Flow boiling heat transfer to carbon dioxide: general prediction method

An updated version of the Kattan–Thome–Favrat flow pattern based, flow boiling heat transfer model for horizontal tubes has been developed specifically for CO₂. Because CO₂ has a low critical temperature and hence high evaporating pressures compared to our previous database, it was found necessary to first correct the nucleate pool boiling correlation to better describe CO₂ at high reduced pressures and secondly to include a boiling suppression factor on the nucleate boiling heat transfer coefficient to capture the trends in the flow boiling data. The new method predicts 73% of the CO₂ database (404 data points) to within ±20% and 86% to within ±30% over the vapor quality range of 2–91%. The database covers five tube diameters from 0.79 to 10.06 mm, mass velocities from 85 to 1440 kg m⁻² s⁻¹, heat fluxes from 5 to 36 kW m⁻², saturation temperatures from −25 °C to +25 °C and saturation pressures from 1.7 to 6.4 MPa (reduced pressures up to 0.87).     

Heat transfer and flow pattern during two-phase flow boiling of R-134a in horizontal smooth and microfin tubes

Flow pattern and heat transfer during evaporation in a 10.7 mm diameter smooth tube and a micro-fin tube are presented. The tubes were tested in the ranges of mass flux between 163 and 408 kg m⁻² s⁻¹, and heat flux between 2200 and 56 000 W m⁻². The evaporation temperature was 6 °C. Flow maps for both the tubes are plotted in the coordinates of mass flux and vapor quality. The relations of flow pattern and local heat transfer coefficient are discussed. The heat transfer coefficients for intermittent and annular flows in both the smooth tube and the micro-fin tube are shown to agree well with Gungor and Winterton's correlation with modified constants.     

Flow boiling of ammonia and hydrocarbons: A state-of-the-art review

A comprehensive review of flow boiling heat transfer, two-phase pressure drops and flow patterns of ammonia and hydrocarbons applied in air-conditioning, refrigeration and heat pump systems is presented in this paper. First, experimental studies of flow boiling of ammonia and hydrocarbons are addressed. Then, the prediction methods for flow boiling heat transfer, two-phase pressure drops and flow patterns are described. Next, comparisons of four flow boiling heat transfer and four two-phase pressure drop methods to the experimental data in smooth tubes derived from the available studies are presented. In addition, comparison of flow patterns to a flow map is presented. Based on the comparisons and analysis, recommendations on these methods are given. Furthermore, research needs on flow boiling and two-phase flow of ammonia and hydrocarbons have been identified. It is suggested that more experimental data be obtained through well conducted experiments and new prediction methods or modified ones based on the available methods be made for ammonia and hydrocarbons. In addition, the effect of oil on ammonia and hydrocarbon flow boiling and two-phase flow should be studied in order to have conclusive evidence of its effect.     

Flow boiling heat transfer characteristics of CO2 at low temperatures

This paper presents an overview of the flow boiling heat transfer characteristics and the special thermo-physical properties of CO₂ at low temperatures (down to −30 °C). Subsequently, the boiling heat transfer of CO₂ at low temperatures is experimentally investigated in a horizontal tube with inner diameter of 4.57 mm. Due to the large surface tension, the boiling heat transfer coefficient of CO₂ is found to be much lower at low temperatures but it increases with vapour quality (until dryout), which is contrary to the trend at high temperatures around 0 °C. None of the empirical correlations from open literature were able to predict the boiling heat transfer coefficient for CO₂ in good agreement with the experimental data, suggesting the need for further research in this area.     

Nucleate boiling heat transfer coefficients of pure halogenated refrigerants

Nuclate pool boiling heat transfer coefficients (HTCs) of HCFC123, CFC11, HCFC142b, HFC134a, CFC12, HCFC22, HFC125 and HFC32 on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accomodate high vapor pressure refrigerants such as HFC32 and HFC125 with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool of 7 °C. Test results showed that HTCs of HFC125 and HFC32 were 50–70% higher than those of HCFC22 while HTCs of HCFC123 and HFC134a were similar to those of CFC11 and CFC12 respectively. It was also found that nucleate boiling heat transfer correlations available in the literature were not good for certain alternative refrigerants such as HFC32 and HCFC142b. Hence, a new correlation was developed by a regression analysis taking into account the variation of the exponent to the heat flux term as a function of reduced pressure and some other properties. The new correlation showed a good agreement with all measured data including those of new refrigerants of significantly varying vapor pressures with a mean deviation of less than 7%.     

Use of fluorescence to measure the lubricant excess surface density during pool boiling

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. 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 measurement technique was used to confirm the existence of 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 lubricant excess density on the heat transfer surface 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 both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.

Résumé

This paper presents what are believed to be the first measurements of the non-adiabatic lubricant excess surface density on a roughened, flat, plain horizontal pool-boiling surface. Pool boiling heat transfer data is given for pure R123 and a R123/lubricant mixture. Lubricant excess surface density data are given for the boiling R123/lubricant mixture. 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 measurement technique was used to confirm the existence of 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 lubricant excess density on the heat transfer surface 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 both pure R123 and an R123/lubricant (1.8% lubricant mass fraction) mixture at 277.6 K. The heat transfer data shows that the lubricant excess causes an average degradation of 12% in the heat flux for a given superheat.     

Heat transfer characteristics of the ice slurry at melting process in a tube flow

The heat transfer characteristics were experimentally investigated for ice slurry made from 6.5% ethylene glycol–water solution flow in a 13.84 mm internal diameter, 1500 mm long horizontal copper tube. The ice slurry was heated by hot water circulated at the annulus gap of the test section. Experiments of the melting process were conducted with changing the ice slurry mass flux and the ice fraction from 800 to 3500 kg/m² s and 0–25%, respectively. During the experiment, it was found that the measured heat transfer rates increase with the mass flow rate and ice fraction; however, the effect of ice fraction appears not to be significant at high mass flow rate. At the region of low mass flow rates, a sharp increase in the heat transfer coefficient was observed when the ice fraction was more than 10%.     

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.     

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.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.     

Enhancement of condensation heat transfer by means of passive and active condensate drainage techniques

A short state-of-the-art review on the passive and active enhancement of condensation heat transfer techniques developed recently is presented in the paper. The particular attention is paid to the methods involving the augmentation of the condensate drainage. As an example of the passive technique the method of condensate drainage enhancement by using the drainage strip is presented. For an active method of heat transfer enhancement a novel EHD technique is described. For both methods the own experimental results as well as theoretical models are provided.     

A review of hydrocarbon two-phase heat transfer in compact heat exchangers and enhanced geometries

Hydrocarbons are considered as alternative fluids for refrigeration, air-conditioning and heat pump applications. Pure butane, propane or their mixtures can be adopted, but due to their flammable properties, the systems have to be designed in such a way that the refrigerant charge is minimized. Therefore, compact heat exchangers and enhanced geometries are adopted in such systems. In this paper, the current state of the art for two-phase heat transfer calculations for pure hydrocarbons and their mixtures is reviewed and analysed. Recommendations are proposed for estimating evaporation and condensation heat transfer in various geometries including enhanced tubes as well as compact heat exchangers.     

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.     

Evaluation of heat and mass transfer coefficients for falling-films on tubular absorbers

A coupled heat and mass transfer model is developed to extract the transfer coefficients for falling-films from the measurements on a tubular absorber. The mass transfer coefficients obtained from the coupled model and the log-mean-difference approach agree within about 10%. For the heat transfer coefficient, the values given by the two models can differ quite significantly. The cooling water temperature distribution predicted by the coupled model agrees well with measurements. The transfer coefficients obtained from experimental measurements using the various methods reported in the literature show wide variations.     

Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm⁻² and mass fluxes of 200 to 530 kgm⁻² s⁻¹. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed.     

Optimal shape of the multi-passage branching system in a single-phase parallel-flow heat exchanger

A numerical simulation is performed to examine the heat and fluid flow characteristics of the branching system in a single-phase parallel-flow heat exchanger (PFHE) and to obtain its optimal shape. The relative importance of the design parameters [injection angle of the working fluid (Θ), inlet shape and location (Y_c), and height of the protruding flat tube (Y_b)] is determined to decide the optimization sequence. The optimal geometric parameters are obtained as follows: Θ=−21°, Type A, Y_c=0 and Y_b=0. The heat transfer rate of the optimum model compared to that of the reference model is increased by about 55%. The optimal values of the parameters can be applicable to the Reynolds number ranging from 5000 to 20,000.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

Pool boiling heat transfer to hydrocarbons and ammonia: A state-of-the-art review

Interest has grown in recent years to extend the use of hydrocarbons and ammonia as working fluids in refrigeration to new domains of application, despite their flammability. In the context of pool boiling heat transfer, this has created increasing research activities, particularly with regard to hydrocarbons. In contrast with this, only a few new experimental results have been added to the data set existing for ammonia in the literature. So this review will concentrate on hydrocarbons, while ammonia will be treated in a comparatively brief part.The review starts with the state-of-the-art that had been reached at about 1990. It continues with the data set for propane being taken as an example to highlight various reasons for the experimental data scatter that is found when different sources are compared for the same substance. In the main part, new results of 12 (aliphatic) hydrocarbons are discussed regarding the influence of heat flux q and reduced saturation pressure p^* = p_s/p_c on the heat transfer coefficient α, and also the variation in α₀ caused by the differences in the thermophysical properties of the 12 hydrocarbons at constant q₀ and . It is shown that the dependencies of the heat transfer coefficient α on heat flux q and reduced pressure p^*, and on the thermophysical properties of the various fluids at constant values q₀ and can be correlated by general semi-empirical functions with comparatively narrow limits of error that do not reach far beyond the experimental scatter occurring when different sources are compared for the same substance. Before treating ammonia in a final section, the review on hydrocarbons closes with short discussions for mixtures of hydrocarbons, for the bundle effect, and for the behaviour of enhanced tubes.     

An experimental study on heat transfer and pressure drop characteristics of carbon dioxide during gas cooling process in a horizontal tube

The heat transfer coefficient and pressure drop during gas cooling process of CO₂ (R744) in a horizontal tube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a gas cooler (test section). The water loop consists of a variable speed pump, an isothermal tank, and a flow meter. The refrigerant, circulated by the variable-speed pump, condenses in the inner tube while water flows in the annulus. The gas cooler of tube diameter is 6000 mm in length, and it is divided into 12 subsections.The pressure drop of CO₂ in the gas cooler shows a relatively good agreement with those predicted by Blasius's correlation. The local heat transfer coefficient of CO₂ agrees well with the correlation by Bringer–Smith. However, at the region near Pseudo-critical temperature, the experiments indicate higher values than the Bringer–Smith correlation. Based on the experimental data presented in this paper, a new correlation to predict the heat transfer coefficient of supercritical CO₂ during in-tube cooling has been developed. The majority of the experimental values are within 18% of the values predicted by the new correlation.