The hydrogen flow characteristics of the multistage hydrogen Knudsen compressor based on the thermal transpiration effect

Multistage hydrogen Knudsen compressor based on the thermal transpiration effect has very exciting prospect for the hydrogen transmission in the micro devices. Understanding of the hydrogen flow characteristic is the key issue for the designs and applications of the hydrogen energy systems. Firstly, the numerical models of the multistage hydrogen Knudsen compressor are established. The distributions of the rarefaction, velocity and temperature at different stages of the hydrogen flow are calculated and presented. Moreover, the dimensional pressure increases of the hydrogen gas flow are analyzed, and the flow behaviors in the microchannel and the connection channel are discussed. Secondly, the numerical simulation at different connection channel height is implemented, and the hydrogen gas flow characteristics in the connection are analyzed. Especially, the performances of the pressure drop in the connection channel under different channel heights are studied, and the hydrogen gas compression characteristics of different cases are compared and discussed. Also, the effect of the connection channel height on the hydrogen gas pressure increase in the microchannel is investigated. The studies presented in this paper could be greatly beneficial for the hydrogen detection and transmission.     

Effect of the microchannel obstacles on the pressure performance and flow behaviors of the hydrogen Knudsen compressor

The hydrogen Knudsen compressor has potential applications on the hydrogen transmission for the microdevices and systems. In this paper, the numerical model of the hydrogen Knudsen compressor was established, combining the NS continuity equations with the slip boundary conditions. The effect of structures on the performance of the hydrogen Knudsen compressor is studied by generating different obstacles in the microchannels. This paper is mainly concerned on the rectangular and the triangular obstacles, and the influence of the obstacles length and height are investigated, respectively. The Knudsen number distribution and the rarefaction of the hydrogen gas flow are analyzed. Also, the characteristic of the pressure increase for the compressor under different parameters are investigated and discussed. The effect of the structure parameters on the flow velocity distributions are detailed described, as well as the velocity contour and the vortex distributions. Moreover, the variation of the Knudsen layers of the hydrogen gas flow in the hydrogen Knudsen compressor is presented, and the key factor of the Knudsen layers is analyzed and discussed. The results is significantly beneficial for the applications and designs of hydrogen Knudsen compressor.     

Characteristics of thermal transpiration effect and the hydrogen flow behaviors in the microchannel with semicircular obstacle

The thermal transpiration effect has great potential applications for the hydrogen energy. In this paper, the thermal transpiration effect and the hydrogen flow behaviors are studied in the microchannel with the semicircular obstacles. Firstly, the slip boundary model is used in the simulation of the flow performance in the microchannel. The validity of the model at different Kn is verified by comparing with some previous work. Then, the hydrogen flow characteristics of the thermal transpiration effect with the semicircular obstacle are investigated. The result shows that as the size of the semicircular obstacle increases, the hydrogen flow path of the thermal transpiration effect becomes longer, and the temperature gradient decreases. As the characteristic length of the hydrogen flow decreases, there is an obviously negative influence on the thermal transpiration flow. A deeper analysis shows that the thermal driven flow and the pressure driven flow will produce y-component velocity, which leads to a backflow under the effect of semicircles, and the semicircular obstacles make the Knudsen layer spread to the channel center.     

Research on sealing performance and self-acting valve reliability in high-pressure oil-free hydrogen compressors for hydrogen refueling stations

Piston ring sealing and valve design play an important role in high-pressure oil-free reciprocating compressors for hydrogen refueling stations. The severe non-uniformity of the pressure distribution was suggested to be the root cause of the premature failure of the sealing rings, and therefore a mathematical model was established to simulate the unsteady flow within the gaps of piston rings, based on which the pressure distribution was obtained and the mechanism of the non-uniform abrasion of the rings was disclosed. The method to equalize the pressure difference through each ring was proposed by re-distributing the cut size of each ring, and it was validated experimentally. Aiming at the problem that the self-acting valves in hydrogen compressors could be easily destroyed by severe impact, this paper investigated the motion and impact of valves theoretically and experimentally, based on which the methodology was explored to design the parameters of valves for hydrogen compressors.     

Flow and transmission characteristics of the multistage hydrogen Knudsen pump in the micro-power system

The multistage hydrogen Knudsen pump based on the thermal transpiration effect has exciting application prospects for hydrogen transport in the micro-power system. The multistage hydrogen Knudsen pump with the silica microchannel is beneficial to its temperature control, which can accurately provide hydrogen transport and storage for the micro-power system. In this paper, the model of the multistage hydrogen Knudsen pump with the silica microchannel is established. The effects of the microchannel height, width and parallel number on the flow and transmission characteristics of the multistage hydrogen Knudsen pump are studied by using the method of N–S equations with the slip boundary. The temperature difference, Knudsen number, thermal transpiration effect, maximum mass flow rate, maximum pressure difference and performance curve under different microchannel parameters are analyzed in detail. The results show that the thermal transpiration effect increases with the microchannel height and decreases with the microchannel width. As the number of parallel microchannels increases, the microchannel is closer to the silicon cantilever, and the thermal transpiration effect becomes stronger. The pumping performance increases with the microchannel height, width and parallel number. The pressurization performance increases with the microchannel height and parallel number. The research results have important guiding significance for the application and design of the multistage hydrogen Knudsen pump in the micro-power system.     

Evaluation of hydrogen concentration effect on the natural gas properties and flow performance

The natural gas flowing through transmission pipeline is impure and has a wide range of non-hydrocarbons components at different concentrations like hydrogen. The presence of hydrogen in the natural gas mixture influences its properties and flow performance. The effect of hydrogen concentration on the natural gas flowing through a transportation pipeline has not been adequately investigated and widely comprehended. In this paper, several mixtures flow through pipeline include typical natural gas and hydrogen at different concentrations up to 10% are evaluated to demonstrate their impact on the flow assurance and the natural gas properties. The string Ruswil – Griespass part from the Transitgas project with 94 km length is simulated applying Aspen Hysys Version 9 and validated using Aspen Plus. The simulation specifications were 1.228 1 10⁶ kg/h mass flowrate, 1200 mm and 1164 mm the outer and inner diameters, and 75 bar and 29.4 °C operating pressure, and temperature. The effect of different hydrogen concentrations has been examined and the differences from the typical mixture are estimated. The results show that the presence of hydrogen in the natural gas mixture reduces its density, 10% hydrogen content records 11.78% reduction in the density of typical natural gas. Interestingly, it has been found that up to 2% of hydrogen concentration turns in elevating the viscosity of the typical natural gas while the viscosity decreases at the point that hydrogen content increases above 2%. In addition, the pressure losses over the transmission pipeline increases due to the presence of hydrogen, 10% hydrogen concentration turns in 5.39% increase in the pressure drop of the natural gas mixture. Also, the temperature drop across the pipeline decreases as the hydrogen concentration increases; 10% hydrogen content can result in a 6.14% reduction in the temperature drop across the pipeline. As well as, the findings prove that the hydrogen strongly impacts the phase envelope by changing from size symmetric to size asymmetric diagram. The effect of pipeline elevations has been investigated by changing the elevation up to 25 m uphill and 25 m downhill. The results state that increase the pipeline elevation turns in increasing the pressure losses over the pipeline length. Along with this, the results illustrate that the presence of hydrogen in the mixture elevates the critical pressure and reduces the critical temperature.     

Study on the high-pressure hydrogen gas flow characteristics of the needle valve with different spool shapes

The needle valve is a critical control unit for high-pressure hydrogen systems such as hydrogen refueling stations, which is the infrastructure of hydrogen energy. As an important part of the needle valve, the valve spool affects the flow characteristics of hydrogen in the valve and then affects the working performance and safety of the high-pressure hydrogen valve. In this paper, based on the real hydrogen gas model and the finite volume method, a CFD model of the high-pressure hydrogen needle valve is constructed to find out the influence of the valve spool shape on the performance and flow characteristics of the high-pressure hydrogen needle valve. The results show that high-pressure hydrogen will produce a sudden change in pressure around the valve spool and there will be a local high-speed area, and the turbulent intensity will also increase. The arc cone spool can increase the flow by 2%–8% at different openings of the valve, and reduce the maximum speed at the spool by 15% at small openings. In addition, the sudden change of pressure and the eddy current have also been improved. Flat-bottomed cone spool reduces turbulence intensity and energy consumption. Therefore, it can be concluded that changing the shape of the valve spool to have a larger flow area at a small opening can make the high-pressure hydrogen valve have a better flow field distribution. Flattening the cone angle of the spool can improve the turbulent flow in the valve. The research in this paper can provide research accumulation and theoretical support for the optimization design of the needle valve of the high-pressure hydrogen system.     

Effect of hydrogen addition on the detonation performances of methane/oxygen at different equivalence ratios

Detonation performances of methane/hydrogen/oxygen (CH₄/H₂/O₂) mixtures were investigated experimentally in a 3000 mm long tube with an inner diameter of 30 mm at different initial pressures $p_0$ (ranging from 10 kPa to 50.5 kPa). Mixtures with different proportions of H₂ in the total fuel $\alpha$ (0%, 14.29% and 25%) and different equivalence ratios $\Phi$ (0.8, 1.0 and 1.2) were tested. Signals of flame front and pressure were obtained by ion probes and high frequency pressure transducers, respectively. Results showed that with the increase of $p_0$, $\alpha$and $\Phi$, the average velocity of steady detonation $V_{ave}$ increased. For mixtures with the given $\alpha$, when $\Phi$ increased by 0.2, $V_{ave}$ increased by 100 m/s. In the present study, velocity deficits were found to be within 5%, and when $p_0$ was higher than 20 kPa, the velocity deficits were within 2%. The average peak pressure of steady detonation $p_{ave}$ was close to the von Neumann pressure $p_{vN}$. Both the increase of $p_0$ and $\Phi$ led to the increase of the $p_{ave}$. But the addition of H₂ led to the decrease of $p_{ave}$, and $p_{ave}$ decreased with the increased of $\alpha$.     

一维多级轴流压气机性能的解析优化

用一维理论对轴流压气机的初步设计作进一步的研究，导出了多级轴流压气机特性关系，建立了在给定轴向分速时最优化设计的数学模型，得到了解析关系，所得结论具有一定的普适性，对多级轴流压气机的初步设计有一定的指导作用。     

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ?. A wide range of ? ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ? exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP₀, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ? vs. IP. In certain cases (e.g. the combinations of ? = 0.6 and IP₅₀₀ or IP₇₀₀), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ? gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ?. More specifically, in the leanest (? = 0.4) and the richest (? = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (? = 0.8–3.0), periodical pressure oscillations were only observed for IP₉₀₀. The maximum pressure peaks P_max were reached at ? = 1.25 rather than at the highest reactivity ? = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ?.     

Three dimensional numerical computations on the fast filling of a hydrogen tank under different conditions

Hydrogen-fueled vehicles offer a clean and efficient alternative for transportation. Compressed gas in high pressure tanks is a popular storage mode for hydrogen fuel. Time required for filling a hydrogen tank for vehicular applications should be short. But quick filling of hydrogen tanks at high pressures can result in high gas temperatures which can damage the tank and lead to its rupture. Hence the real time monitoring of gas temperature is essential during filling. This paper reports the findings of numerical simulation of filling process of hydrogen tanks. Real gas effects are considered. Local temperature distribution in the tank is obtained at different durations of the fill. Effect of changes in ambient temperature and initial and inlet gas temperatures is studied. Results of the study can aid in optimizing the filling time and in identifying the most suitable locations for the feedback devices within on-board hydrogen tanks.     

Transient flow performance and heat transfer characteristic in the cylinder of hydraulic driving piston hydrogen compressor during compression stroke

With the advantages of large flow capacity and high pressure, the use of hydraulic driving piston compressors in hydrogen refueling stations is becoming the development trend. Understanding transient flow and heat transfer characteristic is the key issue for the design and application of hydrogen compressors. The transient model of the hydraulic driving piston compressor is constructed by dynamic mesh and the National Institute of Standards and Technology (NIST) real hydrogen model, which accurately predicts flow field and heat transfer. Moreover, the effect of piston reciprocating cycle frequency on hydrogen parameters variation and heat transfer characteristic is investigated. Adiabatic compression theory is commonly applied in the design of reciprocating compressors. The results show that due to the heat transfer, the exhaust temperature predicted by the adiabatic compression theory is 6.29 K higher than the actual value. This study provides beneficial references for the design optimization and reliable operation of hydraulic driving piston hydrogen compressors.     

Salinity,temperature and pressure effect on hydrogen wettability of carbonate rocks

Hydrogen had been injected into the geologic formations, and the geologic formation wettability would influence the hydrogen storage. Hydrogen wettability of sandstone reservoirs (quartz), mica and other rocks have been explored in the previous study. However, the research on hydrogen wettability of carbonate rocks was lacked. In this study, we studied the carbonate rock wettability alteration when exposed to the hydrogen environments. Salinity, temperature and pressure effect on H₂/carbonate rock/brine wettability were explored. When the solutions ions concentration increased, the advancing/receding contact angle would increase, and divalent ions could make the contact angle higher than monovalent ion, which was because ions could compress the electric double layer. The carbonate rock powder in brine showed negative charge, and the zeta potential increased with higher ions concentration. When temperature increased and the pressure decreased, the contact angle would decrease, which was related to the H₂ gas density and molecular interactions.     

Theoretical study of the dynamic characteristics of a self-commutating liquid piston hydrogen compressor

Hydrogen is being more and more widely deployed in various fields for its ‘clean’ character. For applications in automobiles where hydrogen has already been adopted for years, higher pressure means better mileage. To improve the pressure of the hydrogen compressor, a novel self-commutating liquid piston hydrogen compressor is proposed in the present study. A two-stage hydrogen booster is designed on both sides of the hydraulic cylinder piston, which is driven by a spool installed in the cylinder piston. The benefits of the novel hydrogen compressor are reducing the throttling loss and enhancing the response of the piston. Furthermore, the principle of the hydrogen compressor is illustrated, based on which a dynamic model is established while taking oil compressibility, leakage and flow force in the compression process into consideration. Moreover, system simulation model is established by applying the simulation software, verifying the feasibility and validity of the novel structure. Accordingly, the energy efficiency on the mechanical-hydraulic structure is improved.     

Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Study on the performance and characteristics of fuel cell coupling cathode channel with cooling channel

Water flooding in the cathode channel of the proton exchange membrane fuel cell (PEMFC), which reduce the current density output and affect fuel cell lifetime. Hence, to suppress water flooding, a novel channel is proposed in this study, that is to perforate hole between the cooling channel and cathode channel. A 3D numerical model is used to investigate the influence of the parameters including the hole's dimension, position, numbers, the operation conditions of the PEMFC and the slope angle (θ) of the incline cooling channel. The numerical results indicate that the optimal single hole parameters are 0.4 mm long, 0.5 mm wide and 20 mm position, which can maximum the current density output of the PEMFC. Increasing the hole numbers for novel channels can improve water removal. In addition, in comparison with the conventional channel with θ = 0.20° at 1.8 cathode stoichiometry, the H5 (novel channel with five holes) with θ = 0.20° decreases by 43.10% in the maximum water saturation of cathode channel, while increases by 12.54% in current density output. What's more, all the novel channel structure research hardly raises the pressure drop of channels.     

Numerical investigation on the characteristics of mass transport and performance of PEMFC with baffle plates installed in the flow channel

A 3D numerical model of proton exchange membrane fuel cell (PEMFC) with the installation of baffle plates is developed. The majority of the conservation equations and physical parameters are implemented through the user defined functions (UDFs) in the FLUENT software. The characteristics of mass transport and performance of PEMFC are investigated. The results reveal that the baffle plate can enhance the mass transport efficiency and the performance of PEMFC. The baffle plate installed in the PEMFC flow channel increases the local gas velocity, which can promote the reactant gas transport and the liquid water removal in the porous electrode. As a result, the reactant gas concentration is larger in the porous electrode, which enhances the fuel cell performance for decreasing the over-potential of concentration. The fuel cell output power increases with the blockage ratio of the baffle plate. Considering the extra pumping power resulted from pressure loss caused by the baffle plate, the fuel cell with the blockage ratio of 0.8 is found to perform best in terms of the fuel cell net power generation. The fuel cell performance increases first with the baffle plate number, due to the better reactant distribution and water management, but decreases when the baffle plate number is too large, due to the excessive blockage for the reactant gas transport to the channel downstream. The PEMFC investigated with 5 baffle plates in the channel is found to be optimal. A channel design to achieve gradually increasing blockage ratios is also proposed, which exhibits better cell performance than the design with even blockage ratios.     

Evaluation of ionic liquids as replacements for the solid piston in conventional hydrogen reciprocating compressors: A review

Substituting the solid piston of conventional reciprocating compressors used for the compression of hydrogen with a suitable ionic liquid will solve many practical problems and limitations that conventional reciprocating compressors face. However, because of the large number of cation and anion combinations and many studies on the unique properties of ionic liquids and the role of ionic liquid cations and anions in determining these properties, a systematic review is required to narrow down the choice of ionic liquids. Therefore, in the present review, a comprehensive study to find the most appropriate ionic liquid candidate to replace the solid piston in reciprocating compressors for compressing hydrogen is reported.Specific criteria concerning the applications of ionic liquids are determined and the roles of the cations and anions, as well as the effect of temperature, are extensively reviewed to identify the most suitable ionic liquid that can fulfill the requirements. As a next step, the options are narrowed down to five ionic liquids with the triflate and bis(trifluoromethylsulfonyl)imide as the anion choices and three different cation types, imidazolium-, phosphonium-, and ammonium-based, as the cation choices. Finally, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is recommended as the best candidate that can be safely used as a replacement for the solid piston in reciprocating compressors for compressing hydrogen in hydrogen stations.     

The ignition characteristics of the pre-chamber turbulent jet ignition of the hydrogen and methane based on different orifices

In this paper, the combustion characteristics of premixed CH₄-air and H₂-air mixtures with different excess air coefficients ignited by hot jet or jet flame are investigated experimentally in a constant volume combustion chamber (CVCC). The small volume pre-chambers with different orifices (2 or 3 mm in diameter) in the passive or active pre-chamber were selected. Both the high-speed Schlieren and OH1 chemiluminescence imaging are applied to visualize the turbulent jet ignition (TJI) process in the main chamber. Results show that the variation of orifice has diverse influences on the turbulent jet ignitions of methane and hydrogen. Smaller orifices will reduce the temperature of the jet due to the stronger stretch and throttling effect, including change of lean flammability limit, ignition delay, and re-ignition location. Furthermore, shock waves and pressure oscillations were captured in the experiments with hydrogen jets. The former is related to the jet velocity, while the latter is mainly affected by the mixture thermodynamic states in the main chamber. Furthermore, the re-ignition location is discussed. If the mixture reactivity and the jet energy are sufficiently high, the reaction will be initiated at the tip of the jet in a short time. On the contrary, a relatively long time is required to prepare the mixture during the entrainment when the reactivity is not high enough, and the corresponding re-ignition location will move towards the orifice exit owing to the temperature decline at the tip. Finally, the ignition mode transition of hydrogen jet in lean cases with a 2 mm orifice is explained.     

Effects of hydrogen addition on combustion and flame propagation characteristics of laser ignited methane/air mixtures

In this study, effects of hydrogen addition on combustion and flame propagation characteristics of methane/air mixtures were investigated in a constant volume combustion chamber. Tested gas mixtures are 100% CH₄, 05% H₂ – 95% CH₄, 10% H₂ – 90% CH₄ and 15% H₂ – 85% CH₄, and such mixtures were ignited using a passively Q-switched Nd:YAG laser ignitor which has a pulse energy of 12.3 mJ, pulse duration of 2.4 ns and wave length of 1064 nm. A Schlieren setup coupled with a high-speed camera enabled evaluating flame propagation behavior, while pressure curve analysis provided necessary data for characterization of combustion properties. Additionally, lean flammability limits of gas mixtures were also determined at the test conditions. The unique properties of hydrogen (such as low density, high reactivity, high diffusivity etc) widened lean flammability limit. Rate of pressure rise and measured pressure values increased with hydrogen addition, regardless of the air-fuel equivalence ratio (λ). Lastly, hydrogen addition uniformly affected flame propagation characteristics and flame luminosity. Combustion process became more stable with hydrogen addition.