【作者】 郭兰兰；

【导师】 杜广生；

【作者基本信息】 山东大学 ， 热能工程， 2014， 博士

【摘要】 流量计量是计量科学技术的重要组成部分,流量的准确测量对提高产品的质量与产量,保障生产安全,改进操作工艺,改善生产条件及科学实验等方面具有重要意义。作为流量单位量值统一与传递的标准,流量标准装置的研究、建立和应用在国内外引起普遍重视。在流量标准装置中,水流量标准装置的使用最为广泛,很多流量计量的理论研究、原理实验以及流量仪表的检定都是在水流量标准装置上进行的。可见,水流量标准装置性能的好坏,对流量仪表计量的准确性及流量量值的统一性将产生直接影响,因此对影响水流量标准装置性能的关键问题展开研究十分必要。从水力学角度来说,水流量标准装置的性能主要受到稳压容器、阀门、弯管、及变径管道等局部阻力件处水流特性的影响。因此本文在对这些影响水流量标准装置性能的水力学问题进行论述的基础上,采用理论分析和数值模拟方法对稳压容器动态响应及结构特性、阀门关闭过程中产生的水击现象及弯管内流动特性等进行了研究,为水流量标准装置的设计和运行提供了一定理论依据,具体有：基于流体网络法建立了横隔板式稳压容器的数学模型,并用matlab进行动态仿真,计算了不同工况下稳压容器的插入损失和传递损失,并以此二者为指标对所设计的横隔板式容器的稳压效果进行了评价。结果表明：横隔板稳压容器对高频压力脉动的衰减效果较好；稳压容器内气体腔室体积的变化对稳压效果能够产生影响,可以通过改变其工作体积,使稳压容器达到较好的衰减效果；稳压容器的安装位置对稳压效果也会产生影响。采用CFD数值计算方法对竖隔板式稳压容器的结构特性进行了研究。为实现瞬态工况下稳压容器内脉动流场的准确模拟,建立了包含稳压容器、管路和水箱的整体流道模型,使用用户自定义程序UDF (User Defined Functions)编写了入口流量脉动信号,并采用VOF多相流模型对气液界面进行跟踪,计算得到了不同工况下稳压容器出口处的流量波动系数。进而以其作为评价标准,分析了脉动频率、初始水位高度和隔板结构等对流量稳定性的影响。结果表明,竖隔板式稳压容器对高频压力脉动有较好的衰减效果,当频率低于10Hz时,脉动衰减率急剧下降；初始水位高度存在最佳值,以气、水容积比值在0.5左右时稳压效果最好；隔板位置以位于容器前部1/3处为佳,隔板高度在不影响气相空间的前提上可取最高值。隔板上开孔位置以隔板中上部为宜。隔板圆孔区应保证一定的流通面积,在进水腔和出水腔液位差较小的前提下,尽量减小圆孔的直径,以增加对流量脉动的吸收和缓冲作用。采用滑移网格技术实现了球阀的动态关闭过程,并用UDF控制阀门转动速度,对简单直管道内由关阀引起的水击现象进行了数值模拟研究。计算得到了水为常密度、变密度(压强的函数)和气液两相混合密度(空化模型)三种情况下阀前压强随时间的变化关系曲线,并与实验曲线和特征线法计算结果曲线进行了比较。结果表明,变密度情况下的计算结果与实验值更为接近,说明在考虑水的可压缩性的情况下对阀门完全关闭情况下的水击现象进行模拟是可行的。在此基础上,模拟计算了不同关阀时间和关阀方式下的水击压强随时间的变化关系,结果表明,增大关阀时间能够减小水击压强,但大于某个时间点后效果将不再明显；在关阀时间一定的情况下,先快后慢的减速关阀方式能有效降低最大水击压强。另外,本文还对水流经过弯管时的流动情况进行了模拟计算,对二次流的产生原因及发展过程进行了分析,并比较了弯管内加装导流片前后管内流动特性的变化情况,分析了导流片对二次流的消减作用和影响范围。更多还原

【Abstract】 Flow measurement is an important part of measurement science and technology. The accurate measurement of flow rate is of great significance to improve the yield and quality of the product, ensure productive safety, improve the operation process and production condition and so on. As the consistence and transfer standard of the measurement value of flow rate unit, the research and application of the.flow calibration facility is caused widespread attention at home and abroad, of which the water flow calibration facility is most widely used. Many theoretical research, principle experiment and flow instrument calibration of flow measurement are conducted in the water flow standard facility, so the performance of which has a direct influence on the flow measurement accuracy and consistence and transfer of flowrate measurement value. So it is very necessary to carry study on the key questions to the influence on the performance of a water flowrate calibration facility.From the perspective of hydraulic, the performance of the water flow ratef calibration acility is mainly influenced by the water flow characteristic at surge tank, valve, elbow pipe and varying-area channel, etc. So in this paper, on the basis of the discussion on hydraulics problems that affect the water flow calibration facility, the methods of theoretical analysis and numerical simulation are used to study the dynamic response and structural characteristics of the surge tank, the water hammer phenomenon occurring in the process of valve closing and the flow characteristics of the elbow pipe, and so on. And it provided a certain theoretical basis for the design and operation of the water flow calibration facility. The following researches have been done in this paper.Based on the fluid network method, this paper establishes a mathematical model of the horizontal-baffle type surge tank, and the insertion loss and transmission loss are calculated through the dynamic simulation by the Matlab software. The results show that surge tank has a good attenuation effect on the high-frequency pulsation; the volume of gas chamber in container can influence the stability of flow, the container can reach better attenuation effect by changing the gas volume and the installation position of the container can also influence the stability of flow.Structural characteristics of surge tank with vertical baffle are studied using CFD numerical calculation method. To realize the accurate transient simulation of pulsating flow field, a three-dimensional model, including a surge tank, the pipeline and the water tank is built. The inlet flow pulsation is defined by the UDF (User Defined Functions), and the VOF model of fluent is used to track the gas-liquid interface. By calculating the flow fluctuation coefficient of variation of different flow conditions, the influence of pulse frequency, initial water level height and baffle plate structure on the flow stability are analyzed. Results show that surge tank with vertical baffle has a good attenuation effect on the high-frequency pulsation, when the frequency is below lOHz, the pulsation attenuation ratio decreases rapidly; there is an optimal initial water level to suppress fluctuations, the best volume ratio of air to water is about0.5; the preferable position of the baffle is at the front1/3of the surge tank; the height of the baffle can take the highest value if not affecting the air space; the preferable positon of the holes is in the upper part of the baffle; the round hole’s part of baffle should ensure certain circulation area, on the premise of little water level difference between the inlet and outlet water chambers, the bore diameter should be proper decreased to increase the damping effect.Sliding mesh is used to realize the dynamic closed process of ball valve and the rotational speed of which is controlled by the UDF, and the water hammer phenomenon in simple straight pipe caused by the closing of the valve is numerical simulated. The constant density, variable density (function of the pressure) and gas-liquid two-phase mixture density are used to calculate individually to get pressure curve during valve closing. The results are compared with the experimental curve and the method of characteristic line curve. Results show that the variable density of calculation results is more close to experimental value and prove the water hammer phenomenon is feasible to simulate in the case of considering the compressibility of water on the valve fully closed condition. The relationship between the water hammer pressure and time is obtained under condition of different time and different way to close the valve through simulation, the results show that increasing valve time can reduce the water hammer pressure, and the effect is unconspicuous after certain time; when the valve closing time is fixed, the deceleration way can effectively reduce the water hammer pressure.In addition, the flow in the90degree elbow pipe is simulated, and the reason for the generation of the secondary flow and its development process are analyzed. Then it compares the changes of flow characteristic before and after the installation of the deflector in the elbow pipe, and analyzes the weaken effect and influence region of the secondary flow.更多还原