Improvement and uncertainty evaluation of mercury sealed piston prover using laser interferometer

Many NMIs (National Measurement Institute) and calibration laboratories are using a mercury sealed piston prover as calibration system for gas flow meter. But it has problems in deciding measuring volume, traveling time of piston and generating low flow rate below 10 cc/min. In this study, a new structure of a piston prover is designed and its flow measurement uncertainty is evaluated according to ISO/IEC 17025. A laser interferometer, instead of optical sensors used in a typical piston prover, is employed to measure testing time and moving distance of the piston, accurately. A new material of the piston is used to eliminate contaminated particles from the material of the piston body. Uncertainty is calculated by evaluating various uncertainty factors which have influence on gas flow measurement. The expanded uncertainty of the piston prover is 0.11% at the confidence level of 95%. The uncertainty evaluation procedure of this study would be useful in flow measurement uncertainty determination of other types of gas flow measurement systems.     

Modeling of the flow comparator as calibration device for high pressure natural gas flow metering in Modelica

The German national metrological institute, Physikalisch-Technische Bundesanstalt, is developing a new concept a for a volumetric primary standard to calibrate high pressure gas flow meters. The TUHH is supporting these Research & Development activities with its competence to elaborate computational models for detailed analysis of complex electromechanical systems including fluid flow aspects. The new primary standard is called Flow Comparator and uses an actively driven piston prover to measure the gas flow rate using the time the piston needs to displace a defined enclosed volume of gas in a cylinder. A computational model is developed in Modelica® to investigate the Flow Comparator’s dynamic behavior and interaction with other components in the loop. The validation of the developed model shows good compliance with measured piston velocity and differential pressure at the piston. The model is used to optimize the frequency inverter’s control voltage trajectory to increase the available calibration time.     

径向柱塞泵柱塞副泄漏流量的计算

考虑到流体的粘度随压力及温度变化，柱塞内油液压力随吸油压力和排油压力交变，密封长度也随柱塞在缸孔中的位置而变化，及柱塞运动引起的剪切流量等因素，通过积分方法，对新型径向柱塞泵中柱塞副的环形间隙泄漏流量计算公式进行修正，得到了柱塞副泄漏流量修正系数。     

宽温域下电静液作动器的液压缸活塞格莱圈动密封性能分析

为了分析宽温域下电静液作动器的液压缸活塞格莱圈密封性能,基于流体动压润滑理论,建立了考虑油液黏温特性的最大启动压力和泄漏流量的数学模型。利用有限元软件ABAQUS计算得到活塞密封接触面间的接触压力,通过逆解法求解一维雷诺方程得到密封接触面间的油膜厚度,从而定量计算出密封结构单行程的最大启动压力和泄漏流量,并分析了介质压力和温度对最大启动压力和泄漏量的影响。仿真结果表明,介质压力升高会导致活塞动密封有效工作的温域变窄,最大启动压力降低,泄漏流量增大;温度每增加20 ℃,泄漏流量降低量不低于25%,最大启动压力降低量不低于15%,即温度适当升高有利于降低最大启动压力和泄漏流量,但过高的温度将增加格莱圈失效的风险。     

Analysis,Design, and Comparison of Different Types of Pistons for Sealless Pneumatic Cylinders and Valves

This article presents a theoretical and experimental analysis of sealless pistons featuring a special geometry capable of eliminating friction in pneumatic cylinders and valves. Three different piston geometries were investigated: tapered, stepped, and slot-pocket. Performance was evaluated for each geometry in terms of pressure distribution, leakage flow rate, and the self-centering force on the piston. The piston configuration that provides the best tradeoff between self-centering thrust and leakage flow rate in the air gap was also determined. The theoretical and experimental results obtained with the different piston designs were then compared.     

双作用斯特林热机活塞密封流阻特性数值计算

考虑活塞前部环形间隙的影响,建立斯特林发动机活塞密封结构的流阻模型,分别展开动、静态下数值计算,得到活塞各密封段内压力与流量的关系,分析工质随结构参数变化的流动特性。结果表明：动、静态下活塞密封处流阻呈现不同特点,动态时流阻随流量增加而增大,静态时流阻为一恒定值;活塞环泄漏面积对流阻的影响较大,可以通过使用组合式活塞环以增大流阻而减小泄漏;而前部间隙宽度对流阻的影响较小,适当加大间隙宽度能在不影响密封的前提下减小活塞与气缸的摩擦。     

轴向柱塞泵低气压运行特性建模与试验分析

空化是影响液压系统动态特性的重要因素,为此开展了轴向柱塞泵低压环境下的工况研究。考虑气液两相混合油液的密度、体积弹性模量和黏度的影响,限制入口油腔的最低压力,建立轴向柱塞泵的压力流量模型,计算获得轴向柱塞泵在不同工况下的流量特性,并通过试验验证。研究表明:负载增大导致更严重的空化以及泄漏,并使容积效率降低;轴向柱塞泵在达到临界流量之后,转速提升只会加剧空化,而不能提升流量;最大容积效率出现在临界流量产生之前。为轴向柱塞泵低气压性能预测提供了理论支撑。     

车辆减振器油液微小内泄漏分析

针对车辆减振器油液内泄漏问题,对其内部油液微小内泄漏开展仿真与试验分析。通过数学模型对活塞与缸筒环形缝隙中流体进行理论受力分析,运用Autodesk Inventor软件建立减振器内部环形间隙流体几何模型,利用CFD仿真技术对环形间隙流体三维模型开展仿真分析,通过改变流场速度、压力、湍流动能及温度参数,分析得到影响减振器油液微小内泄漏的主要影响因素;采用伺服示功机对不同活塞速度和环形间隙下的油液内泄漏进行试验测试。结果表明:活塞静止时,节流口速度、压力、湍流动能的变化对环形间隙油液内泄漏影响较大,温度变化影响较小;活塞运动时,泄漏量随活塞速度、活塞与缸筒之间的间隙的增大而增大,因此在加工精度允许条件下,可通过减少活塞与缸筒间的间隙来减小泄漏量。     

The humidity effect on air flow rates in a critical flow venturi nozzle

A Critical Flow Venturi Nozzle (CFVN) is usually calibrated using dry air. Yet CFVNs in industrial and calibration service centers are often used to measure flow rates of humid air. Therefore, ISO 9300 provides the calculation method for the humidity effect on discharge coefficients of CFVN. However, since this method is only due to a theoretical analysis, it is important to confirm and check the ISO calculation method for the humidity effect on CFVN with its isentropic analysis by means of an experimental method.In this experiment, three CFVNs with diameters of 0.4, 0.8 and 1.6 mm were calibrated with dry air (with the dew point −40 °C), in a primary air flow standard system with a mercury sealed piston prover, installed at the Korea Research Institute of Standards and Science (KRISS). Another piston prover, a portable dry piston prover, was used as a reference meter and was also calibrated in the primary standard system using dry air. The repeatability of this dry piston prover was confirmed with the deviation being less than 0.05%. The CFVNs were tested with this dry piston prover, using humid air. For air types with high humidity, the humidity effect on flow rates through the CFVNs showed quite significant difference between the experimental results and those from the ISO method with isentropic analysis. But for air types with low humidity, its effect was relatively insignificant.     

Experimental investigation of an oscillating circular piston positive displacement flowmeter: II – Leakage flows and wear tests

Experimental data from an oscillating circular piston positive displacement flowmeter are described which focused on leakage flows and wear. This is the second part of a two part paper on the experimental tests, the first part concerned piston movement and pressure loss. In this paper the leakage across the meter is reported for key parameters including: flow rate, liquid density and viscosity, mass of circular piston, surface coating and length of connection pipes. In addition to the average leakage over a period of time, the leakage past a stationary circular piston was measured during a simulated oscillation. Wear tests were run for both stainless steel and carbon pistons to identify the affected areas of piston and chamber.     

直线电机驱动水液压柱塞泵运动规划及仿真分析

水液压柱塞泵是水液压系统的关键元件,由于水介质的理化特性差异导致其泄漏、摩擦磨损、腐蚀、气蚀等现象比油压柱塞泵严重,为解决传统斜盘式水液压柱塞泵流量脉动大的问题,提出了一种新型的直线电机驱动水液压柱塞泵结构。通过研究恒流量直线电机驱动柱塞泵的可行运动规划,选取了直线电机以三角波间隔T/4相位差的运动方案,以实现双直线电机双作用水压柱塞泵实际输出较小的流量脉动。应用AMESim软件,构建了两种不同配流方式的双直线电机双作用柱塞泵系统的仿真模型。仿真发现,柱塞配流电机柱塞泵相比阀配流,其压力和流量脉动很小,其压力脉动幅度小于2%,流量脉动率仅为0.008。     

变粘度条件下柱塞摩擦副泄漏流量的计算

在变粘度的条件下，并同时考虑柱塞的偏心和变密封长度问题，对轴向柱塞泵中校塞与缸孔之间的特殊圆环缝隙的泄漏流量公式进行了推导，得到了该圆环缝隙泄漏流量计算公式的修正系数，为高速高压轴向柱塞泵的设计提供了一定的理论依据。     

Analysis and design of an axial piston water-pump with piston valve

In this paper, we present a novel axial piston water-pump with piston valve, which carries out the flow distribution to drain and sucks water by the interaction of each piston. This novel water-pump could achieve change in-phase between flow rate and the passage areas of the intake and outlet ports. In order to completely achieve the in-phase change, the pressure that is assigned a fixed value in the piston chamber is optimized. On these grounds, the optimum shapes of the intake and outlet ports have been found to be rectangular according to theoretical analysis. The simulation results indicated that the optimum intake and outlet port shapes keeps the pressure in the piston chamber constant, and there is almost no pressure surge except for a very small pressure wave in the piston chamber. Finally, the test of model machine showed that the pressures of the intake and outlet port are very steady, but the leakage is large and the volumetric efficiency is 74.7%.     

转速对EHA泵柱塞副柱塞位姿及泄漏量影响仿真分析

斜轴式轴向柱塞泵作为电动静液压作动器（Electro-hydrostatic actuator,EHA）液压系统的重要元件,高转速、高工作压力是实现其小型化、轻量化的重要手段。柱塞副是斜轴式轴向柱塞泵中重要的两对摩擦副之一,对斜轴式轴向柱塞泵的工作状况起着直接影响。随着缸体转动,柱塞在缸体柱塞孔中的位姿朝着各个方向不断变化,柱塞副泄漏量也随之变化。为准确得到高速高压下柱塞副的泄漏量并给出解释,充分考虑柱塞受力情况,通过对离散化的柱塞副油膜雷诺方程和力平衡方程进行迭代求解,使用柱塞端面偏移量来确切描述了柱塞在缸体柱塞孔中的位姿随转速的变化情况,据此给出了柱塞副泄漏量模型,分析了转速变化对柱塞位姿和柱塞副泄漏量的变化的影响。结论为EHA泵的设计提供了适当的理论指导。     

A11VLO190轴向柱塞泵柱塞副泄漏与润滑特性研究

将轴向柱塞泵柱塞副的泄漏看做偏心圆环缝隙的流动,使用Matlab进行仿真,分析得到了柱塞副泄漏量与负载压力、配合间隙、工作转速和斜盘倾角的关系。采用有限差分法求解二维雷诺方程,该文在考虑油膜动压效应和挤压效应的条件下,得到了柱塞在缸体内的倾角、柱塞自转速度、进口压力对柱塞副油膜分布压力的影响,为提高柱塞副的效率、改善润滑、减少摩擦磨损和增加使用寿命提供了一些参考意见。     

汽轮机中压平衡活塞汽封间隙及漏汽量在线监测的原理及应用

现有的测定中压平衡活塞汽封漏汽量的变汽温试验法存在一定的误差且不能实现在线监测。分析变汽温试验法存在的误差,并考虑中压平衡活塞汽封漏汽的影响,对某300 MW汽轮机进行变工况详细计算。结果表明,中压缸效率基本不受再热蒸汽流量、中压平衡活塞汽封漏汽量及再热蒸汽温度的影响。基于相邻两工况下中压平衡活塞汽封间隙一定及中压缸相对内效率相等的原理,提出中压平衡活塞汽封间隙、漏汽量及中压缸相对内效率的计算方法。利用变工况详细计算数据进行检验,以及与变汽温试验法测定结果进行比较,证明所提出的计算方法的有效性。利用所提出的计算方法,对某高中压合缸汽轮机在某一运行时段内的中压平衡活塞汽封间隙、中压平衡活塞汽封漏汽量、再热蒸汽流量及中压缸相对内效率进行实例计算。     

Flow Ripple of Axial Piston Pump with Computational Fluid Dynamic Simulation Using Compressible Hydraulic Oil

The flow ripple, which is the source of noise in an axial piston pump, is widely studied today with the computational fluid dynamic(CFD) technology development. In the traditional CFD modeling, the fluid compressibility, which strongly influences the accuracy of the flow ripple simulation results, is often neglected. So a compressible sub-model was added with user defined function(UDF) in the CFD model to predict the flow ripple. At the same time, a test rig of flow ripple was built to study the validity of simulation. The flow ripple of pump was tested with different working parameters, including the rotation speed and the working pressure. The comparisons with experimental results show that the validity of the CFD model with compressible hydraulic oil is acceptable in analyzing the flow ripple characteristics. In this paper, the improved CFD model increases the accuracy of flow ripple rate to about one-magnitude order. Therefore, the compressible model of hydraulic oil is necessary in the flow ripple investigation of CFD simulation. The compressibility of hydraulic oil has significant effect on flow ripple, and the compression ripple takes about 88% of the total flow ripple of pump. Leakage ripple has the lowest proportion of about 4%, and geometrical ripple leakage ripple takes the remnant 8%. Besides, the influence of working parameters was investigated through the CFD simulations and experimental measurements. Comparison results show that the amplitude of flow ripple grows with the increasing of rotation speed and working pressure, and the flow ripple rate is independent of the rotation speed. However, flow ripple rate of piston pump grows with the increasing of working pressure, because the leakage ripple will increase with the pressure growing. The investigation on flow ripple of an axial piston pump using compressible hydraulic oil provides a more validity simulation model for the CFD analyzing and is beneficial to further understanding of the flow ripple characteristics in an axial piston pump.     

Modified mathematic model of a parallel two-dimensional piston flowmeter

The parallel two-dimensional piston (2D piston) flowmeter is a novel kind of positive displacement flowmeter that has been recently invented and proposed by our research group. Following an overview of the working principle of this flowmeter, a modified mathematic model was carefully established in this paper based on the analysis of the shortcomings of the presented primary model. The churning loss, as the first of two ignored parts in the previous research, is caused by the rollers rotation in oil, and it is considered a part of the torque balance equation in the pressure loss model. Besides, the flow rate of the transient leakage, which comes from the change in the leakage flow rate equation when the 2D piston rotates at a specific angle, was analytically modeled and added to the flow rate distribution equation. By comparing the experimental results from the last study, the modified mathematical model showed to be precise and became more suitable for predicting the flowmeter's performance and enhancing its measuring accuracy.     

Experimental investigation of an oscillating circular piston positive displacement flowmeter: I – Piston movement and pressure losses

Tests of an oscillating circular piston positive displacement flowmeter are described which focused on the effect on pressure drop across the meter of variation in key parameters. These included flow rate, liquid density and viscosity, mass of piston and length of connecting pipes. In addition to the average pressure loss, the pressure loss variation during the oscillation cycle was measured and found to vary with an amplitude dependent on the various parameters. A companion paper reviews data on leakage and wear.     

双滑片椭圆转子压缩机径向间隙泄漏研究

建立了双滑片椭圆转子压缩机的径向间隙泄漏模型,对不同径向间隙的泄漏量进行了计算和分析,并和相同规格的滚动转子压缩机进行比较。结果表明:椭圆转子压缩机的径向间隙泄漏量在相同条件下大于滚动转子压缩机,两者的差值随着间隙的增大而增大,在间隙为80μm时,椭圆转子压缩机的径向间隙泄漏量最多可比滚动转子压缩机大2.352 g/s;压比对泄漏量的影响较大,在压比一定时,转速和转子椭圆面短轴和长轴之比对泄漏的影响并不明显。严格控制的径向间隙是提高椭圆转子压缩机容积效率的主要因素。