泡状入流条件下旋流泵气液两相流动特性

为研究泡状入流条件下旋流泵内气液两相流动特性规律,对旋流泵进行了全流道数值模拟,并通过试验对其进行验证,分析不同入口体积含气率及不同液相流量对旋流泵内部流动结构演化的影响,并探讨流动结构与外特性的关联。结果表明,在1%～20%入口体积含气率条件下,气泡进入旋流泵后大部分聚集在叶轮轮毂附近,泵内气液两相流型主要有3种,分别为凝聚气泡流、气囊流和气液分离流;当入口体积含气率为1%时,液相介质中少量气体的混入有助于泵压升和效率的提高,而当含气率增大到5%时,泵内循环流旋涡数量增多,并且向无叶腔内聚集,从而增加了能量耗散,导致泵做功能力降低,进而降低了泵的性能。研究结果可为气液两相条件下旋流泵的优化设计和选型提供参考。     

低含液率气液两相流涡街探头最佳安装位置研究

通过数值仿真试验研究,验证利用FLUENT仿真软件进行数值仿真的可行性,并且得出在单相流以及气为连续相的低含液率气液两相流这两种情况下,50 mm口径涡街流量计探头的相对最佳安装位置,其中单相流的涡街探头最佳安装位置在发生体后0.4D处,气为连续相的低含液率气液两相流涡街探头的最佳安装位置在发生体后1D处。利用FLUENT仿真软件的后处理功能对流场进行分析,得出低含液率下采集的信号在发生体后1D处最优的本质原因——在发生体后1D处,旋涡达到成熟,涡街信号最强,流场信息最为丰富稳定。以含液率0.04%的气液两相流流场与单相流流场进行对比,分析得出单相流和低含液率两种情况下,涡街探头最佳安装位置不同的原因。     

气液单向阀弹簧刚度对开启特性的仿真分析

利用VOF法建立了混输泵出口单向阀内流场的气—液两相流CFD模型,采用UDF及动网格技术,研究了介质不同含气率下弹簧刚度对出口单向阀开启特性的影响。仿真结果表明:在一定范围内,随着弹簧刚度的增大,单向阀最大开启高度降低,开启滞后时间稍有增加;介质含气率变化对阀芯开启滞后影响明显,随含气率的增加,阀最大开启高度大幅降低,开启滞后时间增加,且随含气率增大,弹簧刚度的影响减弱。     

气液脉冲两相流耦合激振数值模拟与试验

提出了一种气液耦合激振方式,通过控制气路和液路交替产生的高压脉冲两相振荡流,利用其产生的激振力实现对液压系统管道内壁的污染物去除。首先,建立了高压脉冲两相流动力学模型,开发了气液脉冲两相流试验系统,利用Ansys Fluent模块进行数值模拟与仿真分析;其次,湍流模型采用k-ε二方程模型,气液两相流采用流体体积函数(volume of fluid,简称VOF)模型,用Simple算法对双流体控制方程组进行迭代求解,对气液脉冲两相振荡流的压力场、速度场及流态进行了分析;最后,采用压力变送器和数据采集卡对气液入口处不同流体压力下管道中部的混合流体激振压力进行测量,对实测压力信号进行滤波,并与数值模拟进行对比。分析表明:混合流体激振压力随着进气口和进油口流体压力的增大而增大,变化趋势与数值模拟基本吻合;在气液交替混合过程中,随着通气时间的增加,混合流体激振压力逐渐增大。数值模拟和试验研究揭示了气液脉冲两相流的动力学特性,为气液两相流激振的可控性提供了理论依据和试验基础。     

不同含气率下球阀内部流动特性试验及其数值模拟

为研究不同含气率对油气混输泵泵阀内部流场的影响,运用CFD软件对油气混输泵球阀内部气液两相分布、速度场及进出口压差进行了模拟仿真。同时对球阀进行试验研究,测量出球阀进出口的压力值,以验证模拟结果的准确性。试验表明:模拟值与试验值数据误差不超过15%,模拟较为准确。在相同的开启高度、进出口流量下,随着含气率的增加,球阀内流体的压差降低幅度高达90%以上,且其降低较均匀。与此同时,流体密度降低并没有使流量系数和阻力系数有明显的变化。观察两相云图可以发现,在阀球尾部A处产生了旋涡,旋涡随含气率的增加略微增多;突扩界面C处产生了二次回流,使得该处的气相也较多,而在阀球四周内壁,气相分布较少。同时可观察出,流体含气率对阀球间隙处速度的影响不大,但是对流动状态有一定的影响。     

含气介质用机械密封端面两相流动特性*

气液混输条件下,密封腔内的含气率较高将会使得密封液膜中有气体进入,从而导致密封环出现“失稳”现象。为探讨含气介质对机械密封性能的影响,通过建立端面螺旋槽型液膜模型,基于Mixture多相流模型,对端面液膜中气液两相分布及机械密封密封性能进行研究。结果表明：液膜内气相体积分数随气泡直径的减小而增大;不同入口含气率下密封端面两相分布规律相近,含气率较高的位置出现在槽根半径处;随着含气率、转速、压差的升高,〖JP2〗槽根处的压力随之升高,从而影响密封性能;在相同含气率、转速及压差下,随膜厚的增加,泄漏量增大,开启力减小,且较小的膜厚对工况参数的改变更为敏感,槽深与膜厚的相关性较强,优化机械密封结构时需综合考虑两者的影响。     

气相压缩性对气液混输泵设计工况点性能的影响

为研究气相压缩性对气液混输泵性能的影响规律，以螺旋轴流式气液混输泵为研究对象，采用欧拉-欧拉双流体模型，在20%～80%进口含气率范围内对混输泵设计工况点进行数值计算，分析气相压缩性对混输泵外特性曲线和内流场的影响。研究表明，在高含气率下气相可压缩性对混输泵的外特性预测结果影响明显增大；考虑气相压缩性后，混输泵叶轮单个流道内最先出现气堵问题，并且气相压缩性明显影响叶轮叶片上压力载荷分布以及叶轮内的气液两相的相间速度差分布。     

水轮机过流部件出水边夹角对卡门涡频率的影响

利用数值模拟方法,模拟具有相同出水边厚度、不同出水边夹角的一组水轮机过流部件下游侧的卡门涡现象,研究出水边夹角对卡门涡频率的影响,结果表明,随着出水边夹角的增加,卡门涡频率逐渐降低;当出水边夹角增加到30°时,卡门涡频率模拟值大约降低到圆柱绕流卡门涡频率理论公式计算值的1/3。通过对数值模拟结果和理论公式计算值进行对比分析以及数据处理,获得斯特劳哈尔数与过流部件出水边夹角的关系曲线和近似的直线拟合公式。以某水轮机固定导叶出水边卡门涡振动测试数据为基础,对比验证水轮机过流部件卡门涡数值模拟方法的准确性,以及获得的斯特劳哈尔数取值计算公式的准确性。研究结果对合理设计水轮机过流部件出水边形状、避免发生卡门涡共振问题具有较高的指导意义和参考价值。     

射流式离心泵气液两相流数值分析

射流式离心泵是一种特殊的流体机械,采用射流器与离心泵组合的方式设计,以牺牲一部分效率来提高离心泵抗空化性能,增大吸程。基于CFD技术,针对射流式离心泵进行了定常气液两相流数值求解。设定进口处气体的容积含气率分别为5%,10%,15%和20%,气泡直径为0.1 mm,在单相计算收敛后数值模拟了气液两相全流场。结果表明:当含气率0.05时,泵性能不会发生较明显的变化;喷嘴到叶轮进口20～90 mm之间液相速度大于气相速度,气液两相速度差为1～2 m/s;在喷嘴到叶轮进口90～150 mm之间时气相速度大于液相速度,两相最大速度差为4 m/s。     

叶片式混输泵入口段气液两相流场可视化试验

气液两相混合流体在叶片式混输泵内的流动与入口段气液混合程度有直接的关系,故在混输泵入口前端设置了自行设计的缓冲均化器,并通过可视化试验探索入口段气液两相流型及气泡直径随转速和入口含气率的变化规律。研究发现:经过缓冲均化器后,气液两相流体在混输泵入口段表现为均匀的泡状流,无大团气泡聚集现象,说明缓冲均化器结构及多孔管开孔方案合理,能够起到均匀混合气液两相流体的作用;在同一转速和液相流量下,混输泵入口段气泡直径变化规律呈正态分布,随着入口含气率的增加(0~50%),气体总流量增加,在均化器中与水混合后形成的气泡初始直径逐渐增加,使得混输泵入口段气泡直径也逐渐增加;在同一液相流量和入口含气率工况下,气泡的初始直径相同,而随着混输泵转速的增加(1 800~2 700 r/min),入口段流体旋转角速度增大,导致液相对气相的拖曳力也相应增大,最终导致气泡直径变小;绘制了泵入口气泡直径随入口含气率及转速的变化规律曲线,可以为混输泵内流场数值模拟中入口气液两相流型及平均气泡直径的设置提供参考。     

Research on signal amplitude of the Kármán vortex street in gas–liquid two-phase flow with high void fraction

Based on Biot–Savart law and single-phase flow Kármán vortex characteristics, flow field has been analyzed when gas–liquid flow past a fixed bluff body with high void fraction. Vortex signal characteristics have been studied for stratified two-phase flow on atmospheric conditions in a horizontal pipe. To discuss the relation between void fraction and vortex signal amplitude spectrum, this paper sets up the vortex-induced pressure field model for gas–liquid two-phase flow and gives the relationship between void fraction and relative amplitude spectrum of two-phase flow to single-phase flow. An algorithm is proposed for predicting the two-phase flow parameters. Experiments were performed using air–water as working fluid along with a test tube diameter of 50 mm, at gas volume flow rate of 20–68 m³/h, and void fraction of 0.9–1. The results indicate that calculations by the vortex-induced pressure field model on the amplitude spectrum of vortex signal are in good agreement with the experimental data, and relative errors of the algorithm predictions on gas volume flow rate and liquid volume flow rate are 0.08 and 0.56, respectively.     

Influence of blockage and shape of a bluff body on the performance of vortex flowmeter with wall pressure measurement

Experimental and numerical investigations are carried out with various bluff body shapes to identify an appropriate shape which can be used for vortex flowmeter application. In both the cases vortex shedding frequency is inferred from the fluctuation of wall pressure. The numerical simulations are carried out with cylindrical and triangular bluff bodies to understand the vortex shedding phenomenon and to identify an appropriate turbulence model for this class of flows with wall pressure measurement. The simulations reveal that the k-ε RNG model predicts the Strouhal number closer to the experimental results than other models. The experimental investigations are carried out with several bluff body shapes, such as triangular, trapezoidal, conical, cylindrical and ring shapes, with water as the working medium. In this context, the effects of sampling rate, tap location and blockage effects are explored. The results suggest that the axisymmetric tapping is better than differential pressure tapping in terms of signal amplitude. The non-dimensional location of the static pressure tap is found to be 0.714 times diameter of pipe times blockage. The trapezoidal bluff body is found to be the best among all the bluff bodies investigated in terms of signal amplitude and constancy of Strouhal number. The vortex flowmeter performance is also measured under disturbed flow conditions created by using gate valve and bends. These results are significant because they provide an optimum bluff body shape and blockage, and also present the performance of vortex flow meter under disturbed flow conditions which is rather seldom reported in the literature.     

The Effects of Orifice Nozzle Number on Heating and Cooling Performance of Vortex Tubes: An Experimental Study

Abstract

In this study, the effects of the orifice nozzle number and the inlet pressure on the heating and cooling performance of the counter flow type vortex tube is investigated experimentally. The orifices are produced using the polyamide plastic material. Five orifices with 2, 3, 4, 5, and 6 nozzles are manufactured. In this study, air is used as a fluid. In the experiments, for each of the orifices, inlet pressures were adjusted from 150 kPa to 700 kPa with 50 kPa increments and the energy separation was investigated. During the experiments, cold mass fraction of the vortex tube that L/D ratio is 15, is held constant at 0.5. At the end of the experimental study, it is noted that the temperature gradient between the cold and hot fluid is decreased with increasing of the orifice nozzle number.     

气液混输漩涡泵内部气相分布与性能预测研究

为研究漩涡泵的气液混输性能,采用Fluent计算软件对泵内部气液两相流场进行了数值模拟。模拟结果初步揭示了该泵内气液两相流动特征,由此可知泵流道内的气泡主要集中在叶片压力面根部,气泡聚集程度随含气率的增加而增加。泵性能预测曲线与试验曲线较为吻合,说明了所采用的计算模型是基本可行的。     

涡流管制冷分析

对涡流管制冷本质并进行相关的试验研究,进行理论分析,以及对四流道喷嘴涡流管的速度和温度分布进行数值模拟,结果表明:涡流管内实际流场由轴向,径向和漩涡运动组成,其流动形态在轴向上为阿基米德螺线,在横截面上为强制涡—自由涡的模型;管内流体能量的分离主要发生在涡流室区域附近,且气体从喷嘴出来后有少量直接进入冷端孔与冷气流混合,从而影响涡流管制冷效率。     

双钝体涡街流量计流体振动特性的试验研究

研究了双钝体旋涡脱离的流体动力学特性以及利用双钝体增强流体振动的最佳组合结构。给出了多种钝体组合结构的斯特劳哈尔数的试验结果,表明双钝体结构具有常定的斯特劳哈尔数。用50 mm口径涡街流量计进行的试验证明,双钝体组合在一定的条件下能获得理想的旋涡重叠,从而增强流体的振动。在钝体的轴对称点上,还观察到强度和频率相同,相位相差180°的流体振动点,利用优化的双钝体结构和差动传感技术,研制出小流量灵敏度和抗干扰性能很好的新型旋涡流量计。     

Numerical and experimental investigation of vortex breaker effectiveness on the improvement in launch vehicle ballistic parameters

The focus of the present study is to investigate the effectiveness of installing vortex breakers at the outlet of launch vehicle tanks on postponing vortex formation and decreasing the critical height of propellants while discharging. Analytical results in the absence of a vortex breaker show that the effects of the Weber and Reynolds numbers in the flow field can be ignored for values greater than 720 and 1.1 × 10⁵, respectively; and critical height can be considered as a function of Froude number under aforementioned conditions. The analytical criteria are verified by two-dimensional, axis symmetrical, transient and two-phase numerical model. Eventually, some experiments are conducted to examine the effectiveness of the applied vortex breakers in reduction of the critical height of propellant. Experimental results show that a 30% decrease can be achieved in critical height by using a particular type of vortex breaker. Additionally, the carried out simulations for an existing two-stage launch vehicle indicate a 13% increase in orbital altitude, which in turn proves the considerable improvement in launch vehicle mass/energetic capabilities.     

旋流泵内液固两相流场的CFD-PBM耦合计算

为获得旋流泵内更为符合物理真实的液固两相流动特征,在传统欧拉(Euler)双流体模型基础上加载群体平衡模型(PBM),以考虑实际存在的颗粒聚并、破碎等动力学行为,与CFD耦合计算了不同流量、颗粒粒径及浓度下的液固两相流场,分析了颗粒存在对泵外特性的影响规律。计算结果表明:从进口到出口,叶片背面附近颗粒粒径明显增大;在叶轮出口位置,同一半径上,从叶片工作面到背面附近存在粒径梯度;在外缘部,沿轴向形成粒径梯度。与Euler模型计算结果对比发现:加载PBM模型后,颗粒总体浓度分布特征存在差异;同一轴截面上,颗粒浓度在中心部的分布基本相同,而在中间和外缘部位置出现差异。PBM模型计算得到的泵扬程、效率曲线更接近于实验值,证明基于CFD-PBM耦合计算的预测精度更符合实际。     

Installation effects on vortex shedding flowmeters

Numerous measurements of the effects of pipe fittngs on vortex shedding flowmeters are carried out as a contribution to flow metering standards. A water test line of 150 mm diameter is used in the experiments covering a Reynolds number range of about 2 × 10⁵ to 10⁶. The effects of six kinds of piping configurations are examined at various upstream straight pipe lengths and all four kinds of liquid vortex shedding flowmeters, which were commercially available in Japan, are tested. The vortex shedding flowmeters are compared with a turbine meter in experiments designed to evaluate reproducibility of measurements. The uncertainty of the measured data is estimated at about 0.1%. It is found that the magnitude of each installation effect strongly depends on the design of the flowmeter. The experimental results are presented in detail and a table is given of the minimum upstream straight pipe lengths needed to suppress the effects to less than 0.5% for each of the tested flowmeters. This can be used as guidance in the installation of vortex shedding flowmeters.     

High GVF and low pressure gas–liquid two-phase flow measurement based on dual-cone flowmeter

Parameter measurement of gas–liquid two-phase flows with a high gas volume fraction (GVF) has received great attention in the research field of multiphase flow. The cone meter, as a new proposed differential pressure (DP) meter, is increasingly being applied in flowrate measurement of gas–liquid two-phase flow. A dual-parameter measurement method of gas–liquid two-phase flow based on a dual-cone meter is proposed. The two-phase flow is investigated in a horizontal pipeline with high GVF and low pressure, and exists in the form of annular flow. By adding a second cone meter, both gas mass fraction (GMF) and mass flowrate are measured. The pressure drop performances of five different sized cones have been discussed to make a cooperating cone selection and efficiently position the dual-cone in the pipe. Dual-cone flowmeter experiments of 0.45 and 0.65 equivalent diameter ratio combination, and 0.65 and 0.85 equivalent diameter ratio combination are respectively carried out to analyze the linearity of two-phase flow multiplier with Lockhart–Martinelli parameter and obtain the dual-parameter measurement results. The relative experiment error of GMF, gas mass flowrate and total mass flowrate are respectively within ±7%, ±5% and ±10%. The relative error of the liquid phase is within ±10% when the liquid mass fraction is beyond 40%. The experimental results show that it is efficient to utilize this dual-cone method for high GVF and low pressure gas–liquid two-phase flow measurement.