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.     

基于CFD数值模拟下的蒸汽喷射器性能预测

建立了蒸汽喷射器热力学模型,并验证了该模型性能及结构计算的可靠性。并针对实际情况中,对该模型进行了动态模拟。结果显示：当工作流体的温度升高时,工质流体的质量流量都会增加,引射系数存在峰值,峰值所对应温度为热力学模型设定温度值;当引射流体的温度升高时,其质量流量也会随之增大,而工作流体的质量流量则较稳定,因此引射流体温度与压力的升高可以改善喷射器的性能;当背压升高时,在一定压力范围内,工质流体的质量流量都趋于稳定,而当背压超过热力学模型设定背压值时,引射流体的质量流量便随背压的升高而急剧下降,喷射器性能严重恶化,故认为该压力值为喷射器的临界背压。本文研究结果对喷射器的设计计算具有一定的指导作用。 关键词：蒸汽喷射器;热力学模型;数值模拟;引射系数     

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。     

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     

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.     

基于CFD的悬臂式多级离心泵径向力与轴向力研究

为提高悬臂式多级离心泵的运行稳定性,以某不锈钢悬臂式多级离心泵为例,采用Pro/E与ICEM软件完成全流场三维模型与网格划分,在ANSYS CFX中选取RNGκ-ε湍流模型方法,研究了以全流场非定常数值计算为基础提取得到的计算模型在多个典型工况下作用在叶轮上的径向力和轴向力。发现外特性曲线的模拟值和试验值吻合良好,说明径向力与轴向力的数值预测规律能够反映泵运转状态下的实际变化规律。分析结果表明,各级叶轮在一个单旋转周期内所受径向力较小,且分布不均匀;随着流量的增大,各级叶轮所受径向力均呈现先减小后增大的趋势,在设计点工况达到最小值;各级叶轮所受到的轴向力随流量增大而逐渐减小,首末两级叶轮较大,第二与第三级叶轮较小;总轴向力大小与流量呈现非线性关系,流量越大轴向力减小越快;随着流量的增加,单个周期内的总轴向力出现脉动频率等于叶轮和导叶叶片数最小公倍数的周期性现象。     

Validation of a 3D multiphase-multicomponent CFD model for accidental liquid and gaseous hydrogen releases

As hydrogen-air mixtures are flammable in a wide range of concentrations and the minimum ignition energy is low compared to hydrocarbon fuels, the safe handling of hydrogen is of utmost importance. Additional hazards may arise with the accidental spill of liquid hydrogen. Such a release of LH2 leads to a formation of a cryogenic pool, a dynamic vaporization process, and consequently a dispersion of gaseous hydrogen into the environment. Several LH2 release experiments as well as modeling approaches address this phenomenology. In contrast to existing approaches a new CFD model capable of simulating liquid and gaseous distribution was developed at Forschungszentrum Jülich. It is validated against existing experiments and yields no substantial lacks in the physical model and reveals a qualitatively consistent prediction. Nevertheless, the deviation between experiment and simulation raises questions on the completeness of the database, in particular with regard to the boundary conditions and available measurements.     

Performance evaluation of PdAg membrane reactor in glycerol steam reforming process: Development of the CFD model

In the present paper, a CFD modeling of palladium membrane reactor, in which hydrogen produced through glycerol steam reforming, is presented. A comprehensive and precise kinetic and permeation model was used. On the basis of the equations and assumptions, an excellent agreement between model prediction and experimental data was achieved. Pressure, velocity and concentration distribution of various component within the Membrane Reactor (MR) were predicted. Moreover, the performance of both a Traditional Reactor (TR) and a MR was compared in various condition. The effects of some operating conditions such as temperature, pressure, feed flow rate and flow pattern on the glycerol conversion, hydrogen recovery and CO selectivity were evaluated. The most effective parameter was pressure: increasing it from 1 to 10 bar in co-current MR, the glycerol conversion, H2 recovery and CO selectivity were shifted from 46%, 17% and 6.6%–81%, 56% and 0.8%, respectively. The CFD model indicates that the performance of glycerol steam reforming improves when MR is used instead of TR. At various operating conditions the glycerol convertion enhanced 10–64% and CO selectivity reduced 7.5–99.0% in the MR when compared with the TR.     

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.     

Thermal design and CFD analysis of the radial inflow turbine for a CO2-based mixture transcritical Rankine cycle

Transcritical carbon dioxide (CO₂) Rankine cycle has exhibited great potential in the field of low-temperature heat utilization. But its application is restricted by the condensing issue and the safety concern due to the relatively low critical temperature and high critical pressure of CO₂. Blending CO₂ with organic fluids for the transcritical Rankine cycle is regarded as an effective method to solve these problems. And the turbine performance has great influence on the performance of transcritical Rankine cycle. In this paper, the thermal design of the CO₂-based mixture turbine is firstly carried out based on the parametric optimization of the system. Then the computational fluid dynamics (CFD) analysis is performed to examine the turbine performance and validate the reliability of thermal design. Furthermore, the effects of blade tip clearance and nozzle-to-rotor clearance on the turbine performance are investigated. Results show that the turbine is well designed with an isentropic efficiency of 84.54%, and the CFD simulation results basically agree with the thermal design results. The influence of leakage flow on mainstream grows significantly as the blade tip clearance increases. When the blade tip clearance is 2 mm, the relative loss of power output could achieve as large as 7.81%. Larger nozzle-to-rotor clearance leads to more uniform distributions of Mach number and pressure, but the flow losses also increase. The effect of trailing edge disturbance on the flow field at the nozzle outlet is almost negligible if the nozzle-to-rotor clearance is 6 mm or more.     

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。     

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.