Numerical analysis of a radial hybrid lobe bearing with gas lubrication

В работе рассмотрены результаты численного исследования модели радиального гибридного подшипника с газовой смазкой. Установлено наличие оптимальных значений параметров, определяющих форму смазочного зазора, получены их значения и закономерности влияния на них различных характеристик подшипника. Рассмотрено распределение давления в смазочном слое при различном количестве секторов лепестков. Подтверждено предположение о наличии оптимальных значений параметров, определяющих форму смазочного зазора. И опровергнуто предположение о том, что оптимальные значения этих параметров будут зависеть от значений других параметров подшипника. Выявлен положительный эффект от совместного воздействия газодинамического и газостатического эффектов. Относительно газостатических параметров – выявлен параметр, имеющий оптимальное значение (расстояние от выбранного торца подшипника до линии сетки, на которой располагаются питатели ряда с номером р), которое сохраняет постоянное значение. На основе полученных результатов разработаны основы методики расчета и оптимизации радиальных гибридных подшипников с газовой смазкой, имеющих профилированную рабочую поверхность. The paper considers the results of numerical study of gas-lubricated hybrid radial bearing model. The presence of optimal values of parameters determining the shape of the lubrication gap is established, their values and regularities of influence of different bearing characteristics on them are obtained. Pressure distribution in the lubricating layer at different number of lobe sectors has been considered. The assumption of optimum values of parameters determining the form of the lubrication gap is confirmed. And the assumption that the optimal values of these parameters will depend on the values of other bearing parameters is refuted. The positive effect of the combined effect of gas-dynamic and gas-static effects has been revealed. Concerning gas-static parameters - the parameter which has optimum value (distance from the chosen end face of a bearing to a grid line on which feeders of a row with number p are located) which keeps constant value has been revealed. On the basis of the obtained results the bases of calculation and optimization methodology of radial hybrid bearings with gas lubrication, having profiled working surface, are developed.

Investigation into gas lubrication performance of porous gas bearing considering velocity slip boundary condition

Porous gas bearings (PGBs) have a proactive application in aerospace and turbomachinery. This study investigates the gas lubrication performance of a PGB with the condition of velocity slip boundary (VSB) owing to the high Knudsen number in the gas film. The Darcy-Forchheimer laws and modified Navier-Stokes equations were adopted to describe the gas flow in the porous layer and gas film region, respectively. An improved bearing experimental platform was established to verify the accuracy of the derived theory and the reliability of the numerical analysis. The effects of various parameters on the pressure distribution, flow cycle, load capacity, mass flow rate, and velocity profile are demonstrated and discussed. The results show that the gas can flow in both directions, from the porous layer to the gas film region, or in reverse. The load capacity of the PGB increases with an increase in speed and inlet pressure and decreases with an increase in permeability. The mass flow rate increases as the inlet pressure and permeability increase. Furthermore, the simulation results using VSB are in agreement with the experimental results, with an average error of 3.4%, which indicates that the model using VSB achieves a high accuracy. The simulation results ignoring the VSB overrate the load capacity by 16.42% and undervalue the mass flow rate by 11.29%. This study may aid in understanding the gas lubrication mechanism in PGBs and the development of novel gas lubricants.

PULSED COMBUSTION OF FUEL-AIR MIXTURE IN THE CAVITY UNDER THE BOAT BOTTOM: SIMULATION AND OUTDOOR TESTS

For reducing the hydrodynamic drag of a boat, a gas cavity can be made under the boat bottom, which will partially isolate the bot- tom from direct contact with water and provide ¤gas lubrication¥ by forced supply of atmospheric air or exhaust gases from a boat motor.

A note on the aerodynamic splashing of droplets

When a drop of a low-viscosity liquid of radius $R$ impacts against an inclined smooth solid substrate at a velocity $V$, a liquid sheet of thickness $H_{t}\ll R$ is expelled at a velocity $V_{t}\gg V$. If the impact velocity is such that $V>V^{\ast }$, with $V^{\ast }$ the critical velocity for splashing, the edge of the expanding liquid sheet lifts off from the wall as a consequence of the gas lubrication force at the wedge region created between the advancing liquid front and the substrate. Here we show that the magnitude of the gas lubrication force is limited by the values of the slip length $\ell _{\unicode[STIX]{x1D707}}$ at the gas–liquid interface and of the slip length $\ell _{g}\propto \unicode[STIX]{x1D706}$ at the solid, with $\unicode[STIX]{x1D706}$ the mean free path of gas molecules. We demonstrate that the splashing regime changes depending on the value of the ratio $\ell _{\unicode[STIX]{x1D707}}/\ell _{g}$ – a fact explaining the spreading–splashing–spreading–splashing transition for a fixed (low) value of the gas pressure as the drop impact velocity increases (Xu et al., Phys. Rev. Lett., vol. 94, 2005, 184505; Hao et al., Phys. Rev. Lett., vol. 122, 2019, 054501). We also provide an expression for $V^{\ast }$ as a function of the inclination angle of the substrate, the drop radius $R$, the material properties of the liquid and the gas, and the mean free path $\unicode[STIX]{x1D706}$, in very good agreement with experiments.