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。     

受热工况截面平均空泡率对漂移流模型分布参数C0的影响分析

空泡率是汽液两相流动的基本参数之一.在计算空泡率的众多模型中,Zuer-Findlay漂移流模型目前应用最为广泛.根据受热工况下局部界面参数径向分布特性实验研究结果,并通过对现有实验数据的处理,分析了受热工况下垂直上升两相流漂移流模型分布参数C0随截面平均空泡率的变化规律.证实了从过冷沸腾泡状流到饱和沸腾弹状流工况,随着空泡率的增加,分布参数C0从一个小于1的值增加到1.0～1.2.     

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.通过与试验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算.试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速分布情况.     

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。     

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。     

内燃机鼻梁区内过冷沸腾两相流研究

为定量研究发动机冷却水腔内的过冷沸腾传热,以模拟通道内过冷沸腾试验为基础,采用欧拉多相流模型,确定了欧拉多相流模型的选用,将壁面温度分别取400K、400K、400K,得出最优沸腾模型适用范围;对比欧拉多相流模型、Mixture模型及单相流模型的计算精度,证实欧拉多相流具有更高计算精度,在绝大部分工况范围与试验数据对比误差低于10%,局部区域误差低于3%;证明湍流离散力在强迫对流过冷沸腾模拟中不可忽略。     

狭缝中流动沸腾传热过冷沸腾起始点的实验研究

以间隙为1．0mm和1．5mm的环形狭缝通道中流动沸腾传热的实验数据为基础，分析了影响过冷沸腾起始点热负荷的主要因素，给出了计算环形狭缝通道中流动沸腾传热过冷沸腾起始点的经验关联式，并将计算结果与实验值进行了比较。该关联式可以用来预测实验范围内的过冷沸腾起始点的热负荷。     

基于VOF两相流的缸盖过冷沸腾模型及试验验证

以流体体积(VOF)两相流方法为基础建立了适用于高强化柴油机缸盖的过冷沸腾传热模型.该模型考虑了过冷流动沸腾中的蒸发和冷凝现象.采用T型管和铸铝缸盖两个传热试验对上述模型进行验证.结果显示:T型管传热试验在高壁面过热度下,模型计算误差能控制在5%以内;铸铝缸盖火力面测温试验结果与模型预测结果之间的平均误差为5.6%.上述结果充分表明,提出的基于VOF两相流方法的过冷沸腾传热模型有较强的适用性,能够预测较宽范围内过冷流动沸腾现象.     

欠热沸腾空泡份额计算

空泡份额α对于蒸汽发生器和核反应堆的设计计算有重要作用。欠热沸腾下的空泡份额计算不同于饱和沸腾,具有热不平衡特点。本文作者根据宏观热平衡概念推导出一个新的计算方法与公式。计算结果与劳哈尼模型的计算结果较一致,并更符合于实验数据。     

常压下狭缝流动沸腾传热的研究

以蒸馏水为工质，在常压下，对间隙为1mm的环形狭缝通道中的流动传热进行了实验研究。分别将狭缝通道中的单相强制对流和过冷沸腾的实验数据与传统的Dittus-Boelter型关系式的计算结果进行了比较。通过分析狭缝通道中流动沸腾的传热特性认为，过冷沸腾传热比单相强制对流传热加强；质量流速对狭缝通道中的流动沸腾传热有较大影响。     

Development of a fast empirical design model for PEM stacks

A simple and fast empirical design model for a 5 kW proton exchange membrane (PEM) stack is presented in this paper. The performance analysis of the PEM stack operating on a membrane humidifying method is made through a series of experiments, including current–voltage–power characteristics, uniformity of cell unit voltages, gas pressure impact and air flux impact. Based on the above analysis, an empirical predicted model for the PEM stack has been developed by the combination of mechanistic and empirical modeling approaches to characterize and predict the voltage–current characteristics without examining in depth all physical/chemical phenomena. The good agreement between the predicted and experimental results covering a range of optimal operating conditions shows that the proposed model provides an accurate representation of the behavior for the PEM stack.     

Performance of PEMFC stack using expanded graphite bipolar plates

The bipolar plates are in weight and volume the major part of PEM fuel cell stack, and also a significant effect to the stack cost. To develop the low-cost and low-weight bipolar plate for PEM fuel cell, we have developed a kind of cheap expanded graphite plate material and a production process for fuel cell bipolar plates. The plates have a high electric conductivity and low density, and can be stamped directly forming fuel cell bipolar plates. Then, 1 and 10 kW stacks using expanded graphite bipolar plates are successfully assembled. The contact resistance of the bipolar plate is investigated and the electrochemical performances of the fuel cell stacks are tested. Good fuel cell performance is obtained and the voltage distribution among every single cell in the stacks is very uniform.     

Design of thermal management subsystem for a 5 kW polymer electrolyte membrane fuel cell system

High-efficiency thermal management subsystem has a key role on the PEM fuel cell performance and durability. In this study, design of thermal management subsystem for a 5 kW PEM fuel cell system is investigated. A numerical model is presented to study the cooling flow field performance. The number of parallel channels in parallel serpentine flow field is selected as the design parameter of the flow field and its optimum value is obtained by compromising between the minimum pressure drop of coolant across the flow field and maximum temperature uniformity within the bipolar plate criteria. The optimum coolant flow rate is also determined by compromising between different criteria. Test results of a 5-cells short stack are presented to verify the numerical simulation results.     

Innovative gasketless carbon composite bipolar plates for PEM fuel cells

Conventional bipolar plates for proton exchange membrane (PEM) fuel cells use extra rubber gaskets to seal the stack, which require an additional curing process at high temperature and increase the manufacturing and assembling time. To reduce the assembling time of fuel cell stacks and achieve gas sealability without using extra gaskets or curing cycles, innovative gasketless carbon composite bipolar plates were developed. To ease the assembling of the cell stacks, special grooves on the edge of the composite bipolar plate are provided for mechanical joining and the behavior of the bipolar plates under the stack compaction pressure conditions was investigated by FE analysis. The mechanical properties of the grooves were measured by the compressive strength test and compared with the FE analysis results. The sealability of the gasketless bipolar plate with grooves was tested to verify the integrity of the design.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Design and manufacturing of end plates of a 5 kW PEM fuel cell

End plate is one of the main components of the proton exchange membrane (PEM) fuel cells. The major role of the end plate is providing uniform pressure distribution between various components of the fuel cell (bipolar plates, etc.) and consequently reducing contact resistance between them. In this study a procedure for design of end plate has been developed. At first a suitable material was selected using various criteria. Then a finite element (FE) analysis was accomplished to analyze end plate deflections and get its optimized thickness. After fabricating the end plates, a single cell was assembled and electrochemical impedance spectroscopy (EIS) tests were carried out to ensure their good operation. A 5 kW fuel cell assembled with these end plates was tested at different operating conditions. The test results show an appropriate assembly pressure distribution inside the stack which indicates good performance of the designed end plates.     

Study on the membrane electrode assembly fabrication with carbon supported cobalt triethylenetetramine as cathode catalyst for proton exchange membrane fuel cell

An improved fabrication technique for conventional hot-pressed membrane electrode assemblies (MEAs) with carbon supported cobalt triethylenetetramine (CoTETA/C) as the cathode catalyst is investigated. The V-I results of PEM single cell tests show that addition of glycol to the cathode catalyst ink leads to significantly higher electrochemical performance and power density than the single cell prepared by the traditional method. SEM analysis shows that the MEAs prepared by the conventional hot-pressed method have cracks between the cathode catalyst layer and Nafion membrane, and the contact problem between cathode catalyst layer and Nafion membrane is greatly suppressed by addition of glycol to the cathode catalyst ink. Current density-voltage curve and impedance studies illuminate that the MEAs prepared by adding glycol to the cathode catalyst ink have a higher electrochemical surface area, lower cell ohmic resistance, and lower charge transfer resistance. The effects of CoTETA/C loading, Nafion content, and Pt loading are also studied. By optimizing the preparation parameters of the MEA, the as-fabricated cell with a Pt loading of 0.15 mg cm⁻² delivers a maximum power density of 181.1 mW cm⁻², and a power density of 126.2 mW cm⁻² at a voltage of 0.4 V.     

Modelling and evaluation of heating strategies for high temperature polymer electrolyte membrane fuel cell stacks

Experiments were conducted on two different cathode air cooled high temperature PEM (HTPEM) fuel cell stacks; a 30 cell 400 W prototype stack using two bipolar plates per cell, and a 65 cell 1 kW commercial stack using one bipolar plate per cell. The work seeks to examine the use of different heating strategies and find a strategy suited for fast start-up of the HTPEM fuel cell stacks. Fast start-up of these high temperature systems enables use in a wide range of applications, such as automotive and auxiliary power units, where immediate system response is needed. The development of a dynamic model to simulate the temperature development of a fuel cell stack during heating can be used for assistance in system and control design. The heating strategies analyzed and tested reduced the start-up time of one of the fuel cell stacks from 1 h to about 6 min.     

1.5 kWe HT-PEFC stack with composite MEA for CHP application

In this work, the performance of a High Temperature (HT) Polymer Electrolyte Fuel Cell (PEFC) stack for co-generation application was investigated. A 3 kW power unit composed of two 1.5 kW modules was designed, manufactured and tested. The module was composed of 40 composite graphite cell with an active area of 150 cm². Composite Membrane Electrode Assemblies (MEAs) based on Nafion/Zirconia membranes were used to explore the behavior of the stack at high temperature (120 °C). Tests were performed in both pure Hydrogen and H₂/CO₂/CO mixture at different humidification grade, simulating the exit gas from a methane fuel processor. The fuel cells stack has generated a maximum power of 2400 W at 105 A with pure hydrogen and fully hydrated gases and 1700 W at 90 A by operating at low humidity grade (95/49 RH% for H₂/Air). In case the stack was fed with reformate simulated stream fully saturated, a maximum power of 2290 W at 105 A was reached: only a power loss of 5% was recorded by using reformate stream instead of pure hydrogen. The humidification grade of Nafion membrane was indicated as the main factor affecting the proton conductivity of Nafion while the addition of the inert compound like YSZ, did not affectthe electrochemical properties of the membrane but, rather has enhanced mechanical resistance at high temperature.     

Effect of clamping pressure on the performance of a PEM fuel cell

Examined were the effects of the clamping pressure on the performance of a proton exchange membrane (PEM) fuel cell. The electro-physical properties of the gas diffusion layer (GDL) such as porosity, gas permeability, electrical resistance and thickness were measured using a special-designed test rig under various clamping pressure levels. Correlations for the gas permeability of the GDL were developed in terms of the clamping pressure. In addition, the contact resistance between the GDL and the bipolar (graphite) plate was measured under various clamping pressures. Results showed that at the low clamping pressure levels (e.g. <5 bar) increasing the clamping pressure reduces the interfacial resistance between the bipolar plate and the GDL that enhances the electrochemical performance of a PEM fuel cell. In contrast, at the high clamping pressure levels (e.g. >10 bar), increasing the clamping pressure not only reduces the above Ohmic resistance but also narrows down the diffusion path for mass transfer from gas channels to the catalyst layers. Comprising the above two effects did not promote the power density too much but reduce the mass-transfer limitation for high current density.