This study proposes a novel design having dense fins with lesser thickness at the upper layer and comparatively spare fins with greater thickness in the lower layer to further improve the overall thermal performance of a double-layer microchannel heat sink. The design can effectively direct more low temperature fluid flow toward the lower layer to improve heat transfer while the sparse fin structure at low layer can ease pressure drop penalty. At the same time, the thicker fins at the lower layer ensure higher fin efficiency to facilitate high heat transfer. Parametric and detailed analysis is conducted for the proposed double-layer microchannel heat sink in comparison with the traditional one. After optimization, the thermal resistance of the proposed double-layer microchannel heat sink at the same pumping power is found to be reduced by 9.42% when compared to the traditional double-layer microchannel heat sink. Yet at the same Reynolds number, the Nusselt number of the proposed design exceeds the traditional value by 13%.

This study numerically investigates the effects of changing vaned diffuser angles and vaneless diffuser widths of a twostage centrifugal refrigerant compressor on the reduction of surge flow rate and the improvement of part-load performance. The first stage has a low-solidity vaned diffuser, while the second stage has a vaneless diffuser. The analysis results show that adjusting the angle of low-solidity vaned diffuser in the first stage can lower the surge flow rate and keeps high efficiencies at low flow rates. The inlet guide vane of the first stage can also lower the surge flow rate; however, their simultaneous actuation cannot lower the surge flow rate further. Reducing the vaneless diffuser width of the second stage can lower the surge flow rate. Meanwhile, simultaneously adjusting the individual inlet guide vanes of the second stage can also effectively lower the surge flow rate. If the diffuser width of the first stage is reduced while the low-solidity vaned diffuser blade angle is fixed, the surge flow rate will be lowered obviously as the inlet guide vane is adjusted. However, reduction in diffuser width decreases the efficiency at low flow rate. Controlling the inlet guide vane and diffuser width well can lower the surge flow rate to 30% of the design flow.

In this study, the heat transfer performance of high-viscosity polyol ester (POE) oil POEA-220 (220 cSt) with low-GWP (global warming potential) refrigerant R-1234ze(E) on enhanced GEWA-B5H tube was investigated at saturation temperatures of 10 °C, 0 °C, and −6 °C. The mass fraction of oil varied from 0.25% to 10%, and all the nucleate pool boiling data were measured at heat fluxes ranging from 10 kW/m2 to 90 kW/m2. The experimental results showed that the heat transfer performance of the R-1234ze(E)/POEA-220 mixtures were superior to the R-1234ze(E)/POEA-68 mixtures. At saturation temperatures of 0 °C and −6 °C, even a 10% mass fraction of the POEA-220 oil showed an enhancement in the HTC (heat transfer coefficient) compared to the pure refrigerant in the moderate heat flux range. On the other hand, for the R-1234ze(E)/POEA-68 mixtures, a 5% mass fraction of oil showed no enhancement in the HTC compared to pure refrigerant at the same saturation temperature. Moreover, at low saturation temperatures (0 °C and −6 °C), the enhancement in the HTC decreased with increasing mass fraction of low-viscosity oil POEA-68, whereas high-viscosity oil POEA-220 showed the highest enhancement in the HTC for a 5% mass fraction of oil at −6 °C saturation temperature compared to the pure refrigerant. The results indicate that for nucleate boiling, the effect of oil viscosity on heat transfer performance is negligible if it contains comparatively high thermal conductivity and low surface tension. In addition, the effect of surface aging on heat transfer performance for the GEAW-B5H tube with pure refrigerant was also reported.

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An active magnetic bearing (AMB) revolutionizes rotating machinery applications by eliminating the friction problem of conventional contact bearings and the need for lubrication. Supporting equipment is necessary as a countermeasure in case the AMB fails to operate or its magnetic field decreases in power, causing the rotor to fall, known as a dropdown event. An auxiliary bearing is necessary to prevent direct contact between the rotor and the AMB, as the AMB itself is not designed to be durable against impact or rotational friction. Therefore, the auxiliary bearing is designed to absorb and withstand the contact force. The aim of this study is to find the best design for the auxiliary bearing to decrease the erratic and whirling motion of the rotor after contact. The nonlubricated Hertzian contact model is used to analyze the rotor drop dynamics, and the finite element method is used as the basis of the flexible rotor model. A simulation is carried out using MATLAB software. The simulation reveals the rotor orbit, rotor response, and contact force, provides a comparison of the stiffness support and damping support effects, and also predicts the bearing life to provide a new approach for bearing selection. The results of this study provide the relevant information for selecting the material of the auxiliary bearing with the most appropriate damping and stiffness properties to disperse the contact force during a rotor drop event.