Research On The Flow Measurement Of The Final Stage Of Differential Pressure Orifice Flowmeter

China is a large agricultural country,and agriculture is a big household in China.With the increase of population,the development of industry,the acceleration of urbanization and the continuous increase of non-agricultural water use,higher requirements have been put forward to strengthen the management of water use in irrigation area,control agricultural water use and improve the efficiency of irrigation water use.Measuring and measuring water in irrigation area is an important means to rationally dispatch water resources and realize efficient and sustainable utilization of water resources.With the gradual advancement of water measuring work in irrigation area,higher requirements have been put forward for water measuring equipment and technology.Based on the Venturi principle,an eccentric orifice flowmeter for measuring water in small irrigation canals is presented in this paper,and the feasibility of using this device to measure water in the last stage of irrigation canals is studied.The main achievements of this paper are as follows:(1)Comparing the calibration data and numerical simulation results of DN200 culvert tube 10 cm orifice eccentric orifice flowmeter,mainly comparing the water depth behind the sluice,the head of weir and the effluent coefficient under the condition of submerged outflow.The results show that the goodness of fit is good,which proves the feasibility of using FLOW-3D for numerical simulation.(2)FLOW-3D was used to simulate the flow measurement of eccentric orifice flowmeters with DN200,DN300 and DN400 culverts.The influence of velocity change and aperture ratio change on the flow field,the influence of velocity change and aperture ratio change on the effluent coefficient,the error analysis of averaging the effluent coefficient of the same orifice plate to calculate the flow rate of the pipeline through the orifice are discussed,and the most suitable orifice plate diameter is selected.The numerical simulation results show that:? The faster the flow velocity in the tube,the more turbulent the flow pattern downstream of orifice plate,the more intense the eddy current,the greater the permanent pressure loss.At the same time,with the increase of the flow velocity,the pressure upstream of orifice plate becomes larger and larger,and the pressure downstream of orifice plate becomes smaller and smaller,so pressure difference before and after orifice plate increases with the increase of the flow velocity.When orifice diameter is small,the effluent coefficient varies little with the flow velocity and keeps stable basically.When orifice diameter is large,the effluent coefficient increases gradually with the increase of flow velocity;? With the increase of aperture ratio,the flow pattern downstream of orifice plate is more stable,the eddy region is smaller,and the permanent pressure loss decreases.At the same time,with the increase of aperture ratio,the pressure at the upstream of orifice plate decreases,the pressure at the downstream of orifice plate increases,the pressure difference decreases before and after orifice plate,and the overall variation trend of flow coefficient increases;? Within a certain orifice plate aperture range,for the same diameter,the same orifice plate aperture,the ratio of the actual flow rate of the channel to the theoretical flow rate calculated by the pressure difference between the upstream and downstream D-D/2 through the orifice plate at the bottom of the pipeline has a fixed coefficient,that is,the flow coefficient value obtained by different flow rates is approximately a constant;? The error analysis shows that the flow measurement error increases with the increase of the aperture ratio.In the range of the aperture studied in this paper,when 0.5D?d?0.75D(d is aperture,D is pipe diameter),the flow measurement error is less than ±5%,when d>0.75D,the flow measurement error is more than ±5%.Therefore,eccentric orifice plate with aperture ratio of 0.75 can be used in the actual channel,which not only meets the measurement accuracy but also does not produce obvious head loss.