Research On Flow Field Characteristics And Measurement Performance Of Momentum Flowmeter

Flow is a widely measured parameter in industrial production.In the measurement of gas flow,the flowmeters on the market have a common shortcoming:it is not suitable to measure the gas containing impurities.Industrial gas often contains impurities,due to the phenomenon of adhesion and blockage,the traditional flowmeters and ultrasonic flowmeter can not measure it reliably for a long time.Momentum flowmeter has a good adaptability to the above media,but there is little research on the momentum flowmeter.The measurement accuracy of the momentum flowmeter depends on the flow field characteristics and measurement performance in the tube.In this thesis,the numerical simulation method is used to explore the flow field characteristics of momentum flowmeter,and data fitting method is used to investigate the measurement performance of momentum flowmeter.In order to study the influences of different shapes of the components on the flow field characteristics and the measurement performance,four kinds of stressed components,whose main shape is circle,square,ellipse and rectangle,are selected.Based on the reasonable prediction of the flow field characteristics and the flow state,the appropriate mesh is chosen and the high Reynolds number Standard k-εturbulence model is applied to calculate the flow field in the pipe.After the model,the mesh and the algorithm are determined,the benchmark model of the stressed components which is placed in the center of the circular tube are firstly simulated and analysed.Based on the analysis of the drag coefficient and the pressure loss,the concepts of the perimeter rate and the surface roundness are put forward.By presenting these geometrical concepts,the drag coefficient and the pressure loss of the other stressed components can be roughly judged without doing simulation and experiments in the future;at the same time,it also provides a way of changing the drag coefficient and reducing the pressure loss by designing component shapes.In the writing of the formula,it is emphasized that the dimension is uniform,and the norm symbol is used to ensure the consistency of the dimension.By analyzing the pressure loss,the concepts of critical velocity and stable loss coefficient is put forward based on the characteristics of local loss coefficient of different components.On the whole,the results show that the pressure loss of stressed components with shapes of ellipse and rectangle which is slender is less.Then the measurement formulas,which is another important aspect of momentum flowmeter,is studied.For this,the essence of the principle of measurement errors is put forward,and the error relations is explored;then the fitting method is studied and improved,which leads to determine that the most appropriate fitting method is the relative square method;after that,several kinds of fitting model are selected and the advantages and disadvantages are analyzed to determine that the simplified fraction fitting model and the simplest fraction fitting model are preferred according to the relationship trendency between the component drag coefficient and fluid velocity,and in this process,the concepts of the best constant drag coefficient and drag coefficient stable value is put forward;finally,the measurement formulas and errors of different stressed components of properly placed in different fitting model are obtained,in which all kinds of fractional fitting model can meet the precision requirement.After different stressed components which are placed in the center of the pipeline are fully researched,deflecting a small angle towards the left side(similar for the right side)and moving a small distance towards the up side which are always occurred in practice are constructed,a new offset drag coefficient is defined,and the influences of left deflection and up movement on the flow field characteristics and the measurement performance are obtained by comparative study with properly placed.The results are that a small angle left deflection only has a little influence on the drag coefficient;while a small distance up movement makes the drag coefficient reduce obviously,one hand by the new definition of drag coefficient,the other hand by the reduction of the average differential pressure.Therefore,the measurement formulas have been changed and the precision is still satisfied the requirement.After that,for elbow pipes,the influence of the length of the straight pipe on the flow field characteristics and the measurement performance is studied by taking one of the stressed components as an example.The drag coefficient increases with the increase of the length of the straight pipe when the elbow is upstream,and the length of the straight pipe has little influence on the drag coefficient when the elbow is downstream,only a little fluctuation.On the curve fitting of elbow pipes,the composite fitting which is interpolated on the basis of the original fitting is put forward,by making the constant in the original fitting model as a function of the length of straight pipe,which makes the final measurement formulas have more extensive scope of application,and the precision can also meet the requirement.In this thesis,a basic research on the momentum flowmeter which is suitable for the measurement of gas have been done.It provides ideas and methods for further research in the future.