低流速净蒸汽产生点模型预测过冷沸腾空泡率

空泡率是汽液两相流动的基本参数之一,而已有过冷沸腾空泡率计算方法研究以高质量流速为主,且大量文献报道现有空泡率模型难以适用于低流速过冷沸腾工况。本文基于低流速过冷沸腾净蒸汽产生点(NVG)理论模型,进一步建立了计算过冷沸腾空泡率的分布拟合模型。在较宽广的压力、质量流速、热流密度和流道尺寸范围内将模型计算结果与现有空泡率实验数据进行了比较,低流速工况下该模型与实验数据符合良好,表明该模型可适用于低流速过冷沸腾工况。     

Investigation on the void fraction of gas–liquid two-phase flows in vertically-downward pipes

A special experimental loop is designed and constructed to study the characteristics of the void fraction of gas–liquid two-phase flow in vertically-downward pipes. The test section is made of transparent pipe with a length of 6 m and an internal diameter of 25 mm. The void fraction ranging from 0.1 to 0.98 widely is measured using quick-closing valve method. It is found that the range of the void fraction could be divided into three regions with different flow patterns and different relationships between the void fraction and the gas–liquid volumetric flow rate ratio. Moreover, 39 correlations for calculating the void fraction collected from present literature, are classified, and evaluated using the experimental data obtained in this study. The prediction of correlations in the literature needs to be improved when the void fraction is small.     

Influence of Void Ratio on Phase Change in Thermal Storage Canister of Heat Pipe Receiver

In this paper, with both void cavity and phase change considered, influence of void ratio on phase change in thermal storage canister of heat pipe receiver under microgravity is numerically simulated. Accordingly, physical and mathematical models are built. A solidification–melting model upon the enthalpy–porosity method is specially provided to deal with phase changes. The change of liquid fraction with respect to void ratio and the liquid fraction distribution of different void ratios in a thermal storage canister of a heat pipe receiver are shown. Numerical results are compared with experimental ones. Research results indicate that the void cavity prevents the process of phase change significantly. Phase-change material (PCM) melts slowly during sunlight periods and freezes slowly during eclipse periods as void ratio increases. The utility ratio of PCM during both sunlight periods and eclipse periods decreases obviously as the void ratio increases. The void cavity prevents the heat transfer between the PCM zone and canister wall. The void cavity blocks the processes of both melting and solidification during cycle orbital periods.     

Hydrodynamics and heat transfer in bubbly flow in the turbulent boundary layer

In the work presented is a new approach to modelling the bubbly flow in the boundary layer. The approach is based on summation of dissipation energy coming from the shearing turbulent flow in the absence of bubbles and the dissipation contribution from the presence of bubbles. As a result we obtain the dissipation of equivalent single phase turbulent flow. The model has been solved using the method of asymptotic correction to provide an explicit differential equation describing the velocity profile. That can be solved with the assumption of constant void fraction distribution to yield the analytical velocity profile. Alternatively, author has developed his own model of lateral void migration, which is distinct from other models by virtue of presence of another rotational velocity. Velocity distributions calculated using the new model have been compared against the experimental data of turbulent bubble flows with small void fraction. A good consistency between calculations performed using a new model and available experimental data has been obtained. Additionally, a solution of the temperature field is also given. In the case of a constant void fraction distribution analytical distribution of the Nusselt number is given or the set of differential equations needs to be solved.     

The influence of bubble size on void fraction distribution in subcooled flow boiling at low pressure

The effect of bubble size on void fraction distribution in subcooled flow boiling in a vertical annular channel at low pressure is studied numerically. It is found that a simple linear formula used by Anglart and Nylund [1] and adopted in Reference [2] for calculation of bubble size as a function of local subcooling lacks a physical and experimental basis limiting the general application of the model for predicting subcooled flow boiling. A bubble size correlation proposed by Zeitoun and Shoukri [3] has been employed in this study. The predictions of void fraction profiles and the bubble size distributions, after incorporating the above bubble size correction, show very good agreement with the experimental data.     

水平管内油气水三相分层流截面含气率的研究

以油气水三相混合物为工质,对水平管内分层流动的平均截面含气率进行了理论与实验研究。通过对分层流动的简化动力学分析,得到了截面含气率的理论模型,计算值与实验值符合良好。结果表明：影响分层流截面含气率的因素不仅包括折算气速和折算液速,还包括油水混合物的含油率。     

Mechanistic study for the interfacial area transport phenomena in an air/water flow condition by using fine-size bubble group model

A set of number density transport equations based on the bubble size are used to predict the void fraction and the interfacial area concentration in an air/water flow conditions. As the closure relations for the number density transport equations, a coalescence due to random collisions and a breakup due to an impact of the turbulent eddies are modified based on previous studies. The bubble expansion term due to a pressure reduction and a coalescence due to a wake entrainment are modeled for the number density transport equation. In order to predict the local experimental data, a computational fluid dynamic (CFD) code coupling the two-fluid model and number density transport equations are developed in this study. As for the results of the numerical analysis, the developed model predicts well the void fraction and interfacial area concentration although some deviations between the prediction and the experiment are shown for the high void fraction conditions.     

Validation of void fraction models and correlations using a flow pattern transition mechanism model in relation to the identification of annular vertical downflow in-tube condensation of R134a

This paper reports the experimental investigation of a model for predicting flow pattern transitions and for the validation of void fraction models and correlations proposed in the authors' previous publications and for the identification of flow regimes in data corresponding to annular flow downward condensation of R134a in a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm. R134a and water are used as working fluids on the tube side and annular side, respectively, of a double tube heat exchanger. Condensation experiments are done at mass fluxes of 260 and 515 kg m^− 2 s^− 1 in the high mass flux region of R134a. The condensing temperatures are between 40 and 50 °C; heat fluxes are between 10.16 and 66.61 kW m^− 2. A mathematical model proposed by Soliman based on the models of Kosky and Lockhart–Martinelli is used to determine the condensation film thickness of R134a. Comparative void fraction values are determined indirectly using the measured data under laminar and turbulent flow conditions together with various void fraction models and correlations reported in the literature. There is good agreement between the void fraction results obtained from the theoretical model and those obtained from the void fraction models of Soliman, Chisholm and Armand, Turner and Wallis, Smith, Spedding and Spence previously proposed in the authors' publications and tested against their experimental database. Various well-known flow regime correlations from the literature are investigated to identify the flow regime occurring in the test tube, the correlations of Taitel and Dukler, Dobson, Akbar et al., Breber et al., Cavallini et al., and Sardesai et al. can provide accurate estimates of the annular flow conditions in spite of their different working conditions.     

Effect of void fraction models on the film thickness of R134a during downward condensation in a vertical smooth tube

In the present study, the void fraction and film thickness of pure R-134a flowing downwards in a vertical condenser tube are indirectly determined using relevant measured data together with an annular flow model and various void fraction models reported in the open literature. The vertical test section is a countercurrent flow double tube heat exchanger with refrigerant flowing down in the inner tube and cooling water flowing upward in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter with a length of 0.5 m. The experimental runs are carried out at average saturated condensing temperatures of 40 and 50 °C, and mass velocities are around 456 kg m^− 2 s^− 1, over the vapour quality range 0.82–0.93, while the heat fluxes are between 45.60 and 50.90 kW m^− 2. Analysis based on simple void fraction models of the annular flow pattern are presented for forced convection condensation of pure R134a, taking into account the effect of the different saturation temperatures at high mass flux conditions. The comparisons of calculated film thickness show that the void fraction models of Spedding and Chen, and Chisholm and Armand are the most accurate ones with the experimental data due to their low deviation with Whalley's annular flow model over 35 void fraction models presented in this paper.     

Heat transfer and void fraction profiles around a horizontal cylinder immersed in a bubbling fluidised bed

The local void fraction profile around a surface immersed in a fluidised bed has a great importance for the estimation of transport phenomena and chemical reaction rates in these units but it is rarely reported in literature. The scope of this work is to demonstrate that heat transfer measurements can be reliably used to derive the void fraction profile once a proper correlation model is available. To this aim, measurements of local heat transfer coefficient around a horizontal cylinder were performed and the angular void fraction profile was reconstructed with a new heat transfer model. The modeled void fraction profile resulted coherent with experimental and numerical evidences available in literature.     

Comparison of Void Fraction Correlations for Different Flow Patterns in Upward Vertical Two-Phase Flow

A comparison of the performance of 52 void fraction correlations was made based on an unbiased experimental data set of 1208 data points. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data for upward vertical two-phase flow. The performance of the correlations in correctly predicting the diverse data set was evaluated. Comparisons between the correlations were made and appropriate recommendations were drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on this analysis, void fraction correlations with the best predictive capability are highlighted.     

壳侧气液两相流沿水平方向横掠水平管束截面含气率的预测

本文在实验研究的基础上,结合前人的研究结果,将预测管内平均截面含气率的理论模型经适当修正后,提出了预测带的流板的管壳式换热器壳侧气液两相流沿水平方向横掠水平管束截面含气率的理论模型。结果表明,该模型能很好地预测壳侧顺排和叉排水平管束中不同物纱两相流体的平均截面含气率。     

An experimental study on local interfacial area concentration using a double-sensor probe

Interfacial area concentration is an important parameter in modeling the interfacial transfer terms in the two-fluid model. In this paper, the interfacial area concentration, void fraction, and bubble Sauter mean diameter for air-water bubbly flow through a vertical transparent pipe with 40 mm internal diameter was investigated experimentally using both digital high-speed camera system and a double-sensor conductivity probe. Based on the experimental data of digital high-speed camera system, the statistical models derived by different researchers for local interfacial area concentration measurement using double-sensor conductivity probe were evaluated. The results show that there are obvious differences among the values of local interfacial area concentration calculated by different statistical models even from the same probe signals. The section-averaged values of the local interfacial area concentration calculated using the statistical model by Kataoka et al. agree best with experimental data of digital high-speed camera system. Therefore, the statistical model developed by Kataoka et al. is recommended for the local measurement of interfacial area concentration using a double-sensor conductivity probe in bubbly two-phase flow. Using the verified double-sensor probe method, we carry out experiment to study the local distribution characteristic of the interfacial area concentration and void fraction in air-water bubbly flow through a vertical pipe.     

Void fraction and pressure drop in two-phase liquid metal flows in channels

Experimental results on void fraction and friction pressure drop in vapour-potassium flows in the high-vapour-quality region up to unity are presentéd. The experimental data obtained and the pertinent results of other authors are generalized, and empirical relationship are suggested to calculate void fraction and pressure drop in two-phase liquid metal flows for channels of various configurations and orientations. The relationships are valid within the range of vapour qualities from almost zero to unit. The experimental data prove the mass velocity to have no influence on the hydrodynamic characteristics within the range of the parameters investigated. It is found in the experiments with heat supply that friction pressure losses are smaller than those for adiabatic conditions. It is shown that this result is in good correspondence with the model of the effect of injection in a boundary layer on the value of shear stresses between cases.     

Acoustic Wave Prediction in Flowing Steam-Water Two-Phase Mixture

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Measurement and Modeling of Void Fraction in High-Pressure Condensing Flows Through Microchannels

Void fraction measurements are obtained using high-speed video for the condensation of R404A in tubes of diameter 0.508, 1.00, and 3.00 mm. Experiments were conducted on refrigerant R404A throughout the entire condensation quality range (0.05–0.95) at varying mass fluxes (200–800 kg m^?2 s^?1) and saturation temperatures from 30 to 60°C (which correspond to the reduced pressure range 0.38–0.77). These high pressures are representative of actual operation of air-conditioning and refrigeration equipment. The influence of saturation temperature on void fraction is most pronounced in the quality range 0.25–0.75. In addition, it was found that the influence of mass flux on void fraction was negligible for all saturation temperatures and tube diameters investigated. Three void fraction models from the literature are compared with the data. Of these, the Baroczy correlation predicted the data the best, with an overall absolute average deviation of 11.2%. A new drift flux void fraction model is developed to predict void fraction for condensing flows in microchannels and compared with the R404A data and R134a void fraction data from Winkler et al. Overall, the model is able to predict 92.3% of the R404A data and 81.6% of all refrigerant data within 25%.     

一种适用于缸盖水腔沸腾传热计算的模型

基于VC 6.0开发了一种单相流沸腾传热模型,通过引入空泡份额的概念将沸腾发生时的流场看作一个气液均匀混合的单相流,从数学上对该模型进行了描述并介绍了模型的数值实现方法。通过与实验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算。实验和计算结果还表明,压力对沸腾传热的影响较为明显。最后以226B型发动机水腔为工程应用对象,计算出了水腔内的空泡份额分布和水腔内的流度分布情况。     

Experimental and numerical investigation of two-phase pressure drop in vertical cross-flow over a horizontal tube bundle

Experimental pressure drop data for vertical two-phase air–water flow across horizontal tubes is presented for gas mass fractions in the range 0.0005–0.6 and mass fluxes in the range 25–700 kg/m² s. The square in-line tube bundle had one column containing ten tubes and two columns of half tubes attached to the walls. The tubes had a diameter of 38 mm and a pitch to diameter ratio of 1.32. This data and air–water and R113 vapour–liquid data available in the literature are compared with the predictions from two kettle reboiler models, the one-dimensional model and a one-dimensional formulation of the two-fluid model. The one-dimensional model was implemented with three separate void fraction correlations and one two-phase friction multiplier correlation. The results show that the two-fluid model predicts air–water void fraction data well but R113 data poorly with pressure drop predictions for both being unsatisfactory. The one-dimensional model is shown to predict pressure drop and void fraction data reasonably well, provided a careful choice is made for the void fraction correlation.     

A fractal model for the coupled heat and mass transfer in porous fibrous media

This paper developed a mathematical model for the coupled heat and mass transfer in porous media based on the fractal characters of the pore size distribution. According to Darcy’s law and Hagen–Poiseuille’s law for liquid flows, the diffusion coefficient of the liquid water, a function of fractal dimension, is obtained theoretically. The liquid flow affected by the surface tension and the gravity, the water vapor sorption/desorption by fibers, the diffusion of the water vapor and the phase changes are all taken into account in this model. With specification of initial and boundary conditions, distributions of water vapor concentration in void spaces, volume fraction of liquid water, distribution of water molecular content in fibers and temperature changes in porous fibrous media are obtained numerically. Effects of porosity of porous fibrous media on heat and mass transfer are analyzed. The theoretical predictions are compared with experimental data and good agreement is observed between the two, indicating that the fractal model is satisfactory.     

Drift flux model for large diameter pipe and new correlation for pool void fraction

A void fraction for a bubbling or boiling pool system is one of the important parameters in analyzing heat and mass transfer processes. Using the drift flux formulation, correlations for the pool void fraction have been developed in comparison with a large number of experimental data. It has been found that the drift velocity in a pool system depends upon vessel diameter, system pressure, gas flux and fluid physical properties. The results show that the relative velocity and void fraction can be quite different from those predicted by conventional correlations. In terms of the rise velocity, four different regimes are identified. These are bubbly, churn-turbulent, slug and cap bubble regimes. The present correlations are shown to agree with the experimental data over a wide range of parameters such as vessel diameter, system pressure, gas flux and physical properties.