音速喷嘴法气体流量标准装置比对分析

本文主要介绍了华北大区五个省级计量实验室的音速喷嘴法气体流量标准装置间的比对,通过比对统一了音速喷嘴法气体流量标准装置的测量方法及不确定度分析,提高了计量实验室的气体流量标准装置的计量水平。     

音速喷嘴气体流量标准装置小流量检测出现的问题及解决方案

摘,,要:对音速喷嘴气体标准装置的研究、设计制造并对装置的校准及应用,发现现有国内大多数音速喷嘴气体标准装置对小流量检测精度低,或标定前的稳定时间太长。经过进一步探究,对装置进行了重新设计和改造,解决了装置检测小流量不准确问题和大大缩短了标定前的稳定时间。     

标准流量积算仪的研究与实现

介绍了标准流量积算仪的基本功能,从硬件构成、仪表接口和控制逻辑几个方面,详细阐述了标准流量积算仪在计量检定和校准装置中的应用。在音速喷嘴气流量标准装置、钟罩式气体流量标准装置、静动态质量法水流量标准装置、静动态容积法水流量标准装置和标准体积管流量标准装置中,可以准确地采集标准计量时间、标准表和被检表的脉冲频率、脉冲累积量以及被检表的电流信号值。通过面板控制,DI端子触发和串口命令3种方式实现仪表的启停控制,以实现智能化采集,并根据数值大小调整小数点的显示位数。经测试比较,标准流量积算仪可以稳定高效地作为标准装置的计量仪表,达到了预期的设计效果。     

亚音速与音速节流装置气体流量测量方法

气体流量是工艺过程中一个十分重要的控制变量,目前可用于气体流量测量的流量计种类繁多,但仍普遍存在精度低、安装要求严和维护多等不足。为满足物料反应系统、小孔流道堵塞检测和流量计标定等诸多应用对气体流量测量的更高要求,为此本文引进了多种亚音速与音速节流装置的流量测量方法,首先分析各种装置的结构、测量原理、压损、流量曲线及适用场合等,再重点分析了音速喷嘴的两种典型应用和应用实例等,为气体流量测量方法的选择提供一定的技术参考。     

热式质量燃气表自动化校准技术研讨

利用热式传感器输出瞬时流量的计量特性,提出一种基于瞬时流量的校准方法,在热式表主板模块上加装红外发射及接收电路,通过红外通信方式与上位机建立数据交互,读取其显示的瞬时流量,以音速喷嘴法气体流量标准装置为标准器,通过下位机读取音速喷嘴处气体的瞬时流量,计算出示值误差。程序设置通信、初校、生成系数修正列表、写入系数、复校、判定一共6个工序,一键实现热式表连续化、批量化、自动化校准。通过用瞬时流量法测试G1.6规格燃气表0.5 m~3/h流量点的示值误差,重复性为0.053%,与累积流量方法比较提高了0.05%,缩短校准时间87%,极大提升工作效率。     

音速喷嘴法检定气体流量分析

随着科技的进步发展,在气体流量检测方面的科学技术在迅猛发展,本文将主要对于音速喷嘴法检定气体流量进行分析,希望能够通过本文的详细介绍,给予大家一些启发。     

基于双速度探头的微小流量热式气体流量计

针对传统热式流量计测量微小流量精度不高的问题,基于对流换热原理,采用2个参数一致的铂热电阻获取流量信号的方法,研制了一种双速度探头热式流量计。分析了流量测量原理以及环境温度补偿,简述了铂热电阻加工工艺,拟合建立了流量测量函数关系式。流量计在音速喷嘴气体流量标准装置上实验的结果表明该方法可实现对微小流量的测量。     

改进取压方式提升音速喷嘴气体流量标准装置的准确度

音速喷嘴气体流量标准装置因其结构简单、性能稳定、准确度较高、无可动部件、维护方便等特点,成为气体流量计量检测的主流标准装置。被检表以及标准器处的气体取压方式有两种:绝压、绝压+差压。通过对装置不确定度评定,测差压的差压变送器量程范围小,降低了最大允许误差;测被检表与标准器处气体的绝压共用同一个绝压变送器,灵敏系数的降低,提高了准确度。同样采用0.2级压力变送器测压的标准装置,前者因压力引起的相对标准不确定度为0.177%,而后者为0.016 6%,仅为前者的1/10,装置的扩展标准不确定度U_r=0.24%,k=2,完全满足准确度等级为1.0级气体流量计的检定要求。采用绝压+差压取压方式,在不提高压力变送器准确度等级的情况下,有效提升装置的准确度。该方法简单可行,适用范围广,可应用于其他气体流量标准装置。     

探讨制约气体流量标准装置流量范围的因素

本文介绍了计量检测中心临界流文丘里喷嘴气体流量标准装置采用比较法开展气体流量计的检定和校准工作中出现的问题,并对影响装置流量范围的问题因素进行了分析。     

小喉径音速喷嘴热效应对流量影响的热边界层分析

随着气体在音速喷嘴中膨胀降温,低温气体与管壁的热交换会产生一系列复杂的影响,称之为"热效应"。用于微小流量测量的小喉径音速喷嘴由于喉部直径很小且精度要求高,热效应的影响更为严重。利用热边界层理论分析不同入口压力、不同位置、不同喉径和不同保冷条件下的管壁温度变化规律,根据试验结果,从理论上分析管壁动态温度分布与热边界层的相互作用关系,利用CFD软件仿真,分析管壁温度变化引起的边界层厚度变化对音速喷嘴流动特性的影响规律,并通过数据拟合得到了雷诺数和管壁温降对流出系数偏差的影响的计算公式。结果表明,以仿真数据为例,当管壁温降15 K时,由热边界层变薄引起流出系数增大可达0.195%,其影响不容忽视。     

Influence of swirl on the discharge coefficients of sonic nozzles

Flow characteristics of a swirl generator are studied using an open circuit flow loop, and influence of upstream swirl on the discharge coefficients of sonic nozzles is investigated using a high pressure gas flow standard measurement system. The open circuit flow loop consists of swirl generator, testsection, sonic nozzle, suction fan and LDV system. The gas flow measurement system comprises two compressors, storage tank, temperature control loop, sonic nozzle testsection, weighing tank, gyroscopic scale and data acquisition system. Experiments are performed at various nozzle throat diameters, inlet pressures and angles of swirl generator. As the angle of swirl generator becomes larger, axial velocities decrease near the wall and rapidly increase in the pipe core, and swirl velocities increase to form swirl flow. Influence of upstream swirl on discharge coefficients becomes greater as the intensity of swirl increases and as the nozzle throat diameter is enlarged. Variation trend with Reynolds number, however, is very similar each other regardless of swirl intensity.     

Non-equilibrium condensation process of water vapor in moist air expansion through a sonic nozzle

Non-equilibrium vapor condensation phenomenon through a sonic nozzle is very complicated and closely related to the flow measurement of the sonic nozzle. The gas–liquid two-phase flow Eulerian models for homogeneous and heterogeneous nucleation of moist gas in transonic nozzle flow were built to investigate the effect of vapor condensation on the mass flow rate of the sonic nozzle. Grid independence was achieved by using a solution-adaptive refinement. The CFD models were carefully validated by published experimental data and analytical results. It was shown that the flow rate of the sonic nozzle is affected by both homogeneous and heterogeneous nucleation. In comparison with the experimental data, the effects of vapor condensation on the mass flow rate of the sonic nozzle were obtained. Besides, experiments on periodic, unsteady condensation flow in the sonic nozzle were also reported. This unsteady flow will also affect the flow rate of the sonic nozzle. The results of experiments accorded well with simulations and semi-empirical formula. All the results can be used to further analyze the effect of unsteady flow induced by vapor condensation on the flow rate of the sonic nozzle.     

Approximate solution for discharge coefficient of the sonic nozzle with surface roughness

Sonic nozzle is widely used in the flow measurement and control. Nowadays, it has been applied to higher Reynolds number flow increasingly. The effect of surface roughness on discharge coefficient of the sonic nozzle should be discussed. An approximate analytic solution for discharge coefficient of the sonic nozzle with surface roughness was proposed in detail. The determination of this coefficient was based on universal logarithmic velocity-distribution law and the principle of equivalent velocity profile. Although there are some apparently approximations, this algebraic method accurately predicts the discharge coefficient of the sonic nozzle with surface roughness in Reynolds number range from 10⁴ to 10⁹, a relative equivalent roughness range of 10⁻⁶ to 10⁻². Some experiments of sonic nozzle conducted by others showed agreement with present algebraic method. Besides, the agreement between this method and the corresponding exact numerical calculation is also good. The present method provides an excellent tool to deeply investigate the roughness effect and promote further improvement of the standard.     

Flow characteristics of pyramidal shaped small sonic nozzles

Four types of pyramidal sonic nozzles made of silicon crystal were studied experimentally. The throat sizes varied from 38 to 140 μm for type A and D nozzles and from 75 to 188 μm for type B and C nozzles. For each of the nozzle types, the results show that the discharge coefficient is proportional to the throat size, and the critical back pressure ratio for choking is insensitive to Reynolds’ number. In parallel, the flow field of a type B nozzle was investigated by numerical simulation. The effect of heat flux coming from the nozzle body was examined and the flow patterns obtained from Spalart-Allmaras and standard k−ω turbulence models were compared. The simulation results indicate the heat flux does not noticeably change the velocity field and discharge coefficient. Also, the flow downstream of the nozzle throat develops into an under-expanded supersonic jet in which expansion and oblique shock waves appear alternately.     

Interference effects of three sonic nozzles of different throat diameters in the same meter tube

For calibration of a large capacity gas flow meter, a sonic nozzle bank may be used as a reference system. International standards (ISO9300:1990) allow installation of a single nozzle in a meter tube as a flow transfer standard. For multiple nozzles in a single tube, the effect of interference between sonic nozzles and the chamber wall must be measured to predict the discharge coefficient of a nozzle array from those of single nozzles. The interference effect between neighboring nozzles can be additional error sources in mass flow measurement. Sonic nozzles with three different throat diameters (d=4.3, 8.1, and 13.4 mm) were tested in a single meter tube in three geometrical arrangements. The mass flow rate was measured against a primary gas flow standard system. Three installation plates for sonic nozzles were made to vary the distance between nozzles and distance from the chamber wall. Discharge coefficients of the three individual nozzles were in agreement with the ISO recommended equation within ±0.2%. Discharge coefficients of the nozzle bank calculated from those of the individual sonic nozzle were the same as the direct measurements within ±0.098% at the 95% confidence level for all cases. For these experiments, the results were not influenced by the proximity of the tube wall or the interaction of the nozzles.     

Uncertainty analysis of the high pressure closed loop gas flow standard facility in NIM

High pressure air flow standard facilities, including the pVTt facility, sonic nozzle facility and closed loop facility were built in NIM at the end of 2014. The high pressure closed loop gas flow facility was the first closed loop facility in China. The system has 4 sets of 100 mm diameter turbine meters for the reference meters with a flow range of (40–1300) m³/h and a pressure range of (190–2500) kPa. To avoid uncertainties introduced during installation, the reference meters were designed to be calibrated in situ using the sonic nozzle facility. The uncertainty in the pressure measurement was reduced by installing an absolute pressure transducer in the manifold upstream of the reference meters, with differential pressure transducers used to measure the pressure drops across the reference flow meter and the test flow meter. The relative expanded uncertainty for the test meter can reach 0.20% (k = 2) as verified by comparison the sonic nozzle facility and the closed loop facility measurements.     

Preliminary considerations in the use of industrial sonic nozzles

Preliminary considerations applying to the necessary instrumentation for the use of sonic nozzles is reviewed. A way to make sonic nozzles for industry, together with a method to determine the discharge coefficient of a convergent nozzle operating in the sonic regime in conditions of ideal flow is suggested. The results obtained for the behaviour of the discharge coefficient of sonic nozzles, in the range from 4×10⁴ to 2×10⁵ are compared with the standard ISO 9300 (Standard ISO 9300. Measurement of gas flow by means critical flow Venturi nozzles, 1990) and the curve proposed by Ishibashi et al. (M. Ishibashi, M. Takamoto, N. Wanatabe, Y. Nakao. Precise calibration of critical nozzles of various shapes at the Reynolds number of 0.2–2, 5×10⁵. Proceedings of FLOMEKO 94).     

Study on the compressible viscous flows through a straight pipe

When sonic nozzles of significantly smaller diameter are used as standard flow meters, the critical backpressure ratio is affected by the boundary layer at the nozzle throat. It is known that, as the flow pathway is complicated in actual valves, the critical pressure ratio differs from the ideal value. However, the effect of the boundary layer thickness on choking criteria is still controversial. Choking phenomenon in a sonic nozzle fitted with a straight circular pipe of variable length is investigated numerically and the results are compared with experimental and theoretical ones. The results show that the actual critical pressure ratio is less than the ideal value and it decreases with increase in pipe length. The effect of boundary layer thickness on main flow Mach number is also made clear in this work. In addition, sonic conductance and pressure loss in actual viscous flows computed are compared with the ISO 6358 standard.     

高效三元流技术对高扬程大流量离心泵叶轮的节能改造

高扬程大流量卧式离心泵运行中存在工况点偏离，效率低下，耗电量大等问题，通过三元流技术对叶轮重新设计制造，在原有水泵及配套电机不变的情况下只更换叶轮，运行中进行参数测试比较，水泵运行效率明显提高，达到显著的节能效果。同时为同类型水泵及系统的高效节能改造提供借鉴途径。     

Piezoelectric diffuser/nozzle micropump with double pump chambers

To eliminate check valve fatigue and valve clogging, diffuser/nozzle elements are used for flow rectification in a valveless diffuser/nozzle micropump instead of valves. However, the application of this type of micropump is restricted because of its pulsating or periodic flow and low pump flux. In this paper, a diffuser/nozzle Si/Glass micropump with two pump chambers by IC and MEMS technology is designed. The fabrication process requires only one mask and one etch step, so that the fabrication has the advantages of low cost, short processing period, and facilitation of miniaturization. The pump is equipped with a glass cover board so as to conveniently observe the flow status. Pump-chambers and diffuser elements are fabricated by the anisotropic KOH-etch technique on the silicone substrate, and the convex corner is designed to compensate for an anisotropic etch. The driving force of the micropump is produced by the PZT piezoelectric actuator. The pump performance with both actuators actuated in anti-or same-phase mode is also researched. The result indicates that the micropump achieves great performance with the actuators working at anti-phase. This may be because the liquid flows steadily, pulse phenomenon is very weak, and the optimal working frequency, pump back pressure, and flow rate are both double that of the pump driven in same-phase.