Effects of lubricating oil on flow and heat transfer characteristics in microchannel: A systematic review and meta-analysis

Abstract

Vapor compression refrigeration and heat pump (VCRHP) systems consume large amounts of energy annually. Therefore, improving the VCRHP system efficiency holds paramount significance in pursuing the dual-carbon goal. The heat transfer performance of heat exchangers, as an essential component of the VCRHP system, has a significant impact on the VCRHP system efficiency. Compared with traditional large-size heat exchangers, microchannel heat exchangers (MCHXs) have garnered substantial attention due to their advantages, such as compact structure, high heat transfer coefficient, and small refrigerant charge. In actual VCRHP systems, oil in compressor is inevitably carried away by refrigerant. Oil can either form a mixture with refrigerant liquid, or exist as a separate oil film in various components of VCRHP system. Depending on internal geometry and operation conditions, MCHX exhibits distinct different oil retention (OR) characteristics. Oil retained in inlet header of MCHX changes the flow pattern within header and affects the pressure of downstream tube, consequently impacting refrigerant distribution characteristics. During flow boiling and cooling/condensation processes, oil changes flow behavior and heat transfer characteristics. However, previous review work has not comprehensively summarized and discussed the impact of oil in MCHX. Therefore, in this study, to provide valuable insights for assessing OR characteristics of MCHX, action mechanism of oil on refrigerant distribution behavior, and effect of oil on flow behavior and heat transfer characteristics during flow boiling and cooling/condensation, a comprehensive review of literature from the past 20 years has been conducted. Considering the limitations of current research, several potential future research directions are proposed: research on the effect of oil on flow and heat transfer in microgravity environment, design of super-oleophobic surface with nanoscale structure, optimal design of MCHX header and development of refrigerant-oil distributor, measurement of thermophysical properties and heat transfer performance of non-azeotropic refrigerant-oil mixture, as well as numerical simulation of flow and heat transfer of refrigerant-oil mixture. This review is intended to serve as a reference for the practical design and optimization of MCHX under oil-bearing conditions.