Choking phenomena of sonic nozzles at low Reynolds numbers

The choking phenomena of sonic nozzles were investigated for the Reynolds number range from 40 to 30 000 for nitrogen gas. The results showed that the critical back pressure ratio is a function of the Reynolds number only, with different characteristics for different nozzle shapes. In ISO-type toroidal-throat Venturi nozzles, the minimum Reynolds number satisfying the choking condition is about 40 and the critical back pressure ratio is only about 0.05 at this minimum Reynolds number. It was also found that the critical back pressure ratio has a local minimum value around Re_th=4000 and that the local maximum value is around Re_th=10 000 due to the change in characteristics of the boundary layer in the diffuser. On the other hand, the critical back pressure ratio in quadrant nozzles decreases monotonically with decreasing Reynolds number, unlike the former nozzle, and the minimum Reynolds number necessary for choking is estimated to be approximately the same as that in the Venturi nozzle.     

Experimental and CFD investigation of 100 mm size cone flow elements

This paper presents performance characteristics of 100 mm line size cone flow elements having beta ratios of 0.4, 0.5, 0.6, 0.7 and 0.8. A magnetic flow meter is used as a reference standard for flow measurement in vertical test section. A series of experiments have been conducted using water at in-house Flow Calibration Facility (FCF) to cover the Reynolds number ranging from 20,000 to 200,000. The performance characteristics of 100 mm line size cone flow elements with different beta values have been evaluated experimentally. It is found that the discharge coefficient of the cone flow element is nearly independent of the specified range of Reynolds number. Testing of the cone flow element in accordance with new API 5.7 is carried out at flow calibration facility. The testing requirements in the standard explain the conditioning effect of the cone flow element having gate valve disturbance upstream of the cone at various locations. The effect of the upstream velocity profile has been investigated by placing a gate valve upstream of the cone flow element at a distance of 0D and 28D and performing experiments at 25%, 50% and 100% opening of gate valve. The value of the discharge coefficient is not affected when the cone is placed at a distance of 0D and for 100% opening of gate valve. The uncertainty results of the cone testing are discussed. For studying pressure and velocity distributions, cone elements are modeled using computational fluid dynamics (CFD) code PHOENICS. Pressure and velocity profiles for different sizes of cone elements are plotted. From the pressure profile, it can be seen that the pressure recovery downstream of the cone is within a distance of 3D. The velocity profile downstream of the cone signifies the use of flow element as a signal conditioner. For measurement of flow through a 100 mm line, differential pressure across the cone is measured using a Differential Pressure Transmitter (DPT). Experiments were repeated by replacing the cone element for obtaining different β values.     

V形内锥流量计关键参数对流出系数的影响

将计算流体动力学仿真试验和实流物理试验相结合,剖析V形内锥流量计的流出系数特性,探索不同等效直径比β、前后锥角以及雷诺数等对流出系数的影响规律。湍流模型为RNG k-ε模型。针对100 mm口径,设计了β值分别为0.50、0.65、0.85,前锥角分别为40º、45º、50º,后锥角分别为120º、130º、140º,共27种组合的V形内锥体,仿真介质为水,温度为293 K。经过数值模拟试验,获得了27条内锥流量计流出系数特性曲线。为进一步验证仿真试验结果,在已有实流试验系统的基础上,建立针对V形内锥流量计的专用自动标定平台。两类试验数据比较发现：内锥流量计的流出系数不仅和雷诺数有关,还与等效直径比β及前后锥角相关。首先,β值越大,流出系数越小,且β值越大,流出系数更易受雷诺数的影响;其次,β值相同时,前锥角对流出系数具有决定性的影响,而后锥角对流出系数的线性度具有一定程度的影响;第三,较大的前锥角可减弱雷诺数对流出系数的影响。数值模拟试验预测平均误差约5%,最大值小于10%。     

EFFECTS OF POLYTROPIC EXPONENT ON DATA PROCESSING METHODS USED IN CONSTANT VOLUME DISCHARGE

In order to obtain the accurate sonic conductance of solenoid valves, the polytropic exponent is used in the data processing of the discharge method for measuring the flow rate characteristics of pneumatic components. Three data processing principles are first introduced, and then the discharge pressure data obtained from the measured solenoid valve are processed to obtain the sonic conductance with three methods： the partial polytropic exponent method, the complete polytropic exponent method and the adiabatic method. By comparison of the obtained results it indicates that the complete polytropic exponent method is the most accurate. However, the partial polytropic exponent method is of a high applicable value, because it is easy and simple to measure and the obtained results are relatively accurate.     

钢液中非金属夹杂物极限上浮速度的计算

为了计算非金属夹杂物在中间包和铸锭模等冶金容器内的上浮速度,根据夹杂物的阻力系数与雷诺数的关系,可将极限上浮速度分为斯托克斯区、过渡区和牛顿定律区等。通过分别计算在不同夹杂物密度、钢水密度、钢水粘度和夹杂物形状情况下夹杂物极限上浮速度与夹杂物当量直径的关系,为计算夹杂物的上浮距离和判断夹杂物能否去除提供理论依据。     

高速轴配流中旋转径向小孔流量系数的实验研究

针对高转速轴配流中旋转径向小孔出流流量系数开展了实验研究。研究中涉及到两种流动方向,即自旋转缸体中心流过径向小孔向外离心运动称为正向流动,反之则为反向流动。结果表明：旋转小孔出流流量系数同时显著地受离心力和科氏力的影响;因离心力的赋能作用,正向流动时流量系数随转速的增加而变大,反向流动时离心力对小孔内流动有抑制作用,使流量系数随转速的增加而变小;两个方向的科氏力综合效果使正向流动流量系数随出流雷诺数增加而变小,反向流动流量系数随出流雷诺数的增加而变大;当出流雷诺数足够大时,两种流动方向、不同孔型的流量系数各自都有一个稳定值;各种孔型的正向流动流量系数稳定值分布均在0.65～0.80范围内。     

Effect of boundary condition history on the symmetry breaking bifurcation of wall-driven cavity flows

A symmetry breaking nonlinear fluid flow in a two-dimensional wall-driven square cavity taking symmetric boundary condition after some transients has been investigated numerically. It has been shown that the symmetry breaking critical Reynolds number is dependent on the time history of the boundary condition. The cavity has at least three stable steady state solutions for Re=300-375, and two stable solutions if Re>400. Also, it has also been showed that a particular solution among several possible solutions can be obtained by a controlled boundary condition.     

边界条件对滑动轴承性能的影响

本文根据双雷诺边界条件计算了滑动轴承在各种偏心率下的轴承性能,同目前普遍使用的压力边界条件的计算结果进行比较,研究各种边界条件对轴承性能的影响,从而为进一步研究大扰动下油膜破裂边界的变化打下基础。     

Theoretical discharge coefficient of a critical circular-arc nozzle with laminar boundary layer and its verification by measurements using super-accurate nozzles

The discharge coefficient of a circular-arc critical Venturi nozzle is derived theoretically by combining theories to calculate mass flow defects caused by the core flow distribution and the laminar boundary layer. The equation obtained is verified by measurements using a constant volume tank system and nozzles of similar shape machined by super accurate lathes, which achieved mirror finish without polishing thus resulting in a machining error of less than 1 μm. The agreement of the theoretical and measured discharge coefficients is better than 0.04% at Reynolds numbers larger than 8×10⁴, the minimum used. The equation derived is in an analytical form, which enables an estimation of the effects of specific heat ratio as well as nozzle shape.     

叶片外部换热实验和数值研究

在短周期跨音速传热风洞上,研究不同雷诺数和压比下无气膜导叶表面换热系数分布,并在相应的数值模拟中分析叶栅通道涡结构的生成与发展,研究其对叶片换热的影响。     

高雷诺数下有限长圆柱绕流阻力特性研究

针对不同长径比的有限长圆柱模型,采用大涡模拟及雷诺平均的方法,对高雷诺数下有限长圆柱绕流阻力特性进行数值模拟和分析,得到了圆柱阻力系数随长径比和雷诺数的变化规律,讨论端面效应对绕流阻力系数的影响。结果表明：在亚临界区内,相同雷诺数下阻力系数随长径比的增大而增大,呈线性变化规律,L/D的对阻力系数的影响明显大于Re对阻力系数的影响;在阻力危机区内,相同雷诺数下阻力系数随长径比的增大而增大,呈二次函数的变化规律,但各工况达到阻力系数“转折点”对应的雷诺数各不相同,基本呈现随雷诺数的增大向前推移的趋势,Re对阻力系数的影响明显大于L/D对阻力系数的影响;在阻力回升区,阻力系数回升的“转折点”随着雷诺数的逐渐增大而向后推移。在各分区内,端面效应对阻力系数的影响随雷诺数增大而更加明显,在高度方向上的最大影响区域约占圆柱总高度的16%。研究结果对有限长圆柱绕流特性的研究及应用具有重要意义和价值。     

高雷诺数水流中悬空输油管道安全性数值分析

穿越河流的输油管道在长距离输油管道工程中占相当比例,在水流长期作用下容易裸露悬空。悬空管道在水流作用下可能产生共振或锁频现象,造成管线失效。通过交错网格系统中的SIMPLE算法求解N-S方程,对均匀来流中的静止管道绕流场进行模拟,验证了模型的可靠性;并应用动网格技术对高雷诺数均匀来流中悬空的横向振动管道绕流场进行了数值模拟,得到不同频率比下的升力系数、阻力系数随时间的变化曲线以及涡量等值线分布图。通过分析涡量等值线分布图得到管道在给定雷诺数流体中的"锁频频率"以及高雷诺数下管道临界长度曲线,结论可用于水流作用下管线的安全分析。