Oil mass fraction measurement of CO2/PAG mixture

In this study, a method of using a capacitance sensor was investigated as a means to measure the mass fraction of a type of PAG oil flowing with CO2 in a transcritical cycle. The test facility equipped with the capacitance sensor was fabricated to establish and maintain a known oil mass fraction and to measure the capacitance of the CO2/oil mixture. By using this facility, the relationship among three parameters (reduced CO2 density (CO2 density divided by the critical density of CO2), oil mass fraction, and relative dielectric constant of the CO2/PAG oil mixture) was developed. For the range of oil mass fraction 0–0.07, the error of new measurement method was within 0.005 for a wide range of pressures and temperatures tested. This study established the method of measuring the oil mass fraction continuously in the transcritical CO2 cycle without affecting the cycle performance. Through this method, the effect of oil mass fraction on the characteristics of the oil circulation behavior and the performance of the transcritical CO2 cycle can be investigated.