Experimental investigation of liquid water in flow field of proton exchange membrane fuel cell by combining X-ray with EIS technologies

The flow field is a pivotal part to manage the transport of water and gas in proton exchange membrane fuel cell. However, the reported water measurement methods (e.g., X-ray and electrochemical impedance spectroscopy (EIS)) cannot give a comprehensive understanding water distribution in the flow field, resulting in challenges in optimizing the channel design and enhancing fuel cell performance. Therefore, we propose a water measurement method combining the X-ray radiography with EIS to investigate the effect of different operating conditions on the growth law and distribution of liquid water in parallel and serpentine flow fields. The attenuation coefficient of liquid water to X-ray is calibrated with constant tube-current and tube-voltage of X-ray generator. Besides, the parallel flow field with hydrophobic treatment is studied. The results show that the water accumulation of the parallel flow field is far more than the serpentine flow field, and the water content of the middle region is higher than that of other regions in the parallel flow field. Furthermore, operating conditions (cathode inlet gas flow rate, inlet gas humidity, and back pressure) have little effect on the liquid water content of the middle region in the parallel flow field. The polarization curve, EIS result, and X-ray radiography show that the performance and water drainage capacity of the hydrophobic parallel flow field are better than the normal one.

     

涡旋膨胀机润滑系统设计及输出性能实验研究

为了研究涡旋膨胀机输出性能规律,将某商用涡旋压缩机改造成涡旋膨胀机,并对其润滑系统进行了设计和精确控制;基于此,搭建了以压缩空气作为工质的测试实验台,对比了膨胀机在有无润滑条件下的性能,并对膨胀机的性能进行了全面的测试。结果表明:润滑系统在机械润滑和膨胀机性能方面都起到良好的效果;在实验工况范围内,膨胀机等熵效率均保持在40%以上,最高等熵效率为54.45%,输出电功率达到了753 W;负载数量一定时,膨胀机输出功率随空气流量的增加而增加,但等熵效率随流量的增大而减小;在不同的负载数量下,受到机械磨损和内部泄漏的影响,等熵效率均随着膨胀机转速的增加先增大后减小,存在最优等熵效率转速区间为2 800~3 200 r/min;而当转速一定时,膨胀机的输出功率取决于进口压力的大小;随负载的增加,膨胀机的输出功率和进口压力成相似的增长趋势。研究结果可为有机朗肯循环系统优化提供参考。     

Overall efficiency optimization of controllable mechanical turbo-compounding system for heavy duty diesel engines

A controllable mechanical turbo-compounding (CMTC) system including continuously variable transmission (CVT) and power turbine bypass valve is proposed to recover waste heat from engine exhaust. The combined matching principle considering swallowing capacity of both charging turbine and power turbine, main gear ratio is investigated at first based on the analysis of individual influence. Then the effects and strategies of CVT and power turbine bypass valve are studied for better performance under off-design conditions. At last, the transient response of intake pressure of engine with CMTC system is researched and the fuel saving potential is tested under driving cycle conditions. The results indicate that the overall fuel efficiency elevates at the off-design conditions if CVT is adopted due to the improvement of power turbine operating efficiency by speed modulation. The diversion of exhaust through power turbine bypass valve under the low load condition is necessary. The back pressure of the charging turbine infuences the transient response of intake pressure for a fixed CMTC configuration. A method featured by the assistance of power turbine bypass valve is tested to improve the transient response of the intake pressure. The fuel consumption reduces by 2% and 3.4% under highway fuel economy test (HWFET) and Tianjin 503 (TJ503) driving cycles respectively.     

Analysis of an electricity-cooling cogeneration system for waste heat recovery of gaseous fuel engines

Waste heat recovery(WHR)is one of the most useful ways to improve the efficiency of internal combustion engines,and an electricity-cooling cogeneration system(ECCS)based on Rankin-absorption refrigeration combined cycle for the WHR of gaseous fuel engines is proposed in the paper.This system can avoid wasting the heat in condenser so that the efficiency of the whole WHR system improves,but the condensing temperature of Rankin cycle(RC)must increase in order to use absorption refrigeration system,which leads to the decrease of RC output power.Therefore,the relationship between the profit of absorption refrigeration system and the loss of RC in this combined system is the mainly studied content in the paper.Because the energy quality of cooling and electricity are different,cooling power in absorption refrigeration is converted to corresponding electrical power consumed by electric cooling system,which is defined as equivalent electrical power.With this method,the effects of some important operation parameters on the performance of the ECCS are researched,and the equivalent efficiency,exergy efficiency and primary energy rate are compared in the paper.     

An effective MBSE approach for constructing industrial robot digital twin system

Recently, the rapid development of digital twin (DT) technology has been regarded significant in Cyber-physical systems (CPS) promotion. Scholars are focusing on the theoretical architecture and implementing applications, in order to establish a high-fidelity, dynamic, and full-lifecycle DT model and achieve a deep fusion of real and virtual. As a typical complex system with multi-disciplines, multi-physics, and multi-domain characteristics, industrial robot (IR) involves various processes and elements from the two other levels of the system: components and production lines. Their complex relationships lead to a huge challenge to build a comprehensive DT model. Current researchers usually concentrates on single-layer services because of limited construction methodology, which results in enormous isolated models, and leads to low reusable system blocks, finite scalability, and high costs of design, adjustment, upgrade, and maintenance. To address these issues, a standardized methodology and a hierarchical, modular, and generic architecture are proposed to depict comprehensive and variable industrial robot digital twin (IRDT). Firstly, the ontology information model is presented by analyzing variable factors systematically. Then, model-based system engineering (MBSE) based methodology is introduced, including construction process and variants management. After modeling process of three levels (problem domain, solution main, and implementation domain) and four viewpoints (requirement, structure, behavior, and parameter), a generic architecture of IRDT is constructed and a feature-based variants management method is described. Besides, a six-axis IRDTS is implemented to illustrate the mapping of logical architecture and physical system as a multi-level elements and processes representation example. And the steps of numerical evaluations consist of system delay and derivation. Finally, results show the effectiveness and the potential of the proposed theoretical methodology for constructing IRDTS and other industrial applications.     

An open-source library for the numerical modeling of mass-transfer in solid oxide fuel cells

The generation of direct current electricity using solid oxide fuel cells (SOFCs) involves several interplaying transport phenomena. Their simulation is crucial for the design and optimization of reliable and competitive equipment, and for the eventual market deployment of this technology. An open-source library for the computational modeling of mass-transport phenomena in SOFCs is presented in this article. It includes several multicomponent mass-transport models (i.e. Fickian, Stefan–Maxwell and Dusty Gas Model), which can be applied both within porous media and in porosity-free domains, and several diffusivity models for gases. The library has been developed for its use with OpenFOAM^®, a widespread open-source code for fluid and continuum mechanics. The library can be used to model any fluid flow configuration involving multicomponent transport phenomena and it is validated in this paper against the analytical solution of one-dimensional test cases. In addition, it is applied for the simulation of a real SOFC and further validated using experimental data.Program summaryProgram title: multiSpeciesTransportModelsCatalogue identifier: AEKB_v1_0Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEKB_v1_0.htmlProgram obtainable from: CPC Program Library, Queen?s University, Belfast, N. IrelandLicensing provisions: GNU General Public LicenseNo. of lines in distributed program, including test data, etc.: 18 140No. of bytes in distributed program, including test data, etc.: 64 285Distribution format: tar.gzProgramming language:: C++Computer: Any x86 (the instructions reported in the paper consider only the 64 bit case for the sake of simplicity)Operating system: Generic Linux (the instructions reported in the paper consider only the open-source Ubuntu distribution for the sake of simplicity)Classification: 12External routines: OpenFOAM® (version 1.6-ext) (http://www.extend-project.de)Nature of problem: This software provides a library of models for the simulation of the steady state mass and momentum transport in a multi-species gas mixture, possibly in a porous medium. The software is particularly designed to be used as the mass-transport library for the modeling of solid oxide fuel cells (SOFC). When supplemented with other sub-models, such as thermal and charge-transport ones, it allows the prediction of the cell polarization curve and hence the cell performance.Solution method: Standard finite volume method (FVM) is used for solving all the conservation equations. The pressure-velocity coupling is solved using the SIMPLE algorithm (possibly adding a porous drag term if required). The mass transport can be calculated using different alternative models, namely Fick, Maxwell–Stefan or dusty gas model. The code adopts a segregated method to solve the resulting linear system of equations. The different regions of the SOFC, namely gas channels, electrodes and electrolyte, are solved independently, and coupled through boundary conditions.Restrictions: When extremely large species fluxes are considered, current implementation of the Neumann and Robin boundary conditions do not avoid negative values of molar and/or mass fractions, which finally end up with numerical instability. However this never happened in the documented runs. Eventually these boundary conditions could be reformulated to become more robust.Running time: From seconds to hours depending on the mesh size and number of species. For example, on a 64 bit machine with Intel Core Duo T8300 and 3 GBytes of RAM, the provided test run requires less than 1 second.     

Large eddy simulation and proper orthogonal decomposition analysis of turbulent flows in a direct injection spark ignition engine: Cyclic variation and effect of valve lift

Large eddy simulation(LES)is used to calculate the in-cylinder turbulent flow field in a direct injection spark ignition(DISI)engine.The computations are carried out for three different maximum valve lifts(MVL)and throughout 100 consecutive engine cycles.The simulated results as well as corresponding particle image velocimetry(PIV)measurement database are analyzed by the proper orthogonal decomposition(POD)method.Through a new developed POD quadruple decomposition the instantaneous in-cylinder flow fields are decomposed into four parts,named mean field,coherent field,transition field and turbulent field,respectively.Then the in-cylinder turbulent flow characteristics and cycle-to-cycle variations(CCV)are studied separately upon the four part flow fields.Results indicate that each part exhibits its specific characteristics and has close connection with others.The mean part contains more than 50%of the total kinetic energy and the energy cascade phenomenon occurs among the four part fields;the coherent field part possesses the highest CCV level which dominates CCV of the bulk flow.In addition,it is observed that a change in MVL affects significantly the in-cylinder flow behavior including CCV,especially for the coherent part.Furthermore,the POD analysis demonstrates that at least 25 sample cycles for the mean velocity and 50 sample cycles for the RMS velocity are necessary for obtaining converged and correct results in CCV.     

Experimental study on the effects of HP and LP EGR on thermal efficiency and emissions of a two-stage turbocharged diesel engine

An experimental study was performed to compare the effects of high- and low-pressure exhaust gas recirculation loops (HP and LP EGR loops) on thermal efficiency and emissions of a diesel engine. Tests were conducted on a 12-L six-cylinder turbocharged diesel engine under various operating conditions. We found that at a low speed of 1100 r/min, 1 MPa BMEP, the LP EGR loop could achieve higher brake thermal efficiency and lower emissions than the HP EGR. This is because the lower enthalpy available at the turbine inlet of the HP EGR loop increased the fuel/oxygen equivalence ratio. For the HP EGR, the gross indicated thermal efficiency was reduced by 1%, but pumping losses were only reduced by 0.5%, compared to the LP EGR loop. At a higher speed of 1600 r/min, 1 MPa BMEP, the HP EGR loop attained a higher brake thermal efficiency and lower emissions because of the relatively sufficient flow through the turbocharger. For the HP EGR loop, the gross indicated thermal efficiency was only reduced by 0.5% and pumping losses were reduced by 1.5%, compared to the LP EGR loop. Lower fuel consumption and a longer ignition delay made the distribution of fuel/oxygen equivalence ratio more homogeneous, leading to lower emissions. Our data also showed that at the high speed of 1600 r/min, 0.55 MPa BMEP, the brake thermal efficiency of the HP EGR loop first increased, then decreased as the EGR rate increased. Therefore, under all conditions, a reasonable match of both EGR loops could achieve a good balance between fuel consumption and emissions of NO _x and soot.     

Vectorizing NURBS surface evaluation with basis functions in power basis

Several known methods for direct evaluation of NURBS curves and surfaces are described. Runtime performance and simplicity of vectorization are discussed. An evaluation method, which uses basis functions precomputed in shifted power basis, is shown to be promising. This method for surfaces is vectorized with SSE2 intrinsics, yielding 3 times performance improvement over the non-vectorized version. Branchless vectorized linear search is proposed for span search, being most efficient for small number of knots. Binary search ending with a small linear search is shown to be most efficient for large number of knots, and good for general case. Performance comparison of the evaluation method and its equivalents from available geometric kernels is included.     

Thermal management in PEMFCs: The respective effects of porous media in the gas flow channel

This study aims to evaluate the convective heat transfer enhancement of the proton exchange membrane fuel cells (PEMFC) numerically. As the higher heat transfer surfaces lead to higher heat transfer rates, a flat plate porous layer is utilized in the gas flow channel (GFC). This enhancement in heat transfer stems from the corresponding modification in the temperature and velocity profiles. The influencing parameters on these profiles are the thickness, permeability, and porosity of the GFC porous layer. After performing the simulations, the results indicate that convective heat transfer has a direct relationship with GFC porous layer's thickness and permeability. However, lower values of porosity lead to the higher Nusselt numbers. Previous investigations have also mentioned the positive impact of the microporous layer (MPL) on the water management of these fuel cells. Therefore, six different sizes of MPL and the gas diffusion layer (GDL) are utilized to evaluate their impacts on the thermal management. Results indicate that although these sizes have negligible effects on the heat transfer, Nu increases by enhancing the total size of MPL and GDL. The results also show that thicker MPLs lead to higher heat transfer rates. The evaluation of the friction factor also indicates the adverse effect of the GFC porous layer, although this undesirable effect is negligible. Finally, all the simulated values are utilized to train an artificial neural network (ANN) model with high precision. This ANN model can produce more data for sensitivity analysis and presenting respective 3D diagrams of the influencing parameters on heat transfer.