循环流化床锅炉循环回路压力平衡理论的分析

通过对循环流化床锅炉循环回路各段压降特性分析 ,建立了包括炉膛、分离器、立管和返料装置等部分的压降关系式 ,并基于循环回路压力平衡特性对所建立的模型进行了求解。计算结果与试验结果相比 ,吻合得较好。图 3参 8     

高压共轨燃油系统喷油诱导的压力波特征试验研究

在油泵试验台架上开展喷油为主动量诱导的高压压力波动特征的试验研究,分析靠近嘴端的高压油管内以及高压油轨的轨压传感器处压力波动规律。研究表明:柴油机高压共轨燃油喷射系统中,喷油引起的压力波不受油泵引起的压力波影响;喷油引起的嘴端压力波动比轨端要显著;嘴端压力波呈现明显的波动衰减,其压力波的频率与燃油属性有关,波动的最大振幅主要与喷射压力、喷孔大小有关;高压油轨处的压力波动特征不明显,单次喷油引起的轨压下降总量不仅与喷射压力、喷孔大小有关,还与喷射脉宽有关。     

Two-phase flow pressure drop hysteresis in parallel channels of a proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in a non-operational PEM fuel cell to understand the effect of stoichiometry, GDL characteristics, operating range, and initial conditions (dry vs. flooded) for flow conditions typical of an operating fuel cell. This hysteresis is noted when the air and water flow rates are increased and then decreased along the same path, exhibiting different pressure drops. When starting from dry conditions, the descending pressure drop tended to be higher than the ascending pressure drop at lower simulated current densities. The hysteresis effect was noted for stoichiometries of 1-4 and was eliminated at a stoichiometry of 5. It was found that the hysteresis was greater when water breakthrough occurred at higher simulated current densities, which is a function of GDL properties. The operating range had to reach a critical simulated current density (800 mA cm⁻² in this case) between the ascending and descending approach to create a pressure drop hysteresis zone. The descending step size does not change the size of the hysteresis effect, but a larger step size leads to lower fluctuations in the pressure drop signal. An initially flooded condition also showed hysteresis, but the ascending approach tended to have a higher pressure drop than the descending approach.     

高压煤粉密相气力输送水平管阻力特性

通过试验和理论计算两种方法研究高压下煤粉密相气力输送水平管稳定段的压力损失情况.理论计算是在Barth气力输送理论基础上,引用Stegmaier的附加压力损失系数的经验关系式进行的.试验发现,尽管输送气体表观速度呈下降趋势,而水平管稳定段的单位压降随固气比增大而增加.显然,较高输送系统压力下气体引起的压降虽然不能忽略,但引起水平管段压降变化的主要因素是输送的煤粉浓度.通过试验值与计算值比较,发现试验与理论计算值吻合得相当好,说明基于Barth气力输送理论的附加压降法对高压煤粉密相气力输送阻力特性的计算具有较好的适应性.     

Hydrogen pressure drop characteristics in a fuel cell stack

Research on hydrogen pressure characteristics was performed for a fuel cell stack to supply a rule of hydrogen pressure drop for flooding diagnostic systems. Some experiments on the hydrogen pressure drop in various operating pressure, temperature, flowrate and stack current conditions were carried out, and hydrodynamic calculation was managed to compare with the experiment results. Results show that the hydrogen pressure drop is strongly affected by liquid water content in the flow channel of fuel cells, and it is not in normal relation with flowrate when the stoichiometric ratio is inconstant. The total pressure drop can be calculated by a frictional pressure loss formula accurately, relating with mixture viscosity, stack temperature, operating pressure, stoichiometric ratio and stack current. The pressure drop characteristics will be useful for predicting liquid water flooding in fuel cell stacks before flow channels have been jammed as a diagnostic tool in electric control systems.     

Numerical Simulation of the Supersonic Flows in the Second Throat Ejector —Diffuser Systems

INTRODUCTIONThe ejector system is a device which employs ahigh-velocity primary motive fluid to enirain and accelerate a slower moving secondary fluid. The resulting kinetic energy of the mixture is subsequently usedfor self-compression to a higher pressure, thus performing the function of a compressor. The ejectorsystem has lOng been applied to jet pumps, vacuumpumps, high-altitude simulators, V/STOLs, etc[lrv4],because of the major advantages of its structural simplicity and reliabili…     

Two-phase flow pressure drop hysteresis in an operating proton exchange membrane fuel cell

Two-phase flow pressure drop hysteresis was studied in an operating PEM fuel cell. The variables studied include air stoichiometry (1.5, 2, 3, 4), temperature (50, 75, 90 °C), and the inclusion of a microporous layer. The cathode channel pressure drops can differ in PEM fuel cells when the current density is increased along a path and then decreased along the same path (pressure drop hysteresis). Generally, the descending pressure drop is greater than the ascending pressure drop at low current densities (<200 mA cm⁻²), and the effect is worse at low stoichiometries and low temperatures. The results show that the hysteresis occurs with or without the inclusion of a microporous layer. Initial results show a modified Lockhart-Martinelli approach seems to be able to predict the two-phase flow pressure drop during the ascending path. The results compare well with photographs taken from the cathode flow field channel of a visualization cell.     

CFD evaluation of pressure drop across a 3-D filter housing for industrial gas turbine plants

This paper investigated the flow distribution and total pressure drop across a designed 3-D filter housing integrated with a 3-stage filtration system using computational fluid dynamics (CFD). The filter housing model was proposed for a heavy-duty industrial gas turbine plant operating at an average ambient temperature of 20°C.The pressure drops across the classes of filters were 652.8 Pa, 2692.2 Pa, 887.8 Pa, 776.2 Pa and 2304.2 Pa for I-GB, GB-GA, GA-FA, FA-HA, and HA-O, respectively. The results obtained indicated an acceptable total pressure drop of 7.2% for the entire filter housing before filter clean-up. Although the CFD simulation result shows that small outlet flow velocity and transonic flows exist at the outlet of the filter housing, the designed filter housing was proved compatible with the studied GT, for inlet flow conditions between 600≤W _air≤610 kg/s and 60≤v _air≤70 m/s for the air flow rate and velocity, respectively. Furthermore, the designed filter housing could be adopted for the studied GT and locations of Usan and Maiduguri in Nigeria, and other locations with similar environmental conditions.     

Analysis on flow structure and improvement of heat transfer in 3D circular tube with varying axial groove turbulator configurations

Using passive devices are an efficient method to enhance streamline behavior when liquid flows through the circular pipe. The interrupted structure groove is usually used to change the flow patterns. In this analysis, a heat performance numerical technique is applied to study the characteristics of fluid flow and heat transfer of the circular pipe using different axial groove geometrical configurations with different axial groove numbers, including 2, 3, and 4, under different conditions. The number of annular grooves and circumferential positions are the important parameters to analyse with varying operating conditions, with the Reynolds number (Re) range from 1500 to 23,000. A three-dimensional coordinate pipe system is applied using tetrahedron grids. The discretization equations are obtained by deriving algebraic approximations to integral conservation equations. Results observed that using this type of passive method has a low effect on pressure dope compared to the normal one (smooth pipe). The flow change occurs near and closed to the axial groove parameters. Moreover, the Nusselt number (Nu) value for the groove turbulators was higher than the normal one, about 14.5%–21%. The friction factor (f) value for the groove turbulators was higher than the normal one, were about 7.5%–24%. Most friction losses are caused by dynamical pressure dissipation owing to more viscous losses closed to the wall surfaces. The improvement of heat performance using this type of passing method was more than 1.2%.     

Prediction of pressure drop in multiphase horizontal pipe flow

Empirical correlations were tested against reliable two phase pipe flow data for the prediction of pressure drop. Correlations are recommended for the prediction with stratified and annular type flows. When these correlations were adapted to three phase gas–water–oil pipe flow in general they predicted for intermittent slug type flows. Momentum balance models could not be successfully adapted to the prediction of pipe three phase pressure drop.     

Two-phase co-current flow in inclined pipe

Data are presented on horizontal and slightly inclined flows at +5 and −5° for a 0.058 m inner diameter (i.d.) pipe with the co-current air–water system.The prediction capabilities of existing flow regime maps were shown to be inadequate. However, the transitions for stratified ripple to role wave, for slug to blow-through-slug, for film plus droplet to stratified, and the modified maps for stratified type to slug flows all gave good prediction performance with horizontal and slightly inclined flows.The largest liquid hold-up occurred in upward flow except at high gas rates and low liquid rates where the downflow condition gave the highest liquid hold-up. The lowest liquid hold-up occurred in downward flow at low gas flow rates and horizontal flow at high gas flow rates. Hold-up prediction proved to be flow regime dependent.The inclined total average pressure drop data crossed over the horizontal data from higher to lower values with increasing gas rate at a gas rate of just under V?_SG = 10 m s⁻¹.Below this gas rate the horizontal pipe gave the lowest pressure drop while above this gas rate the upwardly inclined pipe gave the lowest pressure drop. A pressure loss minimum occurred at V?_SG = 10 m s⁻¹ for upward flows. Below V?_SG = 10 m s⁻¹ the pressure loss for downward flow was virtually independent of gas rate being mainly due to hydrostatic head. As the gas flow approached V?_SG = 50 m s⁻¹ there was very little effect of inclination on the pressure loss.Pressure drop was successfully predicted although the accompanying hold-up prediction was not always reliable.     

Experimental study of convective condensation in an inclined smooth tube. Part II: Inclination effect on pressure drops and void fractions

This article is the second part of a two-part paper, dealing with an experimental study of convective condensation of R134a at a saturation temperature of 40 °C in an 8.38 mm inner diameter smooth tube in inclined orientations. The first part concentrates on the flow pattern and the heat transfer coefficients. This second part presents the pressures drops in the test condenser for different mass fluxes and different vapour qualities for the whole range of inclination angles (downwards and upwards). Pressures drops in a horizontal orientation were compared with correlations available in literature. In a vertical orientation, the experimental results were compared with pressure drop correlations associated with void fraction correlations available in literature. A good agreement was found for vertical upward flows but no correlation predicted correctly the measurements for downward flows. An apparent gravitational pressure drop and an apparent void fraction were defined in order to study the inclination effect on the flow. For upward flows, it seems as if the void fraction and the frictional pressure drop are independent of the inclination angle. Apparent void fractions were successfully compared with correlations in literature. This was not the case for downward flows. The experimental results for stratified downward flows were also successfully compared with the model of Taitel and Dukler.     

变倾角管道气力输送阻力特性研究

在变倾角管道气力输送试验装置中,对平均粒径相同（d=2 mm）、密度不同的空心玻璃珠和小米进行了气力输送试验,研究了气固两相流动在不同倾角管路中的阻力特性,着重分析了表观气速变化、管道倾斜角度和输送固气质量比的变化对气固两相管道流动压力损失的影响。结果表明,固气质量比、输送管道的倾斜角度及空气表观气速的变化对输送管道单位长度上的压差都有较大影响;而固相颗粒物性的变化对其结果也有一定的影响。     

Heat transfer enhancement of round impinging jet by orifice nozzle (Effects of contraction area ratio)

The effects of the nozzle contraction ratio on the flow and heat transfer characteristics of an orifice impinging jet were investigated in this experiment. The nozzle diameter was d_o=10.0 mm=const., and the contraction area ratio CR=(d_o/d_i)², where d_i is the inner pipe diameter was varied from CR=1.00 to 0.11 and the nozzle‐plate distance was varied from H/d_o=2.0 to 5.0. The nozzle Reynolds number was Re=1.5×10⁴=const. The flow characteristics were clarified by measuring the pressure and velocity distributions on the plate and flow visualization. The Nusselt number obtained from measuring the temperature distribution on the plate of an orifice impinging jet with a CR of 0.11 and 0.69 were respectively larger by 19% and 9% than those of a pipe impinging jet (CR=1.00), because the centerline velocity of the orifice jet was larger than that of the pipe jet. Under the same operation power, an orifice impinging jet has improved heat transfer characteristics. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20225     

Experimental investigations on a 20 kWe, solid biomass gasification system

When the objective is to generate motive or electric power via I.C. engine, the overall pressure drop through the suction gasification system in addition to gas quality has become a sensitive issue. This work, therefore, presents an experimental study on a suction gasifier (downdraft) arrangement operating on kiker wood or Acacia nilotica (L). Studies were conducted to investigate the influence of fluid flow rate on pressure drop through the gasifier system for ambient isothermal airflow and ignited mode, pumping power, and air-fuel ratio, gas composition and gasification efficiency. Results of pressure drop, temperature profile, gas composition or calorific value are found to be sensitive with fluid flow rate. Ignited gasifier gives much higher pressure drop when compared against newly charged gasifier bed with isothermal ambient airflow. Higher reaction temperatures in gasifier tends to enhance gasifier performance, while, overall pressure drop and thus pumping power through the system increases. Both ash accumulated gasifier bed and sand bed filters with tar laden quartz particles also show much higher pressure drops.     

Effect of Axisymmetric Sonic Nozzle Geometry on Characteristics of Supersonic Air Jet

The effect of nozzle geometry on sonic line and characteristics of supersonic air jet was studied. Computational fluid dynamics was applied in this study. The axisymmetric nozzle geometries investigated were two different contour converging nozzles, two different conically converging sharp-edged nozzles and a sharp-edged orifice. The results show that the supersonic jet structure, sonic line and streamlines in supersonic jet are strongly influenced by the nozzle geometry, and the total pressure loss increases with the increase of Mach disk diameter. The present numerical simulation is an effective tool to evaluate compressible flows in supersonic air jet.     

垂直管密相输送的数值模拟

基于稠密气体分子运动论和颗粒动力学,应用稠密气固两相流动的欧拉一欧拉双流体数学模型,利用数值方法模拟模拟垂直管内上升流动的行为,得到了气相和颗粒相速度分布以及压力损失.将该模型运用到垂直管高压密相(输送压力3 MPa、最大固气比426 kg/m3)气力输送中,结果表明模型预测的压降梯度与实验测量结果相符合.     

Analytical solution for Newtonian–Bingham plastic two-phase pressure driven stratified flow through the circular ducts

For steady state, stratified, laminar, fully developed two-phase flow which one of them is Newtonian and the other one is Bingham plastic, the motion equations in horizontal pipe with appropriate boundary conditions have been solved analytically. Pressure drop, velocity distribution and location of plug region related to Bingham plastic fluid have been reported. The results show that the non-Newtonian rheological properties have negligible effects on two-phase velocity profile and consequently on pressure gradient in small viscosity ratio of two fluids. With promotion of viscosity ratio, the influence of yield stress on two-phase velocity profile is more considerable.     

Theoretical approach for a pressure drop in two-phase particle-laden flows

The purpose of this research is to develop an analytical model for a pressure drop per unit pipe length due to the turbulence modulations of a carrier phase which results from the presence of a dispersed phase in various types of diluted two-phase flows. The wake behind a particle, a particle size, the loading ratio and the density difference between two phases of a particle-laden flow were considered as significant parameters, which have an influence on the turbulence of a particle-laden flow, and the relative velocity of the laden particles was calculated by using a terminal velocity. The frictional pressure drop was formulated by using the force balance in the control volume by considering the shear stresses due to the presence of particles and an analogy of the shear stresses in the solid surfaces. The numerical results show a good agreement with the available experimental data and the model successfully predicted the mechanism of the pressure drop in the particle-laden flows.