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.