| Pneumatic conveying is commonly used to transport powdery and granular materials in a large number of industrial processes including chemical,metallurgy,electric power,medicine,and food processing,etc.Particle velocity is one of important parameters describing gas-solid two-phase flow.The accurate,reliable,real-time and continuous measurement method of the particle velocity becomes increasingly important for efficient utilization of energy and raw materials,and to optimize particle handling and control process.In this thesis,a novel matrix capacitance sensor was designed,and combined with the principle of optical spatial filtering,for measuring the velocity of the local particles within pneumatic conveying pipeline.The measurement method can overcome the insufficiency of the existing particle velocity measurement technology that cannot reflect the distribution of the velocity field of the pipeline cross-section.This thesis first establishes a mathematical and simulation model for a matrix capacitance sensor.Then the spatial sensitivity distribution of the matrix capacitance sensor is analyzed,and the spatial sensitivity characteristics of the sensor with axial periodic distribution and local distribution of the pipeline cross-section are obtained.Further,the influence of the structural parameters of the sensor on the spatial sensitivity distribution is systematically studied,and the similarity calculation model of the sensitivity of the matrix capacitance sensor is established using the principle of similarity.Then,from the view of the "flow noise" frequency response and statistics,the general theory of measuring the fluid velocity is theoretically deduced based on the capacitance spatial filtering effect.Based on the matrix capacitance sensor designed in this thesis,the theoretical formula of its velocity measurement is deduced and its spatial filtering characteristics are analyzed in detail.The velocity of the local particles within pipeline can be obtained by determining the narrow-band peak frequency in the frequency spectrum of the output capacitance signal.On this basis,the differential matrix capacitance sensor is further utilized to improve the spatial frequency selectivity and to expand the dynamic range of solid particle velocity detection.The theoretical analysis of the peak frequency deviation is given,and the method of compensation correction is also given.By defining the effective sensitivity and frequency bandwidth as evaluation indicators for the filter characteristics,general recommendations are given for the design principles of the structural parameters of the matrix capacitance sensor.Finally,the error of velocity measurement by spatial filtering method is analyzed.Based on the theoretical analysis,the spatial filtering characteristics and performance of the matrix capacitance sensor was evaluated on a gravity-fed solids flow rig in terms of its local sensitivity,measurement accuracy and repeatability.In the inclined test,the transport pipeline was inclined at an angle of 45° to control the particles flowing along the bottom of the pipeline.The experimental results verified that the matrix capacitance sensor had the local sensitivity and the capability for the local particles velocity measurement.In the vertical test,the quantitative evaluation on the performance of the matrix capacitance sensor velocimeter was further conducted.The results indicate that the standard deviations of the repeated measurements were less than ±9%over the velocity range of 2.4～5.8 m/s. |
