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Hydrogen refueling stations (HRSs) are among the most important infrastructures for fuel cell vehicles. However, the safety issue of HRSs has become a key constraint to the wide application and development of hydrogen energy. This article presents a quantitative risk assessment of the first liquid HRS (LHRS) in China and conducts a comprehensive assessment in terms of both individual (IR) and societal risks (SRs). The results showed that both the IRs and SRs related to the LHRS exceeded the risk acceptance criteria. The rupture of the flexible hose of the dispenser and the leak from the compressor are the main contributors to these risks. On the other hand, implementing appropriate mitigation measures on the level of the LHRS dispenser and compressor, including the addition of breakaway couplings in the flexible hose of the dispenser, the installation of hydrogen detection sensors, the arrangement of automatic and manual emergency shutdown buttons, and the elevation of the compressor, is capable of reducing the risk of the LHRS to be within the risk acceptance criteria. [ABSTRACT FROM AUTHOR]

Application of high-precision non-intrusive flow field measurement techniques in transonic cryogenic wind tunnels (CWT) is confined to the lack of suitable tracer particles. The present study explored the feasibility of utilizing non-equilibrium condensing nitrogen droplets as tracer particles. A numerical model of the nitrogen non-equilibrium condensation was established. Accordingly, the non-equilibrium condensation process of nitrogen droplets and feasibility analysis of tracing were investigated. The results reveal that the occurrence of nitrogen droplets as well as the droplet concentration and mean radius can be regulated in the nucleation zone and droplet growth zone by varying gas temperature or pressure. Furthermore, the feasibility of employing nitrogen droplets as tracer particles in the working conditions of CWT is confirmed through the analysis of tracking capability, response time, concentration, and duration of droplet traceability. Formulas correlating used to calculate duration of droplet traceability have been proposed. This study proved the potential application of condensing nitrogen droplets as tracer particles in CWT. [ABSTRACT FROM AUTHOR]

In the area of the heating industry, a heat pump is an efficient alternative technology to achieve energy saving and carbon emission reduction. The conventional heat pump has gradually been applied to replace the traditional direct electrical heating method while the required temperature is below 100 °C. A heat pump with temperatures between 100–140 °C is in the stage of rapid development. However, a heat pump with temperatures above 150 °C has received relatively little attention. In this paper, two systems combining a heat pump and water vapor compression (CHPVC and HPTVC) have been studied for waste heat recovery from 45 °C to a water vapor supply with a temperature above 150 °C. A thermodynamic model has been proposed to analyze the performance of the two systems, and a twin-screw compressor model has been developed to calculate the isentropic efficiency of the compressor applied in the heat pump. Four different parameters have been used to analyze the energy efficiency. The simulation results show that while the inlet water temperature is 45 °C and the required vapor temperature is 150 °C, the optimal COPs of CHPVC and HPTVC are 2.432 and 2.436, respectively. Moreover, CHPVC is more suitable for the large saturation temperature lift, and HPTVC is more suitable for a relatively small temperature difference between the inlet water and the required vapor. Compared with the direct electrical heating method or the conventional two-stage heat pump, these two systems are remarkably efficient and show good energy-saving potential. [ABSTRACT FROM AUTHOR]

Micro-structured surfaces can affect heat transfer mechanisms because of enlarged specific surface areas. However, employing the Leidenfrost effect during liquid nitrogen (LN2) droplet cooling of a heated micro-structured surface possessing a fin with a spacing much smaller than the diameter of the droplet has not yet been explored. In the present work, a direct numerical simulation (DNS) is carried out to investigate heat transfer mechanisms of the LN2 droplet, whose diameter is sufficiently larger than the structured spacing of fin, impinging on a micro-structured surface with variable velocities. For a comparative study, a smooth surface is also employed in numerical simulations. The spreading mechanisms and vaporization behavior of the droplet along with liquid film morphology at various conditions are investigated. Results show that a smaller fin size inhibits LN2 in entering into the grooves between the fins and left the surface untouched by the droplet completely, and eventually, a thinner liquid film is spread out in contrast to the smooth surface. Notably, at a low Weber number, the droplet can be shrunk or even rebounded away from the wall after impinging on the wall. The fastest vaporization behavior for both surfaces, namely smooth and micro-structured, is obtained at a Weber number of 180. Additionally, an effective heat transfer upon the micro-structured surface is observed at a low impinging velocity of the droplet. [ABSTRACT FROM AUTHOR]

Magnetic refrigeration is acknowledged as a potential substitute for the conventional vapor-compression refrigeration technology, owing to its high efficiency and environmental friendliness. Existing magnetic refrigeration systems are mostly based on permanent magnets, owing to the characteristics of lower magnetic field intensity, non-uniform magnetic field distribution, and lower operating frequency due to the moving parts, which results in a low cooling capacity and small temperature difference. Thus, this study proposes the application of a pulsed magnetic field, with a high intensity and frequency, to a magnetic refrigeration system to achieve a high performance. A verified numerical model is established to investigate the thermodynamic cycle and cooling performance of an active magnetic regenerator (AMR). The transient and steady-state performances of AMR under pulsed and permanent magnetic fields are compared. The results suggest that an AMR can establish a stable temperature difference under a pulsed magnetic field that is 40 times faster than that under a permanent magnetic field. The maximum steady-state cooling capacity under a pulsed magnetic field is 2.5 times that under a permanent magnetic field when the temperature difference is 20 K. Additionally, the effects of pulsed magnetic field waveforms, frequency, and intensity on the performance of AMR are investigated under various utilization factors. These results can guide the improvement of room-temperature magnetic refrigerators. [ABSTRACT FROM AUTHOR]

In marine storage and transportation, the sloshing of liquid oxygen disturbs the thermodynamic equilibrium and induces stress on tank walls. Numerous problems are associated with the sloshing mechanism and demand a detailed investigation. In this study, a numerical model is developed by coupling the Eulerian framework and the algebraic interface area density (AIAD) method while considering the interphase drag force to investigate the thermal behavior of sloshing liquid oxygen. The effect of the sloshing frequency on the evaporation performance of liquid oxygen is studied. Moreover, anti-sloshing is conducted by employing a T-shaped baffle. The results show that the sloshing induced a vapor explosion phenomenon due to the invalidation of the surface impedance and thermal destratification to enhance free convection, resulting in rapid depressurization and increased evaporation loss. In addition, maximum evaporation loss occurred under the vapor–liquid coupling excitation condition. The T-shaped baffle has an excellent anti-sloshing effect because of the generating tip vortices and the enhanced shearing effect of the walls, which are regarded as motion damping factors. [ABSTRACT FROM AUTHOR]

Outer edge bending is already used on the axial fan blades of air conditioners, reducing the leakage flow loss at the blade tip and suppressing the tip vortex development, thereby improving fan aerodynamic and acoustic performance. However, there are few studies on the multi-parameter design and optimization of this complicated structure, and most studies only focus on the overall sound pressure level rather than the sound quality when evaluating the fan noise. This study investigated the effects of outer edge bending structure on the aerodynamic performance and sound quality of air conditioners' axial fans by experiments and numerical methods. Based on the orthogonal design method, the effects of three bending parameters, the circumferential starting angle, radial relative position, and the bending degree effects on the performance of the axial flow fan blade were analyzed, and the best efficiency scheme was selected. A comparative analysis of the preferred and the original bending schemes shows that the bending towards the blade suction surface successfully inhibits the development of tip leakage vortex at the blade tip, thereby achieving efficiency enhancement and noise reduction. The experimental results show that the preferred bending scheme with a 10° circumferential starting angle, 90% radial relative position, and 8% bending degree can effectively reduce the fan's broadband noise within 200~1000 Hz by 0.54~2.68 dB (A) at different operating conditions. Additionally, the preferred bending blade with reasonably designed bending effectively reduced the loudness and roughness of the fan noise in the rated conditions, and the sound quality of the studied fan was correspondingly improved. [ABSTRACT FROM AUTHOR]

Atomization with cryogenic fluids is complicated due to the combined effects of both the mechanical breakup and thermodynamic flashing. However, the weight of the two effects on the atomization has not been determined yet. This paper proposes a hybrid mathematical model to evaluate the weight of the thermodynamic effect and aerodynamic effect. The entire process of jet spray breakup, including the primary breakup and secondary atomization, is described in the model. A linear stability analysis is carried out using the dispersion equations for the primary breakup. In the secondary atomization, the Taylor analogy breakup and bubble micro-explosion models are employed to study aerodynamic and thermodynamic effects on the atomization, respectively. The homogeneous nucleation is implemented in the bubble micro-explosion models. A criterion is obtained which can be evaluated to predict the dominant effect. Accordingly, the influence of factors such as ambient pressure and other physical parameters on the atomization process is analyzed. The results reveal that, under low ambient pressure, thermodynamic flashing is stronger. Besides, the mechanical action is restrained when the surface tension is increased. [ABSTRACT FROM AUTHOR]

Chilldown process is usually the initial operation for storing and transporting cryogenic fluids. During this process, film boiling is of great importance in terms of the large time span. In this research, a numerical model was built to study the flow regime and heat transfer characteristics of the film boiling with cryogenic fluids. An algebraic interface area density framework was coupled in the model to calculate the drag force at the liquid-gas interface. Accordingly, the effects of wall superheat and mass flow rate on heat flux and film thickness were investigated. Furthermore, the variance analysis method was employed to evaluate the weight of the effects of superheat and inlet mass flow and their coupling effect on the heat transfer coefficient. The results reveal that the wall superheat effect prevails over the mass flux effect on the heat transfer coefficient. Moreover, the two factors have a relatively weak coupling impact on the heat transfer coefficient. Finally, the features of pressure drop in the tube were further analyzed. [ABSTRACT FROM AUTHOR]

The Digital Particle Image Velocimetry (DPIV) is an efficient method for measuring the internal flow field of a low-speed cooling fan. This paper studied the velocity field by means of PIV technology for a leading edge swept axial-flow fan without casing, and the tip vortex phenomenon was observed. Time-averaged velocity measurements were taken near the pressure surface, the suction surface and the tip of blade, etc. Moreover, the flow characteristics were visualized using numerical techniques. Experimental results showed that this tip vortex existed at the leading edge of the blade. The generating, developing and dissipating evolvement process of the tip vortex from the blade leading edge to downstream were discussed in detail. In addition, by comparing DPIV results and numerical results, a good agreement between them indicated a possibility to predict flow field using CFD tools. The experimental data provided in this paper are reliable for improving the aerodynamic characteristics of the open axial fan. [ABSTRACT FROM AUTHOR]

In this study, a collaborative simulation method is proposed to predict dynamic variations of the thermal environment in an air-conditioned room. The room thermal environment was predicted and analyzed by varying the structural and control parameters of the air conditioner considering the dynamic coupling effect. Connections and regularities were established between the applicable parameters and evaluation indices of the thermal environment. The simulation results demonstrated the interactions among the system structural parameters, control parameters, and the thermal environment. Within a certain parameter range, the evaporator structure exhibited a significant effect on temperature uniformity and vertical air temperature difference, followed by predicted mean vote (PMV) and draught rate (DR). The associated evaluation indices were sensitive to fin spacing, tube spacing, and tube outer diameter, in the same order, which were structural parameters of the evaporator. The effect of the air supply angle on the vertical air temperature difference was evident; however, its influence on the PMV, DR, and temperature uniformity did not indicate consistent variations. [ABSTRACT FROM AUTHOR]