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effect of the oil, both in the direction of a penalization, of an improvement and of a negligible effect of heat transfer, depending on the different testing conditions. Concerning the “pool boiling”, in the 21 considered works, the greatly different conditions, in terms of oil type, refrigerant type, saturation temperature, heat flux, foaming, led to and extremely large scattering of the results, thus making impossible the implementation of a sufficiently consistent semi-empirical model for HTC prediction. In general, the effect of the lubricant was found to be more penalizing on enhanced microfin surfaces. One of the most interesting theory was proposed by Kedzierski (2001): a lubricant film is formed in direct contact with the heated surface. Kedzierski observed that in presence of small oil mass percentage (typically lower than 0.5% with R134a) the HTC tended to increase, while HTC decreased sharply for higher oil concentrations. Concerning the “flow boiling” process, Shen and Groll (2005a), through the analysis of 33 different works, inferred that the presence of oil promotes surface wetting, thus promoting the on-set of annular flow pattern. The studied work clearly indicated that it is fundamental to analyze the experimental data with reference to the relevant flow pattern. However, it is worth noting that the oil-refrigerant mixture viscosity is much higher than the pure refrigerant one. So the lubricant layer in direct contact with the heated surface, causes a marked mass transfer resistance. Also during the “flow boiling” the microfin tubes seem to be more penalized by the presence of oil. The behaviour of a refrigerant that is not completely miscible in the oil must be considered separately. In fact, in this situation the oil can form a separate phase laying directly on the heated surface. The consequent additional thermal resistance can be particularly penalizing in this situation. The leading penalization during condensation of refrigerants with oil seems to be related to the high viscosity of the refrigerant/oil mixture if compared with the pure refrigerant (Shen e Groll, 2005b, 16 papers considered). Accordingly, the penalizing effect is by far higher at high vapour quality (incipient condensation). In this situation, the data available in literature indicate that the oil effect is less penalizing in microfin tubes. Both during evaporation and condensation fluid pressure drops inside the heat exchangers increase because of the presence of oil. As a consequence, as it is well known, the compression work increases and the refrigerating cycle efficiency is reduced. The pressure drop increment is more marked inside minichannels: a review of the data available in the open literature and some comments about the consistency of the available calculation models is presented in Thome and Ribatski (2006) for CO2 evaporation inside tubes. Recently, Field and Hrnjak (2006) measured the pressure drop during two-phase adiabatic flow of carbon dioxide inside a channel with hydraulic diameter of 148.0 μm with R134a and POE ISO 32 oil for different refrigerant mass fluxes and different OCRs. The highest pressure drop increment was observed at high vapour qualities. The supercritical cooling of carbon dioxide in presence of oil should be considered apart from the “traditional” condensation process. Recently, Dang et al. (2007) measured the effect of PAG oil on R744 “gas cooling” process inside smooth tubes with inner diameter from 1 to 6 mm and working pressure from 8 to 10 MPa, mass fluxes from 200 to 1200 kgm-2s-1 and circulating oil mass percentage between 0 to 5%. The Authors pointed out that increasing the OCR also the pressure drops increase while HTC decreases. The effect can be “dramatic” close to thePDF Image | Circulating lubricant and effects of refrigeration circuits
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