Numerical analysis of the static performance of an annular aerostatic gas thrust bearing applied in the cryogenic turbo-expander of the EAST subsystem

The EAST superconducting tokamak, an advanced steady-state plasma physics experimental device, has been built at the Institute of Plasma Physics, Chinese Academy of Sciences. All the toroidal field magnets and poloidal field magnets, made of NbTi/Cu cable-in-conduit conductor, are cooled with forced flow supercritical helium at 3.8 K. The cryogenic system of EAST consists of a 2 kW/4 K helium refrigerator and a helium distribution system for the cooling of coils, structures, thermal shields, bus-lines, etc. The high-speed turbo-expander is an important refrigerating component of the EAST cryogenic system. In the turbo-expander, the axial supporting technology is critical for the smooth operation of the rotor bearing system. In this paper, hydrostatic thrust bearings are designed based on the axial load of the turbo-expander. Thereafter, a computational fluid dynamics-based numerical model of the aerostatic thrust bearing is set up to evaluate the bearing performance. Tilting effect on the pressure distribution and bearing load is analyzed for the thrust beating. Bearing load and stiffness are compared with different static supply pressures. The net force from the thrust bearings can be calculated for different combinations of bearing clearance and supply pressure.

Effect of micro-clearance structure on the collapse of individual liquid hydrogen bubbles

Cavitation occurs in the micro-clearance of liquid-hydrogen-lubricated bearings owing to the pressure drop caused by high-speed shearing.The pressure undulation caused by cavitation collapse results in bearing surface erosion and significantly affects the bearing performance.In this study,a modified Z-G-B cavitation model was used to study the crushing process of a single liquid hydrogen bubble in a shear micro-clearance.Fast Fourier transform(FFT)and wavelet transform(WT)were applied to study the frequency characteristics of the pressure,mass transfer rate,and vapor mass fraction during bubble rupture in shearing micro-clearance.To obtain a deeper insight into the details of the effect of the shear micro-clearance structure on bubble collapse,the relationship between the flow field energy,attenuation rate,and frequency was investigated.The proper orthogonal decomposition(POD)and dynamic mode decomposition(DMD)methods were used to analyze the energy of each order mode of the flow field.The analysis results of the bubble vibration intensity with respect to time and frequency provide a theoretical basis for the optimization of the bearing structure.

Experimental investigation of inflow-outflow asymmetry in induced-charge electro-osmosis

Induced-charge electro-osmosis(ICEO)is a research hotspot in bioengineering and analytical chemistry.Inflow-outflow asymmetry of ICEO was reported in the existing literatures,but systematic study on this phenomenon is insufficient.In this experimental study,we found that in strong electric fields,not only the velocity magnitude but also the vortex positions of ICEO are asymmetrical along the inflow and outflow directions because of the pronounced non-uniform surface electrokinetic transport.On the inflow and outflow directions,the amplitudes of velocities are unequal,ICEO maximum velocity positions change depending on the electric field intensity and sodium chloride(NaCl)concentration.Additionally,the distances between vortex centers are different.At NaCl solution concentration of 0.001 mol⋅L^(-1),the outflow velocity almost vanishes.The asymmetry rises with the increasing electric field intensity.The new discoveries can direct the application of microscale devices.

Numerical prediction of spatio-temporal frosting patterns of curved surface considering varying working parameters

Accurate numerical prediction of frosting patterns is essential for the efficient layout and timing defrosting of heat exchangers under frosting conditions.In this study,a numerical model is developed to predict the spatio-temporal frosting habits on curved surfaces in combination with the correlations of frost density and thermal conductivity.In the model,frost melting is considered.After verification,the frosting and heat transfer characteristics along the flow path are investigated under various structural and operating conditions.Frost thickness along the path is mainly affected by the cooling surface temperature,while the heat and mass transfer rates are strongly correlated with the humidity ratio.The proportions of latent heat and sensible heat are distributed more unevenly in parallel flow case than in counter flow case.Frost deposition is facilitated by a smaller radius of curvature of the cooling surface.More uniform frosting characteristics along the path and smaller heat transfer obstruction are presented with a smaller length-to-height ratio of the flow path.

Numerical simulation on the frosting and heat transfer characteristics of plate-fin heat exchanger considering confinement effect

In refrigerating industry,frost commonly deposits on the confined cold surfaces of heat exchangers,which affects the heat transfer performance.Along the confined flow path of the heat exchanger,the frosting at downstream is affected by the parameters from the upstream.In this study,a numerical model considering the confinement effect has been proposed to predict frosting characteristics in plate-fin heat exchanger.Convection-diffusion equations for humid air and empirical correlations for local frost density are employed in the numerical prediction.Frosting behavior and heat transfer in the confined channel are investigated with different humid air parameters and cold surface temperatures.The results indicate that frost thickness in the confined channel is thicker than that in open space under the same inlet parameters.The frost layer is thicker and fluffier along the confined channel.In addition,the air temperature difference between inlet and outlet of the confined channel enlarges with frosting.Under the same average temperature of upper and lower surfaces,the heat and mass transfer of frosting are enhanced with diminishing temperature difference of upper and lower surfaces.In such condition,frosting is mainly influenced by the cold surface with the lower temperature.