Condensation of pure hydrocarbons and zeotropic mixtures in smooth horizontal tubes
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A study of the condensation of hydrocarbons and zeotropic hydrocarbon mixtures in smooth horizontal tubes was conducted. Measurements of condensation heat transfer coefficients and frictional pressure drop were taken over a range of mass fluxes (G = 150 – 450 kg m-2 s-1), a range of reduced pressures (Pr = 0.25 - 0.95), for two tube diameters (D = 7.75 and 14.45 mm), several working fluid-to-coolant temperature differences (ΔTLM = 3 – 14°C) and temperature glides (ΔTGlide) between 7 - 14°C. The wide range of conditions investigated in this study provides considerable insight on the transport phenomena influencing condensation in pure fluids and their mixtures. The trends in heat transfer coefficient and frictional pressure gradient are discussed and compared with the predictions of correlations from the literature. The results of the experiments, combined with previous flow visualization studies on hydrocarbons, were used to develop physically consistent heat transfer and frictional pressure gradient models that are applicable to pure fluids and zeotropic mixtures. A framework was developed for zeotropic mixture condensation that recommends a specific modeling approach based on the observed trends in the heat transfer coefficient and the points of deviations from pure fluid trends. The documentation of the condensation heat transfer and pressure drop behavior of environmentally friendly refrigerants, and the development of accurate correlations, will facilitate their widespread introduction as a working fluid for refrigeration cycles. Furthermore, the accurate pure fluid models, which serve as a baseline case for zeotropic mixture modeling, yield more effective predictions of zeotropic mixture condensation, which will lead to increased efficiencies of chemical processing plants.