Degradations in porous components of a proton exchange membrane fuel cell under freeze-thaw cycles: Morphology and microstructure effects

In this study, porous components of a proton exchange membrane (PEM) fuel cell, i.e., single-layer gas diffusion layer (GDL, carbon paper), double-layer GDL (microporous layer (MPL) deposited carbon papers), and catalyzed electrodes, are subjected to 60 repetitive freeze-thaw cycles between −40 °C and 30 °C under water-submerged conditions; and their morphological and microstructural characteristics are investigated at each 15 cycles and compared with those of virgin materials. The results indicate that consecutive cycling of temperature causes different degradation patterns in different components. The single-layer GDL shows a unique degradation mechanism, in which macro-scale pores volumetrically expand, and relatively small-scale hollows and cracks form on the polymeric binder and carbon fiber interfaces, respectively. For the double-layer GDL, large-scale surface cracks form on the MPL surface after 15 cycles, and the morphology and microstructure degradation gains momentum with the formation of these cracks, and upon completion of 30 cycles, large-scale carbon/hydrophobic agent flakes start to detach from the surface. For the catalyzed electrodes, due to their inherently cracked surface, the catalyst layers (CLs) degrade first through expansion of the cracks in the in- and through-plane directions, and then through swelling and agglomeration of the ionomer; and combination of these two patterns triggers detachment of large CL flakes from the surface, negatively affecting the microstructure.     

基于ESC测试循环的SCR催化剂动态转化效率温度窗口研究

为了建立选择性催化还原转化器(SCR)台架评价结果与欧洲稳态测试循环(ESC)工况条件下的减排效果之间的联系,进行了催化剂动态转化效率试验,绘制了基于反应平衡时间的SCR催化剂转化效率温度窗口,得到了不同反应温度下催化剂动态转化效率发展过程对反应平衡时间的依赖程度。基于ESC试验排放采样窗口期内发动机排气条件的特征分析,研究了ESC试验中催化剂动态转化效率发展特征,提出了基于该催化剂动态转化效率温度窗口表示方法,分析了温度窗口的试验评价条件和基于实际化学计量比(NSR)控制策略的窗口模拟计算调整方法。使用该温度窗口对ESC工况排放值进行预测,依据ESC名义转化效率温度窗口预测的ESC排放结果与实测值的偏差小于8%。     

高压共轨喷油器可变续流驱动电路的设计研究

基于提升保持(PeakHold)驱动方式不同电流释放阶段对下降速率的需求,进行了可变续流喷油器驱动电路设计与性能研究。电路设计采用高、低边及单电源驱动,在高边MOS源极设置反向二极管及电容接地,在低边MOS漏极设置二极管接驱动电源。通过改变不同阶段高低边MOS开关逻辑,使用不同有效续流回路决定电流下降快慢以满足PeakHold需求。试验表明:与通过匹配阻抗而折中电流下降速率的RD续流(R=10Ω)相比,可变续流驱动电路喷油量控制精度提高,喷油器关闭阶段时间缩短了30μs,喷油量循环变动率在大油量时下降了0.36%,在小油量时降低了2.86%;此外,可变续流驱动电路在电流关闭阶段利用喷油器线圈释放电磁能向驱动电源充电,使喷油器电磁铁驱动能耗降低。在大油量时,可变续流驱动电路驱动电源电压下降幅度减少了1.5V,其恢复时间缩短了0.5ms。由于能耗降低,电路发热量减少,可变续流驱动电路最高温度仅37℃,比RD续流下降了127℃。     

Tetralin pyrolysis under H2 pressure,between 350 and 510°C

The chemistry and kinetics of tetralin thermolysis were experimentally specified under a hydrogen pressure of 8 MPa within a wide temperature range of 350–510°C. The main products are 1-methylindane and n-butylbenzene; out of a total of 16 hydrocarbons identified in the thermolysis products, only benzene, toluene, o-xylene and naphthalene are final. The general order of tetralin thermolysis is 2. The individual orders of isomerization and hydrogenolysis are 0 and 1, respectively. A simplified kinetic model of tetralin thermolysis at the initial stage agrees well with experimental data. The tetralin thermolysis was assumed to begin with a bimolecular transformation into tetralyl and cyclohexadienyl radicals. This is consistent with the facts that the main intermediate products formed from these radicals are 1-methylindane and n-butylbenzene and that the experimentally established activation energy of the total process (59.1 kcal/mol) is close to the thermodynamically evaluated enthalpy of the proposed disproportionation.     

New correlation for liquid hold-up of sodium citrate buffer solution in a Raschig ring packed column

Dynamic liquid hold-up was measured with an air/aqueous sodium citrate buffer solution at 20–40 °C, and an air/water system at 23 °C, in a 0.1 m diameter/1 m high glass column covered by a heat-isolating vacuum jacket and packed with 0.012 m nominal size ceramic Raschig rings. The superficial gas velocity range was extended to 1.2 m s⁻¹. Experimental results of this work were compared with literature data, with different correlations and with a general equation. All expressions were found to be unacceptable for the air/buffer system and useable for the air/water system. In the case of the air/buffer solution a new correlation is recommended on the basis of our measured data and literature values.     

3D lattice Boltzmann modeling of droplet motion in PEM fuel cell channel with realistic GDL microstructure and fluid properties

A 3D multi-component multi-phase lattice Boltzmann model is developed to study the droplet motion in the flow channel of proton exchange membrane fuel cell. The model is capable of reaching realistic density and viscosity ratio, tunable surface tension with low spurious velocity and is also validated by various benchmark tests in both static and dynamic states. For the first time, the effect of realistic microstructure of gas diffusion layer (GDL) on droplet dynamic behavior is comprehensively studied in terms of comparison with smooth channel, contact angle and droplet size with the motion processes clearly illustrated. The simulation results show the GDL microstructure can amplify the material wettability, affect the motion direction and impede the droplet motion. More hydrophobic GDL can effectively accelerate the transport. However, it is observed the droplet may reach the sidewall due to the presence of GDL and the motion is therefore severely impeded regardless of the GDL contact angle or droplet size, which is hard to avoid but deadly for the water management. For this problem, a novel water management strategy is proposed and the results show the hydrophilic side & top wall can effectively remove the liquid water from the GDL surface, decrease pressure drop and prevent reactant maldistribution.     

Hydrogen utilization enhancement of proton exchange membrane fuel cell with anode recirculation system through a purge strategy

Proton exchange membrane (PEM) fuel cells are widely considered as potential alternative energy candidates for internal combustion engines because of their low-temperature start, high energy density, and ease of scale up. However, their low hydrogen utilization rate is one of the main reasons for the limited commercial development. This study focuses on improving the hydrogen utilization rate of PEM fuel cells and system efficiency using optimal active recirculation system (ARS). An anode ARS and purging strategy are introduced to enhance the hydrogen utilization rate of PEM fuel cells. An ARS simulation model with purge strategy model is developed in a MATLAB/Simulink environment. A control-oriented dynamic model is developed to study the hydrogen recirculation system characteristics. The dynamic model is used as basis to propose a proportional integration differentiation controller to maintain the anode hydrogen concentration and increase the hydrogen utilization rate. Several experiments are performed using different purging strategies in conjunction with ARS. The hydrogen utilization rate is the highest when the purge time is 0.3 s and the purge period is 10 s. Simulation results show that the PEM fuel cells with an anode recirculation configuration exhibit a better performance than other configurations in terms of hydrogen utilization. Experimental results also demonstrate the feasibility of the proposed system, the performance of which is also superior to that of other hydrogen supply system.     

COSIMA—a computer program simulating the dynamics of fractal aerosols

The sectional aerosol behavior code COSIMA simulates the time evolution of the structural, dynamical, and optical properties of airborne agglomerate particles as well as their heterogeneous chemical interactions with reactive trace gases utilizing a formalism based on fractal scaling laws. The modeled processes include diffusion to the walls and sedimentational deposition, Brownian and gravitational coagulation, molecular transport from the gas phase to the accessible particle surface, surface adsorption and reactions, gas phase reactions, and dilution effects due to sampling (e.g. during aerosol chamber experiments). The effect of hydrodynamic interactions and shielding on particle mobility is considered within the framework of the Kirkwood–Riseman theory. Rayleigh–Debye–Gans theory is used to deal with light absorption and scattering. The code is validated against new experimental data on the dynamics of Diesel and graphite spark soot as well as recent theoretical and simulation results. Applying the Kirkwood–Riseman formalism to compute the mobility of fractal like agglomerates significantly enhances coagulation rates as well as wall and depositional loss but does not affect the form of the self preserving size distributions attained in the long time regime if Brownian coagulation dominates the aerosol dynamics.     

基于EGR的国Ⅳ重型柴油机技术路线探讨

在一台高压共轨重型柴油机上进行基于EGR技术的排放试验,对比研究了5个不同功率段(升功率分别为23.9、25.5、28.2、29.8和30.7kW/L)柴油机匹配单级增压和二级增压的性能与排放特性,探寻了采用EGR满足不同升功率柴油机的国-Ⅳ技术路线。试验结果表明:随功率段增加,二级增压比单级增压能显著改善中低转速全负荷时NOx与BSFC和碳烟之间的平衡关系,单级增压受EGR引入能力较弱和涡前温度较高双重限制,高功率段中低转速全负荷的NOx比排放难以降低到国-Ⅳ限值以下。ESC测试循环表明:碳烟和CO加权比排放随功率段降低而下降,但HC加权比排放随功率段降低而增加;在中小功率段(23.9、25.5和28.2kW/L),单级增压和二级增压NOx和碳烟加权比排放相当,2种增压方式都能在无后处理情况下使各项排放指标均满足国-Ⅳ排放法规,但单级增压的加权BSFC低于二级增压;随升功率增加,在高功率段(29.8和30.7kW/L),单级增压仅采用EGR技术难以实现国-Ⅳ的重型柴油机排放控制,而采用二级增压耦合EGR仍能在无后处理或简单后处理条件下使各项排放指标达到国-Ⅳ法规要求。     

柴油机燃烧生成芳香烃、碳烟及NO_x的化学动力学研究

在LLNL详细正庚烷氧化反应模型基础上,将苯(A1)和多环芳香烃(A2、A3、A4)的详细反应机理和NOx的生成机理加入其中形成了一个包含芳香烃和NOx的正庚烷氧化反应组合模型,并通过激波管试验验证了该组合模型。在CHEMKIN软件上运用该组合模型对柴油机燃烧反应产物中的芳香烃、碳烟和NOx进行了研究,结果表明:芳香烃的生成在Ф-T图中都呈钟形分布,随着环数的增加它们所对应的生成温度大致呈逐渐升高的趋势。利用芳香烃-Ф-T图可以较准确地预测碳烟-Ф-T图,再结合NOx-Ф-T图可以绘制出芳香烃-碳烟-NOx-Ф-T图,结果表明:芳香烃和碳烟在1 150～2 200K、当量比大于2的范围内大量生成,NOx在温度高于2 350K、当量比小于1.2的范围内大量生成。