A coupling study of ignition position on fuel-air mixing and diffusion of a linear hydrogen engine

Linear hydrogen engine (LHE) is a new technology of hydrogen energy utilization due to its flexible compression ratio coupling with ignition, fuel-air mixing, and combustion to optimize thermal efficiency. Fuel-air mixing in LHE is expected to be promoted by using ignition, which differs with conventional engine. This paper develops a full-cycle model which couples with dynamics, hydrogen-air mixing and combustion to describe the effect of ignition position, meanwhile a loop iterative calculation method is proposed to solve the coupling model for hydrogen-air mixing predication. The results show that ignition position variation can cause the piston trajectory to change significantly, and the higher equivalent speed is obtained in the medium ignition position. Besides, the higher equivalent speed in the injection stage is conducive to the diffusion of hydrogen, but the higher equivalent speed is not conducive to diffusion and mixing in the diffusion stage. More importantly, the equivalence ratio distribution at the ignition position is more uniform for the later ignition position due to the longer mixing stroke, and the mixture uniformity index at the ignition position is inversely proportional to the advance of the ignition position. Therefore, the late ignition is recommended to obtain a uniform hydrogen mixture.     

LES study on flow features of the supersonic mixing layer affected by shock waves

The supersonic mixing layer in a model scramjet has been simulated using large eddy simulation (LES) with the consideration of complex shock waves. A comparison with experimental data demonstrates the accuracy and applicability of the employed numerical models and computational methods based on the OpenFOAM solver. The flow features are examined not only from the visualization of flow field, but also from a fundamental point of fluid motion with a focus on evaluating the impacts of shock waves on the turbulent mixing layer. Four basic types of interaction between the shock waves and the mixing layer are discussed according to the flow features and the field development. The mixing properties are analyzed from the evolution of the mixing layer thickness with emphasis on mixing efficiency and total pressure recovery. Results show that three typical developing regions can be observed for the supersonic mixing layer in the scramjet. Shock waves make contributions to the fuel-air mixing due to the amplification of turbulence and the gain of vorticity. The increase in total pressure losses is unavoidable despite the increased mixing efficiency in the presence of shock waves. Additionally, a significant decrease of the convective Mach number is caused by the expansion-fan/shock-wave pattern at the injector exit, leading to a reduction in compressibility effects, which is an important aspect of the physics for the development of the mixing layer in the model scramjet.     

A numerical study of hydrogen leakage and diffusion in a hydrogen refueling station

Studies focused on the behavior of the hydrogen leakage and diffusion are of great importance for facilitating the large scale application of the hydrogen energy. In this paper, the hydrogen leakage and diffusion in six scenarios which including comparison of different leakage position and different wind effect are analyzed numerically. The studied geometry is derived from the hydrogen refueling station in China. Due to the high pressure in hydrogen storage take, the hydrogen leakage is momentum dominated. The hydrogen volume concentration with the variation of the leakage time in different scenarios is plotted. More importantly, profiles of the flammable gas cloud at the end of the leakage are quantitatively studied. Results indicate that a more narrow space between the leakage hole and the obstacle and a smaller contact area with the obstacle make the profile of the flammable gas cloud more irregular and unpredictable. In addition, results highlight the wind effect on the hydrogen leakage and diffusion. Comparing with scenario which the wind direction consistent with the leakage direction, the opposite wind direction may result in a larger profile of the flammable gas cloud. With wind velocity increasing, the profile of the flammable gas cloud is confined in a smaller range. However, the presence of the wind facilitates the form of the recirculation zone near the obstacle. With an increase of the wind velocity, the recirculation zone moves downward along the obstacle. Thus, the hydrogen accumulation is more prominent near the obstacle.     

A comparative study on effects of homogeneous or stratified hydrogen on combustion and emissions of a gasoline/hydrogen SI engine

A comparative study on effects of homogeneous or stratified hydrogen on combustion and emissions was presented for a gasoline/hydrogen SI engine. Three kinds of injection modes (gasoline, gasoline plus homogeneous hydrogen and gasoline plus stratified hydrogen) and five excess air ratios were applied at low speed and low load on a dual fuel SI engine with hydrogen direct injection (HDI) and gasoline port injection. The results showed that, with the increase of excess air ratio, the brake thermal efficiency increases firstly then decreases and reaches the highest when the excess air ratio is 1.1. In comparison with pure gasoline, hydrogen addition can make the ignition stable and speed up combustion rate to improve the brake thermal efficiency especially under lean burn condition. Furthermore, it can reduce the CO and HC emissions because of more complete combustion, but produce more NO_X emissions due to the higher combustion temperature. Since, in the gasoline plus stratified hydrogen mode, the hydrogen concentration near the sparking plug is denser than that of homogeneous hydrogen, the ignition is more stable and faster, which further speed up the combustion rate and improve the brake thermal efficiency. In the gasoline plus stratified hydrogen mode, the brake thermal efficiency increases by 0.55%, the flame development duration decreases by 1.0°CA, rapid combustion duration decreases by 1.3°CA and the coefficient of variation (COV) decreases by 9.8% on average than that of homogeneous hydrogen. However, in the gasoline plus stratified hydrogen mode, due to the denser hydrogen concentration near the sparking plug and leaner hydrogen concentration near the wall, the combustion temperature and the wall quenching distance increase, which make the NO_X and HC emissions increase by 14.3% and 12.8% on average than that of homogeneous hydrogen.     

Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume

Experiments were carried out inside a 25 m³ vented combustion test facility (CVE) with a fixed vent area sealed by a plastic sheet vent. Inside the CVE, a 0.64 m³ open vent box, called RED-CVE was placed. The vent of the RED-CVE was left open and three different vent area were tested. Two different mixing fans, one for each compartment, were used to establish homogeneous H₂ concentrations. This study examined H₂ concentrations in the range between 8.5% vol. to 12.5% vol. and three different ignition locations, (1) far vent ignition, (2) inside the RED-CVE box ignition and (3) near vent ignition (the vent refers to the CVE vent). Peak overpressures generated inside the test facility and the smaller compartment were measured. The results indicate that the near vent ignition generates negligible peak overpressures inside the test facility as compared to those originated by far vent ignition and ignition inside the RED-CVE box. The experiments with far vent ignition showed a pressure increase with increasing hydrogen concentration which reached a peak value at 11% vol. concentration and then decreased showing a non-monotonic behaviour. The overpressure measured inside the RED-CVE was higher when the ignition was outside the box whereas the flame entered the box through the small vent.     

A pore network study on the role of micro-porous layer in control of liquid water distribution in gas diffusion layer

In proton exchange membrane fuel cell (PEMFC), a hydrophobic micro-porous layer (MPL) is usually placed between catalyst layer (CL) and gas diffusion layer (GDL) to reduce flooding. Recent experimental studies have demonstrated that liquid water saturation in GDL is drastically decreased in the presence of MPL. However, theoretical studies based on traditional continuum two-phase flow models suggest that MPL has no effect on liquid water distribution in GDL. In the present study, a pore network model with invasion percolation algorithm is developed and used to investigate the impacts of the presence of MPL on liquid water distribution in GDL from the viewpoint at the pore level. A uniform pressure and uniform flux boundary conditions are considered for liquid water entering the porous layer in PEMFC. The simulation results reveal that liquid water saturation in GDL is reduced in the presence of MPL, but the reduction depends on the condition of liquid water entering the porous layer in PEMFC.     

The effect of dilution on the diffusive-thermal instability of the rich premixed hydrogen deflagration

The study focuses on the dynamics of rich premixed diluted hydrogen-oxygen flames. The problem of unsteady flame propagation is treated numerically within a 1D framework with a detailed model for the chemical kinetics and mixture averaged molecular diffusion. The effect of mixture dilution on the onset of the diffusive-thermal pulsating instabilities of freely propagating flames is investigated by studying the characteristics of the pulsating instabilities e.g. critical pressure of the onset, period and amplitude of pulsations. The dilution with helium, nitrogen and argon shows quantitatively a similar effect on the flame instability: namely, the critical pressure for the onset of oscillations is essentially reduced, while the period of oscillations increases. It is also shown that the neutral stability boundary is affected to a leading order through the reduction of the adiabatic flame temperature rather than by modification of the species molecular diffusion. The observed reduction of the critical pressure for the onset of oscillations and the sensitivity of the stability boundary of a steady flame propagation regime to the thermo-chemical system parameters open new perspectives for additional experimental validation of mechanisms of chemical kinetics.     

On the effects of non-uniform property distribution due to compression in the gas diffusion layer of a PEMFC

The purpose of the present study is to investigate both experimentally and theoretically the effect of GDL porosity non-uniformity on fuel cell performance due to clamping force. In the experimental study, a unit cell with a single serpentine channel is employed to test the effect of compression force on cell performance. The degree of GDL deformation is achieved by varying the thickness of a gasket spacer. In the numerical simulations, a three-dimensional model of the same geometry as the test cell is developed to simulate coupled electrochemical kinetics, current distribution, hydrodynamics, and multi-component transport. The properties of the GDL used in the simulation are expressed as functions of the compression ratio, which is defined as the ratio of compressed GDL thickness versus its uncompressed thickness. The simulation results are found to be in good agreement with experimental data in overall fuel cell performance. Numerical results obtained by using uniformly distributed GDL properties are compared with the results with non-uniform properties and it is found that although the overall cell performance is similar, local distributions from both models are significantly different. Based on the computational model, numerical simulations are performed to investigate the effects of compression ratio on local species, temperature and current distributions as well as the effects on overall cell performance. The distributions of temperature, heat flux, species concentration, current density and saturation are found to be highly oscillating in nature between the local rib and channel locations. Furthermore, the higher the compression ratio, the better is the cell performance and the larger is the fluctuation amplitude. Finally, the higher the compression ratio, the more are the saturation, water flooding and hydrogen deficiency downstream.     

First-principles study on the dissolution and diffusion behavior of hydrogen in carbide precipitates

To understand the hydrogen (H) behavior in the carbide precipitates, the dissolution and diffusion properties of interstitial H in the transition metal carbide (TMC; TM = Hf, Nb, Ta, Ti, V, and Zr) were studied by first-principles calculations. In these carbides, it can be seen that H tends to occupy the trigonal site (tri2-site) surrounded by three transition metal atoms and one carbon atom rather than the face center (fc-site) and the body center (bc-site) which with the larger space. We found that the bonding interaction between H atom and the nearest-neighbor (1NN) carbon atom is the dominant influence on the stability of H dissolution. Besides, we obtained the temperature-dependent solubility and diffusion coefficients of H in TMC and pure vanadium through Sievert's law and transition state theory. Compared with pure vanadium, H shows the worse solubility in TMC, and it is more difficult for hydrogen to migrate in TMC, but segregate toward the interface. Furthermore, it is interesting to note that, the diffusion barrier and the H solution energy show a linear relationship for transition metal carbides in the same period. These results can help us deepen the understanding of H behavior in vanadium alloys strengthened by carbide precipitates, and furtherly providing the theoretical guidance for the design of alloys with excellent performance.     

A novel multi-porous and hydrophilic anode diffusion layer for DMFC

The effect of hydrophilic treatment within the anode diffusion layer for direct methanol fuel cell (DMFC) has been investigated. By nitrated treatment, the surface structure and wettability of diffusion layer can be tuned. The anode micro-porous surface of carbon paper with hydrophilic adhesive after nitrated treatment presents more multi-hole structures with about 30 μm large pores and about 5 μm small pores, which were significantly larger than commercial carbon cloth and carbon paper without nitrated treatment. FTIR and EDS show that the surface of micro-porous layer has more oxygenic groups and the contact angles test also indicates that it becomes more hydrophilic after nitrated treatment. It is indicated that the anode charge transfer resistance and internal resistance dramatically decrease after nitrated treatment on the EIS test. The performance of assembled cell is also evaluated of which the power density of cell using novel diffusion layer (260 mW/cm²) is significantly higher than cell using commercial diffusion layer. The results indicate that this novel multi-porous and hydrophilic anode diffusion layer is suitable to DMFC.     

A study on the transport process in gas diffusion layer of proton exchange membrane fuel cells

Gas diffusion layer(GDL) plays a great important role in proton exchange membrane fuel cell(PEMFC).Water transport mechanism in GDL is still not clear.In the present study,an ex-situ transparent setup is built to visualize the transport phenomena and to measure the threshold pressure of water in GDL at different temperatures.It is found that the relationship between the breakthrough pressure and the temperature is nearly linear(i.e.the pressure decreases linearly with the increase of temperature).To avoid the problems faced by the continuum models,the pore network model is developed to simulate the liquid water transport through the carbon paper.A uniform pressure boundary condition is used in simulation and the results are similar to the ones obtained in the experiment.The reason is that the contact angle and surface tension coefficient of water in GDLs change accordingly with the change of temperature.     

Release of chemical energy by combustion in a supersonic mixing layer of hydrogen and air

The process involved in chemical energy release by combustion in a supersonic, constant pressure, hydrogen-air laminar mixing layer was studied computationally, with a chemical kinetics model involving nineteen reactions and eight species. To try to find out the physical reason for the different trends of the pressure curves observed in an experimental supersonic combustor at two different initial air stream temperatures. Two initial air stream temperatures corresponding to the two experimental cases are chosen such that the higher temperature yielded a shorter ignition distance, and the lower temperature yielded a longer ignition distance. For both cases the stream wise rate of energy release rises rapidly to a peak after ignition then falls to a post-ignition value which decreases very slowly with distance. A single premixed flame occurs at ignition for both cases, but then develops into a triple flame structure in the high temperature case, and a flame with only two branches in the low temperature case. The flames move from the airside to hydrogen side consuming the oxygen as they go, until the post-ignition phase is reached. There the dominant energy release arises from the formation of a diffusion flame. In the high temperature case a narrow lean premixed flame accompanies this diffusion flame on the airside. The flame structure, but not the energy release, is effected by the initial temperature distribution across the mixing layer, which is found to be influenced by the velocity difference between the faster air stream and the slower hydrogen stream. Increasing the concentration of oxygen atoms in the oncoming air stream was found to cause substantial reduction in the ignition distance, but did not significantly effect the flame structure, or the rate of heat release in the post-ignition phase. Finally, the different trends of pressure curves observed in the experiment can be reconstructed when pressure variation was considered in this model. Thus we can conclude that the difference in the trends of the pressure curves is caused by the difference in the initial air stream temperature.     

Numerical study on the compression effect of gas diffusion layer on PEMFC performance

A numerical method is developed to study the effect of the compression deformation of the gas diffusion layer (GDL) on the performance of the proton exchange membrane fuel cell (PEMFC). The GDL compression deformation, caused by the clamping force, plays an important role in controlling the performance of PEMFC since the compression deformation affects the contact resistance, the GDL porosity distribution, and the cross-section area of the gas channel. In the present paper, finite element method (FEM) is used to first analyze the ohmic contact resistance between the bipolar plate and the GDL, the GDL deformation, and the GDL porosity distribution. Then, finite volume method is used to analyze the transport of the reactants and products. We investigate the effects of the GDL compression deformation, the ohmic contact resistivity, the air relative humidity, and the thickness of the catalyst layer (CL) on the performance of the PEMFC. The numerical results show that the fuel cell performance decreases with increasing compression deformation if the contact resistance is negligible, but an optimal compression deformation exists if the contact resistance is considerable.     

Ca-decorated borophene as potential candidates for hydrogen storage: A first-principle study

Based on density functional theory calculation, we have investigated the hydrogen storage properties of three types of Ca-decorated borophene which have been recently synthesized experimentally. It is found that Ca atoms have strong bonding strength with borophene without the problem of aggregation. The configuration of borophene has an important effect on the H₂ storage capacity. Ca-decorated borophene of S2 and S3 are promising materials system for high-capacity hydrogen storage with gravimetric hydrogen density 9.5 wt% and 7.2 wt% while that of S1 are not. Not only the polarization mechanism but also the orbital hybridization are involved in the adsorption of H₂. The temperature and pressure have also been taken into consideration. All the hydrogen adsorbed structures of S2 and S3 are stable at room temperature under mild pressure. Our results indicate that Ca-decorated borophene of S2 and S3 structures could be expected to be promising H₂ storage materials for H₂ fuel battery.     

Experimental study on diffusion combustion of high-speed hydrogen round microjets

Experimental data on the phenomenon of nozzle choking at diffusion combustion of a high-speed hydrogen microjet at its ignition close to the nozzle are presented. As is found, such a phenomenon is due to the nozzle heating by the «bottleneck flame region» which is generated at the origin of microjet. This flow region persists up to transonic velocities of the microjet preventing from cooling of the nozzle and the transition to supersonic speed. In the case of hydrogen ignition far from the nozzle exit in supersonic conditions, the «bottleneck flame region» is suppressed, the flame becomes detached from the nozzle which is no longer heated so that the supersonic range is attained. The subsonic combustion of hydrogen microjet is stabilized by the «bottleneck flame region» while the supersonic one becomes more stable at the generation of shock cells. The results of the present study provide new details on the combustion of hydrogen microjets and could by useful for the operation of different burners.     

The Parametric Study on the Development of the Large-Scale Coherent Structures in a Mixing Layer

IntroductionDuring the past two decades, free shear layers haveattracted increasing attention afer the discovery of the1arge-scale coherent strUctllres which are the prondnentfeatures of such flows during their eary developmentand transition to ful1y turbulent flow.Wnant & Browand[l] observed the repeatedfOrmation and pairing of vortices in the spatiallydeveloping Inixing layer at moderate Reyno1ds numbers.They also fOund that the growth of a mixing layer iscontrolled by the pairing mechanis…     

An intercomparison exercise on the capabilities of CFD models to reproduce a large-scale hydrogen deflagration in open atmosphere

Within the HySafe Network of Excellence, several organizations with experience in numerical combustion modeling participated to Standard Benchmark Exercise Problem 2 (SBEP-V2), trying to reproduce numerically the explosion of a stoichiometric hydrogen-air mixture in a 10 m radius balloon. Different codes and models have been applied in the validation exercise. The simulation results and experimental data for the flame speed, the maximum pressures, the rate of pressure rise and the maximum impulse are discussed and compared by means of statistical analysis. An overall satisfactory agreement for the flame speed and maximum pressure is found.     

Compressive stress and its impact on the gas diffusion layer: A review

The gas diffusion layer is subjected to several mechanical stresses during the manufacturing and application processes of fuel cell. The deformation of gas diffusion layer under compression affects the mass transfer and electron migration between the flow channel and reaction area. This paper mainly reviews the published research on stress distribution of the gas diffusion layer, the effects of compressive stress on the bulk and pore characteristics of the gas diffusion layer, and the performance of fuel cell. Furthermore, the relationship between stress and strain of the gas diffusion layer is summarized. The impacts of compression on the gas diffusion layer present the characteristics of multi-parameters, non-linearly, small variation and strong coupling. This review suggests that the compression affects the pore and bulk characteristics of gas diffusion layer should be comprehensively investigated.     

Effects of gas diffusion layer structure on the open circuit voltage and hydrogen crossover of polymer electrolyte membrane fuel cells

The clamping pressure of polymer electrolyte membrane fuel cells for vehicle applications should be typically high enough to minimize contact resistance. However, an excessive compression pressure may cause a durability problem. In this study, the effects of gas diffusion layer (GDL) structure on the open circuit voltage (OCV) and hydrogen crossover have been closely examined. Results show that the performances of fuel cells with GDL-1 (a carbon fiber felt substrate with MPL having rough surface) and GDL-3 (a carbon fiber paper substrate with MPL having smooth surface) are higher than that with GDL-2 (a carbon fiber felt substrate with MPL having smooth surface) under low clamping torque conditions, whereas when clamping torque is high, the GDL-1 sample shows the largest decrease in cell performance. Hydrogen crossover for all GDL samples increases with the increase of clamping torque, especially the degree of increase of GDL-1 is much greater than that of the other two GDL samples. The OCV reduction of GDL-1 is much greater than that of GDL-2 and GDL-3. It is concluded that the GDL-3 is better than the other two GDLs in terms of fuel cell durability, because the GDL-3 shows the minimum OCV reduction.