Two-Phase Flow Characteristics During Convective Boiling of Halocarbon Refrigerants Inside Horizontal Small-Diameter Tubes

Two-phase flow observations were performed for R134a and R245fa in 1.1- and 2.32-mm ID horizontal tubes. The tests were run for mass velocities ranging from 100 to 600 kg/m²-s and saturation temperatures of 22, 31, and 41°C. Additionally, an objective method to characterize two-phase flow patterns was developed. This method is based on simultaneous processing of signals from the following devices: a pair of diodes laser-sensors with a transparent tube between them within which the two-phase flow occurs, a micro-piezoelectric pressure transducer to determine the variation in the local pressure, and a microthermocouple within the fluid. The method was developed based on the k-means clustering algorithm, which consists of the gradual agglomeration of data of similar average characteristics. Simultaneous images of two-phase flow were obtained through a high-speed camera (10,000 frames/s) and used to identify the following flow patterns: bubbly, elongated bubbles, churn, and annular flows. The maps obtained by the objective method were compared against flow pattern results segregated based on flow visualization and a reasonable agreement was obtained between them. The vapor quality for the transition between churn and annular flow pattern decreases with decreasing the tube diameter, whereas the vapor quality for the transition between elongated bubbles and churn flow decreases with increasing tube diameter. Effects of saturation temperature and mass velocity were also verified. Additionally, elongated bubble velocities, frequencies, and lengths were determined based on the analysis of high-speed videos and the processing of signals of the diode/laser-sensor. The elongated bubble velocity was correlated as a linear function of the two-phase superficial velocity. A new image treatment method was developed to automatically identify the entrainment frequencies, which were segregated according to two groups: low and high frequency. The former group was characterized by frequencies lower than 20 Hz and the later by 50–500 Hz frequency ranges.     

Experimental Investigation of M-Shape Heat Transfer Coefficient Distribution of R123 Flow Boiling in Small-Diameter Tubes

This article presents a study of flow boiling of R123 in two small-diameter silver tubes with inner diameters of 1.15 mm and 2.3 mm. The experiments have been accomplished for a wide range of quality variation (0.01–0.9), mass flow rate (650–3000 kg/m²s), and heat fluxes (40–80 kW/m²). The saturation temperature ranged from 30 to 70°C. In the experiments a peculiar distribution of heat transfer coefficient leading to development of two maxima in its distribution with respect to quality has been observed. Such behavior was seen in both sizes of tubes.     

Prediction of Two-Phase Heat Transfer Coefficient of Flow Boiling in Minichannels

Abstract

The present study mainly concentrates on the prediction of two-phase heat transfer coefficient for saturated flow boiling conditions in minichannels. The experimental database has been generated through systematic experiments on the basis of the effect of aspect ratio. In the experiments, deionized water is used as the working fluid and five single rectangular minichannels with different aspect ratios but with the same hydraulic diameter of 1.2?mm have been tested. The database (120 data points) contains mass fluxes of 70???310?kg m^?2s^?1, wall heat fluxes of 216.2???1,117.6 kWm^?2, vapor qualities of 0.012???0.788, liquid Reynolds numbers of 59.6???1,201.7, and aspect ratios of 0.25???4.00. The data obtained has been used to evaluate the previous correlations proposed for different scales (macro???mini???micro), and, then, a new empirical correlation has been developed. This new correlation presents clearly a good performance with an overall mean absolute error of 9.2%, and all the predictions fall within ±30% error band.     

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m²s and the heat flux from as low as 1.6 to 135 kW/m². Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.     

Heat Transfer Correlations for Single-Phase Flow,Condensation, and Boiling in Microfin Tubes

Experimental single-phase, condensation and flow boiling heat transfer data from the literature and our previous studies were collected to evaluate existing heat transfer correlations for microfin tubes of different geometries. The Ravigururajan and Bergles correlation modified by using the hydraulic diameter proposed by Li et al. (2012) can predict single-phase heat transfer data relatively well. Among the four reviewed condensation heat transfer correlations, the Yu and Koyama (1998) correlation presents the best prediction. However, all the four condensation correlations are prone to overpredict the carbon dioxide data. For flow boiling in microfin tubes, the general semiempirical correlation developed by Wu et al. (2013), applicable for intermittent and annular flow patterns, is the most reliable predictive method among the five evaluated correlations. It can predict 90% of the overall 754 data points within a ±30% error band, with a mean absolute deviation and a standard deviation equal to 18.2% and 21.9%, respectively, covering pure halogenated refrigerants, near azeotropic refrigerant mixtures, and carbon dioxide with the following applicable range: fin root diameter 2.1 to 14.8 mm, mass flux 100 to 800 kg/m²s, heat flux 4.5 to 59 kW/m², and reduced pressure 0.07 to 0.7.     

Confined Boiling Heat Transfer,Two-Phase Flow Patterns,and Jet Impingement in a Hele-Shaw Cell

ABSTRACT

Experiments were carried out to study heat transfer and two-phase flow patterns during boiling in a Hele-Shaw cell filled with pure water vapor at atmospheric pressure and with a central inlet of a liquid jet. The Hele-Shaw cell was based on a circular copper rod surface and a polycarbonate plate permitting optical access and thus high-speed cinematography. The diameter of the heated copper rod was 10 mm, the jet diameters were 0.5 and 1 mm, and spacing was varied between 50, 100, and 200 μm. The heat was applied through 4 cartridge heaters with a maximum heat flux of 327 W/cm². Results showed how high-volume flow rates for the liquid jet led to jet impingement heat transfer while low flow rates led to a Hele-Shaw flow boiling system. The relationship between the volume flow rate and the temperature difference differed significantly between these two regimes. Different flow patterns and evaporation fronts were observed using high-speed cinematography. They strongly depended on jet properties, applied heat flux, and gap spacing. The efficiency of the Hele-Shaw flow boiling system during high heat flux levels was attributed to high interface velocities, combined with viscous fingering at the interface. This combination led to high wetting rates with substantial microlayer evaporation. Good results regarding the heat transfer and the pressure drop were obtained with the final configuration of a 10-mm copper rod diameter, a jet diameter of 1 mm, and a spacing of 0.1 mm. A rather surprising observation was the existence of a stable rotation of an evaporating liquid jet in the Hele-Shaw boiling chamber. The driving mechanism for the rotation with a frequency of 105 Hz was the rapid microlayer evaporation at the rear side of the rotating liquid jet.     

强化管内沸腾换热实验研究

主要研究在低过热度下微槽对流动沸腾换热特性的影响,分别以单工质甲醇和甲醇与甲苯的混合物为工质对不同流量情况下光管、直槽管和螺旋槽管的流动沸腾换热特性进行了实验研究。研究结果表明：对单工质甲醇来说,螺旋槽管可以明显起到强化传热作用,而且流量越低,强化传热效果越明显。对混合工质来说,当流量较低时,螺旋槽管强化传热效果不明显,而在流量较高时,强化传热效果比较明显。无论是单工质还是混合工质,直槽管在实验所能达到的壁面温度条件下不能起到明显的强化传热效果。还给出了螺旋槽管强化传热的定性解释。     

Two-Phase Flow Distribution to Tubes of Parallel Flow Air-Cooled Heat Exchangers

Abstract

This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modification of the design.     

Heat Transfer and Two-Phase Flow during Shell-side Condensation

This paper surveys the evolution of power condenser tube bundle arrangements and examines present-day designs. Condensation heat transfer during shell-side flow is reviewed, including the effects of vapor shear, condensate inundation, noncondensable gases, and enhancement techniques. The difficulties experienced in calculating vapor pressure drop through tube bundles are described, as well as recent attempts to obtain more reliable correlations. The modeling of these phenomena to predict shell-side condenser performance is reviewed, as well as the use of one- and two-dimensional computer codes. Appropriate topics for future research are identified.     

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m^?2 s^?1, heat flux from 0 to 55 kW m^?2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m^?1 and from 1 to 7 kW m^?2 K^?1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the experimental flow boiling heat transfer data with an average error around 19%.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

A General Correlation for Heat Transfer During Dispersed-Flow Film Boiling in Tubes

A general correlation for heat transfer during film boiling in tubes is presented. It is based on the two-step model. It has been verified with data for nine fluids flowing up in tubes. The fluids include water, cryogens, refrigerants, and chemicals. The range of data includes pressures from 1 to 215 bar, reduced pressures from 0.0046 to 0.97, mass velocities from 4 to 5,176 kg/m² s, tube diameters from 1.1 to 24.3 mm, and qualities from 0.1 to 2.4. The 546 data points are predicted with a mean deviation of 15.2%. Deviation is defined as the difference between the measured and predicted heat transfer coefficients divided by the measured heat transfer coefficient, the heat transfer coefficients being based on the saturation temperature. Three other well-known correlations are also compared to the same data and found to have much larger deviations. The correlation is also compared with a limited amount of data from horizontal tubes; the results are encouraging.     

Bubble Dynamics and Heat Transfer during Pool and Flow Boiling

A large number of studies of bubble growth rate and departure diameter have been reported in the literature. Because of uncertainty in defining the shape of an evolving interface, empirical constants are invariably used to match the model predictions with data. This is especially true when force balance is made on a vapor bubble to determine the departure diameter. In this paper, the results of an alternate approach based on a complete numerical simulation of the process are given. Single and multiple bubbles are considered for both pool and flow boiling. The simulations are based on the solution of the conservation equations of mass, momentum, and energy for both phases. Interface shape is captured through a level set function. A comparison of bubble shape during evolution, bubble diameter at departure, and bubble growth period is made with data from well-controlled experiments. Among other variables, the effect of magnitude of gravity and contact angle is explicitly investigated.     

On the Prediction of Heat Transfer in Micro-Scale Flow Boiling

Xu et al. have recently published a set of results for boiling heat transfer measurements in a multi-channel micro-scale evaporator for flow boiling of acetone in triangular cross-section channels (hydraulic diameter of 155.4 mm). In the present collaboration, we assess our current capability to predict this independent flow boiling data set with a fluid not in the original database and also much smaller in size using the phenomenological three-zone model of Thome, Dupont, and Jacobi. The method models boiling in small diameter channels in the elongated bubble/slug flow regime. The boiling data falling in this regime are identified here using a new micro-scale flow pattern map proposed by Revellin in order to utilize only test data corresponding to the elongated bubble flow mode. The decrease of the measured wall temperature due to the heat spread by longitudinal conduction through the heat sink was investigated through a finite differences analysis. In addition, a data reduction procedure different than that one used by Xu et al. was used and, consequently, some differences in the heat transfer behavior were found. Based on the present database, a new set of empirical parameters for the three-zone model was proposed. The conjugated effect of flow pattern and bubble/slug frequency on the heat transfer coefficient was also investigated.     

On Recent Advances in Modeling of Two-Phase Flow and Heat Transfer

General thermal design methods for two-phase heat exchangers are emerging that are based on local two-phase flow patterns and the flow structure of the two-phases. These methods promise to be much more accurate and reliable for predicting two-phase heat transfer coefficients and pressure drops than the older, statistically-derived empirical design methods that completely ignore flow regime effects or simply treat flows as stratified (gravity-controlled) or nonstratified (shear-controlled) flows, which greatly limits their accuracy, validity, and reliability and often results in prediction errors surpassing 100% within their supposed range of application. These new flow pattern and flow structure types of design methods are particularly suited for use in modern heat exchanger design software, which are typically incremental and hence require local methods that capture the real trends in experimental data. The status of these new developments is reviewed here for intube two-phase flow and heat transfer processes.     

A General Predictive Technique for Heat Transfer during Saturated Film Boiling in Tubes

Abstract

A simple predictive technique for heat transfer during film boiling in tubes is presented. This technique is based on the two-step model and consists of a graphic correlation for nonequilibrium quality and an equation for liquid droplet cooling at high pressures. It has been developed from and verified with data for water, nitrogen, para-hydrogen, R-113, methane, and propane. The range of data includes equilibrium qualities from 0.1 to 2.9, pressures from 1.4 to 215 bar, reduced pressures from 0.01 to 0.97, mass flux from 30 to 3442 kg/m² s, tube diameters from 2.5 to 14.9 mm, heat flux from 0.012 to 2.1 [Macute]W/m², and wall temperatures from 81 to 1112 K. For all 722 data points analyzed, heat transfer coefficients based on actual vapor temperatures are correlated with a root-mean-square error of 15%.     

Heat Transfer and Pressure Drop Measurements During Boiling in Vertical Tubes with Coiled-Wire Inserts

ABSTRACT

The paper presents thermal and flow analyses of the boiling process of R507, R410 and R407 C refrigerants inside vertical tubes (21 mm) with coiled-wire inserts and various coil diameters (20; 20.5 mm), coil pitches (26; 44 mm) and wire diameters (1.5; 2 mm). The study differs from other publications as regards the conditions under which the experiment was conducted. It focuses on the boiling process in two long vertical tube sections (2 m), paired in an in-line arrangement. The study was conducted within a moderate range of mass flux densities 80–240 kgm^?2s^?1 and at low heat flux densities 5–11 kWm^?2, corresponding to the operating conditions of air coolers. The study examined the influence of vapour quality, mass flux density, geometrical parameters of the inserts and the impact of temperature glide on heat transfer coefficient and flow resistance increases as compared with a plain tube. The obtained increase ratios of heat transfer coefficients amounted to 1.1-1.7 for an azeotropic agent and to 1.1-1.3 for zeotropic agents, with the relative increase in flow resistances amounting to 1.8-4.5. New equations are proposed in the paper for the calculation of heat transfer coefficient and flow resistance values for boiling inside vertical tubes with spiral inserts.     

Influence of Tube's Diameter on Boiling Heat TransferPerformance in Small Diameter Tubes

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Bubble Dynamics,Flow, and Heat Transfer during Flow Boiling in Parallel Microchannels

Significant efforts have recently been made to investigate flow boiling in microchannels, which is considered an effective cooling method for high-power microelectronic devices. However, a fundamental understanding of the bubble motion and flow reversal observed during flow boiling in parallel microchannels is lacking in the literature. In this study, complete numerical simulations are performed to further clarify the boiling process by using the level-set method for tracking the liquid–vapor interface which is modified to treat an immersed solid surface. The effects of contact angle, wall superheat, and the number of channels on the bubble growth, reverse flow, and heat transfer are analyzed.