Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A |
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Authors: | Stefan Bonekamp Konrad Bier |
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Affiliation: | Institut für Technische Thermodynamik und Kältetechnik, Universität Karlsruhe (TH), Karlsruhe, Germany |
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Abstract: | The effect of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134a has been investigated in a wide range of heat flux and saturation pressure. The enhancement of the heat transfer coefficient, which can be achieved by ultrasound, is much more pronounced for mixtures than for pure substances. It is, however, limited to rather small heat fluxes (). Especially remarkable is the fact, that the maximum influence of ultrasound on the heat transfer coefficient of the mixtures occurs at medium saturation pressures (p/pc ≈ 0.2); the effect is markedly less for higher and for lower saturation pressures. Obviously, the improvement of the heat transfer to mixtures is mainly caused by a decrease of the local saturation temperature near the heating wall, due to a better mixing in the liquid boundary. This explanation is supported by evaluating important parameters of bubble formation from high-speed photographs of the heating surface. It is further noticeable, that the well known hysteresis effect at the beginning of pool boiling is reduced to a great extent by exposure to ultrasound. |
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Keywords: | heat transfer mass transfer pool boiling refrigerant mixture R23 R134a ultrasound measurementtransfert de chaleur transfert de masse ébullition libre frigorigène mélange R23 R134a ultrason mesure |
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