Influences of refrigerant-based nanofluid composition and heating condition on the migration of nanoparticles during pool boiling. Part I: Experimental measurement

Influences of refrigerant-based nanofluid composition and heating condition on the migration of nanoparticles during pool boiling were investigated experimentally. The nanoparticles include Cu (average diameters of 20, 50 and 80 nm), Al and Al₂O₃ (average diameters of 20 nm), and CuO (average diameter of 40 nm). The refrigerants include R113, R141b and n-pentane. The mass fraction of lubricating oil RB68EP is from 0 to 10 wt%, the heat flux is from 10 to 100 kW m⁻², and the initial liquid-level height is from 1.3 to 3.4 cm. The experimental results show that the migration ratio of nanoparticles during the pool boiling of refrigerant-based nanofluid increases with the decrease of nanoparticle density, nanoparticle size, dynamic viscosity of refrigerant, mass fraction of lubricating oil or heat flux; while increases with the increase of liquid-phase density of refrigerant or initial liquid-level height.     

Review on pool boiling heat transfer of carbon dioxide

Although application of carbon dioxide as working fluid in many fields of refrigeration technology has been recommended very often in the recent past, the data on nucleate boiling heat transfer of carbon dioxide in free convection are very scarce in the open literature and new investigations are almost entirely focussed on forced convective flow boiling. In the interpretation of the respective results, heat transfer to carbon dioxide is often characterized as being superior to other refrigerants due to the outstandingly favourable thermophysical properties of carbon dioxide for boiling heat transfer. Different from this view, the discussion of recent results on pool boiling heat transfer of carbon dioxide in this review demonstrates that the high heat transfer coefficients measured for carbon dioxide in comparison to hydrocarbon or halocarbon refrigerants are mainly due to the fact that application of carbon dioxide is mostly envisaged for conditions where reduced saturation pressure p*=p_s/p_c (p_c, critical pressure) is higher than for common refrigerants.

In the first part of the review, the three main influences—by heat flux, saturation pressure and fluid properties—on pool boiling of carbon dioxide are discussed using recent measurements for CO₂ by Kotthoff et al. [S. Kotthoff, U. Chandra, D. Gorenflo, A. Luke, New measurements of pool boiling heat transfer for carbon dioxide in a wide temperature range, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 2/A/3.30]; see also S. Kotthoff, U. Chandra, D. Gorenflo, Neue Messungen zum Behältersieden von Kohlendioxid in einem grösseren Temperaturbereich, DKV-Tagungsbericht 22 (2004) [Bd.II. 1] 233–256 and other organic substances (Gorenflo et al.) [D. Gorenflo, S. Kotthoff, U. Chandra, New measurements of pool boiling heat transfer with hydrocarbons and other organics for update of VDI—Heat Atlas calculation method, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/C/1.00]; Kotthoff and Gorenflo, [S. Kotthoff, D. Gorenflo, Influence of the fluid on pool boiling heat transfer of refrigerants and other organic substances, Proceedings of the IIR-Commission B1 Conference, Vicenza, 2005 [paper #TP-98]. In the second part, a comparison is given with the few former data available and with new results of Loebl and Kraus [S. Loebl, W.E. Kraus, Pool boiling heat transfer of carbon dioxide on a horizontal tube, Proceedings of the Sixth IIR-Gustav Lorentzen Conference, Glasgow, 2004 [paper 1/A/1.20]; S. Loebl, W.E. Kraus, Zum Wärmeübergang bei der Verdampfung von Kohlendioxid am horizontalen Rohr, DKV-Tagungsbericht 22 (2004) [Bd.II.1] 219–232 on the influence of the heating wall (material and surface roughness). Finally, analogies between nucleate pool boiling and new flow boiling data are shown for those domains of flow boiling in which nucleation provides the dominant contribution to heat transfer and convective effects are of secondary importance.     

Influence of heat flux and pressure on heat transfer associated with fully developed nucleate pool boiling

Heat transfer in the pool boiling of helium is investigated experimentally. The dependence of the heat-transfer coefficient on the heat flux and pressure is determined for the fully developed nucleate boiling regime.Notation q heat flux, W/m² - T temperature differential, °K - heat-transfer coefficient, W/m²·°K - P pressure, N/m² - P_cr critical pressure, N/m² - P_* reference pressure, N/m² - n a power exponent - C a proportionality factor - , F₁ special functions Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 5, pp. 828–831, November, 1977.     

Entropy generation during flow boiling of pure refrigerant and refrigerant-oil mixture

Optimization of evaporators in refrigeration systems, for instance, can be conducted using entropy generation as a criterion. The latter can be used for choosing a technology (smooth tubes, enhanced tubes, tube diameter, etc) or selecting a fluid. Once this optimization is performed, the refrigerant charge can be evaluated. In this paper, different entropy generation expressions are presented for a diabatic two-phase flow of a pure refrigerant and a refrigerant-oil mixture. These expressions are developed based on the separated flow model. Depending on the boundary conditions, the equations are different and the conclusions differ from one to the other. It is shown that for a heat flux condition at the tube wall, the use of enhanced tubes is recommended at low mass velocities whereas, the use of smooth tubes is better at higher mass velocities. On the contrary, for a constant wall temperature, the use of enhanced tubes is always better than smooth tubes. The role of oil is also emphasized. The higher the oil concentration in the refrigerant, the higher the entropy generation.     

制冷剂-润滑油互溶混合物系流动沸腾特性测试装置的研制

在对已有相关测试装置设计技术的利弊分析基础上,提出了一种测试制冷剂-润滑油互溶混合流体管内流动沸腾特性的装置,其特征是通过与制冷剂流动沸腾换热测量回路并联接入一开式的润滑油回路,从而实现灵活控制和调节实验段内润滑油量的目的.通过在压缩机出口串连接入3个油分离器进行三级油分,从而实现压缩机出口的润滑油零排放.该装置具有连续在线注油、达到稳态时间短、操作灵活、控制简单等优点.     

Effect of graphene oxide coating on bubble dynamics and nucleate pool boiling heat transfer

Materials coating has been proved to be an effective mean to increase the number of active nucleating sites, and therefore generate more vapor bubbles and lead to better pool boiling heat transfer performance. In this work, graphene oxide (GO) is coated on a boiling surface by self-assembly method, to enhance critical heat flux (CHF). The pool boiling is carried out on a smooth copper surface to study the effect of GO coating using distilled water as the working fluid along with bubble dynamic visualization. GO coating facilitates bubble nucleation by providing numerous microscale cavities. The visualization investigation of bubble dynamic behavior shows that the CO-coated surface exhibits a higher bubble departure frequency, a smaller bubble departure diameter and smaller bubble diameters in the pool, indicating greatly enhanced heat transfer effects. Meanwhile, the GO-coated surface exhibited a smaller contact angle than the copper surface, revealing that surface becomes more hydrophilic after GO coating. Consequently, GO-coated surface with a coating time of 4 h provides a CHF of 224.3 W/cm² and a heat transfer coefficient (HTC) of 8.79 W/(cm²·K), representing an improvement of 94.0% in CHF and 83.5% in HTC compared to smooth copper surface.     

液氮核态池沸腾CFD模拟和可视化实验

为探究低温流体池内核态沸腾机理,对液氮池内核态沸腾进行了计算流体力学(CFD)建模及实验研究。除了探究过热度和热流关系,重点分析过热度对气泡脱离直径和频率影响。根据实验观测,将核态沸腾过程分为3个阶段:低热流阶段;过渡沸腾阶段;完全核态沸腾(FDNB)阶段。基于得到的沸腾过程气泡直径及频率,构建了核态沸腾CFD数值模型,得到的过热度及热流密度关系,与实验测量得到的数据吻合。     

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.     

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 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.     

Influence of thermophysical properties on pool boiling heat transfer of refrigerants

The correct prediction of the heat transfer performance of the boiling liquid within the evaporator of a refrigeration unit is one of the essential features for the successful operation of the whole unit. A theoretically consistent calculation method for the heat transfer coefficient α in nucleate boiling, which should be based on the physical phenomena connected with vapour bubbles growing, departing and sliding on the wall and with the interactions of bubbles and of neighbouring nucleation sites within the microstructure of the heating surface, does not yet exist, despite the increasing number of papers on the subject in the recent past. Instead, the predictive methods for α available at present are empirical or semiempirical, especially for heat transfer conditions relevant in practice. Many of these correlations have been established in the form of power laws in which the relative influences of the main groups of variables on α are treated by separate factors. One of these may stand for the influence of the thermophysical properties of the boiling liquid or these properties will be included in several of the factors.New experimental results are presented for pool boiling heat transfer from a single horizontal copper tube (8 mm OD) to HFC-refrigerants (R32, 125, 134a, 143a, 152a, 227ea) and hydrocarbons (propane, i-butane). The results are compared to experimental data from the literature, and methods are discussed, how to incorporate the data in semiempirical correlations to describe the influence of the thermophysical properties of the fluids on the heat transfer performance.     

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.     

Transient nucleate boiling of liquid nitrogen with a stepwise change of heat flux

Results of experimental and theoretical investigations of unsteady nucleate boiling of liquid nitrogen are presented. A strip of niobium foil placed along the axis of a tube served simultaneously as a heater and as a resistance thermometer. Noted deviations from correlations for the steady nucleate boiling are explained by a new methematical model, which considers the process of transient nucleate boiling as stochastic.     

Influence of the source and sink temperatures on the optimal refrigerant charge of a water-to-water heat pump

This paper presents the results of a study carried out to elucidate the influence of the source and sink temperatures on the optimal charge of a propane water-to-water 16 kW heat pump which does not incorporate any liquid receiver. The unit had been fully tested along a previous experimental study, at various refrigerant charges and different condensing temperatures. A detailed mathematical model was then employed to simulate the unit performance. The predicted results were in very good agreement with the experiments, and furthermore, showed the same trends found in a similar unit tested at the KTH (Sweden) when the evaporation temperature was progressively decreased. Then, the model was employed to study the influence of the source and sink temperatures on the optimal charge of the unit. The simulation showed that the great variation of the optimal charge with the variation of the evaporation temperature is mainly due to the variation of the amount of refrigerant in the compressor oil.     

Investigations on the heat transfer of boiling binary refrigerant mixtures

Heat transfer during nucleate pool boiling was experimentally determined for the mixtures R-12/R-113, R-22/R-12, R-13/R-12, R-13/R-22 and R-23/R-13. For purposes of comparison, the respective five pure refrigerants were also investigated. Dependent upon the mixture, the measurements were made at boiling pressures of p = 0.1 to 2 MPa within the temperature region of t = 198 to 333 (−75° + 60°C) and at heat fluxes of Q = 4 × 10³ to 10⁵ W m⁻². A horizontal, electronically heated copper plate with A = 3 cm² was used. The following quantities were measured: pressure; temperature difference between the heating surface and the boiling liquid; composition and temperature in the liquid and vapour phases; and heat flow rate. The mean error of the heat transfer coefficients found was ± 5%.The results clearly show that the heat transfer for an evaporating mixture deteriorates as compared to the pure components. Essential parameters influencing this reduction are pressure, difference between vapour and liquid composition and heat flux. The fundamental relations and characteristic differences between the individual mixtures are illustrated by figures. The heat transfer coefficients measured can be represented within the whole region studied by a modified relation according to Körner.Observation of the process of evaporation has shown that by agitation (increase of convection) the heat transfer in mixtures can be improved. Additional experiments with evaporation during fluid flow in a pipe are presently in progress.     

Flow boiling heat transfer coefficients at cryogenic temperatures for multi-component refrigerant mixtures of nitrogen–hydrocarbons

The recuperative heat exchanger governs the overall performance of the mixed refrigerant Joule–Thomson cryocooler. In these heat exchangers, the non-azeotropic refrigerant mixture of nitrogen–hydrocarbons undergoes boiling and condensation simultaneously at cryogenic temperature. Hence, the design of such heat exchanger is crucial. However, due to lack of empirical correlations to predict two-phase heat transfer coefficients of multi-component mixtures at low temperature, the design of such heat exchanger is difficult.The present study aims to assess the existing methods for prediction of flow boiling heat transfer coefficients. Many correlations are evaluated against available experimental data of flow boiling of refrigerant mixtures. Silver-Bell-Ghaly correlation and Granryd correlation are found to be more suitable to estimate local heat transfer coefficients. A modified Granryd correlation is recommended for further use.     

Experimental correlation of pool boiling heat transfer for HFC134a on enhanced tubes: Turbo-E

The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (d_o = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively.     

沸腾表面对液氮临界热流密度的影响实验

采用液氮作为沸腾工质,通过可视化液氮沸腾实验台,对不同材料和直径的沸腾表面在液氮中进行相关稳态沸腾实验研究,总结分析沸腾表面材料、表面直径对临界热流密度及其对应过热度的影响规律.     

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.     

Impact of selected parameters on refrigerant flow boiling heat transfer and pressure drop in minichannels

This paper presents results concerning flow boiling heat transfer in a rectangular minichannel 1 mm deep, 40 mm wide and 360 mm long. The refrigerant flowing in the minichannel, Fluorinert FC-72, was heated by a thin foil microstructured on the side in contact with the fluid. Two types of microstructured surfaces were used: one with evenly distributed microcavities and the other with non-uniformly distributed minicavities. Liquid crystal thermography was applied to determine the temperature of the smooth side of the foil. The paper analyses mainly the impact of the microstructured heating surface and orientation of the minichanel on the heat transfer coefficient and two phase pressure drop. This required calculating the local values of heat transfer coefficient and measuring the pressure drop for different positions of the minichannel with enhanced heating wall. Moreover, the effects of selected thermal and flow parameters (mass flux density and inlet pressure), the geometric parameters, and the type of cooling liquid on the nucleate boiling heat transfer is studied. From the measurement results it is evident that applying a microstructured surface caused an increase in the heat transfer coefficient, which was approximately twice as high as that reported for the smooth surface. The highest values of the coefficient were observed for position 90° (the vertical minichannel) and position 0° (the horizontal minichannel), whereas the lowest were reported for position 180° (the horizontal minichannel). The experimental data concerning the two-phase flow pressure drop was compared with the calculation results obtained by applying nine correlations known from the literature. It is reported that most of the correlations can be used to predict the two-phase flow pressure drop gradient within an acceptable error limit (±30%) only for positions 90° and 135° (the vertical and inclined minichannels, respectively). The lowest agreement between the experimental data and the theoretical predictions was reported for the horizontal positions of the minichannel.