Analysis of mass transfer effects on the performance of journal bearings using micropolar lubricant

In this paper the static and dynamic characteristics of a finite hydrodynamic journal bearing with micropolar lubricant are analyzed. The effects of mass transfer of solid additives and contaminants in the lubricant oil, on the bearing characteristics are considered in this study. A generalized form of Reynolds equation is derived from the fluid flow equations with the effects of mass transfer across the fluid film considered. The generalized Reynolds equation is solved using Galerkin's weighted-residual finite element method to obtain the fluid pressure distribution in the bearing. The various static and dynamic characteristics are subsequently obtained and presented.     

低速下润滑接触区补充供油机制的研究

在滚动轴承运行的过程中,滚道上的润滑剂在滚动体的反复碾压下,厚度不断减小,轴承最终进入乏油润滑状态.为了解释长期工作在乏油条件下的轴承依旧能够保持较长时间的良好工作状态,有必要研究在轴承中是否存在某种自发的补充供油机制.本文作者基于球盘接触模型,分别考虑毛细力和分离压力在润滑油迁移过程中的作用,建立赫兹接触区附近油层分布模型,并以此修正弹流计算中的入口供油条件,采用统一Reynolds方程法计算在静止或低速条件下的润滑油膜厚度和压力分布,研究毛细力和分离压力的补充供油机制对润滑条件的改善作用.     

航空发动机轴承腔润滑的气液两相均匀流研究

基于轴承腔中润滑油的两相均匀流动模型,采用湍流模式和有限差分数值方法计算轴承腔内三维定常N-S方程,对腔内润滑油的气液两相均匀流动特性进行研究,以获得气液两相均匀流条件下润滑油流场、压力场和速度场在轴承腔内的分布情况,分析转子转速、含气率和润滑油进口速度对润滑油出口压力以及润滑油与壁面之间剪切力的影响,同时对单相流和两相均匀流润滑性能差异进行比较.结果表明,转子转速、含气率和润滑油进口速度对润滑油出口压力和腔内壁面与润滑油间的平均剪切力具有不同影响,而采用2种流动模型计算出的轴承腔润滑油出口压力的差异较大,同时支持了开展航空发动机轴承腔润滑两相流动分析的必要性.     

Experimental study of the blockage boundary for dense-phase pneumatic conveying of powders through a horizontal slit

The estimation of the blockage boundary for pneumatic conveying through a slit is of significant importance. In this paper, we investigate the characteristics for blockage of powder (48 μm average diameter) through a horizontal slit (1.6 m × 0.05 m × 0.002 m). The results show that the required critical solid mass flow rate increases as the superficial air velocity increases superficial air velocity. The solid loading ratio and superficial air velocity displayed a decreasing power law relationship. This finding agrees with existing theory and experimental results. However, a minimum inlet solid loading ratio exists. When the air velocity is greater than the corresponding air velocity of the minimum solid loading ratio, the solid loading ratio exhibits an increasing trend in power law. We also found that when the inlet conveying pressure increased, the critical solid mass flow rate required for blockage, the inlet solid loading ratio, and the minimum inlet solid loading ratio increased.     

考虑滚道表面油层分布的滚动轴承润滑分析

研究表明供油量对弹流润滑性能产生显著影响.滚动轴承中由于离心力和滚动体的反复滚压,滚道表面上的润滑剂呈现出非均匀分布的特点.大多数润滑剂被推挤到滚道的两侧,致使接触区的入口间隙不能被完全充满,导致乏油润滑,滚动体与滚道间接触压力接近于赫兹压力分布,膜厚较全膜润滑有明显的减小.本文基于润滑剂的流量连续建立滚道表面油层厚度分布模型,考虑润滑接触压力的影响,计算滚道上的侧流量以预测轴承滚道上补给油层厚度及形状随时间的变化规律;进而以此作为滚动体和滚道接触区的入口油层厚度,采用统一Reynolds方程法数值模拟计算每个时刻轴承滚道与滚动体之间的润滑油膜厚度,压力分布等参数,分析轴承在点接触乏油条件下运行的润滑性能.     

NUMERICAL SIMULATION OF THREE DIMENSIONAL GAS-PARTICLE FLOW IN A SPIRAL CYCLONE

The three-dimension gas-particle flow in a spiral cyclone is simulated numerically in this paper. The gas flow field was obtained by solving the three-dimension Navier-Stokes equations with Reynolds Stress Model (RSM). It is shown that there are two regions in the cyclone, the steadily tangential flow in the spiral channel and the combined vortex flow in the centre. Numerical results for particles trajectories show that the initial position of the particle at the inlet plane substantially affects its trajectory in the cyclone. The particle collection efficiency curves at different inlet velocities were obtained and the effects of inlet flow rate on the performance of the spiral cyclone were presented. Numerical results also show that the increase of flow rate leads to the increase of particles collection efficiency, but the pressure drop increases sharply.     

Performance of hydromagnetic squeeze films between conducting porous rough conical plates

An endeavor has been made to discuss the behavior of hydromagnetic squeeze film between two conducting rough porous conical plates. The plates are considered to be electrically conducting and the clearance space between them is filled by an electrically conducting lubricant. A transverse magnetic field is applied between the plates. Efforts have been made to solve the concerned Reynolds’ equation with the associated boundary conditions to get the pressure distribution. This in turn, is used to obtain the expression for load carrying capacity leading to the calculation of the response time. The results are presented graphically as well as in tabular form. It is suggested by the results that the bearing system records an enhanced performance as compared to that of a bearing system working with a conventional lubricant. It is noticed that the pressure, load carrying capacity and the response time increase steadily with increasing values of the magnetization parameter. In general, the bearing suffers owing to transverse surface roughness. However, the negatively skewed roughness tends to better the performance of the bearing system marginally. This performance gets further improved especially, when the negative variance is involved. It is observed that the semi-vertical angle increases the load carrying capacity. Besides, the conductivity also increases the load carrying capacity significantly. In addition, it is revealed that the negative effect induced by the porosity can be neutralized to a nominal extent by the positive effect of the magnetization parameter in the case of negatively skewed roughness in the presence of negative variance. Thus, this study provides ample scopes for improving the performance of the bearing system considerably by choosing a suitable combination of magnetization parameter, semi-vertical angle and the conductivities of the plates.     

Comparison Between Numerically Simulated and Experimentally Measured Flowfield Quantities Behind a Pulsejet

Pulsed combustion is receiving renewed interest as a potential route to higher performance in air breathing propulsion and ground based power generation systems. Pulsejets offer a simple experimental device with which to study unsteady combustion phenomena and validate simulations. Previous computational fluid dynamics (CFD) simulations focused primarily on pulsejet combustion and exhaust processes. This paper describes a new inlet sub-model which simulates the fluidic and mechanical operation of a valved pulsejet head. The governing equations for this sub-model are described. Sub-model validation is provided through comparisons of simulated and experimentally measured reed valve motion, and time averaged inlet mass flow rate. The updated pulsejet simulation, with the inlet sub-model implemented, is validated through comparison with experimentally measured combustion chamber pressure, inlet mass flow rate, operational frequency, and thrust. Additionally, the simulated pulsejet exhaust flowfield, which is dominated by a starting vortex ring, is compared with particle imaging velocimetry (PIV) measurements on the bases of velocity, vorticity, and vortex location. The results show good agreement between simulated and experimental data. The inlet sub-model is shown to be critical for the successful modeling of pulsejet operation. This sub-model correctly predicts both the inlet mass flow rate and its phase relationship with the combustion chamber pressure. As a result, the predicted pulsejet thrust agrees very well with experimental data.     

Correlations of flow maldistribution parameters in an air cooled heat exchanger

The present paper provides correlations of flow maldistribution parameters in air‐cooled heat exchangers. The flow field in the inlet header was obtained through the numerical solution of the governing partial differential equations including the conservation equations of mass and momentum in addition to the equations of the turbulence model. The results were obtained for different number of nozzles of 2–4, different inlet flow velocities of 1–2.5m/s and different nozzle geometries in addition to incorporation of a second header. The results are presented in terms of mass flow rate distributions in the tubes of the heat exchanger and their standard deviations. The results indicate that the inlet flow velocity has insignificant influence on maldistribution while the nozzle geometry shape has a slight effect. Also, the results indicate that reducing the nozzle diameter results in an increase in the flow maldistribution. A 25% increase is obtained in the standard deviation as a result of decreasing the diameter by 25%. Increasing the number of nozzles has a significant influence on the maldistribution. A reduction of 62.5% in the standard deviation of the mass flow rate inside the tubes is achieved by increasing the number of nozzles from 2 to 4. The results indicate that incorporating a second header results in a significant reduction in the flow maldistribution. A 50% decrease in the standard deviation is achieved as a result of incorporation of a second header of seven holes. It is also found that the hole‐diameter distribution at the exit of the second header has a slight influence on the flow maldistribution. Correlations of the flow maldistribution in terms of the investigated parameters are presented. Copyright © 2007 John Wiley & Sons, Ltd.     

Inertia effects in externally pressurized bearing with viscoelastic lubricant

A theoretical investigation of inertia effects on the load capacity of an externally pressurized bearing with a visco-elastic lubricant is presented. The methods of iteration and averaged inertia have been employed to find an approximate solution of the resulting non linear differential equation. Graphical representation of results together with those for the case without inertia has been shown. It is found that the elasticity of the liquid increases the load bearing capacity and the pressure at a point in the lubricant film. There is a negative contribution to the load capacity by inertia forces.     

基于N-S方程的多孔质气膜悬浮力学模型研究

为了研究多孔质气浮承载系统设计参数对气膜厚度的影响特性,以平行圆板模型为对象,建立了包含多孔质特性和缝隙流特性的气膜悬浮静态模型,分析了承载系统多孔质部分的受力特性,得出了气膜厚度控制方程,并利用改进弦截法进行求解。理论计算和实验结果对比表明,基于气膜简化模型的数值计算结果与实验值吻合较好,验证了理论模型的有效性。通过数值计算结果,探讨了通气量和承载力变化对气膜厚度的影响规律。研究表明,气膜厚度受通气量及承载力的影响,可通过对通气量及承载力进行最优控制,以使气膜厚度满足环境使用要求。     

A two-phase model for laminar film condensation from steam-air mixtures in vertical parallel-plate channels

Detailed results are presented for laminar film condensation from steam-air mixtures flowing downward in vertical flat-plate channels. The mixture flow is laminar and saturation conditions prevail at the inlet. A fully coupled implicit numerical approach is used that achieves excellent convergence behavior, even for high inlet gas mass fractions. The detailed results include velocity, temperature, and gas mass fraction profiles, as well as axial variations of film thickness, pressure gradient and Nusselt number. The effects of a wide range of changes in the four independent variables (the inlet-to-wall temperature difference and the inlet values of gas concentration, Reynolds number, and pressure) on the film thickness, axial pressure gradient, and the local and average Nusselt numbers are carefully examined. It was found that increases in inlet concentration of noncondensable gas caused significant decreases in the film thickness, local Nusselt number, and axial pressure gradient. An analytical solution for the film thickness and velocity field at the end of condensation path was developed and shown to be the asymptotic value of the numerical results for large distances along the channel.     

A STUDY ON DROPLET VELOCITY OF ETHANOL DURING ELECTROSPRAYING PROCESS AT CONE-JET MODE

研究液滴在静电喷雾下的速度特性是理解喷雾形态的形成及演化的关键.结合锥射流模式下乙醇静电喷雾实验数据,建立了静电喷雾二维轴对称模型.基于离散相液滴运动方程、连续相空气运动方程、电场方程以及用户自定义函数,进行了数值求解,获得了锥射流模式下的乙醇静电喷雾形态、空间电场分布以及液滴速度场分布.考虑了不同空气入口流速的影响,得到了乙醇/空气同轴射流静电喷雾形态的变化规律.结果表明,喷雾外围液滴与空气流场有较强的相互作用,导致喷雾中轴线附近的液滴速度分布变化较小,而在喷雾外围处的液滴速度分布沿径向剧烈变化;随着空气入口速度的增大,乙醇/空气同轴射流静电喷雾形态先趋于发散,当空气入口速度大于喷雾外围液滴轴向速度时,喷雾形态则趋于聚拢.因此,除改变施加电压、液体流量和电极结构外,通过控制空气入口速度来影响喷雾液滴速度场,也可获得不同的静电喷雾效果.     

Heat transfer characteristics of a new helically coiled crimped spiral finned tube heat exchanger

In the present study, the heat transfer characteristics in dry surface conditions of a new type of heat exchanger, namely a helically coiled finned tube heat exchanger, is experimentally investigated. The test section, which is a helically coiled fined tube heat exchanger, consists of a shell and a helical coil unit. The helical coil unit consists of four concentric helically coiled tubes of different diameters. Each tube is constructed by bending straight copper tube into a helical coil. Aluminium crimped spiral fins with thickness of 0.5 mm and outer diameter of 28.25 mm are placed around the tube. The edge of fin at the inner diameter is corrugated. Ambient air is used as a working fluid in the shell side while hot water is used for the tube-side. The test runs are done at air mass flow rates ranging between 0.04 and 0.13 kg/s. The water mass flow rates are between 0.2 and 0.4 kg/s. The water temperatures are between 40 and 50°C. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. The air-side heat transfer coefficient presented in term of the Colburn J factor is proportional to inlet-water temperature and water mass flow rate. The heat exchanger effectiveness tends to increase with increasing water mass flow rate and also slightly increases with increasing inlet water temperature.     

The Effect of Bubbly Oil on the THD Lubrication of Tilting-pad Journal Bearings Subjected to Rotating Unbalance Load

Little work has been published on the contribution of entrained air and/or gas bubbles on the lubricant viscosity and hence on the hydrodynamic performance of bearings. In this work, a thermo-hydrodynamic analysis of the performance of dynamically loaded tilting-pad journal bearing lubricated by bubbly oil is carried out. The non-steady Reynolds equation for compressible fluid and the energy equation are solved iteratively using finite difference method to study the effect of air bubbles on the bearing performance characteristics. The effect of bubbles content on the lubricant viscosity is considered.The results showed that the value of maximum pressure slightly increases with increasing the bubble ratio up to 0.2, while the film thickness deteriorates at very high bubble content. The pressure peak moves in the downstream direction with the increase of bubble ratio ₁, which would be of great importance in deciding the pivot location in tilting-pad bearings.     

Thermal behavior of spiral fin-and-tube heat exchanger having fly ash deposit

This research investigates the effect of fly-ash deposit on thermal performance of a cross-flow heat exchanger having a set of spiral finned-tubes as a heat transfer surface. A stream of warm air having high content of fly-ash is exchanging heat with a cool water stream in the tubes. In this study, the temperature of the heat exchanger surface is lower than the dew point temperature of air, thus there is condensation of moisture in the air stream on the heat exchanger surface. The affecting parameters such as the fin spacing, the air mass flow rate, the fly-ash mass flow rate and the inlet temperature of warm air are varied while the volume flow rate and the inlet temperature of the cold water stream are kept constant at 10 l/min and 5 °C, respectively.

From the experiment, it is found that as the testing period is shorter than 8 h the thermal resistance due to the fouling increases with time. Moreover, the deposit of fly-ash on the heat transfer surface is directly proportional to the dust–air ratio and the amount of condensate on heat exchange surface. However, the deposit of fly-ash is inversely proportional to the fin spacing. The empirical model for evaluating the thermal resistance is also developed in this work and the simulated results agree well with those of the measured data.     

Heat transfer study in rotating cascade using IR thermography and CFD analyses

This paper presents an experimental and CFD numerical study of convective heat transfer in a rotating cascade. Infrared thermography was used to measure surface temperature distribution on a rotating hollow blade, heated internally by secondary air. A CFD numerical model was made according to the actual test rig geometry and operating conditions. Tests were carried out in an iposonic flow regime at relatively low fluid temperatures, with the rotational and Reynolds numbers varied and hot-to-cool air mass flow ratio kept constant. Experimental and numerical results for the blade pressure side are compared in terms of surface temperature 2D distribution and Nusselt number one-dimensional distribution along the blade midspan, providing a reasonable agreement.     

Microchannel membrane separation applied to confined thin film desorption

The concept of a confined thin film to enhance the desorption process is based on a reduced mass diffusion resistance. A wide thin film is formed into a microchannel by using a porous membrane as one wall of the channel enabling vapor extraction along the flow. Heat added to the channel results in vapor generation and subsequent extraction through the membrane. This experimental study investigates the performance of vapor extraction as a function of confined thin film thickness, pressure difference across the membrane and inlet concentration to the microchannel. In addition, heat added to the system was varied and results are presented in terms of the wall superheat temperature relative to the inlet saturated conditions of the binary fluid. The test section was equipped with a transparent window to observe bubble formation and vapor extraction. Results show that the performance, measured by the vapor release rate, increases for reduced channel thickness, for increased pressure difference across the membrane, and for lower inlet concentration. Results show that lower wall superheat correspond to higher heat transfer coefficients. Trends of Nusselt number and Sherwood number versus both channel Reynolds number and the product of the Reynolds number and Schmidt number are presented. Bubble formation in the channel does not degrade overall performance provided a critical heat flux condition does not occur.     

Proposing a novel combined cycle for optimal exergy recovery of liquefied natural gas

The effective utilization of the cryogenic exergy associated with liquefied natural gas (LNG) vaporization is important. In this paper, a novel combined power cycle is proposed which utilizes LNG in different ways to enhance the power generation of a power plant. In addition to the direct expansion in the appropriate expander, LNG is used as a low-temperature heat sink for a middle-pressure gas cycle which uses nitrogen as working fluid. Also, LNG is used to cool the inlet air of an open Brayton gas turbine cycle. These measures are accomplished to improve the exergy recovery of LNG. In order to analyze the performance of the system, the influence of several key parameters such as pressure ratio of LNG turbine, ratio of the mass flow rate of LNG to the mass flow rate of air, pressure ratio of different compressors, LNG pressure and inlet pressure of nitrogen compressor, on the thermal efficiency and exergy efficiency of the offered cycle is investigated. Finally, the proposed combined cycle is optimized on the basis of first and second laws of thermodynamics.     

Cold rolling mill process: a numerical procedure for industrial applications

The paper proposes a model for a cold rolling mill process in the full-film regime that uses lubricant emulsion sprayed on at the entrance of the strip. The aim of the model is to forecast the reduction of strip thickness versus the flow rate of lubricant given the other operation parameters. The model includes strip plastic deformation, lubricant flow and lubricant viscosity depending on pressure. The mathematical problem is a free boundary one and a numerical procedure, applied to an industrial plant, is presented with some results.