Coupling of 3D Eulerian and Lagrangian Spray Approaches in Industrial Combustion Engine Simulations

The Lagrangian DDM （discrete droplet model） is state-of-the-art for CFD （computational fluid dynamics） simulations of mixture formation and combustion in industrial engines. A commonly known drawback of the DDM approach is the attenuated validity in the dense spray, where the bulk liquid disintegrates into droplets. There the assumption of single droplets surrounded by a homogenous gas field is not reasonable. In this region, the Eulerian-Eulerian multi-phase approach performs better because instead of parcels the spray is represented by the volume fractions of one bulk liquid and several droplet size class phases. A further drawback of the DDM approach is that increasing the spatial resolution of the computational grid leads to a reduced statistical convergence, since the number of spray parcels per computational cell becomes smaller. It is desirable to combine the benefits of both spray approaches in coupled CFD simulations. Therefore, the dense spray region is simulated separately with the Eulerian spray approach on a highly resolved mesh covering only the region close to the nozzle orifice. The entire engine domain with combustion and emission models is simulated with the Eulerian-Lagrangian spray approach for the dilute spray region. The two simulations are coupled through exchange of boundary conditions and model source terms. An on-line coupling interface manages the data transfer between the two simulation clients, i.e., Eulerian spray and engine client. The aim of this work is to extend the coupled spray approach in terms of exchanging combustion related heat and species sources, and consequently creating the link between Eulerian spray and combustion models. The results show mixture formation and combustion in real-case engine simulations, and demonstrate the feasibility of spray model combination in engineering applications.     

Design performance and economics of a low-cost solar wax melter for the candle industry

Solar thermal energy is a viable energy option for low-temperature applications (< 100 °C), especially in developing countries, where at present fossil fuels (coal and petroleum products) and firewood are inefficiently being used for such applications. The candle-making industry requires solid wax to be melted at about 62 °C. Flat-plate solar thermal collectors can be advantageously used for this purpose. This article describes the conventional method of melting wax and the solar alternative proposed. Experiments conducted using the fabricated solar wax melter of collector area 0·64 m² shows that about 13–15 kg of solid wax could be melted per day. An economic analysis done to compare the cost of the solar wax melter with that of the conventional methods of melting wax indicates the economic viability of the proposed system. It has also been shown how such renewable energy devices could be used for generating gainful employment in rural areas owing to its simple technology, easy use and economics.     

A droplet size dependent multiphase mixture model for two phase flow in PEMFCs

A droplet size dependent multiphase mixture model is developed in this paper, and the droplet size in the gas channel can be considered as a parameter in this multiphase mixture model, which includes the effect of gas diffusion layer (GDL) properties and the gas drag function and cannot be considered in the commonly used multiphase mixture model in the references. The three-dimensional two phase and non-isothermal simulation of the PEMFCs with a straight flow field is performed. The effect of droplet size on the liquid remove, the effect of liquid water on the heat transfer and the effect of gas flow pattern on the heat and mass transfer are mainly investigated. The simulation results show that the large droplet is hard to be dragged by the gas, so it produces large water saturation. The results of the heat transfer show that the liquid water hinders the heat transfer in the GDL and catalyst layer, so it produces the large relative high temperature area, and there are large temperature difference and water saturation in the PEMFCs operated with coflow pattern compared with counter flow pattern.     

采用欧拉和拉格朗日混合模型对浓相颗粒流的研究

论述了一种组合了欧拉／欧拉和欧拉／拉格朗日方法的研究浓相气固流的新数值模拟。模型用基于Chapman-Enskog浓相气体理论的微元流体动理学方法计及了欧拉坐标系中的颗粒间相互作用。该模型在计算拉格朗日坐标系中颗粒湍流扩散时采取颗粒随机分布模型。用该模型得出的计算结果与已发表的实验结果进行了比较,能较好地吻合。图7参12     

Particle transport in turbulent flow using both Lagrangian and Eulerian formulations

Lagrangian and Eulerian models for particle transport by a turbulent fluid phase are presented. In both methods, particle distribution results from the action of applied forces (buoyancy, inertial, added mass and drag forces) and turbulent effects are shown. The carrier phase flow – which is solved by finite element method using a k–ε turbulence model – is assumed not to depend on the particles' motion. In the Lagrangian formulation the dynamic equation for the particles is solved. A discrete random walk model is used to account for the turbulent effects. In the Eulerian formulation, the particle concentration is calculated from a convection–diffusion equation using the terminal particles' velocity and turbulent diffusivity. Both models are compared to experimental measurements and analytical results; a good agreement is observed.     

A numerical investigation of two-phase steam flow around a 2-D turbine's rotor tip

In flow of steam during the divergence nozzles, expansion and decreasing the enthalpy, brings the flow near the saturation conditions. After supercooling, nucleation forms in the flow and the second phase appears. This phenomenon occurs specially during the last stages of steam turbines as low-pressure case and nuclear reactors as high-pressure. In this research, a numerical scheme for transonic two-phase flow within the passages of 2-D rotor-tip section with various backpressures is applied and an Eulerian–Eulerian reference frame is employed for both phases. A classical homogenous nucleation model applied for the mass transfer in the transonic conditions. Five deferent cases have been tested and through the results, pressure profiles around the blades are compared with the experimental data and good agreement is observed. Results show that the most condensation is on the suction surface of blade and it grows by decreasing the downstream pressure.     

Modeling and simulation of a simple glass window

This paper presents a mathematical model with numerical simulations of the heat transfer across a simple glass window. The model is two-dimensional, transient based upon the energy equation with a source term to account for the solar radiation absorbed through the glass sheet. Variable incident solar radiation and external ambient temperature are considered in the numerical simulations. The governing equations and the associated boundary conditions are discretized by the finite difference approach and the ADI scheme. Numerical simulations are realized for the cases of clear and absorbing glass to show the effect of the glass thickness on the total heat gain, the solar heat gain and the shading coefficient.     

Direct numerical simulation of interphase mass transfer in gas-liquid multiphase systems

This paper presents the Direct Numerical Simulation of interphase mass transfer in gas liquid multiphase system. The volume-of-fluid (VOF) method in conjunction with mass transfer model has been used. In order to study the process of interphase mass transfer two numerical simulation methods are presented. Two common mass transfer mechanisms, Diffusion through Stagnant Film (DTSF) and Equi-Molal Counter Diffusion (EMCD), are investigated. Two benchmarks, the Stefan diffusion problem and the diffusion in water and methanol Gas-Liquid system, have been used to validate the numerical methods. Afterwards two proposed numerical solution for different mass transfer mechanisms have been investigated in stratified gas liquid flows between two parallel plates. The results show by different approaches in numerical solution, the accuracy of mass transfer simulation is different.     

Numerical study of two-phase flow patterns in the gas channel of PEM fuel cells with tapered flow field design

In this study the air–water two-phase flow in a tapered channel of a PEMFC was numerically simulated using the volume of fluid (VOF) method. In particular, a 3D mathematical model of the fuel cell flow channel was used to obtain a reliable evaluation of the fuel cell performance for different taper angles and different temperatures and to calculate the total amount of water produced. This information was then used as boundary conditions to simulate the two-phase flow in the cell channel through a 2D VOF model. Typical operating conditions were assigned and the numerical mesh was constructed to represent the real fuel cell configuration. The results show that tapering the channel downstream enhances the water removal due to increased airflow velocity. In the rectangular channel no film formation is noted with a marked predominance of slug flow. In contrast, as the taper angle is increased the predominant two-phase flow pattern is film flow. Finally many contact angles have been used to simulate the effect of the hydrophobicity of a GDL surface on the motion of the water. As the hydrophobicity of a GDL surface is decreased the presence of film is more evident even for less tapered channels.     

Numerical study of cavitating flow characteristics of liquid helium in a pipe

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium in a vertical pipe near the lambda point are numerically investigated to realize the further development and high performance of new cryogenic superfluid cooling systems. It is found that the phase transition of the normal fluid to the superfluid and the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid to gas phase change with cavitation actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.     

Update on numerical model for the performance prediction of a PEM Fuel Cell

A Computational Fluid Dynamics (CFD) model developed for a 50 cm² Fuel Cell with parallel and serpentine flow field bipolar plates was presented in an article published in the International Journal of Hydrogen Energy 35 (2010) 11,533-11,550 [1]. The experimental validation details were presented as well in an article published in the International Journal of Hydrogen Energy 35 (2010) 11,437-11,447 [2]. A good agreement between numerical results and experimental measurements were obtained except for the high current density region where mass-transport limitations dominate the voltage loss. This short communication presents an update on the last simulations performed, where an improved prediction of the polarization curve is obtained. The physical and computational aspects of the reasons underlying the improvement of the results are discussed.     

石油管道插入构件流场的CFD模拟及冲蚀预测

采用三维雷诺平均守恒Nayier-Stokes方程及标准k-ε方程湍流模型，对管道插入构件的流动影响及冲蚀作用进行了流体动力学(CFD)模拟。主要通过对插入物不同的构件管径、插入深度、斜度、安装位置和工况等的数值模拟结果进行比较分析，获得了关于管道插入构件附近的具体流动状况及其对管道冲蚀的实际影响。     

Large eddy simulation of hydrogen combustion in supersonic flows using an Eulerian stochastic fields method

An Eulerian Monte-Carlo approach, the so-called Eulerian stochastic fields (ESF) method is implemented and evaluated for simulation of non-premixed hydrogen/air combustion in supersonic flows. The ESF method is integrated into a compressible flow large eddy simulation (LES) solver, and validated on a supersonic combustor with a strut as flame-holder. Comparison with experimental data and with results from a well-stirred reactor (WSR) model demonstrates the advantage of the LES-ESF method for simulation of local-extinction and re-ignition phenomena. The hydrogen/air flame structure and the stabilization of the combustion process in the supersonic combustor are analysed based on the present LES-ESF method. Oscillation of the recirculation zones is found to be the dominant mechanism for the local-extinction/re-ignition and the flame stabilization under the present condition.     

One-dimensional drift-flux model for two-phase flow in a large diameter pipe

In view of the practical importance of the drift-flux model for two-phase-flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for vertical upward two-phase flow in a large diameter pipe. One of the important flow characteristics in a large diameter pipe is a liquid recirculation induced at low mixture volumetric flux. Since the liquid recirculation may affect the liquid velocity profile and promote the formation of cap or slug bubbles, the distribution parameter and the drift velocity in a large diameter pipe can be quite different from those in a small diameter pipe where the liquid recirculation may not be significant. A flow regime at a test section inlet may also affect the liquid recirculation pattern, resulting in the inlet-flow-regime dependent distribution parameter and drift velocity. Based on the above detailed discussions, two types of inlet-flow-regime dependent drift-flux correlations have been developed for two-phase flow in a large diameter pipe at low mixture volumetric flux. A comparison of the newly developed correlations with various data at low mixture volumetric flux shows a satisfactory agreement. As the drift-flux correlations in a large diameter pipe at high mixture volumetric flux, the drift-flux correlations developed by Kataoka-Ishii, and Ishii have been recommended for cap bubbly flow, and churn and annular flows, respectively, based on the comparisons of the correlations with existing experimental data.     

Measurement and numerical simulation of a small centrifugal compressor characteristics at small or negative flow rate

For centrifugal compressors used in automotive turbochargers,the extension of the surge margin is demanded because of lower engine speed.In order to estimate the surge line exactly,it is required to acquire the compressor characteristics at small or negative flow rate.In this paper,measurement and numerical simulation of the characteristics at small or negative flow rate are carried out.In the measurement,an experimental facility with a valve immediately downstream of the compressor is used to suppress the surge.In the numerical work,a new boundary condition that specifies mass flow rate at the outlet boundary is used to simulate the characteristics around the zero flow rate region.Furthermore,flow field analyses at small or negative flow rate are performed with the numerical results.The separated and re-circulated flow fields are investigated by visualization to identify the origin of losses.     

Experiment and computational fluid dynamics simulation of in-depth system hydrodynamics in dual-bed gasifier

Dual-bed gasifier is a new gasifier system with separated combustion and gasification zones. The two-zone separation makes it possible to increase calorific value of the producer gas. In order to develop and improve the process operation, understanding of system dynamics and parameters that describe the in-depth hydrodynamics are essential. Computational fluid dynamics is a tool that can be used to explain the complex multiphase system behavior. The considered dual-bed gasifier had 3.00 m height and the maximum width diameters of riser and downcomer were 0.14 and 0.40 m, respectively. Conservation equations of mass, momentum, energy and species for each phase were solved coupling with the kinetic theory of granular flow using ANSYS FLUENT version 12.1. Here, two-dimensional simulation had been successfully determined the flow pattern and chemical reaction corresponding with actual experimental and theoretical data. The calculated results of the solid volume fraction in the riser section showed the bubbling and slugging flow patterns. The product gas composition and gas temperature inside dual-bed gasifer reflected the advantages for this type of reactor over the other conventional gasifiers. The system turbulences were firstly explored in dual-bed system which were normal Reynolds stresses and granular temperatures. For the effect of interphase exchange coefficient model, the pressure-loop using drag force model proposed by Gidaspow was in good agreement with the experiment than the ones proposed by Wen-Yu and Syamlal-O'Brien.     

Numerical simulation of air flow field in high-pressure fan with splitter blades

For a deeper understanding of the flow characteristics in the high-pressure centrifugal blower of a fan of Model 9–26 with splitter blades, a three dimensional (3-D) numerical simulation of air flows in the fan was conducted with FLUENT software. The standard k-ε turbulent model and unstructured grids were used. The computational fluid dynamics (CFD) results showed that the performance of a fan could be improved by adding the splitter blades in the channel among the leaf blades. Under operational conditions, with the presence of splitter blades, the air flow rate of the fan increased about 5% and the total pressure at the outlet of the fan increased about 10% on average. It was also found that the length of the splitter blades affected the air flow and pressure drop. There is an optimal value for the length. The simulation results provide helpful information for improving the fan performance. __________ Translated from Fluid Machinery, 2007, 35(10): 29–32 [译自:流体机械]     

Drift-flux model for downward two-phase flow

In view of the practical importance of the drift-flux model for two-phase flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for downward two-phase flows. The constitutive equation that specifies the distribution parameter in the downward flow has been derived by taking into account the effect of the downward mixture volumetric flux on the phase distribution. It was assumed that the constitutive equation for the drift velocity developed by Ishii for a vertical upward churn-turbulent flow determined the drift velocity for the downward flow over all of flow regimes. To evaluate the drift-flux model with newly developed constitutive equations, area-averaged void fraction measurement has been extensively performed by employing an impedance void meter for an adiabatic vertical co-current downward air-water two-phase flow in 25.4-mm and 50.8-mm inner diameter round tubes. The newly developed drift-flux model has been validated by 462 data sets obtained in the present study and literatures under various experimental conditions. These data sets cover extensive experimental conditions such as flow system (air-water and steam-water), channel diameter (16-102.3 mm), pressure (0.1-1.5 MPa), and mixture volumetric flux (−0.45 to −24.6 m/s). An excellent agreement has been obtained between the newly developed drift-flux model and the data within an average relative deviation of ±15.4%.     

CFD数值模拟在660MW燃煤发电厂烟气脱硝工程中的应用

结合华东地区某电厂2×660 MW机组SCR烟气脱硝工程,通过CFD数值模拟,对脱硝系统烟道中的均流装置进行优化设计,使喷氨格栅及催化剂层入口烟气流场达到了设计要求,系统阻力降也控制在设计范围之内。     

Pore-scale numerical study of multiphase reactive transport processes in cathode catalyst layers of proton exchange membrane fuel cells

understanding interactions between multiphase flow and reactive transport processes in catalyst layers (CL) of proton exchange membrane fuel cells is crucial for obtaining better performance and lower cost. In this study, a pore-scale model is developed to simulate coupled processes occurring in CLs, including oxygen diffusion, electrochemical reaction, and air-liquid two phase flow. Simulation conducted in an idealized local CL structures shows that the pore-scale model successfully captures dynamic behaviors of liquid water including generation, growth and subsequent migration, as well as the interaction between multiphase flow and reactive transport. Pore-scale simulation is then conducted in hydrophobic CLs with complicated structures where carbon, platinum, ionomer and pores are resolved. It is found that filling modes of the liquid water in the CLs are different. Before forming the continuous flow paths in CLs, liquid water presents as tiny droplets in pores surrounding relative large pores. After the continuous flow paths are formed, liquid water dynamic behaviors follow the capillary fingering mechanism. The multiphase flow and reactive transport processes are closely coupled with each other, and as liquid water saturation increases the reaction rate decreases. Increasing the hydrophobicity can alleviate the water flooding, accelerate the water breakthrough, and facilitate the water evaporation.