Influence of the heat flux in mixture boiling: experiments and correlations

Heat transfer at nucleate pool boiling of the binary and ternary refrigerant mixtures R404A, R407C and R507 at the outside of a horizontal tube with emery ground surface has been investigated in a wide range of pressures and heat fluxes. Together with experimental data of Bednar and Bier for wide boiling ethane/n-butane mixtures, the results of these comparatively narrow boiling mixtures are used to investigate the influence of heat flux q on the heat transfer coefficient as predicted by various correlations for nucleate boiling of mixtures. At comparatively high saturation pressures with experimental -values markedly smaller than the molar average of the pure components, the ,q-relationships predicted differ significantly from the experimental, particularly for wide boiling mixtures.     

Deterioration of Heat Transfer in Boiling of Emulsions with a Low-Boiling Disperse Phase

The paper presents the results of an experimental study of heat transfer in the boiling of emulsions consisting of water dispersed in organosilicon fluids, namely, in PMS-20 or PES-4 silicon oil. The evaporation process during boiling was studied with a high-speed filming technique. It is demonstrated that the transition from boiling of the emulsion disperse phase to simultaneous boiling with the dispersion medium is accompanied by deterioration in heat transfer due to the disperse phase nucleate boiling crisis.     

Influence of oil on nucleate pool boiling heat transfer of refrigerant on metal foam covers

Nucleate pool boiling heat transfer characteristics of refrigerant/oil mixture on metal foam covers were experimentally investigated. The refrigerant is R113, and the oil is VG68. The copper foams, having ppi (pores per inch) of 10 and 20, porosity from 90% to 98%, and thickness of 5 mm, are selected in this study. Experimental conditions include a saturation pressure of 101 kPa, oil concentrations from 0 to 5%, and heat fluxes from 0 to 80 kW m⁻². The experimental results indicate that the nucleate pool boiling heat transfer coefficient on copper foam covers is larger than that on flat heated surface by a maximum of 160% under the present experimental conditions; the presence of oil deteriorates the nucleate pool boiling heat transfer on copper foam covers by a maximum of 15% under the present experimental conditions, and the deterioration of oil on nucleate pool boiling heat transfer on copper foam covers is lower than that on a flat heated surface. A correlation for predicting the nucleate pool boiling heat transfer coefficient of refrigerant/oil mixture on copper foam cover is developed, and it agrees with 95% of the experimental data within a deviation of ±20%.     

Effect of CuO nanolubricant on R134a pool boiling heat transfer

This paper quantifies the influence of CuO nanoparticles on the boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. A lubricant based nanofluid (nanolubricant) was made with a synthetic ester and CuO particles. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) of between 50% and 275%. A smaller enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 19% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in a still smaller boiling heat transfer improvement of approximately 12% on average. Thermal conductivity measurements and a refrigerant/lubricant mixture pool-boiling model were used to suggest that increased thermal conductivity is responsible for only a small portion of the heat transfer enhancement due to nanoparticles.     

Heat transfer near the contact line during boiling in microgravity

Although significant progress has been made, the mechanisms of heat transfer responsible for enhancement during nucleate boiling are not fully understood. The primary barriers to a complete understanding of the heat transfer during boiling are due to difficulties associated with performing rapid microscale measurements, ambiguity due to the interaction of previous and neighbouring bubbles, uncertainty with regard to gravitational effects and the complexity associated with theoretical modelling that often results in empiricism/adjustable parameters being introduced into the theoretical/numerical works. In this investigation, a numerical simulation of hemispherical bubble growth on a heated surface in μ-gravity has been performed. The simulations agree well with R113 μ-gravity measurements provided by [1]. The simulations indicate that during the bubble expansion phase, the heat transfer coefficient along the boiling surface in the vicinity of the triple interface is substantial. The maximum heat transfer coefficient reaches values over 3500 W/m²K and decays with radial distance from the vapour-liquid interface. This is over 10 times larger than the effective thermal conductance due to diffusion in the undisturbed region. The increase in the local heat transfer coefficient is complimented by the fact that the effective area of enhancement around the bubble increases with bubble radius enhancing the thermal conductance of single bubble events.     

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.     

Nucleate boiling heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 °C on a low fin tube of 1023 fins per meter, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. Flammable refrigerants' data showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutene, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those with higher reduced pressure such as propylene and propane. Finally, Thermoexcel-E showed the highest heat transfer enhancement ratios of 2.3–9.4 among the tubes tested due to its sub-channels and re-entrant cavities.     

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.     

多元混合工质池核沸腾传热实验研究

对R134a、丙烷（C3H8）、异丁烷（iC4H10）三种纯工质以及它们相应的二元混合物和三元混合物做了池核沸腾传热实验研究。加热面为紫铜表面,压力范围在0．1～0．6MPa。在纯工质实验数据基础上,分析物性、压力对沸腾传热系数的影响。相对于纯工质,非共沸混合工质沸腾传热系数有所降低,并且在高热流下趋势更明显。最后拟合出纯工质和混合工质沸腾传热关联式。     

Nucleate boiling heat transfer coefficients of HCFC22, HFC134a, HFC125, and HFC32 on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of HCFC22, HFC134a, HFC125, HFC32 were measured on a low fin, Turbo-B, and Thermoexcel-E tubes. All data were taken at the liquid pool temperature of 7 °C on horizontal tubes of 152 mm length and 18.6–18.8 mm outside diameter at heat fluxes of 10–80 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. For a plain and low fin tubes, refrigerants with higher vapor pressures showed higher nucleate boiling HTCs consistently. This was due to the fact that the wall superheat required to activate given size cavities became smaller as pressure increased. For Turbo-B and Thermoexcel-E tubes, HFC125 showed a peculiar behavior exhibiting much reduced HTCs due to its high reduced pressure. The heat transfer enhancement ratios of the low fin, Turbo-B, and Thermoexcel-E tubes were 1.09–1.68, 1.77–5.41, 1.64–8.77 respectively in the range of heat fluxes tested.     

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.     

ISS experiments for the clarification of boiling and two-phase flow in microgravity and for the development of high-performance space cold plates

Inflow boiling, gravity effects on the distribution of both phases are observed in a heated tube and heat transfer coefficients due to two-phase forced convection is deteriorated in microgravity. In narrow channels between heated and unheated plates, the increase in subcooling enlarges a size of flattened bubble and reduces the frequency of detachment under microgravity conditions resulting the emphasis of heat transfer deterioration. To clarify reasons for the unknown behaviors of interfacial distribution and corresponding characteristics in heat transfer not easily be clarified through the experiments on ground, the opportunity on the experiments utilizing long-term microgravity duration realized in ISS is required. The experiments on microgravity boiling and two-phase flow are proposed by the collaboration of researchers in five countries. A common test loop is designed to conduct multiple experiments by the interchangeable structures of test sections; a transparent heated tube for the visualized flow boiling, a stainless tube for the measurement of CHF data, a copper surface for the heat transfer data of nucleate boiling with superimposed liquid flows in a duct, a glass heated plate with multiple array of small temperature sensors and transparent heaters for the clarification of mechanisms in nucleate boiling heat transfer, and one or two models of cold plates for practical applications. A direction of researches in the present discipline is proposed based on the existing experimental results and on the idea developed by the present authors.     

Heat transfer to helium-I during different boiling regimes at high centrifugal accelerations

Heat transfer during nucleate and film boiling of helium and also the boiling crises up to relative accelerations of (1 ÷ 2) × 10³ have been investigated. The heat transfer surface was a flat copper heater. The heat transfer during nucleate boiling proved to be independent of the relative accelerations. For film boiling, it was found that $\text{α ～ η}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$. The dependence of the peak nucleate boiling heat flux and the minimum film boiling flux on the relative acceleration is non-monotonic.     

A numerical investigation of flow boiling of non-azeotropic and near-azeotropic binary mixtures

The flow and heat transfer characteristics of binary refrigerant mixtures in a heated horizontal tube were investigated numerically. The pressure drop, temperature profile, and heat transfer coefficient for non-azeotropic and near-azeotropic mixtures of different bulk compositions were obtained. It is found that the non-linear physical properties of the mixtures strongly affect the pressure drop characteristics. Both the fluid properties and mass transfer resistance are responsible for the heat transfer characteristics. The mass transfer resistance has a more significant influence on the nucleate boiling than the convective evaporation for non-azeotropic mixtures, while the resistance can be neglected for near-azeotropic mixtures.     

Effect of Al2O3 nanolubricant on R134a pool boiling heat transfer

This paper quantifies the influence of Al₂O₃ nanoparticles on the pool-boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface. The nanoparticles enhanced the boiling heat transfer relative to that for R134a/polyolester mixtures without nanoparticles for the three lubricant mass fractions that were tested. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) as large as 400% for the lowest heat flux. The average heat flux improvement for heat fluxes less than 40 kW m⁻² was approximately 105%, 49%, and 155% for the 0.5%, the 1%, and the 2% mass fractions, respectively. A semi-empirical model was developed to predict the boiling enhancement as caused by the interaction of the nanoparticles with the bubbles. The model suggests that small particle size and large nanoparticle volume fraction improve boiling enhancement.     

竖直狭缝通道中液氮沸腾强化换热的实验研究

实验表明，狭缝间隙对液氮自然对流核态沸腾换热有明显的影响，在低热流密度下，间隙小的狭缝沸腾换热比间隙大的狭缝明显增强，当狭缝间隙小于实验压力下气泡的脱离直径时，对于同样的热流密度，传热温差减小一个数量级以上，沸腾换热系数提高十几倍到二十倍以上，当热流密度增加一定程度（＞4W/cm^2)时，间隙小的狭缝沸腾换热比间隙大的狭缝有所减弱。     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

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.     

Enhancing flow boiling heat transfer in microchannels for thermal management with monolithically-integrated silicon nanowires

Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.