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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. • The flow evolution in the serrated hydrogen Knudsen compressor can be divided into four stages. • The main factors affecting the non-equilibrium evolution are analyzed. • The serrated structure has a great influence on the local flow in the serrated microchannel. • The pressure difference between the two containers increases first and then decreases. [ABSTRACT FROM AUTHOR]

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. • Hydrogen temperature inside cylinder is nonuniform during the compression stroke. • Exhaust temperature predicted by adiabatic compression theory is higher than actual value. • As reciprocating cycle frequency increases, maximum wall temperature decreases. [ABSTRACT FROM AUTHOR]

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. • The hydrogen flow behaviors is investigated in the multistage hydrogen Knudsen compressor. • The multistage hydrogen Knudsen compressor has a single-stage replication characteristic. • The pressure drop phenomenon of the hydrogen flow in the connection channel is presented in this paper. • The increase of the connection channel height is beneficial to promote the pressure increase of the hydrogen flow. [ABSTRACT FROM AUTHOR]

• A mathematical model has been presented to study the thermodynamic performance of the rolling piston compressor with EVI. • The suction backflow process and the injection process have been considered. • The errors between calculated and experimental results are within 4%. • The optimal injection pressure for the maximum heating capacity is slightly higher than the theoretical optimal injection pressure. • The optimal injection pressure for the maximum COP is slightly lower than the theoretical optimal injection pressure. The rolling piston compressor with enhanced vapor injection (EVI) is widely used in the household air-source heat pump system. To study the thermodynamic performance and the optimal injection pressure, this paper presents a mathematical method to simulate the whole working process of rolling piston compressor with EVI, which considers suction backflow process, discharge process, vapor injection process and leakage process. According to the calculation, the effects of injection pressure on discharge mass flow rate, p-V diagram and indicated power have been analyzed. In order to verify the accuracy of simulation, an experiment has been carried out to obtain the discharge mass flow rate and the errors between simulated and experimental results are within 4%, which shows that the proposed method is precise enough. Additionally, the experimental results show that EVI can not only improve the heating capacity and COP but also decrease the discharge temperature. However, the optimal injection pressure corresponding to the maximum heating capacity is different from that corresponding to the maximum COP. From the perspective of the heating capacity, the optimal injection pressure is a little higher than the geometric mean of suction pressure and discharge pressure but from the perspective of COP , the optimal injection pressure is a little lower than the geometric mean. [ABSTRACT FROM AUTHOR]

• Two calculation methods of the indicated power have been presented to ensure the accuracy of the simulation results. • Operation speed has a great effect on the power consumption per revolution. • Adding discharge port without valve in swing compressor can reduce the indicated power. Pressure variation during the discharge process is important to the discharge flow loss and indicated power of swing compressor. Due to the small effective discharge area and the large discharge volume flow rate at the beginning of discharge process, pressure shoots up to a high value, which is 25%-35% higher than the nominal discharge pressure. Using a suitable discharge method is beneficial to reduce the maximum pressure and indicated power. This paper presents a numerical model to study the indicated power and pressure variation of three different discharge methods. The power consumption per revolution has been discussed to analyze the discharge flow loss. And the effects of over-compression, under-compression and operation speed have been analyzed. Simulation results show that the errors of indicated power are within 3% and the errors of mass flow rates are within 2%, which proves that the accuracy of the presented model is good enough. Furthermore, the major reason affecting the discharge flow loss is the effective discharge area rather than over-compression or under-compression. And compared with the discharge port with valve, the discharge port without valve has a larger effective discharge area. Adding discharge port without valve in swing compressor can reduce the indicated power. [ABSTRACT FROM AUTHOR]

In this paper, a novel swing compressor with two discharge channels (SCTC) is introduced, which can be an alternative compressor applied in household air-conditioner. SCTC has two discharge ports. The first one without discharge valve operates at all shaft speed and the other one with a high stiffness discharge valve operates at high shaft speed, which is helpful to improve the service life of compressor and discharge valve. Additionally, the discharge close volume is eliminated and the liquid strike can be prevented. In order to obtain the performance of SCTC, a prototype has been tested at three different shaft speeds and its volumetric efficiency, adiabatic efficiency, COP and noise characteristic have been analyzed. The performance of SCTC is always better than that of the swing compressor with no valve (SCNV). Besides, compared with the conventional swing compressor, the COP of SCTC increases by 2.26% at the shaft speed of 1800 r•min−1. [ABSTRACT FROM AUTHOR]