Experimental measurements for condensation of downward-flowing R123/R134a in a staggered bundle of horizontal low-finned tubes with four fin geometriesMesures expérimentales de la condensation lors de l'éoulement vertical d'un mélange de R123 et de R134a sur un faisceau de tubes en quinconce munis de quatre types d'ailettes

Experiments were conducted to obtain row-by-row heat and mass transfer data during condensation of downward-flowing zeotropic mixture R123/R134a in a staggered bundle of horizontal low-finned tubes. The vapor temperature and the mass fraction of R134a at the tube bundle inlet were about 50°C and 14%, respectively. The refrigerant mass velocity ranged from 9 to 34 kg m⁻² s⁻¹, and the condensation temperature difference from 1.9 to 12 K. Four kinds of low-finned tubes with different fin geometry were tested. The highest heat transfer coefficient was obtained with a tube which showed the highest performance for R123. However, the diference among the tubes was much smaller for the mixture than for R123. The heat transfer coefficient and the vapor-phase mass transfer coefficient decreased significantly with decreasing mass velocity. The mass transfer coefficient increased with condensation temperature difference, which was due to the effect of suction associated with condensation. On the basis of the analogy between heat and mass transfer, a dimensionless correlation of the mass transfer coefficient was developed for each tube.     

Condensation heat transfer coefficients of binary HFC mixtures on low fin and Turbo-C tubes

In this study, external condensation heat transfer coefficients (HTCs) are measured for nonazeotropic refrigerant mixtures (NARMs) of HFC32/HFC134a and HFC134a/HCFC123 on a low fin and Turbo-C tubes. All measurements are taken at the vapor temperature of 39 °C with the wall subcooling of 3–8 °C. Test results showed that condensation HTCs of NARMs on enhanced tubes were severely degraded from the ideal values showing up to 96% decrease. HTCs of the mixtures on Turbo-C tube were degraded more than those on low fin tube such that HTCs of the mixtures at the same composition were similar regardless of the tube. The mixture with larger gliding temperature differences (GTDs), HFC134a/HCFC123, showed a larger heat transfer reduction from the ideal values than the mixture with smaller GTDs, HFC32/HFC134a. Heat transfer enhancement ratios of the enhanced tubes with NARMs were almost 2 times lower than those with pure refrigerants and they decreased more as the GTDs of the mixtures increased.     

Condensation heat transfer coefficients of halogenated binary refrigerant mixtures on a smooth tube

In this study, external condensation heat transfer coefficients (HTCs) of nonazeotropic refrigerant mixtures of HFC32/HFC134a and HFC134a/HCFC123 at various compositions were measured on a horizontal smooth tube of a 19 mm outside diameter. All data were taken at the vapor temperature of 39 °C with a wall subcooling of 3–8 °C. Test results showed that HTCs of the tested mixtures were 19.4–85.1% lower than the ideal values calculated by the mole fraction weighting of the HTCs of the pure components. A thermal resistance due to the diffusion vapor film seemed to be partly responsible for the significant reduction of HTCs with these nonazeotropic mixtures.     

Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

Condensation heat transfer of a pure fluid and binary mixture outside a bundle of smooth horizontal tubes. Comparison of experimental results and a classical model

The condensation of pure HFC134a and different zeotropic mixtures with pure HFC134a and HFC23 on the outside of a bundle of smooth tubes was studied. The local heat transfer coefficient for each row was experimentally determined using a test section composed by a 13×3 staggered bundle of smooth copper tubes, measuring cooling water temperature in the inlet and the outlet of each tube, and measuring the vapour temperature along the bundle. All data were taken at the inlet vapour temperature of 40°C with a wall subcooling ranging from 4 to 26 K. The heat flux was varied from 5 to 30 kW/m² and the cooling water flow rate from 120 to 300 l/h for each tube. The visualisation of the HFC134a condensate flow by means of transparent glass tubes reveals specific flow patterns and explains the difference between the measured values of the heat transfer coefficient and the calculated values from Nusselt's theory. On the other hand, the experimental heat transfer data with the binary mixtures HFC23-HFC134a show the important effects of temperature glide and the strong decrease of the heat transfer coefficient in comparison with the pure HFC134a data. The measured values with the different zeotropic mixtures were compared with the data calculated with the classical condensation model based on the equilibrium model. An improvement of this model is proposed.     

In-tube condensation of mixture of R134a and ester oil: empirical correlations

This paper presents a comparative study of the condensation heat transfer coefficients in a smooth tube when operating with pure refrigerant R134a and its mixture with lubricant Castrol “icematic sw”. The lubricant is synthetic polyol ester based oil commonly used in lubricating the compressors. Two concentrations of R134a-oil mixtures of 2% and 5% oil (by mass) were analysed for a range of saturation temperatures of refrigerant R134a between 35 °C and 45 °C. The mass flow rate of the refrigerant and the mixtures was carefully maintained at 1 g/s, with a vapour quality varying between 1.0 and 0. The effects of vapour quality, flow rate, saturation temperature and temperature difference between saturation and tube wall on the heat transfer coefficient are investigated by analysing the experimental data. The experimental results were then compared with predictions from earlier models [Int J Heat Mass Transfer (1979), 185; 6th Int Heat Transfer Congress 3 (1974) 309; Int J Refrig 18 (1995) 524; Trans ASME 120 (1998) 193]. Finally two new empirical models were developed to predict the two-phase condensation heat transfer coefficient for pure refrigerant R134a and a mixture of refrigerant R134a with Castrol “icematic sw”.     

Refrigerant charge, pressure drop, and condensation heat transfer in flattened tubes

Horizontal smooth and microfinned copper tubes with an approximate diameter of 9 mm were successively flattened in order to determine changes in flow field characteristics as a round tube is altered into a flattened tube profile. Refrigerants R134a and R410A were investigated over a mass flux range from 75 to 400 kg m⁻² s⁻¹ and a quality range from approximately 10–80%. For a given refrigerant mass flow rate, the results show that a significant reduction in refrigerant charge is possible. Pressure drop results show increases of pressure drop at a given mass flux and quality as a tube profile is flattened. Heat transfer results indicate enhancement of the condensation heat transfer coefficient as a tube is flattened. Flattened tubes with an 18° helix angle displayed the highest heat transfer coefficients. Smooth tubes and axial microfin tubes displayed similar levels of heat transfer enhancement. Heat transfer enhancement is dependent on the mass flux, quality and tube profile.     

Transferts de chaleur lors de la condensation du mélange HFC23/HFC134a à l'intérieur d'un tube lisse horizontal

The environmental effects of the depletion of stratospheric ozone due to refrigerants containing chlorine, have resulted in international treaties, laws and amendments (Copenhagen, 1992, to the Montreal protocol, 1987) to phase out and eliminate many common refrigerants. HCFC22 is one of these refrigerants and no such single component alternative has been discovered for this fluid. Zeotropic refrigerant mixtures (binary or ternary) are being considered as potential replacements for HCFC22. Evaporation and condensation heat-transfer characteristics, and inside tubes of heat exchangers, due to the use of zeotropic refrigerant mixtures, have been a subject of fundamental importance in evaluating the heat exchanger performances in the refrigeration and air-conditioning industry.In this study, it is proposed to determine the heat transfer and pressure drop coefficients during in-tube condensation of zeotropic mixture HFC23/HFC134a in a smooth copper tube with an inside diameter of 8.92 mm. The test section of three passes of 2 m each; it is a counter flow double-pipe heat exchanger with water flowing in the annulus and refrigerant in the inner tube. This test section is instrumented with temperature and pressure sensors. We have tested HCFC22, HFC134a, and three refrigerant mixtures of HFC23/HFC134a at different compositions to appreciate the effect of glide on heat transfer. The quality was from 1 to 80%, the heat flux ranged from 2 to 50 kW m⁻² and mass flux varied from 80 to 480 kg m⁻²s⁻¹. In these conditions, no effect of a glide on the heat-transfer coefficient was observed; this result was confirmed by using an equilibrium condensation curve analysis. The pressure drop can be calculated with classical correlations but with physical properties of the mixture.     

Flow condensation heat transfer coefficients of pure refrigerants

Flow condensation heat transfer coefficients (HTCs) of R12, R22, R32, R123, R125, R134a, and R142b were measured experimentally on a horizontal plain tube. The experimental apparatus was composed of three main parts; a refrigerant loop, a water loop and a water-glycol loop. The test section in the refrigerant loop was made of a copper tube with an outside diameter of 9.52 mm and 1 m length. The refrigerant was cooled by cold water passing through an annulus surrounding the test section. All tests were performed at a fixed refrigerant saturation temperature of 40 °C with mass fluxes of 100, 200, 300 kg m⁻² s⁻¹ and heat flux of 7.3–7.7 kW m⁻². Experimental results showed that flow condensation HTCs increase as the quality and mass flux increase. At the same mass flux, the HTCs of R142b and R32 are higher than those of R22 by 8–34% while HTCs of R134a and R123 are similar to those of R22. On the other hand, HTCs of R12 and R125 are lower than those of R22 by 24–30%. Previous correlations predicted the present data satisfactorily with mean deviations of less than 20% substantiating indirectly the reliability of the present data. Finally, a new correlation was developed by modifying Dobson and Chato's correlation with an introduction of a heat and mass flux ratio combined with latent heat of condensation. The correlation showed a mean deviation of 10.7% for all pure halogenated refrigerants' data obtained in this study.     

The effect of lubricant concentration, miscibility, and viscosity on R134a pool boiling

This paper presents pool boiling heat transfer data for 12 different R134a/lubricant mixtures and pure R134a on a Turbo-BII™-HP surface. The mixtures were designed to examine the effects of lubricant mass fraction, viscosity, and miscibility on the heat transfer performance of R134a. The magnitude of the effect of each parameter on the heat transfer was quantified with a regression analysis. The mechanistic cause of each effect was given based on new theoretical interpretation and/or one from the literature. The model illustrates that large improvements over pure R134a heat transfer can be obtained for R134a/lubricant mixtures with small lubricant mass fraction, high lubricant viscosity, and a large critical solution temperature (CST). The ratio of the heat flux of the R134a/lubricant mixture to that of the pure R134a for fixed wall superheat was given as a function of pure R134a heat flux for all 12 mixtures. The lubricant that had the largest CST with R134a exhibited the greatest heat transfer: 100%±20% greater than that of pure R134a. By contrast, the heat transfer of the mixture with the lubricant that had the smallest viscosity and the smallest CST with R134a was 55%±9% less than that of pure R134a. High-speed films of the pure and mixture pool boiling were taken to observe the effect of the lubricant on the nucleate boiling.     

Effect of bulk lubricant concentration on the excess surface density during R134a pool boiling

This paper investigates the effect that the bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R134a and three different mixtures of R134a and a polyolester lubricant (POE). A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of an R134a/POE lubricant mixture on a test surface. The lubricant is preferentially drawn out of the bulk refrigerant/lubricant mixture by the boiling process and accumulates on the surface in excess of the bulk concentration. The excess lubricant resides in an approximately 40 μm layer on the surface and influences the boiling performance. The lubricant excess surface density measurements were used to modify an existing dimensionless excess surface density parameter so that it is valid for different reduced pressures. The dimensionless parameter is a key component for a refrigerant/lubricant pool-boiling model given in the literature. In support of improving the boiling model, both the excess measurements and heat transfer data are provided for pure R134a and three R134a/lubricant mixtures at 277.6 K. The heat transfer data show that the lubricant excess layer causes an average enhancement of the heat flux of approximately 24% for the 0.5% lubricant mass fraction mixture relative to pure R134a heat fluxes between 5 and 20 kW/m². Conversely, both 1 and 2% lubricant mass fraction mixtures experienced an average degradation of approximately 60% in the heat flux relative to pure R134a heat fluxes between approximately 4 and 20 kW/m². This study is an effort toward generating data to support a boiling model to predict whether lubricants degrade or improve boiling performance.     

Performance and heat transfer characteristics of hydrocarbon refrigerants in a heat pump systemPerformance et caractéristiques de transfert de chaleur des frigorigènes hydrocarbures utilisés dans un système de pompe à chaleur

Performance of a heat pump system using hydrocarbon refrigerants has been investigated experimentally. Single component hydrocarbon refrigerants (propane, isobutane, butane and propylene) and binary mixtures of propane/isobutane and propane/butane are considered as working fluids in a heat pump system. The heat pump system consists of compressor, condenser, evaporator, and expansion device with auxiliary facilities such as evacuating and charging unit, the secondary heat transfer fluid circulation unit, and several measurement units. Performance of each refrigerant is compared at several compressor speeds and temperature levels of the secondary heat transfer fluid. Coefficient of performance (COP) and cooling/heating capacity of hydrocarbon refrigerants are presented. Experimental results show that some hydrocarbon refrigerants are comparable to R22. Condensation and evaporation heat transfer coefficients of selected refrigerants are obtained from overall conductance measurements for subsections of heat exchangers, and compared with those of R22. It is found that heat transfer is degraded for hydrocarbon refrigerant mixtures due to composition variation with phase change. Empirical correlations to estimate heat transfer coefficients for pure and mixed hydrocarbons are developed, and they show good agreement with experimental data. Some hydrocarbon refrigerants have better performance characteristics than R22.     

The influence of a low viscosity oil on the pool boiling heat transfer of the refrigerant R507Influence d'une huile à faible viscosité sur le transfert de chaleur lors de l'ébullition libre du frigorigène R507

This paper describes the influence of a low viscosity polyolester based lubricating oil on the pool boiling heat transfer of the refrigerant R507. The pool boiling heat transfer coefficients for this refrigerant–oil mixture are measured on a smooth tube and on an enhanced tube. The investigation is made for oil mass fractions up to 10% and for saturation temperatures between −28.6°C and +20.1°C. For the smooth tube the heat transfer increases for increasing oil mass fractions up to 3% at lower saturation temperatures. At higher saturation temperatures the heat transfer decreases for increasing oil mass fractions for both tubes. For oil mass fractions greater than 1% at the higher saturation temperatures a range of decreasing heat transfer coefficient is found for increasing heat flux. The effect is caused by the different miscibility of the oil and the components of the refrigerant mixture.     

A semi-theoretical model for predicting refrigerant/lubricant mixture pool boiling heat transfer

This paper outlines the framework of a semi-theoretical model for predicting the pool boiling heat transfer of refrigerant/lubricant mixtures on a roughened, horizontal, flat pool-boiling surface. The predictive model is based on the mechanisms involved in the formation of the lubricant excess layer that exists on the heat transfer surface. The lubricant accumulates on the surface in excess of the bulk concentration via preferential evaporation of the refrigerant from the bulk refrigerant/lubricant mixture. As a result, excess lubricant resides in a thin layer on the surface and influences the boiling performance, giving either an enhancement or degradation in heat transfer. A dimensionless excess layer parameter and a thermal boundary layer constant were derived and fitted to data in an attempt to generalize the model to other refrigerant/lubricant mixtures. The model inputs include transport and thermodynamic refrigerant properties and the lubricant composition, viscosity, and critical solution temperature with the refrigerant. The model predicts the boiling heat transfer coefficient of three different mixtures of R123 and lubricant to within ±10%. Comparisons of heat transfer predictions to measurements for 13 different refrigerant/lubricant mixtures were made, including two different refrigerants and three different lubricants.     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R134a vaporisation inside a small brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The BPHE tested consists of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and corrugation amplitude of 2 mm.The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow.The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling and a correlation for frictional pressure drop is proposed.     

Effect of refrigerant oil additive on R134a and R123 boiling heat transfer performance

This paper investigates the effect that an additive had on the boiling performance of an R134a/polyolester lubricant (POE) mixture and an R123/naphthenic mineral oil mixture on a roughened, horizontal flat surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for the R134a/POE (98% mass fraction/2% mass fraction) mixture before and after use of the additive. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of the R134a/POE lubricant mixture before and after use of the additive. The measurements obtained from the spectrofluorometer suggest that the additive increases the total mass of lubricant on the boiling surface. The heat transfer data show that the additive caused an average and a maximum enhancement of the R134a/POE heat flux between 5 kW m⁻² and 22 kW m⁻² of approximately 73% and 95%, respectively. Conversely, for nearly the same heat flux range, the additive caused essentially no change in the pool boiling heat flux of an R123/mineral oil mixture. The lubricant excess surface density and interfacial surface tension measurements of this study were used to form the basis of a hypothesis for predicting when additives will enhance or degrade refrigerant/lubricant pool boiling.     

An experimental comparison of evaporation and condensation heat transfer coefficients for HFC-134a and CFC-12

Experimental heat transfer coefficients are reported for HFC-134a and CFC-12 during in-tube single-phase flow, evaporation and condensation. These heat transfer coefficients were measured in a horizontal, smooth tube with an inner diameter of 8.0 mm and a length of 3.67 m. The refrigerant in the test-tube was heated or cooled by using water flowing through an annulus surrounding the tube. Evaporation tests were performed for a refrigerant temperature range of 5–15°C with inlet and exit qualities of 10 and 90%, respectively. For condensation tests, the refrigerant temperature ranged from 30 to 50°C, with et and exit qualities of 90 and 10%, respectively. The mass flux was varied from 125 to 400 kg m⁻² s⁻¹ for all tests. For similar mass fluxes, the evaporation and condensation heat transfer coefficients for HFC-134a were significantly higher than those of CFC-12. Specifically, HFC-134a showed a 35–45% increase over CFC-12 for evaporation and a 25–35% increase over CFC-12 for condensation.     

Enhancement of R123 pool boiling by the addition of hydrocarbonsAmélioration de l'ébullition libre du R123 par l'ajout d'hydrocarbures

This paper presents pool boiling heat transfer data for 10 different R123/hydrocarbon mixtures. The data consisted of pool boiling performance of a GEWA-T^™ surface for pure R123 and for 10 dilute solutions of five different hydrocarbons: (1) pentane, (2) isopentane, (3) hexane, (4) cyclohexane, and (5) heptane with R123. The heat flux and the wall superheat were measured for each fluid at 277.6 K. A maximum (19±3.5)% increase over the pure R123 heat flux was achieved with the addition of 0.5% mass isopentane to R123. Other mixtures of isopentane, pentane, hexane, and cyclohexane with R123 exhibited smaller maximums than that of the R123/isopentane (99.5/0.5) mixture. Presumably, a layer enriched in hydrocarbon at the heat transfer surface caused the heat transfer enhancement. Conversely, an R123/heptane (99.5/0.5) mixture and an R123/cylcohexane (99.5/0.5) mixture exhibited only degradations with respect to the pure component performance for all test conditions. Several characteristics of the hydrocarbons were examined to determine their influence on the boiling heat transfer performance: molecular weight, molecular structure, composition, surface tension, and vapor pressure.     

Effect of bulk lubricant concentration on the excess surface density during R123 pool boiling

This paper investigates the effect that bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat (plain) pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R123 and three different mixtures of R123 and a naphthenic mineral oil. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent technique was used to measure the effect of bulk lubricant concentration on the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the effect of bulk lubricant composition on the lubricant excess density and in turn the effect on the heat transfer would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for pure R123 and three R123/lubricant mixtures at 277.6 K. For heat fluxes between approximately 25 to 45 kW/m², an average enhancement of the heat flux of 9 and 5% was achieved for the 0.5 and 1% lubricant mass fractions, respectively, and an average degradation of 5% in the heat flux was obtained for the 1.8% lubricant mass fraction mixture.

Résumé

This paper investigates the effect that bulk lubricant concentration has on the non-adiabatic lubricant excess surface density on a roughened, horizontal flat (plain) pool-boiling surface. Both pool boiling heat transfer data and lubricant excess surface density data are given for pure R123 and three different mixtures of R123 and a naphthenic mineral oil. A spectrofluorometer was used to measure the lubricant excess density that was established by the boiling of a R123/lubricant mixture on a test surface. The fluorescent technique was used to measure the effect of bulk lubricant concentration on the lubricant excess layer during refrigerant/lubricant mixture boiling. The refrigerant preferentially boils, thus, concentrating and accumulating the lubricant on the surface in excess of the bulk concentration. The excess lubricant resides in a very thin layer on the surface and influences the boiling performance. Accordingly, the ability to measure the effect of bulk lubricant composition on the lubricant excess density and in turn the effect on the heat transfer would lead to a fundamental understanding of the mechanism by which lubricants can degrade or improve boiling performance. In support of this effort, heat transfer data are provided for pure R123 and three R123/lubricant mixtures at 277.6 K. For heat fluxes between approximately 25 kW/m² to 45 kW/m², an average enhancement of the heat flux of 9% and 5% was achieved for the 0.5% and 1% lubricant mass fractions, respectively, and an average degradation of 5% in the heat flux was obtained for the 1.8% lubricant mass fraction mixture.