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.     

Non-equilibrium evolution and characteristics of the serrated microchannel hydrogen knudsen compressor

The hydrogen Knudsen compressor has great potential to transport hydrogen and provide the required pressure in MEMS and microfluidic systems. The microchannel composed of cold and hot serrated surfaces is beneficial to the temperature control of the multistage Knudsen compressor. In the present study, a serrated hydrogen Knudsen compressor model is established initially, and the non-equilibrium evolution is numerically studied by using the method of N–S equations with the slip boundary. The key factors affecting the non-equilibrium evolution are comprehensively analyzed. The flow behaviors and performance of the serrated hydrogen Knudsen compressor in different times are studied. It is found that the main factors affecting the non-equilibrium evolution are the thermal expansion flow, thermal transpiration flow, and Poiseuille flow. Meanwhile, the serrated structure affects the local flow in the serrated microchannel at different times. Under the interaction of the thermal transpiration flow and the Poiseuille flow, the pressure difference between the two containers first increases rapidly and then decreases slowly, and finally approaches 1886 Pa. The research reveals the flow mechanisms of the hydrogen Knudsen compressor in the non-equilibrium evolution, which provides theoretical support for the safety and reliability of the hydrogen Knudsen compressor.     

Three dimensional channel effect on the flow characteristics and the performance of hydrogen Knudsen compressors

As a new type of the micro fluidic device, Knudsen compressor can provide the potential utilizations on the hydrogen transport in the micro systems. Considering actual structure of the compressor is three-dimensional, flow characteristic studies are the key issue for the performance predictions. Firstly, the model of three-dimensional Knudsen compressor is built, and the validity of the model is proved by comparison with the experimental result. Secondly, the flow behaviors in the three-dimensional model is investigated, and the distributions of pressure and velocity are investigated. Also, the performance of the hydrogen Knudsen compressor in two-dimensional structure and three-dimensional structure are compared and discussed. Thirdly, the three-dimensional hydrogen Knudsen compressors with different width are analyzed, and the pressure increase in different cases of the hydrogen Knudsen compressors are studied.     

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.     

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.     

Effect of pressure ratio on transient flow in hydrogen circulating pump

The pressure ratio has an important influence on the performance and internal flow characteristics of the positive displacement pump. In this paper, the influence of the four pressure ratios 1.1/1.2/1.3/1.4 on the internal flow characteristics of the hydrogen circulating pump is studied, the internal relationship between the change of pressure ratio and the flow pattern in the pump are clarified, and the leakage flow pattern and its coupling mechanism in each gap are revealed. The results show that the gap leakage flow induced by pressure difference is an important reason for flow disorder in the pump, however, the generation and growth of gap leakage flow will be affected not only by the pressure difference, but also by the shear drive. The scale and influence of axial gap leakage are far greater than the other two types of gap leakage. The existence of gap leakage flow makes the rate of flow and pressure presents a large amplitude high-frequency pulsation characteristic. The research results of this paper provide a reference for the efficient and stable operation of hydrogen circulating pump in fuel cell system.     

Study of cyclic performance of V-Ti-Cr alloys employed for hydrogen compressor

The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V₄₀Ti_21.5Cr_38.5 and (2) V₂₀Ti₃₂Cr₄₈, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.     

Thermal transpiration effect on the mass transfer and flow behaviors of the pressure-driven hydrogen gas flow

Thermal transpiration is a rarefied gas effect that drives the gas flow creeping in a microchannel due only to an imposed temperature gradient, which is often encountered in the hydrogen-transportation microfluidic applications such as proton exchange membrane fuel cell (PEMFC). Because of its impact on the pressure-driven flow behavior in the microchannel, this pumping phenomenon needs to be studied in designing and improving microfluidic devices for hydrogen transportation. However, so far little literature has discussed the thermal transpiration effects on the flow behaviors under normal boundary conditions. In this paper, a DSMC-SPH coupled multiscale approach is proposed on the study of the thermal transpiration effect on hydrogen gas multiscale flow behaviors. Various wall temperature distributions are used under a pressure-driven condition. The remarkable influence of thermal transpiration on the multiscale hydrogen gas flow are investigated and discussed. Since the thermal transpiration effect is often occurred in hydrogen transportation, the present simulation results can provide significant insights for designing and improving proton exchange membrane fuel cell (PEMFC).     

Influences of a kind of groove-blade on the flow field in a compressor cascade

This paper presents an experimental investigation of effects of a kind of streamwise-grooved blades on the flowfield in a compressor cascade.The flow field downstream the cascade and the boundary layer on the suctionsurface were measured using a mini 5-hole pressure probe at different incidence angles.The flow field in thegroove cascade was compared with that in the smooth cascade.The measurement results indicate that:(1)thegroove surface can restrain the development of the boundary layer on the suction surface;(2)the grooves canrestrain the radial migration of the low-energy fluids in the boundary layer on the suction surface;(3)the grooveblades can reduce total pressure loss and flow blockage in the cascade at the incidence angles of 0°,5°and 8°;(4)the maximum benefit of 8.6% loss reduction was obtained at the incidence angle of 5° while negative benefit of-3.0% loss reduction occurred at the incidence angle of-5°.     

Effect of back-diffusion on the performance of an electrochemical hydrogen compressor

Hydrogen offers the potential to decarbonize the automotive and stationary power sectors and is therefore expected to play an increasingly significant role in meeting global energy demand. However, due to its low volumetric and gravimetric energy densities, it is important to find methods to efficiently store hydrogen in order to grow the hydrogen economy. Storing hydrogen as a compressed gas could be achieved by electrochemical compression (ECC), which is a membrane-based alternative to conventional mechanical compressors. ECC can be superior to mechanical compressors because of its higher efficiency, lack of moving parts and noiseless operation. Here, we report on the ECC of hydrogen using a Nafion 115 membrane at room temperature. Pressure vs. time curves have been collected at various operating voltages, and a compression ratio of 150 has been achieved with a single cell at an operating voltage of 0.1 V. This work focuses on the loss in electrochemical compression efficiency due to back-diffusion. A theoretical formulation for the ECC process incorporating back-diffusion is proposed and validated by experiments. A robust definition for ECC efficiency that properly accounts for back diffusion is also proposed.     

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.     

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.     

Dynamic and energy analysis of a liquid piston hydrogen compressor

Liquid piston compressor is the most promising compressor to be used for hydrogen-refueling stations. However, their energy transfer and the energy dissipation processes of are poorly studied and not well understood. In this paper, a new energy analysis method for an ionic-liquid type liquid piston compressor is proposed. In the compressor section, porous media is used to promote heat transfer from the hydraulic oil during the compression process. A mathematical model has been formulated considering the heat transfer and damping effects of the porous media on the compressor performance. Moreover, the compressibility of the hydraulic oil and its overflow loss on the compressor performance were also established. In the model, the seven stages of the entire working cycle of the compressor were look into in detail, alongside with its energy efficiency. The results show that the key parameters governing the energy efficiency of the compressor are the heat transfer efficiency of the compressor and the overflow losses of the hydraulic oil.     

Effect of hydraulic oil compressibility on the volumetric efficiency of a diaphragm compressor for hydrogen refueling stations

The low volumetric efficiency of the diaphragm compressor under hydrogen refueling process, which hereby results in poor energy efficiency and high cost of hydrogen applications, should be paid attention to. This paper presents theoretical analysis and experimental investigation of the factors affecting the volumetric efficiency of the diaphragm compressor for hydrogen refueling process, focusing on the influence of hydraulic oil compressibility. A mathematical model was established to estimate the volumetric efficiency of diaphragm compressors, in which the effects of clearance volume, superheating of suction gas and pressure loss were taken into account and the emphasis was focused on the compressibility of hydraulic oil. A test rig was built to validate the theoretical model and further experimental investigations were carried out to identify the factors influencing the oil compressibility and hereby the volumetric efficiency. The volumetric efficiency was measured and compared under varied oil compressibility conditions by varying elastic modulus, oil overflow pressure and oil volume. The results indicated that the measured volumetric efficiency agrees well with the calculated value. The compression and expansion of hydraulic oil have a dominant influence on the volumetric efficiency, resulting in a loss of 37% of volumetric efficiency as compared to 2.4%, 18% and 1%, respectively for losses associated with clearance volume, superheating of suction gas and pressure loss, for a diagram compressor under refueling conditions with suction pressure of 30 MPa and discharge pressure of 90 MPa. The volumetric efficiency reduced rapidly with the increased oil overflow pressure, at a rate of 5% decrease with every 10 MPa rise in oil overflow pressure. As the oil volume increased by 100% of the stroke volume, the volumetric efficiency droped by 5.5%.     

Numerical investigation on the wave transformation in the ionic liquid compressor for the application in hydrogen refuelling stations

The ionic liquid compressor exhibits excellent advantages in hydrogen refuelling stations due to the specific design based on the hydraulic system and the ionic liquid piston. The application of the ionic liquid column results in a complex two-phase flow issue inside the compression chamber. This two-phase flow behaviour is critical for the compressor design as it influences the wave dynamics during the compression, but it is absent in the open literature. In this paper, transit numerical simulations were carried out to investigate the wave transformation during a compression cycle by the volume of fluid (VOF) method under different heights of the ionic liquid piston. The effect of liquid height on the wave transformation, discharged quantity of ionic liquid and hydrogen gas, and the turbulence kinetic energy was analysed. The minimum crest value of the turbulent kinetic energy was observed as 0.54 kJ in the cases of 30 and 40 mm. The optimal height of the ionic liquid piston was recommended 40 mm under the presented design condition based on the simulation results.     

给定轴分速时轴流压气机级性能的解析优化

用一维理论对轴流压气机的初步设计作进一步的研究,导出了轴流压气机基元级的特性关系,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,所得结论具有很好的普适性,并可进一步拓广用于多级轴流压气机优化中。     

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.     

Modelling of a hydrogen thermally driven compressor based on cyclic adsorption-desorption on activated carbon

Thermally driven hydrogen compression by cyclic hydrogen adsorption-desorption on activated carbon is presented therein. Hydrogen compression occurs through heat exchange, which allows physisorbed hydrogen to desorb at higher temperature in a given volume. The physical nature of hydrogen adsorption on porous carbon allows reversible desorption, and a flow of compressed hydrogen is then obtained by running adsorption/desorption cycles repeatedly. We investigated the feasibility of such a system through numerical simulations by taking into account both mass and energy balances, and adsorption thermodynamics. We showed that high-pressure hydrogen, up to 70 MPa, can be obtained by simply lowering and/or increasing the system temperature. Such a system opens new perspectives in the frame of the Hydrogen Supply Chain.     

Influence of air supply on the performance and internal flow characteristics of a cross flow turbine

This study attempts to improve the efficiency of a given type of cross flow turbine by supplying air from air suction holes. A newly developed air supply method is adopted. CFD analysis of the cross flow turbine is carried out to investigate the performance and internal flow characteristics of the turbine in detail. The air layer prevents shock loss between water flow and axis and suppresses recirculation flow in the runner passage. Hence, it is necessary to measure the amount of air layer in the runner passage and examine its effect on the performance of the cross flow turbine. The result shows that the turbine efficiency has improved more as the newly developed air supply method is applied effectively.     

Three-dimensional numerical simulation of the slip flow through triangular microchannels

Three-dimensional numerical analysis for fully developed incompressible fluid flow and heat transfer through triangular microchannels over the slip flow regime is simulated in this paper. In order to study the flow through the channel, the Navier–Stokes equations are solved in conjunction with slip/jump boundary conditions. The influences of Knudsen number (0.001 < Kn < 0.1), aspect ratio (0.2 < A < 4.5), and Reynolds number (1 < Re < 15) on the fluid flow and heat transfer characteristics are extensively investigated in the paper. The numerical results reveal that the rarefaction decreases the Poiseuille number, while its effect on the Nusselt number completely depends on the interaction between velocity slip and temperature jump. It is also found that the aspect ratio has an important role in the analysis, but the variation of Reynolds number is less remarkable.