Investigating Heat Transfer Rates of Refrigerant R-410A and Nanolubricant Mixtures
From ASHRAE Journal Newsletter, June 23, 2020
Air-conditioning systems use lubricants in the compressor to limit friction and wear and act as a sealant. However, lubrication is a contaminant to heat exchangers, which can decrease efficiency and performance, wasting energy.
In a recent Science and Technology for the Built Environment article, researchers explore the potential of using nanolubricants to increase thermal systems’ heat transfer rate while reducing compressor friction and wear. Researchers Pratik S. Deokar, Ph.D., Associate Member ASHRAE, and Lorenzo Cremaschi, Ph.D., Member ASHRAE, discuss the research.
1. What is the significance of this research?
Approximately 6.2 quads (6.54 exajoules) of energy are used annually to provide heating and cooling to buildings, homes and for refrigeration. Lubricant is used in compressors to limit friction and wear and provide sealing. Yet it is a contaminant in the heat exchangers that hinders the performance. Nanoparticle-laden lubricants have exciting potential to revolutionize the field of lubrication. This is because the nanoparticle additives in the base lubricant can be optimized to augment the heat transfer rate in thermal systems while simultaneously reducing friction and wear in compressors. Given that a majority of the buildings for the next 20 to 40 years in the U.S. have already been built, our focus is on heat transfer and friction characterization of nanolubricants with the goal of improving their performance for retrofit HVAC applications. Improvements on heat transfer and reduction of friction, at the component and system levels, have been shown, but the fundamental mechanisms are still not clear.
2. Explain the steps of this research project. What did the process look like?
In the vapor compression cycle of air-conditioning systems, a small amount of lubricant leaves the compressor and oil separator and further circulates through the cycle. This lubricant acts as a contaminant, affecting heat transfer and pressure losses in heat exchangers. The mixture of refrigerant and nanolubricants, that is, nanoparticles dispersed in the lubricant oil, have shown potential to augment the two-phase flow boiling heat transfer in the literature. However, the data is limited and mechanisms of heat transfer enhancement or degradation due to the nanolubricants are still not well understood. Published models of two-phase flow boiling lack experimental validation with refrigerant and nanolubricant mixture. This research addresses this gap by presenting new experimental heat transfer coefficient and pressure drop results for the saturated two-phase flow boiling of R-410A with two nanolubricants in a 9.5 mm (0.4 in.) ID smooth copper tube: a zinc oxide (ZnO) nanoparticle-laden lubricant and a γ- aluminum oxide nanoparticle-laden lubricant. A new physics-based superposition heat transfer model for saturated two-phase flow boiling of refrigerant-lubricant and refrigerant-nanolubricant pairs was also developed by integrating a nanofluid-forced flow convective heat transfer model and a semiempirical pool boiling model for nanolubricants.
The research was both theoretical and experimental and was mainly divided in three phases. In Phase 1, we measured the thermal conductivity and specific heat of the nanolubricants. Collaboration with Dr. Mark Kedzierski at NIST allowed the verification of the thermal and transport proprieties of refrigerant-nanolubricant mixtures. In Phase 2, we constructed a new test apparatus to measure the effects of nanoparticle diameter, aspect ratio and thermal conductivity on the two-phase flow boiling heat transfer coefficient and pressure drop in micro-fin and smooth copper tubes. In Phase 3, we developed a new physics-based heat transfer model for saturated flow boiling and validated the model with the data from our new experiments as well as some observations we found in the literature.
3. Why is it important to explore this topic now?
While prior work on nanofluids clearly indicates the potential for heat transfer augmentation via an increased thermal conductivity and viscosity, nanoparticle-laden lubricants work by significantly different mechanisms. Brownian and thermophoresis slip mechanisms control the nanoparticles’ behavior and interactions within the colloidal solution and with the surface. As a result, abnormal heat transfer enhancements are observed during two-phase flow phase change processes.
Driven by higher energy-efficiency targets, there is a critical need for major heat transfer enhancements in heat exchangers, and nanolubricants have shown potential to address such a need in a cost-neutral manner for both new and retrofit applications. This research is carried out to understand the mechanisms of flow boiling heat transfer of refrigerant-nanolubricant mixtures in air-conditioning systems.
4. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?
Elongated-ZnO-based and spherical Al2O3-based nanolubricants had very similar thermal conductivity in a quiescent state, but the ZnO-based nanolubricant for the same conditions showed a lower heat transfer coefficient than the Al2O3-based nanolubricant. This indicated that the degradation in the heat transfer coefficient was caused by some other phenomenon rather than liquid-nanoparticles mixture phase thermal conductivity. The heat transfer coefficient of R-410A-nanolubricant mixtures decreased by about 20% with respect to that of the R-410A at vapor qualities below 50%, while their performance improved with an increase in the refrigerant vapor quality. The nanoparticle displacement closer to the inner wall in annular flow regime at high vapor quality could support the observed enhancement in heat transfer coefficients and increased frictional pressure drops at high vapor qualities. The pressure drop results also suggested that non-spherical ZnO nanoparticles had more wall shear stress than the spherical Al2O3 nanoparticles. Experiments also showed that long-term flow boiling testing of refrigerant-nanolubricant mixtures resulted in a continuous and gradual increase of the heat transfer coefficient. A possible explanation was that the nanoparticle deposition on the tube’s inner wall and its near-wall interaction led to small but incremental enhancements in the nucleate boiling phenomena.
5. How can this research further the industry's knowledge on this topic?
Practically, the nanolubricant concept is able to turn the undesired mishap of coolant contamination into a useful means of performance enhancement. This stimulates a host of new research and widespread commercial and military applications, including in the HVAC and refrigeration, power production, transportation and drug delivery industries. This research opens a new frontier for nanotechnology applied to two-phase flow heat transfer processes in air-conditioning and refrigeration systems. The research confirmed the flow boiling performance of refrigerant in the presence of lubricant in evaporator tubes. In addition, the research compared the performance of saturated flow boiling performance of refrigerant-nanolubricant pairs with the refrigerant-lubricant pair in terms of heat transfer, pressure drop and flow regimes. The new superposition heat transfer model developed in this work is applicable for saturated flow boiling of both refrigerant-lubricant and refrigerant-nanolubricant pairs. The model included the several physical effects that influenced heat transfer, such as slip mechanisms at the nanoparticles and base fluid interface; diffusion and mass balance of different shape nanoparticles within the laminar sublayer and turbulent layer of the flow; momentum transfer from nanoparticles to the growing bubbles; and formation of lubricant excess concentration at the tube surface and its influence on bubble growth and tube wetting.
6. Were there any surprises or unforeseen challenges for you when preparing this research?
The experimental work started with the aim of studying the flow boiling of refrigerant-nanolubricants in internally enhanced fin copper tubes. These experimental results on an internally finned copper tube were promising and motivated the present work with a smooth copper tube. Because the goal of this research was to advance the fundamental understanding of the effects of the nanoparticles in the two-phase flow of refrigerant and nanolubricant mixtures, this particular research project intentionally considered smooth copper tubes. By eliminating the internal fins of the tubes, we were able to decouple their influence on the nanoparticles’ migration and lubricant flow near the inner walls of the tubes. This approach allowed researchers to isolate and quantify the mechanisms on the two-phase flow heat transfer coefficient due to the nanoparticle-laden lubricants.