Nucleate Boiling Heat Transfer of Refrigerant-Oil Mixtures on Various Modified Surfaces

Research Overview

This study experimentally investigates nucleate boiling heat transfer of refrigerant-oil mixtures on various modified surfaces. R134a-POE and R1234ze(E)-POE mixtures were tested on five surfaces: smooth surface, initial laser-ablated surface, stabilized laser-ablated surface, machined surface, and composite processed surface. The effects of oil concentration, surface microstructure, and surface wettability on heat transfer coefficient and bubble behavior were clarified.

Graphical Abstract

Figure: One-page graphical summary of refrigerant-oil nucleate boiling in refrigeration and heat-pump evaporators, five modified surfaces, pool boiling experiment, oil concentration, wettability, surface microstructure, bubble behavior, HTC variation, and heat exchanger design guidance.

Background and Objective

In refrigeration and heat pump systems, evaporator performance directly affects system efficiency. Lubricating oil used in compressors inevitably circulates with the refrigerant and enters the evaporator.

Lubricating oil changes refrigerant properties, bubble nucleation, bubble growth, bubble departure, wall wettability, and local thermal resistance. Therefore, evaporator design based only on pure refrigerant may deviate from real system performance.

The objective of this study is to clarify whether surface microstructure or surface wettability plays a more dominant role in nucleate boiling heat transfer of refrigerant-oil mixtures and to provide design guidance for modified evaporator surfaces.

Key Features of This Study

Practical evaporator problem: The study focuses on real refrigeration and heat pump systems where oil enters the evaporator with refrigerant.
Two refrigerant-oil mixtures: R134a-POE and R1234ze(E)-POE mixtures were tested.
Five modified surfaces: SS, ILAS, SLAS, MS, and CPS were compared to separate the effects of surface microstructure and wettability.
Bubble visualization: Foaming, bubble density, bubble diameter, and tornado-shaped bubble groups were observed using high-speed imaging.
Design guidance: The study emphasizes alternating regions of high and low nucleation-site density to mitigate bubble interaction and coalescence.

Proposed Method and Working Mechanism

1. Surface preparation

Smooth, initial laser-ablated, stabilized laser-ablated, machined, and composite processed surfaces were prepared. SEM, surface roughness measurement, and static contact angle tests were used to characterize their structures and wettability.

2. Pool boiling experiment

Nucleate boiling of R134a-POE and R1234ze(E)-POE mixtures was measured using a copper heating block. The saturation temperature was 10 °C, and oil concentrations were 0, 1, 3, and 5 wt%.

3. HTC and bubble behavior evaluation

Heat flux was calculated from thermocouple temperatures using Fourier’s law, and HTC was obtained from wall superheat. Bubble nucleation, growth, departure, and bubble-group formation were recorded using a high-speed camera.

4. Comparison of surface structure, wettability, and oil concentration

HTC on different surfaces was compared to clarify how microstructure, wettability, nucleation-site density, bubble coalescence, and local oil concentration affect heat transfer.

Main Findings

HTC rankingThe HTC of refrigerant-oil mixtures generally followed the order CPS > ILAS > SLAS > MS > SS.

Surface microstructure is dominantSurface microstructure had a greater impact on NBHT than wettability. Although the contact angle of CPS was about 17° larger than that of ILAS, the HTC of CPS was up to 152% higher.

Dual role of oil additionOil addition promoted foaming and induced tornado-shaped bubble groups, which can facilitate heat transfer. However, the adverse effects of increased viscosity and surface tension were more pronounced.

HTC decreases with oilAs oil concentration and heat flux increased, HTC further decreased because local oil enrichment increased viscosity, surface tension, and thermal resistance.

Modified-surface difference narrows at high oil concentrationMore nucleation sites on modified surfaces led to higher local oil concentration. Therefore, at high oil concentrations, the HTC differences among modified surfaces became smaller.

Representative deteriorationAt q = 24 kW/m² for R134a-POE with ω = 5%, HTC decreased by 40.1%, 59.2%, 49.9%, 43.8%, and 60.3% on SS, ILAS, SLAS, MS, and CPS, respectively, compared with pure R134a.

Future Prospects

This study shows that improving wettability alone is not sufficient for enhancing nucleate boiling heat transfer of refrigerant-oil mixtures. Surface microstructure design should be the primary focus.

Future work should examine flow boiling under practical evaporator conditions, other refrigerant-oil combinations, long-term surface degradation, oil accumulation, local oil-concentration distribution, and durability of modified surfaces.

Potential Applications

The findings are useful for designing evaporators and heat exchangers where oil-bearing refrigerants undergo boiling.

Refrigeration systemsHeat pump evaporatorsAir-conditioning heat exchangersLow-GWP refrigerant systemsModified-surface heat exchangersHigh-efficiency evaporator design

Summary

This study compared nucleate boiling heat transfer of refrigerant-POE mixtures on five modified surfaces and clarified the roles of microstructure, wettability, and oil concentration.

Oil promotes foaming but increases viscosity and surface tension, ultimately reducing HTC. For modified-surface design, constructing dense nucleation-site microstructures is the first priority, followed by wettability optimization.

          Conclusion：
          For nucleate boiling of refrigerant-oil mixtures, surface microstructure is more dominant than wettability, and oil addition ultimately reduces HTC because increased viscosity and surface tension outweigh the positive effects of foaming and tornado-shaped bubble groups.
        