Evaporation du réfrigérant HFC 407C sur des tubes lisses ou à surface améliorée.

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles for heat recovery, requires good methods for predicting heat transfer of substitute fluids. The measurements in the LENI test facility (concentric tubes with water flowing in a countercurrent flow) with HFC 407C, HFC 134a, HCFC 123, HFC 404A and HFC/HCFC 402A provide a new data band for new refrigerants, and allow a coherent comparison with old refrigerants CFC 11, CFC 12, CFC/HCFC 502 and with existing correlations. The existing correlations were found to be inadequate. Because of this work, an improved flow pattern map and flow boiling model were developed, which resulted in a substantial progress in the accurate predict of heat transfer in plain, horizontal tubes for refigerants without oil. The Kattan et al. (1996, [6]) correlation was programmed to calculate and compare predicted heat transfer coefficients to the new HFC 407C test data. The flow pattern map proposed by Kattan et al. (1996) was also programmed and compared to flow regimes observed for HFC 407C. It predicted the HFC 407C flow pattern data quite accurately. The original objective of the HFC 407C measurements was the validation of the Kattan et al. (1996) correlation applied to a zeotropic refrigerant blend. Local flow boiling heat transfer coefficients were measured for HFC 407C evaporating inside a microfin and plain tube. In addition, microfin heat transfer augmentation relative to a plain tube was investigated. The presence of oil in the evaporator had a complex effect on heat transfer coefficients. Local flow boiling heat transfer coefficients were measured for refrigerant HFC 407C ester oil mixtures (Mobil EAL Arctic 68). A new thermodynamic approach for modeling mixtures of zeotropic refrigerant blends and lubricating oils was also developed. A very high accuracy, straight vibrating tube type of density flowmeter was used to measure oil concentrations of flowing HFC 407C oil mixtures. The test covered oil concentration from 0.5-5 wt.% oil. A method was proposed to predict thermodynamic and transport properties of the refrigerant-oil mixtures. In addition, a first empirical approach was proposed for predicting heat transfer for boiling of refrigerant-oil mixtures. Based on this work, the following phenomena important to the problem were identified that require further investigation: • oil retention in the evaporator tubes that was found and clearly proved in the present project. • the formation of foam in the HFC 407C/oil mixture (but not in the previous HFC 134a/oil tests) •the effect of oil on flow patterns, in particular stratified and stratified-wavy flows.