An improved prediction model of vortex shedding noise from blades of fans

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (L_A), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.     

CFD for prediction of k-factors of duct fittings

The k-factors of duct fittings provided by CIBSE and ASHRAE are used extensively by HVAC system designers. Existing guides published by the two institutions do not include many of the duct fittings used in HVAC systems and they neglect to consider parameters, such as Reynolds numbers, which affect k-factors. More importantly, there are large errors in some of the data. This paper presents a study in which the computational fluid dynamics or CFD was applied to predict k-factors. Flow fields in duct fittings have been simulated by solving three-dimensional Navier-Stokes equations with the standard k-ε model. The finite volume approach was adopted and the SIMPLE algorithm has been used. It was found that the accuracy of k-factors is sensitively dependent on the presure sampling position, which may vary with many variables such as duct dimensions, type of fitting and flow speed. The predicted k-factors were compared with the CIBSE and ASHRAE data and although some close agreement were encountered for individual cases, in general there is no consistent agreement with either the CIBSE or the ASHRAE data set. It was shown that the knowledge of detailed flow patterns in individual duct fittings is crucial for accurate measurements of k-factors and should be acquired before the test is set up and carried out. Such an approach is important both for improving k-factor data accuracy and for providing a reliable basis for the development of CFD application.     

A CFD model of autothermal reforming

A numerical model based on computational fluid dynamics (CFD) was developed and validated to simulate the performance of a catalytic monolith reformer for the production of hydrogen that could be used in fuel cell systems. The whole reactor was modeled as porous media for the process of autothermal reforming with n-hexadecane feed. CFD results provided an adequate match to experimental data from literature with respect to temperature and the mole fractions of H₂, CO₂ and CO products. The percentage difference between each experimental measurement of the mole fraction of hydrogen and the corresponding CFD prediction was less than 16.8%. It was found that the thermal conductivity of the solid catalyst substrate affected the temperature profile in the reactor, but its effect on product hydrogen concentration was negligible. The calculated reforming efficiency based on hydrogen decreased by 11.8% as power input was increased from 1.7 to 8.4 kW.     

基于CFD分析的散热器结构优化

利用STAR—CCM＋软件对某款设计中的散热器进行了流动与传热性能的分析,分析结果显示,按空气来流方向为散热器芯体前端,空气侧的对流换热系数远远高于其后端。这表明散热器芯体前端的利用率较高。将散热器芯体变薄之后,分别计算空气来流速度为5m／s、10m／s和15m／s三工况下其单芯体散热量。在来流速度为15m／s的工况下修改后散热器的散热量为原散热器的75％左右,并且随着来流速度的降低,该数值逐渐的增大。这样在相同的散热需求时,新的结构可以较大的节约材料。     

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.     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

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。     

The Darrieus wind turbine: Proposal for a new performance prediction model based on CFD

This paper presents a CFD model for the evaluation of energy performance and aerodynamic forces acting on a straight-bladed vertical-axis Darrieus wind turbine. The basic principles which are currently applied to BE-M theory for rotor performance prediction are transferred to the CFD code, allowing the correlation between flow geometric characteristics (such as blade angles of attack) and dynamic quantities (such as rotor torque and blade tangential and normal forces). The model is proposed as a powerful design and optimization tool for the development of new rotor architectures for which test data is not available.After describing and validating the computational model against experimental data, a full campaign of simulation is proposed for a classical NACA 0021 three-bladed rotor.Flow field characteristics are investigated for several values of tip speed ratio, allowing a comparison among rotor operation at optimum and lower C_p values, so that a better understanding of vertical-axis wind turbines basic physics is obtained.     

Model for predicting performance of cooling fans for thermal design of electronic equipment (Modeling and evaluation of effects from electronic enclosure and inlet sizes)

This study describes the performance of cooling fans in terms of the P–Q curve and the maximum flow rate under various environmental conditions. It focuses on the relationship between fan performance and configuration factors such as the electronic enclosure. The presence of an enclosure wall increased the pressure characteristic of the fan performance. The presence of a narrow inlet decreased the flow rate. When the inlet area of the enclosure became smaller than twice the fan flow area, the flow rate was decreased. The maximum flow rate depended on the ratio of the inlet area to the fan flow area. A model for predicting pressure rise and flow rates in the enclosure is proposed. The model is used in a thermal analysis of a PCB model set in an enclosure. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20347     

基于CFD方法的柴油机气缸盖穴蚀改进

分析某船用柴油机气缸盖内部穴蚀导致的漏水的问题,得出气缸盖穴蚀的原因为死水区的存在不利于散热并加剧气泡的产生导致穴蚀。利用计算流体动力学(computational fluid dynamics,CFD)软件对气缸盖内部冷却水流道进行流体动力学仿真,根据仿真结果对柴油机的气缸盖内部流道改进,设计改进型的水封套管,消除死水区并改善气缸盖内冷却水的流动状态,解决气缸盖内部穴蚀的问题。     

A point model for the design of a sulfur trioxide decomposer for the SI cycle and comparison with a CFD model

Operating under the harsh environment with the significant energy consumption, the sulfur trioxide decomposer is one of the most important components, yet challenging tasks for the designers of an efficient SI cycle. We developed a point model to provide important guidelines for designers of a sulfur trioxide decomposer through estimating outlet physical quantities, such as outlet decomposition ratio, outlet temperature, and pressure drop of a sulfur trioxide decomposer. Then, results of the point model were compared to independent predictions obtained using a CFD model over a wide range of conditions with good agreement. The model indicates that decomposition ratio is a function of the representative non-dimensional design parameter and inlet flow composition. As inlet flow composition rarely affects outlet decomposition ratio, we found out that outlet decomposition ratio can be approximated solely as a function of the non-dimensional design parameter. We demonstrated that the model can provide general guidelines for designers of a sulfur trioxide decomposer to achieve a target decomposition ratio with an economical design. It turns out that an increase in operating pressure and catalyst surface area leads to an increase in outlet decomposition ratio while the reverse is true for an increase in mass flow rate.     

基于CFD的矿车空调风道风速仿真分析与试验研究

利用三维不可压缩流体的k一占湍流运动数学模型,以计算流体力学为（CFD）理论基础,在Fluent环境下对所设计的矿车空调风道系统气流的三维流动特性模型进行进、出口风速的模拟仿真分析,并进行试验验证。试验结果表明,模型建立正确,模拟仿真分析结论可靠,对缩短矿车空调风道系统设计周期、提高产品设计可靠性具有指导意义。     

K-factor data of interacting duct fittings based on measurement and CFD modelling

Energy use and the environment in mechanically-ventilated buildings are strongly influenced by the performance of heating, ventilation and air-conditioning (HVAC) systems, which is in turn governed by the accurate prediction of pressure loss. This paper presents the results of an investigation of pressure loss and associated loss coefficient (k-factor) for a number of interacting duct fittings in close proximity as commonly found in HVAC systems. The constant-injection tracer-gas technique and pitot-tube were used to measure the mean air velocity in ducts and the pressure distribution along the ducts was measured using static pressure tappings. k-Factors were calculated from the measured pressure loss and air velocity for each interacting duct fitting. Computational fluid dynamics (CFD) was used for prediction of air flow and pressure distribution in ducts. Predictions were compared with results obtained experimentally and CFD was found to be a useful method for the prediction of the k-factors of duct fittings. It will also be shown that duct fittings in close proximity interact in such a way that it is difficult to predict their behaviour based on evidence of the fittings' individual characteristics. The behaviour of interacting duct fittings as described in this investigation appears to be isolated to fittings of similar individual loss coefficients placed in close proximity without the effect of spacers. To establish this characteristic and fully to understand the complexity of interacting duct fittings there is an obvious need for further work in this area.     

Investigating the effect of crevice flow on internal combustion engines using a new simple crevice model implemented in a CFD code

A theoretical investigation is conducted to examine the way the crevice regions affect the mean cylinder pressure, the in-cylinder temperature, and the velocity field of internal combustion engines running at motoring conditions. For the calculation of the wall heat flux, a wall heat transfer formulation developed by the authors is used, while for the simulation of the crevices and the blow-by a newly developed simplified simulation model is presented herein. These sub-models are incorporated into an in-house Computational Fluid Dynamics (CFD) code. The main advantage of the new crevice model is that it can be applied in cases where no detailed information of the ring-pack configuration is available, which is important as this information is rarely known or may have been altered during the engine’s life. Thus, an adequate estimation of the blow-by effect on the cylinder pressure can be drawn. To validate the new model, the measured in-cylinder pressure traces of a diesel engine, located at the authors’ laboratory, running under motoring conditions at four engine speeds were used as reference, together with measured velocity profiles and turbulence data of a motored spark-ignition engine. Comparing the predicted and measured cylinder pressure traces of the diesel engine for all cases examined, it is observed that by incorporating the new crevice sub-model into the in-house CFD code, significant improvements on the predictive accuracy of the model is obtained. The calculated cylinder pressure traces almost coincide with the measured ones, thus avoiding the use of any calibration constants as would have been the case with the crevice effect omitted. Concerning the radial and swirl velocity profiles and the turbulent kinetic energy measured in the spark-ignition engine, the validation process revealed that the developed crevice model has a minor influence on the aforementioned parameters. The theoretical study has been extended by investigating in the same spark-ignition engine, during the induction and compression strokes, the way crevice flow affects the thermodynamic properties of the air trapped in the cylinder.     

Numerical model for the performance prediction of a PEM fuel cell. Model results and experimental validation

This work presents a Computational Fluid Dynamics (CFD) model developed for a 50 cm² fuel cell with parallel and serpentine flow field bipolar plates, and its validation against experimental measurements. The numerical CFD model was developed using the commercial ANSYS FLUENT software, and the results obtained were compared with the experimental results in order to perform a model validation. A single parameter, namely the reference exchange current density, was fitted to calibrate the model results. All other model parameters were determined from technical data sheets, literature survey, or experimental measurements. A discussion on different validation issues and model parameters is provided. The results of the numerical model show a good agreement with the experimental measurements for the different bipolar plates and range of operating conditions analysed. However, inaccuracies in the results in the mass-transport polarization region were observed, presumably when liquid water in the channels produces a blockage effect that cannot be modelled with the multiphase flow model currently implemented.     

A three-dimensional full-cell CFD model used to investigate the effects of different flow channel designs on PEMFC performance

A three-dimensional “full-cell” computational fluid dynamics (CFD) model is proposed in this paper to investigate the effects of different flow channel designs on the performance of proton exchange membrane fuel cells (PEMFC). The flow channel designs selected in this work include the parallel and serpentine flow channels, single-path and multi-path flow channels, and uniform depth and step-wise depth flow channels. This model is validated by the experiments conducted in the fuel cell center of Yuan Ze University, showing that the present model can investigate the characteristics of flow channel for the PEMFC and assist in the optima designs of flow channels. The effects of different flow channel designs on the PEMFC performance obtained by the model predictions agree well with those obtained by experiments. Based on the simulation results, which are also confirmed by the experimental data, the parallel flow channel with the step-wise depth design significantly promotes the PEMFC performance. However, the performance of PEMFC with the serpentine flow channel is insensitive to these different depth designs. In addition, the distribution characteristics of fuel gases and current density for the PEMFC with different flow channels can be also reasonably captured by the present model.     

CFD在水泵设计中的应用

利用三维CFD程序进行了一台单级轴流水泵动叶轮的流场计算,根据计算结果对该水泵的性能进行了分析,从而概括了利用CFD对水泵进行性能分析的方法,对于从事流体机械设计的工程技术人员有一定的参考价值。     

Validation of a CFD model for the simulation of heat transfer in a tubes-in-tank PCM storage unit

A computational fluid dynamics (CFD) model was developed for the simulation of a phase change thermal energy storage process in a 100 l cylindrical tank, horizontally placed. The model is validated with experimental data obtained for the same configuration. The cold storage unit was charged using water as the heat transfer medium, flowing inside a horizontal tube bundle, and the selected phase change material (PCM) was microencapsulated slurry in 45% w/w concentration. The mathematical model is based on the three-dimensional transient Navier–Stokes equations with nonlinear temperature dependent thermo-physical properties of the PCM during the phase change range. These properties were experimentally determined using analytical methods. The governing equations were solved using the ANSYS/FLUENT commercial software package. The mathematical model is validated with experimental data for three different flow rates of the heat transfer fluid during the charging process. Bulk temperature, heat transfer rate and amount of energy stored were used as performance indicators. It was found that the PCM bulk temperatures were predicted within 5% of the experimental data. The results have also shown that the total accumulated energy was within 10% of the observed value, and thus it can be concluded that the model predicts the heat transfer inside the storage system with good accuracy.     

基于公交车发动机舱的热管理系统CFD分析

针对某款设计中的公交车,利用Fluent对其热管理系铳进行了分析,结果显示该车在计算工况下冷却模块性能较差,发动机舱内局部区域空气温度已达100°以上。其主要原因包括高温空气的回流,上隔板对空气流动的阻挡和中冷器上气室对散热器的不良影响。结构改进后,冷却模块工作条件得到很大改善,满足设计要求。