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• Employing motor-driven turboexpander compressor provides cooling below −60 °C. • System performance under variable rotating speeds and refrigeration conditions. • Operating characteristics of motor-driven turboexpander compressor are explored. • Advanced exergy analysis shows great potential for the refrigerator improvement. Turbo-refrigerator based on reverse Brayton cycle is an attractive solution for providing refrigeration at ultra-low temperatures below −60 °C for deep freezing. Employing a motor-driven turboexpander compressor (MTEC) not only allows the turbo-refrigerator to achieve high compactness, reliability and flexibility, but also leads to new operating characteristics especially in a closed cycle. This paper aims to investigate the operating characteristics of a closed-cycle single-stage turbo-refrigerator with a MTEC. A high-speed turbo-refrigerator system with the design refrigeration temperature of −86 °C was manufactured and then tested under variable rotating speeds and refrigeration conditions. Test results show that a refrigeration temperature below −96.4 °C was obtained in a pull-down progress. Compared with a fixed-speed operation, variable-speed operation enables the independent control of the refrigeration temperature and the cooling capacity. With the decrease in the refrigeration temperature (or in the cooling capacity), the rotating speed has a greater effect on the COP than that on the cooling capacity (or on the refrigeration temperature). The mass flow rate along with the compression and expansion work shows a decreasing trend as the refrigeration temperature decreases at a fixed speed of the MTEC. The advanced exergy analysis shows a total exergy efficiency of 0.42 (with a COP of 0.62 at refrigeration temperature of −86 °C) could be expected with component improvement in the near future, and it is of the priority to increase the efficiency of the turboexpander. [ABSTRACT FROM AUTHOR]

A high-speed turboexpander compressor (TEC) for small reverse Brayton air refrigerator is tested and analyzed in the present work. A TEC consists of an expander and a compressor, which are coupled together and interact with each other directly. Meanwhile, the expander and compressor have different effects on the refrigerator. The TEC overall efficiency, which contains effects of the expander's expansion, the compressor's pre-compression, and the pressure drop between them, was proved. It unifies influences of both compression and expansion processes on the COP of refrigerator and could be used to evaluate the TEC overall performance. Then, the coupling parameters were analyzed, which shows that for a TEC, the expander efficiency should be fully utilized first, followed by the compressor pressure ratio. Experiments were carried out to test the TEC coupling performances. The results indicated that, the TEC overall efficiency could reach 67.2%, and meanwhile 22.3% of the energy output was recycled. [ABSTRACT FROM AUTHOR]

A small cryogenic reverse Brayton air refrigerator with turboexpander compressor (TEC) is presented in this study. Because of stable process, simple matching between expander and brake blower, and easy regulation, a turboexpander with brake blower is usually used in small reverse Brayton refrigerator. However, a turboexpander with brake blower just consumes and wastes the output energy during the enthalpy drop. In contrast, the output energy of TEC is absorbed by its coupled compressor for recycling. Thus when employing a TEC, the reverse Brayton refrigerator will achieve lower refrigeration temperature, larger cooling capacity and more effective energy use. TEC overall performance, which has an important impact on the refrigerator thermal performance, is mainly determined by the coupling between expander and compressor. In a TEC, the compressor and expander should seek balance among energy, rotating speed, mass flow rate and pressure, though restricted by individual working characteristics. The coupling relations among compressor efficiency, expander efficiency, compressor pressure ratio and expander expansion ratio are quite complex. In this study, theoretical coupling analysis between expander and compressor was conducted. The aerodynamic performances of compressor and expander were calculated using CFX simulation with SST model. The performance curves of compressor and expander were obtained through simulation results, which were validated by experimental data. Based on the coupling analysis and numerical simulations, the automatic coupling model between compression process and expansion process was established via Matlab code. The refrigerator was tested under varied coupling parameters (compressor inlet pressures and expander inlet temperatures). The calculated coupling performance was validated by experimental data. With good coupling, the cooling capacity of refrigerator reached 48.0 W at 99.6 K. The coupling performance was calculated through the coupling model, and mutual relations and interactions among coupling parameters were quantitatively described and clarified. The coupling model could effectively predict the coupling performance and therefore allow for the further design and optimization of TEC. [ABSTRACT FROM AUTHOR]

The stability characteristics of a 25 mm rotor turboexpander with bump-foil journal-gas-bearing pairs and a pair of pressurized thrust bearing are investigated in this study. The journal-gas-bearing pairs in various inner diameters supply radial support for the rotor with different hydrodynamic gas-film thicknesses, and the pair of pressurized thrust bearings balances the axial forces. The experimental results reveal that the stability of the bump foil gas bearing is achieved over a wide range of diametric clearances and that an optimal nominal diametric clearance exists, thus resulting in high rotation speed with small vibration amplitudes. The maximum turboexpander speed reached 93.3 krpm under the nominal diametric clearance of 0 µm with a maximum synchronous amplitude of less than 20 µm. [ABSTRACT FROM AUTHOR]

• Three mean-line methods for the design of turbo-coolers are evaluated. • A coupling model is established to predict the performance and volume size. • Matching characteristics of three coaxial-impellers was quantitatively described. • SS-CR, PR and VC approach gives 84.0%, 83% and 80.9% efficiency respectively. • The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger. High-speed turbo-cooler of coaxial expander-compressor is a promising solution for reverse Brayton cycle refrigeration systems to improve compactness, power recovery and efficiency. The performance curves of coaxial impellers are crucial for their matching characteristics as well as the system efficiency under off-design conditions. In this paper, three mean-line methods for the design of high-speed turbo-coolers are evaluated by comparing the matching characteristics of three coaxial-impellers, including the specific speed and characteristic ratio (SS-CR) approach, the power ratio (PR) approach and velocity coefficient (VC) approach. A coupling model is established to predict the off-design performance of turbo-cooler and the system size. The results show that the rotating speed and mass flow rate by all three methods lie within 2% of each other. At the design point, SS-CR approach gives 84.0% expander efficiency compared to 83.0% by PR approach and 80.9% by VC approach, but the PR approach has better performance at speeds below 35 krpm. The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger designed with all the three methods. However, SS-CR gives 6.0% advantage in system volume 5.6% over VC approach and 3.7% over PR approach. When the cooling capacity is controlled at 2000 W, SS-CR approach gives 79.3% expander efficiency compared to 78.9% by PR approach and 76.4% by VC approach. [ABSTRACT FROM AUTHOR]

Efficient liquefaction of hydrogen is a crucial part for the large-scale storage and long-distance transportation of hydrogen. Helium Brayton cycle based on high-speed turbo-expanders has been widely employed in small and medium hydrogen liquefiers. In present study, a coupled model is proposed to predict the performance and cooling-down process of helium turbo-expanders with brake blower, and validation experiments were performed on a helium turbo-expander of a 2 t/d hydrogen liquefier. Experimental results indicated that the characteristic ratio of expander varied significantly during the cooling-down process which led to a large deviation from the optimal efficiency. The impact of brake pressure on the characteristic ratio and efficiency of the helium turbo-expander is studied, and a variable pressure control method is proposed for the efficient operation of turbo-expanders during the cooling-down process of a hydrogen liquefier. Compared with the constant brake pressure control method, the variable pressure control method can increase the expander efficiency by 5%–10% during the cooling-down process in the high temperature zone. • A coupled model is proposed to predict the performance of helium turbo-expanders. • Low efficiency during cool-down process is due to small characteristic ratio. • Variable pressure control can increase efficiency by 10% during cool-down process. [ABSTRACT FROM AUTHOR]

Abstract Accurate calculation of the transient cooling performance is crucial for the operation and control of a reverse Brayton refrigerator. Components of the refrigerator have complex working characteristics individually and interact each other mutually. To solve the problem easily, the turboexpander matching characteristics were usually ignored and relations among components were simplified. In this study, a cryogenic reverse Brayton air refrigerator equipped with gas bearing turboexpander and plate-fin heat regenerator was presented. The ultimate refrigerating temperature was proposed through analysis. The transient cooling characteristics of the cryogenic part in refrigerator were resolved into the turboexpander matching performance and the regenerator transient cooling characteristics. The regenerator was simulated through numerical heat transfer and computational fluid dynamics by considering the axial conduction and cold loss. The matching model was improved by adopting a significant method of constant rotating speed. Using the dual non-steady time steps, a transient cooling model of the cryogenic part was explored via C++ code, and verified by experiment. Through the model, the refrigerator cooling performances were evaluated under different operation modes, and the energy utilization efficiency was determined. It can be used to evaluate the operation strategy of refrigerators and help to promote energy efficiency. Highlights • Ultimate temperature of the cryogenic part in refrigerator was proposed and analyzed. • The regenerator was modeled through numerical heat transfer and CFD via C++. • Matching model was improved by adopting the method of constant rotating speed. • Transient cooling model was built through the dual non-steady time steps. • Energy efficiency and relations among components were quantitatively clarified. [ABSTRACT FROM AUTHOR]

The recuperative matrix heat exchanger (MHE) has advantages of large specific surface area, compactness, high effectiveness, and low flow resistance, which make it promising for applications in Joule-Thomson cryocooler and reverse Brayton cryocooler. In this study, a numerical model is developed to predict the thermal performance of MHEs. The effects of axial conduction of both inner and outer walls as well as radial heat conduction through mesh-screens and the heat leakage are considered. The temperature distribution over the fluids and mesh screens in MHEs can be determined using this model. The model is validated by analytical solution of effectiveness-NTU method and the results of the experimental tests on two MHEs with different structures at cryogenic temperature. Using the proper correlations for heat transfer coefficients, the relative deviations of the ineffectiveness predictions from experimental results are below 13.2% in the mass flow range of 0.4 g/s – 2.1 g/s. The numerical results show that the axial conduction of inner and outer walls has similar effects on the thermal performance of MHEs. The increase of axial conduction can substantially lower the effectiveness of the MHEs at mass flow rates below 1 g/s, while the influence of the number of transfer unit (NTU) on the MHE effectiveness becomes dominant at mass flow rates above 1 g/s. [ABSTRACT FROM AUTHOR]

A small reverse Brayton cycle air refrigerator was designed and fabricated. Bump-type air journal foil bearing, pressurized thrust gas bearing and centrifugal blower as brake were employed in the turboexpander. Usually, constant brake inlet pressure is set in a reverse Brayton refrigerator. However, the unchanged brake inlet pressure cannot adapt to the changing temperature and expansion ratio during the cooling down process, which could go against the system performance. In this article, the relationship between the turboexpander operation parameters and brake pressure was disclosed through theoretical analysis. The performance curve was analyzed through numerical simulation using CFX. A matching model was established based on the theoretical analysis and numerical simulation. Brake pressure feedback control was then proposed and applied in the experimental study. Thermal performance of the refrigerator was tested under varied operating conditions (different expansion ratios, temperatures and brake pressures). The results indicated that the appropriate brake pressure facilitated system good thermal performance under both design and off-design conditions, and the theoretical results agreed well with the experimental data. [ABSTRACT FROM AUTHOR]