Search

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. [Display omitted] • A two-phase flow model with a phase change is implemented. • The two-phase flow model and single-phase flow model are compared. • The phase change characteristics and liquid phase distribution are presented. • The effect of the phase change on the ejector performance is analysed. [ABSTRACT FROM AUTHOR]

The hydrogen recirculation ejector, known for its low cost and simple structure, has garnered widespread attention in PEM fuel cell applications. As a fluid-driven passive component, the fluid dynamics inside the ejector play a pivotal role in determining its entrainment performance. This study explores the impact of the primary flow temperature on phase change characteristics based on the Euler-Lagrange model. Results indicate that at a primary flow temperature of −40 °C, the mass flow rate of droplets at the outlet reaches 285.2 mg/s, representing a condensation efficiency of 24.1%. Concurrently, condensation elevates the outlet temperature by 31.2 °C, resulting in an 8.8% reduction in the entrainment ratio. The mass flow rate of droplets decreases significantly as the primary flow temperature increases. As the temperature rises to 60 °C, homogeneous droplets vanish entirely. Simultaneously, foreign heterogeneous droplets evaporate by 42.0%, attributed to the negative subcooling degree at the diffuser and outlet pipe. This study emphasizes the significance of controlling the primary flow temperature for the hydrogen recirculation ejector. [Display omitted] • Condensation efficiency increases with decreasing primary flow temperature. • 24.1% of the water vapor condenses at a primary flow temperature of −40 °C. • Condensation reduces the entrainment ratio and enhances outlet temperature. • Evaporation occurs when the primary flow temperature rises to 60 °C. [ABSTRACT FROM AUTHOR]

Abstract The scroll pump has a great potential to recirculate hydrogen in a fuel-cell vehicle (FCV) because of its high efficiency, low noise and vibration, reliable operation, and a wide range of adjustable flow. This paper presents three-dimensional transient computational fluid dynamics (CFD) modelling of a scroll-type hydrogen pump used in FCVs, including leakage flow through both the radial clearance (RC) and axial clearance (AC). A dynamic mesh was generated for the moving orbiting scroll, and high-quality hexahedral structured grids with sufficient grid-density were applied to the clearances to solve the multi-scale problem. The pressure and velocity fields were obtained at different rotating angles to reveal the dynamic characteristics in the compression chambers. The simulation results showed that the radial leakage through AC has more significant influence on the volumetric efficiency than the tangential leakage through RC, especially on scroll-type hydrogen pumps. The presented modelling and simulation methods were validated experimentally by operating a scroll air compressor at different speeds and pressure ratios. The volumetric efficiency of the scroll pump was 85.39% with 0.02 mm AC and 0.02 mm RC, 81.43% with 0.02 mm AC and 0.04 mm RC, and 70.17% with 0.04 mm AC and 0.02 mm RC. Further, it was found that the performance of scroll-type hydrogen pumps is more sensitive to rotating speed than air scroll pumps under the same conditions. With hydrogen, the volumetric efficiency increased by 30.68% when the rotating speed was increased from 3000 r·min−1 to 6000 r·min−1; with air, the volumetric efficiency increased by 12.81%. Therefore, it is necessary to consider both AC and RC in the CFD modelling of scroll machines, particularly in the case of hydrogen scroll pumps. Highlights • 3D transient CFD numerical simulation with dynamic meshing was presented. • Sufficient grid-density mesh was obtained in multi-scale fluid domains. • The dynamic pressure and velocity fields at different angles were obtained. • Leakage characteristics of the radial clearance and axial clearance were revealed. [ABSTRACT FROM AUTHOR]

This paper presents the root cause analysis of the blade fracture of the motor cooling fan in a reciprocating compressor package. The dynamic analyses results showed that the first-order torsional natural frequency of the fan was within ±5% of the fifth-order excitation frequency. In addition, the maximum dynamic stress of the fan operating in conditions of torsional resonance was 40.88 MPa, and exceeded the allowable stress of 36.12 MPa. Furthermore, the maximum dynamic stress occurred at the same locations where the cracks initially emerged. It was concluded that the excessive dynamic stress caused by the torsional resonance was the root cause of the fatigue fracture of the fan blade. Design improvements were implemented to adjust the natural frequencies of the fan, and included reducing the height of the blades, increasing the number of blades, and changing the weld locations. After the implementation of these modifications, the first-order torsional natural frequency of the new fan avoided the resonance regions, and the maximum dynamic stress of the new fan was 0.69 MPa, which was considerably lower than the allowable stress. No fracture problems occurred on the new fan. [ABSTRACT FROM AUTHOR]

In this study, the oil droplets separation process in a swirling flow was investigated using numerical simulations and verification experiments. Numerical models were established to investigate the effects that the collision and breakup of droplets and the collision between droplets and wall on separation efficiency, respectively. The separation efficiencies were calculated using different models under various inlet velocities, from 10 to 18 m/s. The simulated results showed that droplets colliding could improve the separation efficiencies with up to 22.4% increments at the 10 m/s inlet velocity for droplets with a diameter of 10 μm. The oil–gas separation efficiency was reduced up to 17.4% due to the oil droplet–wall interaction in swirling flow when the simulation model involved droplet–wall collisions. The simulated separation efficiencies showed little change when oil droplet breakups were considered in the model under different inlet velocities and droplet diameters. A test rig was built to validate the simulation results. It showed good agreement between the experimental data and simulated results. Thus, the numerical simulation model could be used to predict separation performance and understand the separation process in a swirling flow. [ABSTRACT FROM AUTHOR]

Ionic liquid compressors are suitable for high-pressure hydrogen refuelling stations. The initial liquid piston height is a critical parameter in designing ionic liquid compressors. Sufficient initial liquid column height can effectively prevent hydrogen leakage and exposure of the solid piston. However, there is a lack of basis for selecting the initial ionic liquid column height in the compression cylinder. The gas-liquid interface in the multi-stage compression is investigated by an experimentally verified numerical simulation model. The results show that in the low-pressure stage of the ionic liquid compressor, the initial liquid piston height can equal to the piston stroke, but in the high-pressure stage, the solid piston could be exposed to the hydrogen, so it is necessary to increase the initial liquid piston height. By analyzing the fluctuation characteristics in the cylinder, it is found that there is a minimum initial liquid piston height in each stage to ensure effective sealing. The minimum liquid piston height for the first to the fifth stage is 52 mm, 40 mm, 44 mm, 51 mm, and 54 mm, respectively. • The compression cylinders of a five-stage ionic liquid compressor were designed. • A numerical method was established and verified by experiment data. • The characteristics of the gas-liquid interface were analyzed. • The minimum initial ionic liquid height in each stage was recommended. [ABSTRACT FROM AUTHOR]

Regenerative pumps are considered a promising option for hydrogen recirculation in proton exchange membrane fuel cell (PEMFC) systems. The geometry of the impeller exerts a direct influence on the pressurization process within the regenerative pumps. However, the influence has not been explored systematically in any available publications. This study investigates the effects of geometric parameters on pump performance and provides design guidelines for hydrogen regenerative pumps. A three-dimensional (3D) numerical model is developed and validated. The effects of critical geometric parameters, including the number of blades, impeller radius, and impeller radius ratio, on the performance of the regenerative pumps are then analyzed. Based on the analysis, this work recommends 35 blades and an impeller radius ratio of 0.65. To establish a correlation between operational performance and micro-flow mechanisms, the fluid velocity distribution at the interface of the rotating and stationary domains is quantitatively analyzed. Results indicate that a reduction in the flow rate results in increased axial velocity and decreased tangential velocity. The increased axial velocity enhances mass transfer, thus increasing the pressure and diminishing the efficiency, whereas the decreased tangential velocity intensifies the impact of the fluid on the blades, leading to increased shock losses and reduced efficiency. • The effects of the number of blades, impeller radius, and impeller radius ratio on the performance of hydrogen regenerative pumps are revealed. • The optimal values of the number of blades and impeller radius ratio are proposed. • The fluid flow velocities at the interface of rotating and stationary domains are quantitatively analyzed. • The flow mechanism of regenerative machines is elucidated through numerical methods. [ABSTRACT FROM AUTHOR]

• Experiments of the oil droplet impact on the inclined surface were performed. • Phenomenological description of droplet spreading under various inclined angle and We. • Effects of the surface roughness on the normalized droplet spreading. • The orders of the normalized droplet spreading under various parameters. This study experimentally investigated the wetting process of an oil droplet impacting on the dry surface with different surface roughness (0.3 μm, 1.6μm, 2.9μm, 5.2 μm) from the front view. Experiments were conducted under three impacting Weber ranges with the inclined angles of 37.0°, 63.0°, and 75.5°. The droplet spread as a 'pancake' on the surface when the inclination angle was 37.0°, while the shape of an 'avocado' of the flowing oil layer was observed on higher inclined angle surfaces. The upper part of the 'avocado' became narrower when the inclined angle increased from 63.0° to 75.5°, which then grew longer with the increase in the impact We. The normalized transverse and longitudinal spreading displacement β x and β y were defined, whose evolution with normalized time was then obtained. Three stages of the β x were divided and analyzed. Under the same We range and inclined angle, the largest value of the maximum normalized droplet transverse spreading displacement (β x m a x) was obtained with the smoothest surface (0.3 μm) followed by the roughest surface (5.2 μm). β x m a x was affected by the surface roughness, which also influences the contact angle of the surface. When the surface roughness and the contact angle were regarded as two separate factors, it was found that for the transverse spreading, the lower the inclined angle, the more obvious effect of the surface roughness than that of the contact angle. [ABSTRACT FROM AUTHOR]

• Experiments of the oil–gas impingement onto the vertical wall were performed. • Effects of the surface roughness on the entrainment height were studied by tests. • The oil–gas impinging mixture was characterized by the Weber number. • Effects of the Weber number on the entrainment height are investigated. • The lowest entrainment height was obtained at the roughness level of 1.1 μm. The entrainment of oil–gas mixture from the wall significantly affects the performances of inertial separators. By spraying the oil–gas mixture onto a vertical wall, the effect of surface roughness on entrainment height is investigated in this study. The oil droplet size distribution on the impingement surface is measured using a Malvern particle size analyser, whereas the impingement velocity field is measured using particle image velocimetry technique. The impingement and entrainment profiles are captured through high-speed photography, and the entrainment height is identified from digital image processing. Results show that the entrainment heights range from 2 to 12 mm on the surface with a roughness of 0.3-4.5 μm when the impinge velocity is 3–5 m/s and the Sauter mean diameter of the impinging droplets is approximately 40 μm. The lowest entrainment height in all operating conditions is obtained at the surface roughness level of 1.1 μm. [ABSTRACT FROM AUTHOR]

The startup-shutdown behavior of the gas foil bearings-rotor system is vital for proton exchange membrane fuel cells due to the inherently frequent start-stop of hydrogen fuel cell vehicles, which significantly affects the sustainable power generation and efficiency of the fuel cells. In this paper, the effects of three systematic factors including bump foil structure, nominal clearance, and coating on the dynamic response of the bearings-rotor system are first investigated based on startup-shutdown experiments. The mathematical model considering both gas film and thermo-hydrodynamic characteristics is presented to investigate the startup-shutdown behavior further. The influencing mechanisms of the factors on performance are analyzed. The energetic results demonstrate that the bearing structure parameters have a significant influence on the startup-shutdown performance and energy consumption. Considering optimal startup-shutdown performance, a better bearings-rotor system is proposed. Compared to the average values in all cases, the take-off speed, temperature rise, and average energy consumption of the system are decreased by 51.3%, 16.2%, and 60.7%, respectively. This indicates that the startup-shutdown improvement of the bearing-rotor system has great significance for enhancing the energy efficiency of the fuel cell. [ABSTRACT FROM AUTHOR]

• Homogeneous nuclei manifest when stack power exceeds 31 kW. • The average median diameter of heterogeneous droplets increases by 26.5 %. • A short residence time yields minimal droplets of 1.79 mg/s at 100 kW. • Condensation induces a maximum 4.18 % reduction in the entrainment ratio. The hydrogen recirculation ejector plays a pivotal role in proton exchange membrane fuel cell systems. Nevertheless, a comprehensive grasp of the intricate two-phase flow characteristics within hydrogen recirculation ejectors remains elusive. This investigation delves into the condensation and droplet behaviors of an ejector designed for a 100 kW fuel cell stack. The analysis relies on a two-phase flow model that takes into account both homogeneous and heterogeneous condensation. The findings reveal that homogeneous condensation nuclei manifest within the mixing chamber when the stack power surpasses 31 kW. The average median diameter of homogeneous droplets at the ejector outlet measures 0.367 μm, whereas that of heterogeneous droplets stands at 1.265 μm, signifying a 26.5 % augmentation compared to their initial size. Additionally, two crucial factors impact droplet size: residence time and subcooling degree. At the 100 kW condition, the droplet residence time is a mere 0.272 ms, yielding a meager droplet flow rate of merely 1.79 mg/s. In contrast, at the 53 kW condition, the downstream of the mixing chamber exhibits the maximum subcooling degree, resulting in the maximum droplet flow rate of 22.12 mg/s. A temperature elevation of 2.68 K at the ejector outlet is observed due to the latent heat of condensation. Furthermore, condensation has a marginal impact on the entrainment ratio, with the most significant reduction being 4.18 %, averaging at 1.42 %. [ABSTRACT FROM AUTHOR]

The economy and stability of the hydrogen recirculation system in high-power fuel cells significantly impact the performance of the stack system. However, there is no consensus on the optimal hydrogen recirculation mode to be employed. To determine the most effective solution across the entire power range of the stack, this study compares the performance of various coupling modes. The dynamic performance variations of the system were investigated under different modes by the Computational Fluid Dynamics (CFD) method. The simulation results demonstrated that within the low and medium power range of the stack, the PUE (Pump Upstream of Ejector) mode had the lowest power consumption of all modes, at 112 W. The power of the PUE mode had a reduction of 56.3% compared to the full-power single hydrogen pump mode, and a reduction of 22.8% compared to the PDE (Pump Downstream of Ejector) mode. [Display omitted] • Various coupling models of hydrogen pumps and ejectors are studied. • The pressure rise distribution of coupling modes are compared and analyzed. • Coupling mode reduces power consumption by more than 50%. • The optimal solution for the full power range of the stack is derived. [ABSTRACT FROM AUTHOR]

The hydrogen pump is an important component in the hydrogen circulation system of fuel cells. Excessive pulsation at the outlet of the pump is challenging when the Roots hydrogen pump is used, which has a negative impact on the operation performance. To reduce the impact of excessive fluctuations in the hydrogen circulation system, a new structure of reflow grooves is proposed in this paper, which connects the different chambers and adjusts the pressure using a diversion method. The internal flow of the two pumps was studied for comparison, using a three-dimensional simulation model established by Computational Fluid Dynamics (CFD) method. Results showed that the reflow grooves reduce the exhaust pressure and mass flow pulsations, which has a positive effect on the pressure stabilization of the hydrogen circulation system and the anode inlet pressure of the fuel cell stack, contributing to the long durability and stability of the fuel cell stack. [Display omitted] • A novel reflow structure is proposed for the Roots hydrogen pump. • The internal flow of the pump is simulated by the dynamic grid method. • The gas pressure variation in one compression in the Roots pump was analysed. • The reductions in flow fluctuations at different rotational speeds are compared. • Pressure fluctuation is supressed at high rotation speed by the reflow structure. [ABSTRACT FROM AUTHOR]

The start-stop performance of gas foil bearings (GFBs)-rotor system directly affects the operation range and energy consumption of the air compressor for hydrogen fuel cell vehicles which has the inherent characteristic of frequent start and stop. In this paper, the effects of three factors including cooling gas flow, cooling gas pressure, and cooling gas type on the start-stop performance of the GFBs-rotor system are experimentally investigated for the first time. Based on the start-stop response analysis, the take-off speed of 6576 rpm and power consumption of 0.61 kW for the optimum GFBs-rotor system are obtained. The performance of the proposed system for the centrifugal air compressor is compared with the commercial products. The comparison shows that the lowest speed can be decreased by 34.2% while the operating range is expanded by 8.6%, which can in turn improve the performance of the centrifugal air compressors in hydrogen fuel cell vehicles. [Display omitted] • Effects of cooling gas on the start-stop performance were considered. • Start-stop response of the GFBs-rotor system in start-stop processes was studied. • Optimal operating parameters of the GFBs-rotor system were proposed. • Optimal system with minimum takeoff speed and power consumed was identified. • Verification of the optimum GFBs-rotor system was carried out. [ABSTRACT FROM AUTHOR]

• The liquid film was in the form of many slant streams in the middle region of the annular channel. • Four types of droplet impact outcomes were observed in the annular channel. • The impact outcomes were shown to undergo a transition from bouncing to coalescence as the droplet Weber number increased. • The transition boundary from bouncing to coalescence was 2500 under different liquid volume fractions. The separation of the gas and liquid in the gas-liquid cyclone separator is achieved through the swirling flow. The liquid film flow characteristics and the dynamics of droplets impacting the liquid film are very important for the separation performance, but these phenomena have not been well understood in the swirling flow. In this work, an experiment was conducted to investigate the liquid film flow and droplet impingement in the annular channel of the gas-liquid cyclone separator. Three regions of the annular channel could be divided based on the film flow patterns, which were the inlet region, the annular region, and the outlet region; and four typical droplet impact outcomes could be observed, which were splashing, coalescence, bouncing, and jet breakup. According to the characteristics of the liquid film and the outcome of droplet liquid film impingement, if the droplet splashing in the inlet region could be weakened and the length of the annular region was lengthened, it would be beneficial for gas-liquid separation. The impact outcomes were shown to undergo a transition from bouncing to coalescence as the droplet Weber number increased. Droplet coalescence was affected by the disturbance of gas flow and the evenness of the liquid film, so when the superficial velocity of the gas flow was fixed, the transition boundary was constant whatever the liquid volume fraction in the channel, and when the Reynolds number of the gas flow increased, the transition boundary of droplet Weber number increased first and then decreased. This work may provide a basis for a comprehensive understanding of the liquid film flow pattern and droplet behavior in the cyclone separator, and the results have the potential to be applied in other multiphase studies. [ABSTRACT FROM AUTHOR]

Centrifugal air compressor is critical to hydrogen fuel cell vehicles due to it guarantees the air provided to the stack is higher pressure ratios and a larger flow rate. However, the compressor has low efficiency and high energy consumption. In this paper, the multi-objective optimization was performed based on experimental data, which considered both the maximum isentropic efficiency and minimum total power consumption. The effects of the factors on the performance of the compressor were obtained by mean S/N ratios. The contributions of the factors to the optimization objectives of the compressor were investigated by analysis of variance. The optimal parameters combination providing the larger isentropic efficiency and lower total power consumption was found by grey relational analysis. The isentropic efficiency increased by 34.5% and total power consumption decreased by 48.5% in comparison with the average value in all Taguchi cases for the optimal compressor. [Display omitted] • Optimal operating parameters of the centrifugal air compressor were proposed. • Effects of operating parameters on performance of compressor were analyzed. • The contributions of operating parameters to the optimal objectives were found. • Grey relational analysis was used for performance analysis for the first time. [ABSTRACT FROM AUTHOR]

One-dimensional (1D) design and optimization of the impeller plays a significant role in performance improvement of the centrifugal compressor. However, most of the concentration has been paid to three-dimensional (3D) optimization of blades, few attention was focused on main control parameters determining aerodynamic performance and their optimal combination. Thus, this study innovatively developed a multidisciplinary design method combined with empirical 1D loss models, statistical analysis, and multi-optimization theory. The preliminary design of 1D parameters was developed based on empirical loss models. Besides, the analysis of variance of signal to noise ratio (SNR) was applied to find the main control parameters according to their contributions. To maximize the total pressure ratio and isentropic efficiency, the multi-objective optimization based on grey relational grade (GRG) was used to find the optimal combination of 1D parameters. The results showed that the impeller outlet width and impeller outlet radius are the most sensitive parameters affecting compressor performance. The optimal combination of 1D parameters is obtained. Compared to the initial design, the optimal impeller can reduce consumed power of 2.99%, enhance the isentropic efficiency of 1.24% at design point, and obtain the maximum increment of isentropic efficiency of 2.16% at 50 g/s operating point at 70,000 rpm. • New multidisciplinary design and multi-objective optimization method was proposed. • Analysis of variance of SNR was used to identify main control parameters affecting performance. • The optimal combination was obtained by multi-objective optimization based on GRC. • Optimal impeller has higher isentropic efficiency at design or off-design points. [ABSTRACT FROM AUTHOR]

The rotor profile is a key factor for improving the performance of Roots pumps. The geometric constraints of a novel Roots profile are derived and discussed in this paper. The available range of four independent design parameters, including lobe number n , ratio R of the center distance to the rotor diameter, base circle coefficient ε 1 , and contact ratio ε 2 of the involute segment, were determined according to the design constraints. The area utilization efficiency η A was defined to evaluate the theoretical working capacity of the Roots pump. From the perspective of geometry, the internal leakages due to the radial and meshing gaps were assessed using the tip concentric arc angle α 1 and the geometric leakage index η C derived from carryovers between the two intermeshing rotors. The results of the design domain indicated that more lobes produced a larger design domain and reduced the maximum available area utilization efficiency. The variations in the three performance indices within the feasible design domain reflected three optimizing directions: a larger α 1 , a higher η A , and a lower η C. The goal of area utilization efficiency contradicts that of the geometric leakage index. [Display omitted] • A novel profile for Roots pumps applied in fuel cell heavy-duty trucks was developed. • A geometric index was introduced to evaluate the carryover of the new profile. • The performance of the new profile was analyzed within its feasible design region. [ABSTRACT FROM AUTHOR]

Micrometer-sized droplets impacting on a vertical wall with different surface roughness were numerically studied, with both the single droplet impact and the multiple droplets impact being investigated. The phenomena of the multiple droplets impact were simplified to two droplets successive impact and simultaneous impact. The simulation model was based on the CLSVOF method with the dynamic contact angle considered. The morphology of the single droplet and the droplet pair were compared in the transverse and longitudinal directions. It was found that the surface roughness had a greater effect on the transverse direction, and the rougher the surface, the less the deformation. The depth of the droplets infiltrated into the rough peak was then analyzed. The result indicated that the infiltration depth increased with the impacting velocity, and the interactions between the two successive droplets could affect their forward infiltration into the wall. • Oil droplets' impact on the vertical wall was numerically studied by the CLSVOF model with the dynamic contact angle considered. • Multiple droplets impact were simplified to two droplets successive impact and simultaneous impact. • Effects of the surface roughness on the transverse and longitudinal directions were analyzed. • The infiltration depth of the droplets was compared under different impact parameters. [ABSTRACT FROM AUTHOR]

Disc couplings are widely used in compressors, gas turbines, and aerospace applications because of their flexibility and their consequent ability to compensate in almost all directions. The torsional stiffness of disc coupling has a great influence on shaft torsional vibration, and in many applications low-order torsional resonance may occur due to insufficient stiffness in the disc coupling. Investigation of the factors influencing the torsional stiffness of disc coupling will help to improve the design and control of shaft torsional vibration. In this paper, a 3D finite element (FE) model was built to estimate the stiffness of a disc coupling, taking the behavior of friction and contact into consideration. Based on this model, the curve of torque vs. angular displacement was obtained by applying a series of torsional load steps to the coupling. The effects of diaphragm shape, bolt preload, and fluctuation of torque were evaluated. A test rig was established to validate the simulated results. Both the simulated and experimental results demonstrated a strong nonlinear relationship between the torque and angular displacement during both the loading and unloading processes. The results also showed that the bending and inclining of flanges and diaphragms were mainly responsible for the varied stiffness of the disc coupling. In addition, load fluctuation, bolt preload, and shape of diaphragms could lead to variation in the stiffness of disc couplings. [ABSTRACT FROM AUTHOR]

Ionic liquid compressors are the ideal solution for hydrogen refueling stations, and multi-stage compression is an inevitable choice for achieving high-pressure refueling, such as 90 MPa level. However, the initial filling amount of the ionic liquid in the compression chamber is lacking basis as the characteristics of gas-liquid two-phase flow during the reciprocating movement of the liquid piston are not understood. This study numerically investigates the variation characteristics of the gas-liquid interface in the compression chambers under different structural and operating parameters of a five-stage ionic liquid compressor. Based on the fluctuation feature of the phase interface, the minimum liquid piston heights in each stage that ensure effective sealing for the compression chamber with different stroke-to-diameter ratios (r) are determined. Finally, the mathematical relationship for calculating the minimum ionic liquid filling amount related to structural parameter r and suction pressure is established, which provides guidance for the design of the ionic liquid compressor. [ABSTRACT FROM AUTHOR]

The hydrogen supply of the fuel cell system is realized by the cooperation of multiple components. Transient characteristics of a single component can affect the performance of other components. In this study, a three-dimensional multi-component computational fluid dynamics (CFD) model was developed to investigate the synergistic transient characteristics of the hydrogen recirculation components such as hydrogen injector, ejector, and purge valve in an 80 kW PEMFC. The results show that the entrainment performance of the ejector is reduced under unsteady purge conditions compared with steady conditions. The pressure fluctuation of the secondary flow is significant even under purge closed durations. There are drastic changes in velocity and pressure in the ejector, especially in the mixing chamber. Moreover, an abundant hydrogen supply capacity of the injector is necessary to deal with the excessive anode pressure fluctuation. The feedforward-feedback integrated control of the injector is a more efficient strategy to reduce pressure fluctuations compared with the feedback control. [Display omitted] • A multi-component CFD model is developed to investigate transient characteristics. • The entrainment ratio of the ejector is decreased under unsteady purge conditions. • Drastic changes in velocity and pressure occur in the ejector. • The flow capacity and control strategy of the injector can impact the system performance. [ABSTRACT FROM AUTHOR]

Abstract: A numerical simulation was conducted of the dynamic trajectories and the separation performance of oil droplets, with a focus on the breakup of oil droplets in an oil–gas cyclone separator. The separation efficiency was also studied experimentally, and the oil droplets' diameter distributions before and after the separator were measured with a Malvern particle size analyser to verify the simulation model. Both the experimental and simulation results showed that the breakup of oil droplets occurred in the separation process, clearly influencing the separation efficiency. In addition, the results indicated that inlet velocity played an important role in separation efficiency, as it not only significantly affected the tangential velocity inside the separator, but also determined the possibility and degree of the breakup of oil droplets. [Copyright &y& Elsevier]

[Display omitted] • Experiments of the oil–gas impingement onto the vertical wall were performed. • The spray characteristics were measured by the Malvern particle size analyzer and the PIV technique. • The oil film flow was divided into the initial stage and the steady stage. • The effect of the surface roughness on the separation efficiency was analyzed. • The wall of the middle surface roughness of 1.1–2.5 μm performed better on oil–gas separation. This paper investigated the effect of the surface roughness (0.3 μm, 1.1 μm, 2.5 μm, and 4.5 μm) on the oil–gas separation efficiency. Through spraying the oil–gas mixture onto the vertical wall, the droplets impingement in inertial separators was imitated. The impact separation efficiency on a wall was calculated by comparing the volume of the collected oil flowing down from the wall with the volume of the impinging oil. Experiments were conducted three different impingement conditions. The impinging velocity and droplets size distributions on the impingement surface were characterized by the Malvern particle size analyzer and the particle image velocimetry (PIV) technique, respectively. The droplets impinging velocities varied in the range of 3−5 m/s, and the Sauter mean diameter was observed around 40 μm. The collecting process can be divided into the initial stage and the steady stage according to the film flow state. It was found that the surface roughness has both advantages and disadvantages effect on the film flow. These two opposite effects are of different importance at different roughness levels. Under the studied impingement conditions, the separation efficiency was all above 80%, and the surface of the middle roughness (1.1–2.5 μm) performed better than the smoothest 0.3 μm surface and the roughest 4.5 μm surface. [ABSTRACT FROM AUTHOR]

The startup performance is crucial to the high-efficient operation of the sustainable power generation system. Current literature usually focuses on catalysts and startup strategy to improve startup performance of the proton exchange membrane fuel cell, which ignores the gas foil bearings-rotor system that directly affects startup response. Therefore, in this paper, the effects of bump foil structure, coating, and nominal clearance on the startup response of the bearings-rotor system are first investigated experimentally. Multi-objectives optimization mathematical model considering simultaneously lower power consumption, takeoff speed, and vibration is presented based on grey relational analysis. The optimal bearings-rotor system with radial trisection bump foil, coating MoS 2 , and nominal clearance of 50 μm is identified. Compared to average values, the power consumption of the optimal system is diminished by 27.2% while the takeoff speed and nonlinear vibration are reduced by 16.2% and 47.3%, respectively. These findings can be used to improve energy efficiency and startup performance for fuel cells. The systematic methodology proposed in this study can be extended to other investigations about the startup. • Bump foil type, coating, and nominal clearance are first considered in PEMFCs. • Multi-objective optimization mathematical model is presented. • Optimal gas foil bearings-rotor system is proposed and verified. • Startup performance of the proposed system is improved. • Power consumption of the proposed system is diminished by 27.2%. [ABSTRACT FROM AUTHOR]

As hydrogen circulation pump is one of the key components for hydrogen circulation in Fuel Cell Vehicles (FCVs), the optimization of a hydrogen circulation pump is critical for the performance improvement of a FCV. This study focuses on six factors that have impacts on the performance of a claw-type hydrogen pump, including the rotating speed, pressure ratio, inlet pressure, the radial clearance between the rotor and the casing (RC1), the radial clearance between the rotors (RC2), and the axial clearance (AC). By using the Taguchi method, a series of CFD simulation cases with different levels of the six factors were carried out to determine the optimum conditions for the volumetric efficiency and the shaft power. With the analysis of variance (ANOVA) method, the quantitative contribution of these six factors to volumetric efficiency and shaft power were obtained separately. According to the results, pressure ratio (36.2%), AC (29.4%) and rotating speed (21.5%) had the greatest impact on volumetric efficiency, while shaft power was more sensitive to the pressure ratio (64.6%), rotating speed (23.0%) and inlet pressure (5.2%). Based on the analysis of the ANOVA method, the most significant factors on volumetric efficiency and shaft power were chosen to be input factors of the Neural Network and the data of simulation were adopted to train the Neural Network to predict the performance of a claw pump. The accuracy of the Neural Network was tested and validated. The results can be used as guidelines for the design and selection of claw pumps used for fuel cell systems. • The quantitative effect of 6 main factors was illustrated. • Optimum combinations of the 6 main factors were presented. • Performance prediction model with ANN was established. [ABSTRACT FROM AUTHOR]

The claw-type hydrogen pump has been applied in fuel cell vehicles (FCVs) because of its compact structure, high reliability, and oil-free quality. In this study, a three-dimensional transient computational fluid dynamics (CFD) modelling of a claw-type hydrogen pump used in FCVs was established. Hexahedral structured grids were generated and updated at an increment of 3° in rotating angle to ensure the mesh quality of the whole solving process. The leakage of radial clearance (RC) and axial clearance (AC) was considered. The presented modelling and simulation methods were validated by operating a claw pump at different pressure ratios. The pressure and velocity vector fields in both AC and middle plane, along with the mechanism of the fluid field distribution were analyzed in detail. The in-depth relationship amongst the fluctuation of discharge pressure, outlet mass flow rate and discharge area during the whole working process was revealed. P-θ and V-θ diagram of the whole operating cycle were analyzed. The influence of AC and RCs respectively on the volumetric efficiency of a claw pump was compared and evaluated. It is concluded that back flow in suction pipe happened near 360° as part of the discharge chamber was cut off from the exhaust port and high pressure gas from carryover flowed back into the inlet pipe. The pressure increase during the displacement process, theoretically zero, is actually significant and even comparable to the pressure increase during the compression and discharge process. In addition, volumetric efficiency is most sensitive to axial clearance, followed by radial clearance between rotor and casing, while radial clearance between the rotors has the least influence. • A CFD model with axial clearance and radial clearance was presented. • Leakage and vortex in different clearances were illustrated. • Influence of different clearances on the volumetric efficiency was revealed. • The fluctuation of outlet mass flow rate, P-θ and V-θ diagram were analyzed. [ABSTRACT FROM AUTHOR]

Measuring transient pressure and temperature simultaneously inside a compressor can lead to a thorough characterization of the thermodynamic process in the compressor. However, the rapid changes in airflow temperature make it difficult to measure these conditions precisely, partly due to large thermal inertia of common thermometers such as thermistors and thermocouples. Measurement of rapid temperature changes requires thermometers that have a time constant of a few microseconds or less. Therefore, in this study, two fine wire thermocouples 18 and 35 μm in diameter, respectively, were prepared in advance. A two-thermocouple probe consisting of these two fine wire thermocouples was then used to measure the transient temperature of airflow in the working chamber of a reciprocating compressor. The temperatures detected by the two-thermocouple probe were used to reconstruct the true temperatures. Mean and fluctuating time constants were both evaluated and used to compensate for the raw temperatures. The estimated mean time constants of the two thermocouples were 9.98 and 29.97 ms at 420 rpm, respectively. The maximum difference between the two reconstructed temperatures was 9.7 °C, with an average value of 2.4 °C. Cycle-by-cycle variations of the transient temperatures were also measured. [ABSTRACT FROM AUTHOR]

• An experimental study of the liquid film flow in an annular channel of the vortex separator was performed. • The liquid film pattern was analyzed under different superficial gas velocities and liquid film Reynolds numbers. • A map of critical conditions of liquid film entrainment was given. • Critical entrainment conditions were compared with non-swirling two-phase concurrent film flow. The vortex separator has been widely applied in nuclear reactors to provide high-quality steam to turbines. The entrainment of the liquid film on the wall affects significantly the separator's function but has rarely been concerned. In this study, the entrainment in the annular flow field of the vortex separator was experimentally studied. The inlet gas velocity ranged from 5.6 to 12.2 m/s and the liquid film flow Reynolds number was around 597 ∼ 1392. The critical speeds for the liquid film entrainment that occurred under each liquid film flow condition were identified, and a map of the critical condition for entrainment to take place was given. The performance of the liquid film at different locations of the annular channel indicated that the swirl flow intensified the disturbance of the airflow to the liquid film, and the disturbance would be more severe if the swirl developed more fully. [ABSTRACT FROM AUTHOR]

A Kinetic Monte Carlo (KMC) model with a cluster-multiple labeling technique was proposed to study the effect of growth parameters such as temperature, deposition flux and Si/C ratio on Si–C clusters in the initial stage of SiC (0001) surface nucleation. In this model, Si and C atoms were treated individually and a crystal lattice was established to fix the physical location of atoms and interatomic bonding. The adsorption and diffusion of adatoms on the planar surface, attachment to and detachment from clusters of adatoms, and their diffusion along the edge of clusters were considered in the KMC model. Cluster-multiple labeling technique was used to identify Si–C clusters and the properties of clusters were analyzed. The results showed that larger and less clusters were constructed on growing surface with the increase of temperature and decrease of deposition rate. The saturated clusters density was exponentially related to the ratio of effective diffusion capacity to deposition rate, and the relationship between saturated density of clusters and average growth rate of clusters was obtained. Finally, Si/C ratio affected the characteristics of nucleation, and at C-rich condition, the binding energy of nucleus was larger than that at Si-rich condition, leading to the formation of fewer but larger clusters. • A Kinetic Monte Carlo (KMC) model with Si and C atoms treated individually was proposed. • Cluster-multiple labeling technique was used to identify Si–C clusters. • Effects of growth condition on Si–C clusters in the initial stage of SiC (0001) surface nucleation were analyzed. [ABSTRACT FROM AUTHOR]