temperature level (e.g. 70°C) can be used to produce heat at a high temperature level (e.g. 100°C) when part of the heat is rejected at a low temperature level (e.g. 0°C). For heat storage applications absorption cycles are advantegeous above compression cycles [2] . Commonly used absorbers (and desorbers) fall in two categories: film and bubble absorbers. The first one has to be applied when a low pressure mixture is used (that is when absorber and desorber pressure lie below atmospheric pressure). Bubble absorbers can be used for high pressure mixtures. The low pressure mixture used in this study is LiBr/ZnBr2/CH3OH (at mole fraction LiBr: ZnBr2 = 2:1). The absorber consists of vertical rows of horizontal tubes. Depending on flow rate and vertical spacing of the tubes the liquid flow between the tubes is in the form of a sheet, of jets or of droplets. Droplet flow is believed to enhance transfer rates [3].

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