Effects of Lubricating Oil on Flow and Heat Transfer Characteristics in Microchannels

Research Overview

This paper is a systematic review and meta-analysis on the effects of lubricating oil on flow and heat transfer characteristics in microchannel heat exchangers used in vapor compression refrigeration and heat pump systems. It summarizes research from the past two decades and clarifies how oil affects oil retention, refrigerant distribution, flow behavior, heat transfer, and pressure drop in microchannel evaporators, condensers, and gas coolers.

Graphical Abstract

Figure: One-page graphical summary of oil circulation in vapor compression systems, oil retention and refrigerant maldistribution in microchannel heat exchangers, effects on flow boiling and cooling/condensation, key literature conclusions, and future research directions.

Background and Objective

Vapor compression refrigeration and heat pump systems are widely used in residential air conditioning, refrigeration, heat pumps, cold-chain transportation, food processing, pharmaceutical storage, and transport cooling. Because these systems consume large amounts of energy, improving heat exchanger performance is essential for higher COP and lower carbon emissions.

Microchannel heat exchangers offer compact structure, high heat transfer coefficient, high pressure capacity, and low refrigerant charge. However, they also face challenges such as refrigerant maldistribution, oil retention, flow-pattern transition, and pressure-drop increase. In practical systems, lubricating oil from the compressor is inevitably carried into the heat exchanger by the refrigerant.

The objective of this review is to systematically clarify how lubricating oil affects refrigerant distribution, oil retention, flow boiling, cooling/condensation, heat transfer coefficient, and pressure drop in microchannels, and to identify future research directions for MCHX design and optimization.

Key Features of This Study

Systematic review: The study summarizes research from the past two decades on lubricating-oil effects in microchannel heat exchangers.
Integrated phenomena: Oil retention, refrigerant distribution, flow behavior, flow boiling, cooling/condensation, heat transfer, and pressure drop are discussed together.
Practical component view: The review considers evaporators, condensers, gas coolers, headers, channels, and oil-trap regions in practical VCRHP systems.
Mechanism classification: Oil film, oil droplets, refrigerant–oil mixture, foam layer, viscosity, surface tension, and wettability are identified as key mechanisms.
Future research map: The paper proposes future directions including microgravity, super-oleophobic surfaces, optimized headers, non-azeotropic refrigerant–oil mixtures, and CFD/mechanistic modeling.

Proposed Method and Working Mechanism

1. Organization of reviewed studies

Experimental, numerical, and modeling studies on refrigerant–oil mixtures were reviewed and categorized into four core phenomena: refrigerant distribution, oil retention, flow boiling, and cooling/condensation.

2. Forms of oil in microchannels

Lubricating oil can exist as a refrigerant–oil mixture in the liquid phase, as separated oil droplets, as wall oil films, as foam layers in headers, or as accumulated oil in trap regions. These forms determine how oil affects performance.

3. Extraction of influence mechanisms

Oil modifies viscosity, surface tension, wettability, foaming, local oil concentration, and flow pattern. These changes affect refrigerant maldistribution, oil retention, heat transfer coefficient, and pressure drop.

4. Identification of research gaps

The paper identifies future needs in property measurement, surface engineering, header optimization, refrigerant–oil distribution modeling, and CFD-based prediction of oil retention, heat transfer, and pressure drop.

Main Findings

Effect on refrigerant distributionOil changes flow patterns in headers and affects refrigerant distribution uniformity. Low oil concentration can worsen distribution due to higher viscosity, while high oil concentration can sometimes improve distribution through foam-layer and small-bubble formation.

Major region of oil retentionOil retention mainly occurs in headers. Header structure, tube insertion direction, refrigerant flow direction, mass flow rate, superheat, and oil concentration strongly affect oil retention.

Higher sensitivity of evaporatorsMicrochannel evaporators are generally more sensitive to oil than condensers and gas coolers because lower evaporating temperatures lead to higher oil viscosity.

Flow boiling heat transferDuring flow boiling, oil may enhance HTC in low- and medium-vapor-quality regions through foaming, but it tends to deteriorate HTC in high-quality regions due to local oil concentration increase and oil-film formation.

Cooling/condensation heat transferDuring cooling and condensation, oil generally reduces HTC because oil films and oil droplets introduce additional thermal resistance. Preventing oil-film formation and growth is therefore important.

Effect on pressure dropIn flow boiling, oil tends to increase pressure drop by increasing viscosity, promoting annular flow, increasing wall contact area, and forming oil films. In cooling/condensation, oil can either increase or decrease pressure drop depending on operating conditions.

Future Prospects

This review shows that lubricating oil is not merely a heat-transfer deterioration factor, but a coupled factor affecting oil retention, refrigerant distribution, flow pattern, pressure drop, heat transfer, and system reliability.

Future work should focus on oil behavior under microgravity, super-oleophobic and functional surfaces for suppressing oil adhesion, optimized header and refrigerant–oil distributor design, thermophysical property measurement of non-azeotropic refrigerant–oil mixtures, and CFD/mechanistic models for predicting oil retention, heat transfer, and pressure drop.

These directions are important for designing high-efficiency microchannel heat exchangers with low refrigerant charge, supporting low-GWP refrigerant transition, and improving COP in refrigeration and heat pump systems.

Potential Applications

The findings are useful for the design and evaluation of microchannel heat exchangers operating with oil-bearing refrigerants.

Residential air conditioningHeat pump systemsRefrigeration equipmentCold-chain transportationMedical and food storageLow-GWP refrigerant MCHXs

Summary

This review systematically summarizes the effects of lubricating oil on oil retention, refrigerant distribution, flow boiling, cooling/condensation, heat transfer, and pressure drop in microchannel heat exchangers.

Lubricating oil acts as a hidden governing factor in microchannel heat exchangers and strongly affects system efficiency, COP, reliability, refrigerant distribution, and pressure loss.

          Conclusion：
          Lubricating oil is a key factor governing oil retention, refrigerant distribution, flow behavior, heat transfer, and pressure drop in microchannel heat exchangers, and must be considered in the design and optimization of high-efficiency, low-charge MCHXs.
        