Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Modeling the gas and particle flow inside cyclone separators

This paper reviews the models developed for the flow field inside inverse-flow cyclone separators. In a first part, traditional algebraic models and their foundations are summarized in a unified manner, including the formulae for tangential velocity and pressure drop. The immediate application to the prediction of collection efficiency is also reviewed. The approach is the classical, treating first the dilute limit (clean-gas correlations), and afterwards correcting for “mass loading” effects. Although all these methods have had a remarkable success, more advanced ideas are needed to model cyclones. This is put forward by exploring the work done on the so-called “natural” length of the cyclone, that has led to the discovery of instability and secondary flows. The resort to computational fluid dynamics (CFD) in this case is difficult, however, due to the very nature of the flow structure. A closing section on the subject reviews past and recent CFD simulations of cyclones, both single- and two-phase, steady and unsteady, aiming at delineating the state-of-the-art, present limitations and perspectives of this field of research.     

Application of CFD to closed-wet cooling towers

Computational fluid dynamics (CFD) is applied to predicting the performance of closed-wet cooling towers (CWCTs) for chilled ceilings according to the cooling capacity and pressure loss. The prediction involves the two-phase flow of gas and water droplets. The predicted thermal performance is compared with experimental measurement for a large industrial CWCT and a small prototype cooling tower. CFD is then applied to the design of a new cooling tower for field testing. The accuracy of CFD modelling of the pressure loss for fluid flow over the heat exchanger is assessed for a range of flow velocities applied in CWCTs. The predicted pressure loss for single-phase flow of air over the heat exchanger is in good agreement with the empirical equation for tube bundles. CFD can be used to assess the effect of flow interference on the fluid distribution and pressure loss of single- and multi-phase flow over the heat exchanger.     

Dimensional analysis of thermo-fluid-dynamics of high hydrostatic pressure processes with phase transition

In the current paper the dimensional analysis of thermo-fluid-dynamic mechanisms taking place during high pressure treatment of bio-substances is carried out. Both forced and free convection case is described. Additionally, the phase change in the pressurized medium is considered. The significance of several terms in the conservation equation of momentum and energy is estimated. Especially in systems with low velocity of fluid some terms in the governing equations can be neglected, e.g. irreversible transformation of kinetic in thermal energy. The dimensionless numbers/groups that describe the fluid flow and phase transition at high hydrostatic pressure are determined.     

Systematic assessment of the anode flow field hydrodynamics in a new circular PEM water electrolyser

In this work, we investigated the key underlying flow characteristics of a circular unit cell proton exchange membrane (PEM) water electrolyser. In particular, we focused on investigating anode flow field design using computational fluid dynamics (CFD) tool. Transient, 3D single phase fluid flow simulation results were presented, and in-house experiments were conducted for validation against CFD simulation data identifying key performance parameters of the PEM water electrolyser: uniform water distribution, pressure drop and hydraulic retention time. The effects of the water flow rate, inlet and outlet sizing and different number of inlet and outlet configurations were considered. The main observation from the study was discussed to provide insight into the factors affecting the flow pattern. Among the studied flow field design cases, it was found that the average pressure drop decreased with increase in number of inlets, also flow profile can be grouped into different set, depending on number of inlets. The correlation between pressure drop and mean velocity profile for different inlet and outlet configurations provides a useful basis to properly design the high performance PEM water electrolyser.     

Analysis of influence of physical parameters on vapor-liquid flow behavior up to dryout in a heat-generating porous medium

In the present work the influence of various physical characteristics on the two-phase flow behavior in a self-heated porous medium has been studied using a numerical model, that is, the effects of heat generation rate, of porosity, of particle size, and of system pressure on the dryout process. To analyze the effect of these characteristics, the variation of both liquid volumetric fraction and liquid axial velocity is evaluated at the steady state or at the onset of a first boiled-out region. The analysis of computational results indicate that a qualitative tendency exists between the characteristics such as heat generation rate, porosity, effective particle diameter and the temporal development of the liquid volumetric fraction field up to dryout. In addition to these characteristics, a variation of fluid properties such as phase density, phase viscosity due to a change of system pressure can be used for gaining insight into the nature of two-phase flow behavior up to dryout.     

CFD simulations of hydrodynamic characteristics in a gas–liquid vertical upward slug flow

Computational fluid dynamics (CFD) simulations are conducted using the volume-of-fluid (VOF) method to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO₂ corrosion. The hydrodynamic characteristics are significantly affected by the viscous, interfacial, and inertial forces. In inertia dominated flows, the velocity of fully developed falling liquid film is increased with increased Taylor bubble rising velocity. The developing falling liquid film is formed at about the length of 0.5 diameter from the Taylor bubble nose, the fully developed falling liquid film is reached at about the length of 1.5–2.1 diameter from the Taylor bubble nose. The average mass transfer coefficient in the falling liquid film is always higher than that in the Taylor bubble wake zone. The iron ion near wall mass transfer coefficient is higher than that of hydrogen ion. The wall shear stress is increased with increased Taylor bubble rising velocity in fully developed falling liquid film zone, and the wall shear stress has a large fluctuation due to the chaotic and turbulent vortexes in Taylor bubble wake zone. The formation and the damage mechanism of the corrosion product scale are proposed for the gas–liquid two-phase vertical upward slug flow induced CO₂ corrosion. It is found that the wall shear stress of upward gas–liquid slug flow is alternate with high frequency, which is the key factor resulting in the corrosion product scale fatigue cracking. The CFD simulation results are in satisfactory agreement with previous experimental data and models available in literature.     

Determination of the void fraction and drift velocity in a two-phase flow with a boiling solar collector

This paper presents an approach to determine the void fraction and the drift velocity in a two-phase flow with a boiling solar collector using easily obtained experimental data. The solar collector operates in a thermal siphon circuit, where the working fluid absorbs solar radiation mostly while boiling. The vapor bubbles release their latent heat in a condenser, while heating up a flow of water–glycol. Two numerical procedures are developed to calculate the void fraction because its experimental values cannot be easily measured. The use of a flow meter causes an additional pressure drop in the thermal siphon circuit and, consequently, changes the circulated mass flow rate. The first numerical procedure is based on a force balance in the thermal siphon loop and is used to estimate the total mass flow rate and the void fraction in the circuit. The second uses a drift flux correlation to estimate the void fraction and the drift velocity. Both procedures use the experimental values for the vapor mass flow rate, which is determined by an energy balance in the condenser. The volumetric flow rate of the water–glycol mixture and its temperature difference across the condenser are experimentally measured. The pipe length of the two-phase flow in the solar collector is experimentally determined using 44 thermocouples attached to the back of flow channels in the absorber plate. The results show that the two numerical models compare well and that either one can be used to estimate the void fraction in the two-phase flow solar circuit.     

Numerical study on the behavior and design of a novel multistage hydrogen pressure-reducing valve

Applying hydrogen fuel-cell vehicles (HFCVs) is feasible to achieve net zero carbon emission in transportation sector. The energy density requirements of these vehicles are fulfilled via high-pressure gaseous hydrogen storage; therefore, an effective pressure-reducing system is necessary. In this work, a novel multistage pressure-reducing valve (named as T–M valve) combining a sleeve pressure structure valve and a Tesla-type orifice valve is proposed. A computational fluid dynamics (CFD) model is developed to analyze the influence of operating parameters on pressure and velocity distributions. Results show that the large pressure and velocity gradients’ region is concentrated on the throttling elements. The valve opening and pressure ratio significantly affect energy consumption. In addition, the Mach number in the valve less than one is proposed. This study is conducive to further energy conservation and emission reduction and the research of multistage flow pressure-reducing devices.     

An investigation of two-phase flow pressure drop in helical rectangular channel

Numerical simulations have been carried out to evaluate the two-phase frictional pressure drop for air-water two-phase flow in horizontal helical rectangular channels by varying configurations, inlet velocity and inlet sectional liquid holdup. The investigations performed using eight coils, five different inlet velocity and four different inlet sectional liquid holdups. The effects of curvature, torsion, fluid velocity and inlet sectional liquid holdup on two-phase frictional pressure drop have been illustrated. It is found that the two-phase frictional pressure drop relates strongly to the superficial velocities of air or water, and that the curvature and torsion have some effect on the pressure drop for higher Reynolds number flows in large-scale helical rectangular channel; the inlet sectional liquid holdup only increases the magnitude of pressure drop in helical channel and has no effect on the development of pressure drop. The correlation developed predicts the two-phase frictional pressure drop in helical rectangular channel with acceptable statistical accuracy.     

Computational Fluid Dynamics–Population Balance Modeling of Gas–Liquid Two-Phase Flow in Bubble Column Reactors With an Improved Breakup Kernel Accounting for Bubble Shape Variations

Abstract

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model.     

考虑两相流音速时气固两相流激波研究

1引言超音速冷喷涂(CSC),又称为冷空气动力学喷涂(CGDS),是一种新型的喷涂表面沉积的方法[1~3]。在超音速冷喷涂中,喷涂粉末与气体混合后,在缩放流道内形成超音速流动,与基板碰撞后喷涂粒子在基板上沉积。由于两相介质是超音速流动,此时气固两相流激波起着至关重要的作用。气固     

Analysis of heat transfer with liquid-vapor phase change in a forced-flow fluid moving through porous media

This study focuses on the experimental analysis of transient-regime heat transfer with liquidvapor phase change in a fluid as it flows through a porous media composed of small bronze spheres. Three distinct zones can be observed: liquid, two-phase and superheated vapor. The boundaries between these zones are determined using temperature and pressure fields. An N-shaped profile is observed for the temperature values along the main flow axis. The first local maximum value on the temperature curve corresponds to the boundary between the liquid zone and the two-phase zone. When a local minimum temperature exists, it corresponds to the boundary between the two-phase and the vapor zones. A finite element numerical simulation is used to predict the saturation field, which is numerically determined from the boundaries of the two-phase zone and of the experimental temperature field. The liquid and vapor pressure fields are then deduced for all three phase zones of the porous medium.     

Development of a Multi-Dimensional Fluid Dynamics Code and Its Benchmarking for the Subcooled Boiling Flow

In a two-phase flow analysis, the interfacial area concentration (IAC) is a dominant factor governing the interfacial transfer of the momentum or energy. For a dynamic analysis with the implementation of IAC transport equation, a multi-dimensional computational fluid dynamics code was developed. The code is based on the two-fluid model and the simplified marker and cell algorithm by using the finite volume method, where the conventional approach for a single-phase flow has been modified in order to consider the term for a phase change. As benchmark problems of a single-phase flow and two-phase flow, a natural convection in a rectangular cavity and a subcooled boiling in an annulus channel were selected, respectively. In the calculation for the single-phase flow, the developed code predicted a reasonable behavior for a buoyancy-driven flow depending on the Rayleigh number. In the analysis of the subcooled boiling, the calculation results showed the robustness of code for the analysis of the boiling phenomena and void propagation, where they represented limitations of the one-dimensional IAC model. To conduct a multi-dimensional analysis for the two-phase flow, it is confirmed that the implementation of an IAC transport equation into the code is essential.     

高炉鼓风脱湿系统除雾器挡板结构两相流场数值模拟

对高炉鼓风脱湿系统除雾器的除雾机理进行了详细分析,在此基础上参考实际运行参数,采用计算流体力学(CFD)方法对除雾器挡板结构内的气液两相流动进行了数值模拟,得到了液滴的运动轨迹以及液滴质量浓度、压力、速度和旋涡分布情况,并对除雾器挡板结构内部两相流场进行了深入分析.结果表明:鼓风脱湿系统除雾器能够对携雾气流中的液滴进行有效的分离,从而保证鼓风脱湿系统连续可靠地运行.研究对除雾器的优化设计和运行具有指导意义.     

A two-dimensional model of the kettle reboiler shell side thermal-hydraulics

A two-dimensional two-fluid numerical model is developed for the prediction of two-phase flow thermal-hydraulics on the shell side of the kettle reboiler. The two-phase flow around tubes in the bundle is modeled with the porous media approach. A closure law for the vapour–liquid interfacial friction is based on modified pipe two-phase flow correlations. The tube bundle flow resistance is calculated by applying to each phase stream the correlations for the pressure drop in a single phase flow across tube bundles and by taking into account the separate contribution of each phase to the total pressure drop. Physically based boundary conditions for the velocity field at the two-phase mixture swell level are stated. The system of governing equations is solved numerically with the finite volume approach for two-phase flow built in the commercial computer program. Simulations are performed for available conditions of performed physical experiments. In comparison to the previous kettle reboiler two-dimensional modeling approaches, here presented model is original regarding the applied closure laws for the interfacial friction and bundle flow resistance, as well as applied boundary conditions for the modeling of two-phase mixture free surface. Also, regarding the previous published results, here obtained numerical results are compared with the available measured data of void fraction within the tube bundle and acceptable agreement is shown.     

冷热房间之间隔门通风性能的数值分析

应用数值模拟方法,对冷热房间之间隔门通风情况进行了分析。三维模拟区域主要运用稳态湍流和Boussinesq模型进行模拟,并着重对不同隔门高度和外部条件下流动换热情况作了对比研究。研究表明,两个墙壁间的温差及其引起的密度差是形成温度和速度分布的主要原因。隔门使两个房间之间形成对流,右侧高温房间下部有冷气流进入,而左侧低温房间有热气流由上方进入。当隔门高度降低时,零压差线的相对高度不断升高,对流效应减弱。     

Assessment of heat transfer and flow characteristics of a two-phase closed thermosiphon

In this study, comprehensive modeling and simulations were developed and carried out to perform the investigation of the thermal performance of the enclosed thermosiphon through pool boiling in the evaporator sector and the condensation of the liquid film in the condenser part. To simulate these phenomena, the volume of fluid model was utilized. The simulation modeling using the computational fluid dynamics (CFD) technique was validated with existing experimental results, and a good agreement was reached. The simulation results were presented and evaluated in terms of temperature profiles and contours, the volume of fraction contours, and velocity vector distribution. Moreover, the thermal performance (ie, the heat transfer coefficient and thermal resistance) through the thermosiphon operation was analyzed. From the simulation results, it is found that the thermosiphon performance can be improved by the tilt angle and fill ratio. The results indicated that the optimal performance (ie, a high heat transfer coefficient and a low thermal resistance) was attained at a power input of 250 W, tilt angle of 90°, and fill ratio of 0.5. The established CFD simulations effectively predicted the formation of two-phase flow pattern and boiling and condensation zones with water at a low power input, termed as geyser boiling.     

Analytical solution for Newtonian–Bingham plastic two-phase pressure driven stratified flow through the circular ducts

For steady state, stratified, laminar, fully developed two-phase flow which one of them is Newtonian and the other one is Bingham plastic, the motion equations in horizontal pipe with appropriate boundary conditions have been solved analytically. Pressure drop, velocity distribution and location of plug region related to Bingham plastic fluid have been reported. The results show that the non-Newtonian rheological properties have negligible effects on two-phase velocity profile and consequently on pressure gradient in small viscosity ratio of two fluids. With promotion of viscosity ratio, the influence of yield stress on two-phase velocity profile is more considerable.     

CFD and ERT investigations on two-phase flow regimes in vertical and horizontal tubes

In the present study, different two-phase flow regimes in horizontal and vertical tubes have been studied experimentally and theoretically. A 3-D Computational Fluid Dynamics (CFD) modeling was carried out in order to model gas–liquid two-phase flow using volume of fluid (VOF) model. An Electrical Resistance Tomography (ERT) system was used to visualize these flow regimes, which were produced by change in the gas to liquid flow rate ratio. The reconstructed images from the ERT measurement and corresponding captured photographs for different flow regimes have been compared with the CFD predictions and a good qualitative agreement was observed between them.