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.     

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.     

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.     

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.     

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.     

Experimental investigation of the effect of channel length on performance and water accumulation in a PEMFC parallel flow field

Longer channels within serpentine flow fields are highly effective at removing liquid water slugs and have little water accumulation; however, the long flow path causes a large pressure drop across the cell. This results in both a significant concentration gradient between the inlet and outlet, and high pumping losses. Parallel flow fields have a shorter flow path and smaller pressure drop between the inlet and outlet. This low pressure drop and multiple routes for reactants in parallel channels allows for significant formation of liquid water slugs and water accumulation. To investigate these differences, a polymer electrolyte membrane fuel cell parallel flow field with the ability to modify the length of the channels was designed, fabricated, and tested. Polarization curves and the performance, water accumulation, and pressure drop were measured during 15 min of 0.5 A cm⁻² steady-state operation. An analysis of variance was performed to determine if the channel length had a significant effect on performance. It was found that the longer 25 cm channels had significantly higher and more stable performance than the shorter 5 cm channels with an 18% and an 87% higher maximum power density and maximum current density, respectively. Channel lengths which result in a pressure drop, across the flow field, slightly larger than that required to expel liquid water slugs were found to have minimal water accumulation and high performance, while requiring minimal parasitic pumping power.     

Effect of hydrogen nanobubble addition on combustion characteristics of gasoline engine

Hydrogen is an attractive energy source for improving gasoline engine performance. In this paper, a new hydrogen nanobubble gasoline blend is introduced, and the influence of hydrogen nanobubble on the combustion characteristics of a gasoline engine is experimentally investigated. The test was performed at a constant engine speed of 2000 rpm, and engine load of 40, 60, and 80%. The air-to-fuel equivalence ratio (λ) was adjusted to the stoichiometric (λ = 1), for both gasoline, and the hydrogen nanobubble gasoline blend. The results show that the mean diameter and concentration of hydrogen nanobubble in the gasoline blend are 149 nm and about 11.35 × 10⁸ particles/ml, respectively. The engine test results show that the power of a gasoline engine with hydrogen nanobubble gasoline blend was improved to 4.0% (27.00 kW), in comparison with conventional gasoline (25.96 kW), at the engine load of 40%. Also, the brake specific fuel consumption (BSFC) was improved, from 291.10 g/kWh for the conventional gasoline, to 269.48 g/kWh for the hydrogen nanobubble gasoline blend, at the engine load of 40%.     

Three-dimensional modeling of pressure effect on operating characteristics and performance of solid oxide fuel cell

A three-dimensional (3-D) model for planar, anode-supported, solid oxide fuel cell (SOFC) is developed to investigate the effect of operating pressure on cell characteristics. The results show that the elevated operating pressure can improve cell performance by increasing open circuit voltage and reducing activation overpotential, and enhance the electrochemical reaction in the vicinity of electrolyte. Besides, the high pressure can also change the distributions of species and internal reforming reactions. Compared to the case using syngas as fuel, the operating pressure has more significant effects on temperature gradient along flow direction when partly pre-reformed gas is supplied. In addition, efficient control of cell temperature could be achieved by decreasing fuel utilization in the case of partly pre-reformed gas, but this is achieved at the expense of cell efficiency, especially under high pressure condition. Another way to reduce the temperature gradient is to adopt higher air ratio. Moreover, when partly pre-reformed gas is used, the counter-flow configuration has a better performance due to the higher overall temperature.     

波节管管内流动传热特性的三维数值模拟

利用Fluent三维数值模拟的方法,以水为介质,研究波节管管内流动传热特性,并进行场协同分析。结果表明:波节管波纹段内强回流区的存在,使其管内扰动明显,管内流动与传热特性都呈周期性规律变化,与外界换热效果良好。经计算,在入口流速为2m/s时,波节管的纵截面平均场协同数为0.347,场协同角为69.69°。     

The effect of 3D silver nanodome size on hydrogen evolution activity in alkaline solution

Three-dimensional (3D) Ag nanodomes (AgNDs) having different sizes (400, 800, 1200 and 1600 nm) were fabricated using combination of nanosphere lithography and soft lithography. The surface structures of 3D assembled latex particles, nanovoids and metal nanodomes (ND) were examined using scanning electron microscopy (SEM). Their heights and widths analyses were performed with the help of atomic force microscopy (AFM). The effect of diameter of the NDs on their hydrogen evolution activity was examined in 6 M KOH solution at 298 K using electrochemical techniques. Their activities were compared with the activity of bulk Ag electrode. The preparation of 3D-AgNDs having various diameters and examination of their size effects on the water splitting activity have not been studied yet and are being reported firstly. It was found that very well-structured and very uniformly distributed NDs can be fabricated using this procedure. AgNDs exhibit higher hydrogen evolution activity with respect to bulk Ag. Their hydrogen evolution activity depends on their diameters; 1200 nm NDs were the best among them. The current density at ?1.40 V(Ag/AgCl) which is proportional to the rate of hydrogen releasing reaction increases from 0.70 mA cm^?2 to 44.13 mA cm^?2 at this ND electrode with respect to the bulk Ag electrode. At the same 3D-AgNDs electrode and potential, the resistance against the HER reduces from 148.7 Ω cm² to 1.12 Ω cm² (99.6%) by comparing with the bulk Ag electrode. The average surface roughness factors of bulk Ag, 400 nm, 800 nm, 1200 nm and 1600 nm AgNDs are 8, 123, 100, 291 and 176, respectively. The superior hydrogen evolution performance of this electrode is related to its well-structured surface and large real surface area.     

An experimental study on performance and emission characteristics of a hydrogen fuelled spark ignition engine

In the present paper, the performance and emission characteristics of a conventional four cylinder spark ignition (SI) engine operated on hydrogen and gasoline are investigated experimentally. The compressed hydrogen at 20  MPa has been introduced to the engine adopted to operate on gaseous hydrogen by external mixing. Two regulators have been used to drop the pressure first to 300 kPa, then to atmospheric pressure. The variations of torque, power, brake thermal efficiency, brake mean effective pressure, exhaust gas temperature, and emissions of _NOx${\mathrm{NO}}_x$, CO, _CO2${\mathrm{CO}}_2$, HC, and _O2${\mathrm{O}}_2$ versus engine speed are compared for a carbureted SI engine operating on gasoline and hydrogen. Energy analysis also has studied for comparison purpose. The test results have been demonstrated that power loss occurs at low speed hydrogen operation whereas high speed characteristics compete well with gasoline operation. Fast burning characteristics of hydrogen have permitted high speed engine operation. Less heat loss has occurred for hydrogen than gasoline. _NOx${\mathrm{NO}}_x$ emission of hydrogen fuelled engine is about 10 times lower than gasoline fuelled engine. Finally, both first and second law efficiencies have improved with hydrogen fuelled engine compared to gasoline engine. It has been proved that hydrogen is a very good candidate as an engine fuel. The obtained data are also very useful for operational changes needed to optimize the hydrogen fueled SI engine design.     

A simulation and design tool for hydrogen SI engine systems—Validation of the intake hydrogen flow model

A simulation and design tool applicable to hydrogen powered spark ignition engine systems is introduced in this paper. This software is applicable to single and multi-cylinder engines under steady state or transient operating conditions, and is capable of simulating one-dimensional unsteady chemical species transport through intake and exhaust engine ducting, the induction and combustion of those chemical species, and the engine performance characteristics and emissions which are produced. Results are presented from validation studies carried out on a 1.6 l spark ignition engine converted to operate with manifold-fuelled gaseous hydrogen. These experimental results validate the ability of the simulation to accurately describe the transport of gaseous hydrogen through engine intake ducting, and the displacement of intake air due to hydrogen introduction.     

Engine characteristics of emissions and performance using mixtures of natural gas and hydrogen

An evaluation was performed on the efficiency and emissions from an engine fuelled with compressed natural gas (CNG) and a mixture of natural gas and hydrogen, respectively. The mixtures of CNG and hydrogen were named HCNG.     

Effects of content of hydrogen on the characteristics of co-flow laminar diffusion flame of hydrogen/nitrogen mixture in various flow conditions

Effect of content of hydrogen (H₂) in fuel stream, mole fraction of H₂$\left(X_{H_2}\right)$ in fuel composition, and velocity of fuel and co-flow air $\left(V_{avg}\right)$ on the flame characteristics of a co-flow H₂/N₂ laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with $X_{H_2}$ varying between 0.25 and 1, and $V_{avg}$ between 0.25 and 1 ms^?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either $X_{H_2}$ or $V_{avg}$. Significant effect on the flame temperature and NOx emission are obtained from a higher $X_{H_2}$ in fuel whereas the flame size and heat release are the result of increasing $V_{avg}$. Along with this finding, the role of N₂ and its higher content reducing the flame temperature and NOx emission are presented.     

Numerical simulation of the effect of hydrogen addition fraction on catalytic micro-combustion characteristics of methane-air

Understanding of micro-scale combustion mechanism is very essential to the development of micro-power devices, so hydrogen assisted catalytic combustion of methane on platinum was studied in this paper. The combustion of preheated mixtures of methane-hydrogen-air in a micro-combustor was modeled by a two-dimensional model including an elementary-step surface reaction mechanism. It was demonstrated that the model could predict the effects of changes of hydrogen fraction. It was shown that the mole fraction of H, OH and C(s) increase and ignition time decreases with hydrogen addition. It was also shown that the improving effect of hydrogen on the ignition temperature of the fuel and O(s) coverage is particularly evident at relatively low hydrogen fraction. The promotion of the combustion stability is due to the decrease of coefficient of variation with hydrogen addition. The methane combustion will move toward the more stabilized reaction and there is a great potential to reduce the pressure fluctuation.     

Influence of partial substitution of Mo for Cr on structure and hydrogen storage characteristics of non-stoichiometric Laves phase TiCrB0.9 alloy

The influence of the partial substitution of Mo for Cr on phase composition and hydrogen storage characteristics of non-stoichiometric Laves phase TiCrB_0.9-based alloys is investigated by X-ray diffraction (XRD), pressure composition isotherm (PCT), and scanning electron microscopy (SEM) characterizations. XRD tests reveal that the phase composition of the alloys gradually changes from single TiB_1.07CrB_1.93 Laves phase to the co-existence of Laves phase and Mo-based BCC phase with increasing substitution of Mo for Cr. The phase composition eventually transforms into a single Mo-based BCC phase when the amount of the substitution surpasses a certain level. PCT tests reveal that the maximum hydrogen storage capacity increases with increasing Mo content. The hydrogenation-induced phase changes are also greatly influenced by the substitution of Mo for Cr. SEM tests of the hydrided alloys show that the increasing Mo content enhances hydrogenation-induced pulverization. Finally, hydrogenation-induced phase changes during the course of activation are also investigated.