| The application of heat metering has obvious effect on the adjustment and control of building heat consumption.With the increase of the covering area of heating network and the improvement of heating quality requirements,ultrasonic heat meter has been widely used.Temperature and flow discharge are the basis of heat metering,which are provided by the water flowing through the pipeline and heat meter.In engineering application,the conditions of heating network affects the water flow,which result in the deviation of the metering signal and the decline of the accuracy of heat meter.Therefore,it is of great theoretical and practical significance to study the influence of the water flow characteristics on the metering performance of ultrasonic heat meter.The flowing state of the water in heat meter includes complete turbulence,transitional flow and laminar.The analysis of various types of ultrasonic heat meters shows that the water flow in it is mostly in state of complete turbulence or transitional flow.The flow field distribution of the two flow states has different characteristics,which have different effects on the temperature and flow discharge which can affect the accuracy of heat meter.This paper takes the ultrasonic heat meter used in the condition of complete turbulence and transitional flow as the object,analyzes the influence of water flow characteristics on the metering performance of the ultrasonic heat meter,and provides reference for improving the accuracy of the heat meter.The main tasks are as follows:1.A heat meter test-bed was built,and the TDS-100F ultrasonic heat meter was tested in the heat supply network under multiple working conditions.According to the installation conditions of the heat meter,the influence of water flow on the temperature measurement and heat metering accuracy under different pipeline conditions was analyzed by the temperature data of multiple measuring points.The experimental results show that the change of pipeline conditions affects the water flow,the disturbance of flow field affect the distribution of water temperature field and the heat exchange process of platinum resistor(RTD),which leads to the difference of temperature measurement results of RTD installed at different measuring points.In the test system,the position coefficient of the RTD installed near the elbow is 0.183.When the water flow velocity is 1.06m/s,the maximum total error of the heat meter is to be 4.39%.With the increase of the flow velocity,the influence on the temperature measurement process is reduced,and the measurement accuracy is improved.When the flow velocity is 1.67m/s,the total error is reduced to 3.68%.According to the test data,when the flow rate is 1.67m/s,water supply temperature is 60.2?,return water temperature is 36.2? and outdoor average temperature is 4.6?,it has the greatest impact on the working process of heat meter.2.The internal flow field of heat meter is simulated,and the flow characteristics under different working conditions are analyzed.The influence of straight pipe length and elbow distribution on heat metering accuracy is studied.The flow field disturbance caused by elbow affects the linear/surface velocity ratio,which leads to the increase of heat metering error.The influence of elbow in front of the heat meter is greater than that behind it;when there are elbows in front and behind the heat meter,the influence of opposite side distribution is greater than that of the same side distribution.In all kinds of distributions,the rear elbow has the least influence on the accuracy of flow measurement,and the different side distribution has the most influence.According to the simulation results,the minimum length of the straight pipe section in front and back of the meter is obtained to meet the measurement accuracy under various elbow settings.Under the conditions of meeting the installation requirements,the maximum flow measurement error caused by rear elbow is 2.04%,and the maximum error of the elbow distribution on the opposite side is 2.08%.3.According to the flow state of the water flowing through the small-diameter ultrasonic heat meter is mostly in the transitionnal flow(Reynolds number between 2320 and 13800),a Doppler velocimetry test-bed is built.28 flow states with Reynolds number between 2000 and 16000 are tested.The influence of water flow on the flow process and the flow characteristics in transitional area are analyzed.The relationship between the distribution of dynamic velocity field and Reynolds number is studied.The results show that the nonuniformity coefficient of flow velocity is larger when the Reynolds number is less than 5800.The nonuniformity of velocity distribution affects the line/surface velocity ratio of flow,which leads to the decrease of metering accuracy.When the Reynolds number is 3800,the average line/surface velocity ratio is 0.73 and the flow error is 2.57%.When the Reynolds number is greater than 5800,the non-uniformity of velocity distribution decreases and the linear/surface velocity ratio increases.for example,when the Reynolds number is 12000,it is 0.93 and the flow error decreases to 2.13%.4.Using DNS program to solve the N-S equation directly,several flow states with Reynolds number between 2400 and 5800 in transitional flow are simulated,and the flow field distribution is obtained.The results show that the DNS data are in good agreement with the experimental data,and also in the region close to the pipe wall.With the increase of Reynolds number,the variation characteristics of skewness and flatness of higher-order statistics reflect the enhancement of fluctuation and randomness of flow parameters,and the fluctuation characteristics of flow field also change,which promotes the energy exchange between flow points and reduces the non-uniformity of flow parameters distribution.The simulation results show that when the Reynolds number is 2800,the average value of line surface velocity ratio is 0.71,and the flow error is 2.71%;when the Reynolds number is 5800,it is 0.83 and 2.34%.The better uniformity of the velocity field makes the line velocity closer to the surface velocity,reduces the flow measurement error,and is conducive to improving the measurement accuracy of the heat meter. |
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