Summary

In most vapor compression refrigeration systems, specially those in which the cooling capacity is controlled by the cycling of the hermetic compressor, lubricating oil is necessary in the compressor. While the compressor is off, refrigerant coming from high-pressure parts is ab- sorbed by the oil present in the sump. As the electric motor starts up, the crankcase pressure rapidly decreases, and the oil-refrigerant mixture in the sump becomes supersaturated, which causes refrigerant desorp- tion, bubble cavitation and foam formation. The inflow of liquid from the foam into the compression chamber must be avoided to preserve the integrity of valve system and minimize the undesirable transport of oil from the sump to other parts of the system. In this study, an ex- perimental evaluation of refrigerant desorption and foam formation in mixtures of polyol ester oil and refrigerants 134a and 1234yf under con- trolled conditions is carried out. An experimental facility was designed and built to allow measurement of depressurization rate, foam height and mass flux due to gas expansion and refrigerant desorption from the supersaturated mixture. Quantitative data, together with high-speed video analysis, allowed identification of the main physical mechanisms associated to this problem. Experimental results as a function of time were explored relative to test conditions (temperature and initial mass fraction), showing that there are two characteristic regimes for this phe- nomenon: a growth stage followed by a drainage stage, and that foam height is inversely proportional to temperature. An integral mathemat- ical model was proposed and validated with experimental data, showing good agreement (deviations smaller than 20 %) when the empirical pa- rameters were adjusted specifically for each experimental condition.

Material for download