The compactness and good air-side heat exchange of the microchannel heat exchanger make it widely used in residential and automotive air conditioning systems. However, due to the different properties of refrigerant liquid and vapor, there is phase separation in the header and heat exchange tubes of the microchannel heat exchanger, resulting in uneven distribution of the refrigerant in the parallel microchannel tubes, the heat transfer area utilization rate is reduced, and the system performance is reduced. In addition, the presence of lubricants can also affect refrigerant distribution. The conclusion is that a small amount of lubricating oil will make the refrigerant distribution performance worse, and further adding lubricating oil will change the flow pattern, thereby improving the refrigerant distribution performance. This is because foam is formed after a large amount of oil is introduced to make the flow pattern more uniform. For any given lubricant content, at the same mass flow rate, as the quality increases, the distribution becomes worse because there is less liquid in the top pipe. In the case of the same inlet quality, as the mass flow increases, the high-momentum liquid can reach the top of the header, which is easy to be entrained by the top pipe, and the refrigerant distribution is better. However, although the lubricant improves the distribution of the refrigerant in the parallel microchannel tubes, it also significantly reduces the specific enthalpy difference of the working fluid, which is mainly due to its non-evaporative nature. Therefore, the influence of lubricating oil on the pressure drop and heat transfer characteristics of the microchannel heat exchanger after improving the refrigerant distribution is still an unresolved issue.
Keywords lubricating oil, distribution, microchannel heat exchanger, refrigerate