Effect of fuel jet arrangement on the mixing rate inside trapezoidal cavity flame holder at supersonic flow

Fuel mixing inside the supersonic combustion chamber is a significant process for development of modern scramjets. In this article, computational fluid dynamic (CFD) approach is applied to investigate the effect of various fuel injections on the mixing rate inside the supersonic combustion chamber. The mixing of hydrogen jets with four different arrangements inside the cavity flame holder is comprehensively studied. In order to examine the effect of multi jets within a cavity flameholder, a three-dimensional model is established and Navier-stocks equations are solved to simulate the flow and mixing zone inside a cavity region. Obtained results show that the injection of hydrogen jet from the bottom of cavity flame holder considerable enhances the ignition zone within the cavity. Moreover, the backward fuel injection is more superior to forward fuel injection since low-pressure vortex could significantly distribute the fuel and enlarge the mixing zone inside the cavity flame holder.     

Improvement of proton-exchange membrane fuel cell performance using platinum-loaded carbon black entrapped in crosslinked chitosan

To improve the performance of proton-exchange membrane fuel cells which use hydrogen and oxygen as fuels, the application of small proton-conducting polymer to extend the three-phase boundary into the primary pores of catalyst-loaded carbon black agglomerates is of interest. An alternative and simple crosslinking method is proposed in place of the complicated polymer-grafting methods. Platinum-loaded carbon black is entrapped in epichlorohydrin-crosslinked chitosan of low molecular weight. Morphology and pore analyses of carbon black prior and post treatment are assessed, as well as performances of fuel cells fabricated with the treated and the untreated carbon black at 40 °C and 100% humidity. Results indicate the existence of chitosan chains in the primary pores of the carbon black agglomerates, corresponding to a decline in the activation overvoltage and resulting in significantly better cell performance. An increase in chitosan amount, however, does not necessarily enhance the cell performance because effects of ohmic and concentration losses may become more dominant than that of the raised exchange current density of the cell.     

Coupled computing for reactive hydrogen leakage phenomena with crack propagation in a pressure tank

This research focuses on reactive hydrogen leakage due to the fracturing of high-pressure hydrogen tanks to develop a coupled computing method that can simultaneously perform reactive fluid-structure interaction analyses. The integrated computational approach to reactive hydrogen leakage with combustion due to wall crack propagation was implemented using a hybrid of the coupled particle and Eulerian methods. This computational method provides valuable safety information for predicting crack propagation and hydrogen leakage with combustion reaction in pressure tanks as an essential part of assessing hydrogen as an energy vector. As a result, the effect of crack formation and wall boundary conditions on hydrogen turbulent diffusion and concentration distribution and the thermodynamic behavior of the chemical reaction were predicted computationally. It was found that a combustion reaction does not occur near the streamwise axis at the duct center because there is an excess of hydrogen fuel, which increases scalar dissipation, and that the combustion reaction separated into upper and lower regions as it proceeded.     

Characteristics of direct injection combustion fuelled by natural gas–hydrogen mixtures using a constant volume vessel

The effects of hydrogen addition and turbulence intensity on the natural gas–air turbulent combustion were studied experimentally using a constant volume vessel. Turbulence was generated by injecting the high-pressure fuel into the vessel. Flame propagation images and combustion characteristics via pressure-derived parameters were analyzed at various hydrogen volumetric fractions (from 0% to 40%) and the overall equivalence ratios of 0.6, 0.8 and 1.0. The results showed that the turbulent combustion rate increased remarkably with the increase of hydrogen fraction in fuel blends when hydrogen fraction is over 11%. Combustion rate was increased remarkably with the introduction of turbulence in the bomb and decreased with the decrease of turbulence intensity. The lean flammability limit of natural gas–air turbulent combustion can be extended with increasing hydrogen fraction addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased monotonically with the increase of hydrogen fraction in fuel blends. The sensitivity of natural gas/hydrogen hybrid fuel to the variation of turbulence intensity was decreased while increasing the hydrogen addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased with the increase of turbulent intensity at stoichiometric and lean-burn conditions. However, slight influence on combustion characteristics was presented with variation of hydrogen fraction at the stoichiometric equivalence ratio with and without the turbulence in the bomb.     

Gradation of mechanical properties in gas-diffusion electrode. Part 2: Heterogeneous carbon fiber and damage evolution in cell layers

In PEM fuel cell, gas-diffusion electrode (GDE) plays very significant role in force transmission from bipolar plate to the membrane. This paper investigates the effects of geometrical heterogeneities of gas-diffusion electrode layer (gas-diffusion layer (GDL) and catalyst layer (CL)) on mechanical damage evolution and propagation. We present a structural integrity principle of membrane electrode assembly (MEA) based on the interlayer stress transfer capacity and corresponding cell layer material response. Commonly observable damages such as rupture of hydrophobic coating and breakage of carbon fiber in gas-diffusion layer are attributed to the ductile to brittle phase transition within a single carbon fiber. Effect of material inhomogeneity on change in modulus, hardness, contact stiffness, and electrical contact resistance is also discussed. Fracture statistics of carbon fiber and variations in flexural strength of GDL are studied. The damage propagation in CL is perceived to be influenced by the type of gradation and the vicinity from which crack originates. Cohesive zone model has been proposed based on the traction-separation law to investigate the damage propagation throughout the two interfaces (carbon fiber/CL and CL/membrane).     

Solid-phase temperature measurements in a HTPEM fuel cell

Segmented temperature measurements were performed to better understand the thermal behaviour and thermal interactions between the fluid-(gas)-phase and solid-phase temperature within a working high temperature polymer electrolyte membrane (HTPEM) fuel cell. Three types of flow-fields were studied, and the influence of temperature for no-load and load operating conditions was investigated. Tests were performed under various operating conditions, and the results demonstrate the utility of segmented temperature measurements. A significant difference in the temperature distribution was observed when the HTPEM fuel cell was operated with pure hydrogen and with hydrogen containing carbon monoxide. The findings may lead to improved HTPEM fuel cells and future middle temperature polymer electrolyte membrane (MTPEM) fuel cell designs.     

Thermodynamic analysis of the emptying process of compressed hydrogen tanks

During the driving of fuel cell vehicles, the fast depressurization of compressed hydrogen tanks plus the high storage pressure and the low thermal conductivity of carbon fiber reinforced plastic (CFRP) can lead to significant cooling of the tank. This can result in a temperature below −40 °C inside the compressed hydrogen tanks and cause safety problems. In this paper, a thermodynamic model that incorporates the nature of external natural convection was developed to describe the emptying process of compressed hydrogen tanks and was validated by experiments. Thermodynamic analyses of the emptying process were performed to study the global heat transfer characteristics and the effects of ambient temperature, defueling rate, defueling pattern, initial and final density of hydrogen gas, liner and CFRP thickness and the crosswind velocity on the final temperature decreases of hydrogen gas, the inner wall and the outer wall.     

Sub-nano-Pt cluster supported on graphene nanosheets for CO tolerant catalysts in polymer electrolyte fuel cells

The carbon monoxide (CO) tolerance performance of polymer electrode fuel cells (PEFCs) was studied for a catalyst composed of graphene nanosheets (GNS) with sub-nano-Pt clusters. The Pt catalysts supported on the GNS showed a higher CO tolerance performance in the hydrogen oxidation reaction (HOR), which was significantly different from that of platinum on carbon black (Pt/CB). It is proposed that the presence of the sub-nano-Pt clusters promotes the catalytic activity and that the substrate carbon material alters the catalytic properties of Pt via the interface interactions between the graphene and the Pt.     

Temperature effect in cavitation risk assessments of polymers for hydrogen systems

Hydrogen storage at high pressure is currently attained by the use of different materials, such as elastomers in sealing joints, thermoplastics and thermosetting polymers in high-pressure containers, and metallic tube connections. Hydrogen containers type IV use a thermoplastic polymer for hydrogen tightness and composite materials for mechanical resistance, usually made with thermosetting resins and carbon or glass fibre. International standards impose a wide range of operative temperatures for such containers, from −40 °C to 85 °C.Once saturated with hydrogen at high pressure, a fast depressurisation process can create stress in the polymeric materials, causing its degradation by the formation of cavities. In a previous work, we were able to make a generalization of cavitation risk by the use of non-dimensional parameters, based on a simplified mechanical failure model. We observed that for the model, material's hydrogen diffusivity and yield strength are of upmost importance. In present work, we analyse the effect of temperature on these two properties, as they have an inverse evolution with temperature. Results confirm the pertinence of considering temperature in the whole application range of technology under analyse.     

Effects of fillers on the hydrogen uptake and volume expansion of acrylonitrile butadiene rubber composites exposed to high pressure hydrogen: -Property of polymeric materials for high pressure hydrogen devices (3)-

To develop sealing materials for high-pressure hydrogen devices, the effects of filler type and amount on the hydrogen uptake and volume expansion of rubber composites were evaluated up to 90 MPa. The amount of hydrogen in the rubber matrix and carbon black was elucidated using nuclear magnetic resonance, and the hydrogen elimination behavior of the composites was analyzed by thermal desorption analyses. As hydrogen physically adsorbed on carbon black, the hydrogen uptake of carbon black-filled composites increased. The hydrogen uptake of the composites filled with nonabsorbent silica was smaller, depending on the weight fraction of silica. The ratio of volume expansion to the amount of hydrogen in the matrix of carbon black-filled composites was suppressed by the reinforcement effect of carbon black, which did not expand with hydrogen uptake. The suppression of silica-filled composites was limited by the volume fraction of silica.     

Injection of multi hydrogen jets within cavity flameholder at supersonic flow

Efficient distribution of hydrogen gas inside the supersonic chamber is the main challenge for the increasing the performance of the supersonic vehicles. In this study, the new injection arrangements of the multi hydrogen jets within the cavity flameholder are comprehensively studied at a supersonic free stream. In order to investigate the effect of multi jets within a cavity flameholder, a three-dimensional model is developed and computational technique is used to simulate the flow and mixing zone inside this region. The influence of important parameters such as the pressure of jet and free stream Mach number is investigated to illustrate the flow pattern and evaluate the mixing rate in the supersonic combustion chamber. Obtained results show that the rise of the total pressure of hydrogen jet enlarges the ignition zone within the cavity. Furthermore, the increase of free stream Mach number limited the mixing rate and jet interaction. Our findings confirm that fuel jet with PR = 0.5 significantly enhances the performance of the cavity flameholder inside the scramjet.     

燃料电池车车载储氢系统的技术发展与应用现状

综述了燃料电池车车载储氢系统技术，包括高压氢、液氢、金属氢化物、低温吸附、纳米碳管高压吸附以及液体有机氢化物等的研究进展及其车载应用现状。参照燃料电池车对车载储氢系统单位重量储氢密度与体积储氢密度的目标要求，对目前已应用和处于研发阶段的一些储氢技术的性能指标和存在问题进行了分析讨论。同时对目前该领域的若干新的研究报道，如超高压轻质复合容器、混合储氢容器、b.c．c．储氢合金、超级活性碳和“浆液”双相储氢等，也作了简要介绍。     

Spontaneous ignition of high-pressure hydrogen during its sudden release into hydrogen/air mixtures

This paper investigates the effects of hydrogen additions on spontaneous ignition of high-pressure hydrogen released into hydrogen-air mixture. Hydrogen and air are premixed with different volume concentrations (0%, 5%, 10%, 15% and 20% H₂) in the tube before high-pressure hydrogen is suddenly released. Pressure transducers are employed to detect the shock waves, estimate the mean shock wave speed and record the shock wave overpressure. Light sensors are used to determine the occurrence of high-pressure hydrogen spontaneous ignition in the tube. A high-speed camera is used to capture the flame propagation behavior outside the tube. It is found that only 5% hydrogen addition could decrease the minimum storage pressure required for spontaneous ignition from 4.37 MPa to 2.78 MPa significantly. When 10% or 15% hydrogen is added to the air, the minimum storage pressure decreases to 2.81 MPa and 1.85 MPa, respectively. When hydrogen addition increases to 20%, the spontaneous ignition even takes place at burst pressure as low as 1.79 MPa inside the straight tube.     

Study of cyclic variations of direct-injection combustion fueled with natural gas–hydrogen blends using a constant volume vessel

Cyclic variations of direct-injection combustion fueled with natural gas–hydrogen fuel blends were experimentally studied using a constant volume vessel. Direct-injection combustion was realized by injecting the high-pressure fuel into the vessel. Flame propagating photographs and pressure history in the vessel were recorded at various hydrogen volumetric fractions in the fuel blends (from 0% to 40%) under the same lean-burn conditions where the overall equivalence ratios are 0.6 and 0.8, respectively. The effect of fuel–air mixture inhomogeneous distribution and hydrogen addition on the cyclic variations was analyzed via flame development photographs and pressure-derived combustion parameters. The results indicated that the cyclic variations were initiated at the early stage of flame development. The flame kernel is closely concentric to the spark electrode and flame pattern has less irregular with hydrogen addition. Direct-injection natural gas combustion can achieve the stable lean combustion along with low cyclic variations due to the mixture stratification in the vessel. The cyclic variations decreased with the increase of hydrogen addition and this trend is more obvious at ultra-lean-burn condition. Hydrogen addition weakened the effect from turbulent flow on flame propagating process, thus reduce the cyclic variations related to the gas flow. There exists interdependency between the early combustion stage and the subsequent combustion process for direct-injection combustion.     

Computational investigation of multi-cavity fuel injection on hydrogen mixing at supersonic combustion chamber

Enhancement of the mixing inside the combustor is a significant process for increasing the efficiency of the scramjet. This work applied the computational method for the investigation of the depth of the cavity on the flow feature of the multi hydrogen jet in the supersonic crossflow. The main focus of this research is to evaluate the depth of the cavity on the mixing rate of the hydrogen jets inside the combustion chamber. CFD method with the SST turbulence technique is applied for the simulation of the fluid flow inside the domain. The impact of the depth of the cavity, the pressure of the fuel jet and the number of the jet are comprehensively explained in this study. Our findings show that the rising of the cavity enhances the mixing inside the domain due to more fuel distribution along the spanwise direction. Our results clearly demonstrate that replacing the single jet with 8 equivalent multi jets increases the mixing rate of more than 45% in the vicinity of the jet injection. Attained results revealed that increasing the jet space develops the mixing in far downstream. Obtained results also show that mixing intensifies 15% when jet space of 8 microjets is increased from 4 dj to 10 dj.     

Non-dimensional assessments to estimate decompression failure in polymers for hydrogen systems

Polymer materials subjected to gases at high-pressure can have issues during decompression. For instance, a sudden decompression can promote the formation of cavities inside the material. This phenomenon is known as cavitation or eXposive Decompression Failure (XDF). There is a body of scientific articles discussing different aspects of cavitation phenomenon, which indicate that the degree of damage is proportional to saturation pressure, depressurisation rate, and material thickness, among other parameters.In this article we propose a general approach by non-dimensional parameters to estimate the risk of cavitation. Numerical results were validated with bibliographic evidence of cavitation in polymers, for both thermoplastics and elastomers. Present results can be used as guidelines for design of systems involving polymers under high pressure, such as o-rings or liners in type IV hydrogen containers.     

Highly stable Pt and PtPd hybrid catalysts supported on a nitrogen-modified carbon composite for fuel cell application

The durability and cost of fuel cell cathode catalysts are major technical barriers to the commercialization of fuel cells for vehicle applications. In this work, novel Pt and PtPd hybrid catalysts are developed that use a nitrogen-modified carbon composite (NMCC), which is itself active for the oxygen reduction reaction (ORR), instead of a conventional carbon black support. The fuel cell accelerated stress test (AST) for supports and catalysts demonstrated that the Pt₃Pd₁/NMCC and Pt/NMCC hybrid catalysts possess much higher stability than Pt/C catalysts in polymer electrolyte membrane (PEM) fuel cells. Moreover, the hybrid catalysts exhibit higher mass activity than the Pt/C catalysts. The origin of the hybrid catalysts’ improved performance relative to Pt/C is discussed in light of pore size distribution and surface area analysis, XRD, XPS, and TEM analyses and electrochemical measurements.     

Enhancement of oxygen reduction activity with addition of carbon support for non-precious metal nitrogen doped carbon catalyst

An improved synthesis scheme of non-precious metal N-doped carbon catalysts for oxygen reduction reaction is reported. The non-precious metal N-doped carbon catalysts were prepared by pyrolysis of the mixture (phenol resin, Ketjen black carbon support and cobalt phenanthroline complex). The drastic improvement of distribution state of Ketjen black supported non-precious metal N-doped carbon catalysts was observed by means of transmission electron microscopy (TEM). In addition, the non-precious metal N-doped carbon catalyst synthesized with Ketjen black carbon support showed much higher oxygen reduction reaction (ORR) activity relative to unsupported non-precious metal N-doped carbon catalyst in O₂-saturated 0.5 mol l⁻¹ H₂SO₄ at 35 °C. Moreover, the highest ORR activity was obtained with addition of optimum amount of Ketjen black carbon support was thirtyfold compared to unsupported non-precious metal N-doped carbon catalyst at 0.7 V. Similarly, the performance of a polymer electrolyte fuel cell (PEFC) using the non-precious metal N-doped carbon catalyst as the cathode electrode catalyst was obviously better than that of the non-precious metal N-doped carbon catalyst before optimization. Microstructure, specific surface area and surface composition of the non-precious metal N-doped carbon catalysts were analyzed by XRD, XPS and BET measurement with nitrogen physisorption, respectively.     

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.     

Mechanism of spontaneous ignition of high-pressure hydrogen during its release through a tube with local contraction: A numerical study

The tendency of spontaneous ignition of high-pressure hydrogen during its sudden release into a tube is one of the main threats to the safe application of hydrogen energy. A series of investigations have shown that the tube structure is a key factor affecting the spontaneous ignition of high-pressure hydrogen. In this paper, a numerical study is conducted to reveal the mechanism of spontaneous ignition of high-pressure hydrogen inside the tube with local contraction. Large Eddy Simulation, Renormalization Group, Eddy Dissipation Concept, 37-step detailed hydrogen combustion mechanism and 10-step like opening process of burst disk are employed. Three cases with burst pressures of 3.10, 4.90, and 8.45 MPa are simulated to compare against the pervious experimental study. The spontaneous conditions and positions agree well with the experimental results. The numerical results indicate that shock wave reflection takes place at the upstream vertical wall of contraction part. The interacted-shock-affected region is generated at the tube center because of the subsequent shock wave interaction. The forward reflected shock wave couples with normal shock wave and increases the pressure of leading shock wave. The sudden contraction of tube blocks the propagation of hydrogen jet and decreases the speed from supersonic flow to subsonic flow. More flammable mixture is generated inside the contraction part, as a results, the length of the flame is increased. Two mechanisms are proposed finally.