Numerical investigations on effects of bluff body in flat plate micro thermo photovoltaic combustor with sudden expansion

In order to reveal combustion characteristics of H_2/air mixture in a micro-combustor with and without bluff body, the effects of inlet velocities, equivalence ratios and bluff body's blockage ratios on the temperature field, pressure of the combustor wall, combustion efficiency and blow-off limit were investigated. The numerical results indicate that the sudden expansion plate micro combustor with bluff body could enhance the turbulent disturbance of the mixed gas in the combustion chamber and the combustion condition is improved. Moreover, a low-speed and high temperature recirculation region was formed between the sudden expansion step and the bluff body so that the high and uniform wall temperature(>1000 K) could be gotten. As a result, it could strengthen the mixing process, prolong the residence time of gas, control the flame position effectively and widen the operation range by the synergistic effect of the bluff body and steps. When the blockage ratio ranged from 0.3 to 0.6, it could be found that the bluff body could play a stabilizing effect and expand combustion blow burning limit, and combustion efficiency firstly was increased with the inlet velocity and equivalence ratio, and then was decreased.

EXPERIMENTAL STUDIES ON SWIRLING AND RECIRCULATING TWO-PHASE FLOW FIELD IN A COLD MODEL OF DUAL-INLET SUDDEN-EXPANSION COMBUSTOR

The axial and tangential velocities of gas and particle phases and particle concentration for turbulent swirling and recirculating gas-particle (simulating gas-droplet) flows in a cold model of a dual-inlet sudden-expansion combustor with partially tangential central tubes, proposed by the present authors, were measured by using a 2-D LDV system and a laser optic fiber system combined with a sampling probe. The results show that there are both gas and particle strongly reverse flows and swirling flows in the head part of the combustor. The velocity slip between gas and particle phases is remarkable. The particle concentration is higher near the wall and lower near the axis. There are two peaks in the concentration profiles near the inlet tubes. The above-obtained flow characteristics are favorable to ignition, flame stabilization and combustion. The results can also be used to validate the numerical modeling.

Measurement and simulation of the two-phase velocity correlation in sudden-expansion gas-particle flows

In this paper the present authors measured the gas-particle two-phase velocity correlation in sudden expansion gas-particle flows with a phase Doppler particle anemometer （PDPA） and simulated the system behavior by using both a Reynolds-averaged Navier-Stokes （RANS） model and a large-eddy simulation （LES）. The results of the measurements yield the axial and radial time-averaged velocities as well as the fluctuation velocities of gas and three particle-size groups （30μm, 50μm, and 95μm） and the gasparticle velocity correlation for 30μm and 50μm particles. From the measurements, theoretical analysis, and simulation, it is found that the two-phase velocity correlation of sudden-expansion flows, like that of jet flows, is less than the gas and particle Reynolds stresses. What distinguishes the two-phase velocity correlations of sudden-expansion flow from those of jet and channel flows is the absence of a clear relationship between the two-phase velocity correlation and particle size in sudden-expansion flows. The measurements, theoretical analysis, and numerical simulation all lead to the above-stated conclusions. Quantitatively, the results of the LES are better than those of the RANS model.

Non-Newtonian Two-Phase Flow Characteristics across Sudden Expansion in Horizontal Rectangular Minichannel

In the present paper, in order to clarity the effects of non-Newtonian liquid properties on the flow, similar experiments have been conducted for that of 0.4 wt% polyacrylamide (PAM) aqueous solutions as the working liquid, and air as the working gas. Liquid single-phase and air-liquid two-phase flow experiments were conducted at room temperature using the horizontal rectangular mini-channel with a sudden expansion. The cross-sectional dimensions of the narrow channel upstream from the sudden expansion were 2.79 mm, 3.09 mm and 2.94 mm in the height (H), the width (W) and the hydraulic diameter (DH), while those for the wide channel were 2.95 mm, 5.98 mm and 3.95 mm. The pressure distributions in the channels upstream and downstream from the expansion were measured with calibrated pressure transducer to determine the pressure change due to the expansion. The flow pattern, the bubble velocity, the bubble length, and the void fraction were measured with a high-speed video camera. The flow pattern map is drawn from the observed flow pattern, i.e., bubble flow, slug flow and annular flow in both the wide and the narrow channels. The bubble length data were compared with the calculation by the scaling law proposed by Kanezaki et al. and Kawahara et al. The pressure change data at the expansion were compared with our previous data together with several correlations in literature. Results of such experiment and comparisons are reported in the present paper.

Experimental Determination of Local Resistance Coefficient of Sudden Expansion Tube

Theoretical derivation of local resistance coefficient of sudden expansion tube is presented. Several assumptions are analyzed in the theoretical derivation. That the head loss shall be neglected is affirmed. Experimental data proves that the pressure before and after sudden expansion section is basically the same. That the friction force on the side face of control body is neglected is denied and it is pointed out that such neglect is the main cause for error between theoretical calculation and actual measurement. Experimental device for measuring local resistance coefficient is designed in combination with theoretical derivation process. Optimal gradually varied flow section is selected after sudden expansion pipe in Bernoulli equation based on variation of piezometer tube head. It is pointed out in accordance with experimental data analysis that the value of local resistance coefficient of sudden expansion tube determined through experimental data is closer to the actual situation during pipeline design.

Numerical Study of Heat Transfer and Flow Bifurcation of CuO Nanofluid in Sudden Expansion Microchannel Using Two-Phase Model

In this paper, laminar forced convection of CuO nanofluid is numerically investigated in sudden expansion microchannel with isotherm walls and different expansion ratios (ER). An Eulerian two-fluid model is considered to simulate the nanofluid flow inside the microchannel and the governing mass, momentum and energy equations for both phases are solved using the finite volume method. Eulerian-Eulerian two-phase model is very efficient because of considering the relative velocity and temperature of the phases and the nanoparticle concentration distribution. In solving the flow equations for both phases, the SIMPLE algorithm is modified for the coupling of the velocity and pressure and the continuity equations for both phases are combined in order to create the pressure correction equations. However, the Eulerian-Eulerian modeling results show higher heat transfer enhancement in comparison to pure water, so that for a 2% copper-water nanofluid, it has been observed a 35% increase of the heat transfer. The heat transfer enhancement increases with increase in Reynolds number and nanoparticle volume concentration, while the pressure drop increases only slightly. An investigation of the expansion ratio of microchannel shows that the average Nusselt number increases with decrease in expansion ratio as well as with increase in Reynolds number. Also, the Bifurcation has been occurred in higher Reynolds number that is different for each expansion ratio of the microchannel.

FINITE ELEMENT ANALYSIS OF TRASINENT TWO DIMENSIONAL CHANNEL FLOW WITH A SUDDEN EXPANSION

The finite element method is employed to simulate incompressible viscous unsteady flow in two dimensional channel with a sudden expansion. The streamline patterns and the velocity vector plots are presented at different expansion ratios and at different moment. The results obtained have certain significance to analyze the formation of eddies and energy loss in a sudden expansion channel flow and other complex channel flow.

Platelet adhesion to the surface of a sudden tubular expansion tube under swirling flow condition

The size mismatch in an end-to-end vascular anastomosis between the host vessel and the graft may cause flow disturbance and predispose to thrombosis [1].Although a number of techniques have been employed to reduce the risk of anastomotic thrombosis due to the size mismatch。

Pressure Characteristics Study for the Configuration of Sudden Expansion with Central Restriction and Suction

In this paper, an extensive numerical study on pressure characteristics in the configuration of sudden expansion with central restriction and suction has been carried out. During study, Reynolds numbers (Re) are considered from 50 to 200, suction (S) from 2% to 10% of inlet mass flow, percentage of central restriction (CR) from 0% to 40% and aspect ratio (AR) from 2 to 6. The effects of each variable on average static pressure distribution and average stagnation pressure distribution have been studied in detail. The results have been compared with the configuration of plain sudden expansion, and sudden expansion with central restriction only. From the study, it is noted that maximum magnitude of average static pressure rise from throat increases with the increase in percentage of suction, flow Reynolds number and percentage of central restriction. This magnitude is higher at lower aspect ratio. Also, it is observed that maximum magnitude of average static pressure rise from throat is always more in case of suction configuration compared to the case of configuration of without suction. Average stagnation pressure drop at any section increases with the increase in percentage of suction and percentage of central restriction, but it decreases with the increase in Reynolds number. It is noted that higher pressure drop at any section occurs at higher aspect ratio. This pressure drop at a section is more in case of suction configuration compared to the case of without suction.

阀后管道突扩对控制阀门流阻特性的影响

为准确地描述非均匀流条件下控制阀门阀后采用突扩管道后的流阻特性,针对典型控制阀门(活塞式调流阀与固定式锥形阀),采用物理模型试验与理论分析方法,基于阀后突扩管道内的流场结构及试验段压力特性,研究阀后管道突扩对典型控流调压阀门流阻特性的影响规律。结果表明,不同阀型采用相同突扩体时的流阻特性变化程度不同,阀型的影响主要体现在改变了突扩体的入流、出流条件。在规范公式的基础上,考虑阀门出流流场结构(突扩体入流条件)与水流缩脉(突扩体出流条件)的影响,分析得到管道突扩比与阀门流量系数的定量关系,规范计算值与试验值的最大差异约为8%,优化公式计算值与试验值最大差异约为1%,说明改进后的突扩体流阻系数公式的计算精度较规范公式有所提高。研究结果可为高压差、大流量调流调压系统管路设计提供参考。

Flow patterns and their transition characteristics of the air-water two-phase flow in a horizontal pipe with a sudden-changed cross-section area

Flow patterns in upstream and downstream straight tubes of sudden-changed areas in a horizontal straight pipe were experimentally examined. Both sudden-expansion cross-section (SECS) and sudden-contraction cross-section (SCCS) were investigated. The flow pattern maps upstream and downstream were delineated and compared with those in straight tubes with uniform cross-sections. The effects of the SECS and SCCS on flow patterns were discussed and analyzed. Furthermore, flow pattern transition mechanisms resulting in occurrences of different flow patterns were simply discussed and some transition criteria for the flow pattern transitions were deduced by using the non-dimensionlized analysis method.

典型控流调压阀后突扩体的压力特性

为了研究突扩体对控流调压阀门的增压减振作用,以典型控流调压阀门(活塞式调流阀和固定式锥形阀)为研究对象,采用物模试验和三维数值模拟相结合的研究方法,重点对2类阀门在不同突扩体型、阀门开度及空化强度下的阀后流态及管壁压力特性进行了观测分析,探究突扩比与阀后稳定压力的定量关系,试验阀门通径为150 mm,阀门直径为150~370 mm,突扩体长度为1000 mm.结果表明,突扩体能提升阀后管壁时均压力、减弱压力脉动的基本原理是水垫层效应.综合物模与数模试验结果,拟合得到了突扩比与阀后管壁稳定时均压力的关系式.突扩体可以提升阀门的工作背压,提升程度随突扩比的增大而渐缓.当突扩比为4.00时,阀后管壁稳定时均压力的增幅已非常平缓,趋于稳定.不同阀型试验结果差异小,说明突扩体的增压减振作用具有普适性.研究结果可供输水调压工程阀门段管道系统设计参考借鉴.

燃烧室前缘扩张角对旋转爆震冲压发动机的影响

针对圆柱形隔离段-燃烧室构型的旋转爆震冲压发动机,开展了总温为860 K、马赫数为2的来流条件下的直连式试验,探讨了燃烧室前缘扩张角(θ=30°,45°,60°,90°)对爆震波传播特性、工况范围及压力分布的影响。结果表明:当燃烧室前缘扩张角为90°时,燃烧模态均为爆燃燃烧;随着扩张角的减小,燃烧模态将会向锯齿波和混合模态(包含单波阶段)转换。当燃烧室前缘扩张角为30°时,旋转爆震的自持工况范围最宽且燃烧室压力最高;同时,随着燃烧室前缘扩张角减小,实现混合模态的当量比下限降低。此外,分析了燃烧模态对来流的影响,发现:锯齿波/混合模态燃烧室内存在的周期性高频压力扰动会使隔离段内的激波串位置前移;混合模态对超声速来流的影响最为显著。

侧向膨胀效应下激波与涡轮平面叶栅相互作用机理研究

爆轰燃烧具有更低的熵增率和更高的热循环效率,利用旋转爆轰燃烧室替代涡轮喷气式发动机的主燃烧室可以进一步提高其工作性能。为研究旋转爆轰波组合发动机内部的工作过程,建立激波管与涡轮平面叶栅耦合计算模型,考虑激波传播方向的影响,分析激波在侧向膨胀效应下与涡轮叶片相互作用机理,并在此基础上进一步开展可视化实验研究。结果表明:在侧向膨胀效应影响下,激波波阵面从竖直形状转变为弯曲形状,波后气流在直角台阶处形成一个三角形的局部超声速区域,随后弯曲激波与涡轮叶片的前缘处、压力面、吸力面和尾缘处相互作用,在叶栅通道内形成复杂的波系结构。静子叶栅和转子叶栅对压力和温度有着不同程度的抑制作用,当弯曲激波相对于叶片垂直时,叶栅的抑制更为明显。对比分析了实验与数值模拟获得的激波系在涡轮平面叶栅通道内的传播及演化过程,发现激波演化的时空特性高度一致,实验与数值模拟结果吻合良好。观测到弯曲激波在叶片尾缘处发生衍射,波阵面面积明显增大且激波传播方向改变,随着激波继续传播,叶片尾缘处伴随着脱落涡产生。

突扩整流孔板设计与多孔孔板尾迹发展的数值模拟

为了研究压气机试验台进气系统突扩流动孔板的整流效果,设计了4套不同布局的孔板。采用数值模拟方法研究了突扩流动核心区域(简称“中心区域”)开孔率和整个孔板孔分布对多孔孔板尾迹流和主流掺混过程的影响。结果表明:中心区域开孔率增大会使孔板四周区域分流不足,当中心区域开孔率增大到突扩上下游截面面积比的1.46倍时,中心区域分流流量占比达到其面积占比的1.88倍,稳压箱近壁面区域和角区尾迹涡合并增大,长度达到孔间距的17~24倍;为抑制近壁面区域和角区尾迹涡增大和变长,选取中心区域开孔率与突扩上下游截面面积比相当为宜;缩短中心区域孔后尾迹涡长度的最有效方法是缩小孔间距。

微热光电系统燃烧室内截面突变对燃烧的影响

微燃烧室是微热光电(MTPV)系统的重要组成部分。微燃烧室的外表面必须产生一个高而均匀的温度分布,以尽可能高地输出辐射能。通过对截面突变的微燃烧室的试验研究,得到了不同氢氧混合气流量、氢氧体积混合比下燃烧室内的燃烧状况,壁面和出口端面的温度分布。试验结果表明:采用截面突变的燃烧室能很好地控制火焰中心位置,获得稳定的外壁面温度分布规律,可以在截面突变处产生涡流,增强氢气和氧气的混合。

小通道单相流体突扩和突缩局部阻力特性

采用缝隙测压这种在通道上直接测量压力的方法,测量了内径从0.330mm变化到0.580mm的小通道内突然扩大和突然缩小的局部阻力特性。结果表明,缝隙测压方式是一种可行的压力测量方法;与常规通道的实验结果相比,在层流阶段,小通道内液体流动具有较小的突扩局部阻力系数以及较大的突缩局部阻力系数;而在湍流阶段,小通道内液体流动的突扩和突缩局阻系数与常规通道的实验结果相同。

微燃烧室内截面突变对氢氧预混燃烧的影响

针对微燃烧室内部氢氧预混燃烧过程,建立耦合氢氧气相反应动力学机理的数值计算模型,利用CFD计算软件,在试验验证的基础上得到了带截面突变微燃烧室内的燃烧特性,并分析了截面突变尺寸和入口流速对于燃烧过程的影响.结果表明:采用截面突变的结构使得氢气和氧气混合更充分,燃烧更完全,并能提高微燃烧室内的流体温度;增加截面突变的尺寸,可以增加混合气在微燃烧室内的驻留时间,即使在较低入口流速下也能获得较高的燃烧室内部温度;在较低流速下,增加截面突变的尺寸,微燃烧室内的高温区域更宽;入口流速的增加导致氢气转化率降低,燃烧室内截面突变有利于缓解入口流速增加引起的氢气转化率下降的趋势.

突扩廊道体型减免高水头船闸输水阀门空化研究

高水头船闸存在的主要关键技术问题是船闸输水阀门的空化与空蚀问题.本文在综合研究国内外改善输水阀门空化的措施基础上,提出了突扩廊道体型以及减免输水阀门空化,通过物理模型试验和原型观测研究,充分论证了突扩廊道体型减免空化的效果.结合三峡工程连续五级船闸水力学试验研究,优化了突扩廊道体型,提出了结构简单、具有很强抗空化能力的底扩体型.闽江沙溪口船闸水力学原型观测成果表明,突扩廊道体型减弱了阀门空化强度,起到了保护阀门和廊道边壁免遭空蚀破坏的作用,保证了该船闸的正常运转.

突扩管流动形态的数值模拟

针对油品在管道中运输时常常遇到边界突然扩大的流动状态。分别建立突扩管的物理模型、数学模型,并应用软件模拟不同雷诺数、不同突扩比下的管内流体的流动形态,得出了不同雷诺数下轴向压力的变化趋势以及局部水头损失随突扩比的影响规律。结果可以很好地反映突扩管流的基本特征。对于石油生产中常见的此类问题的研究具有重要的意义和作用。