Research On The Key Questions Of Influence On The Performance Of A Water Flowrate Calibration Facility

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.