Measurement Of Fluid Flow Characteristics And Drying Kinetics In A Fluidized Bed Using Electrostatic Sensor Arrays

In recent years,gas-solid fluidized beds are widely used in industrial processes ranging from power generation,chemical engineering and food processing due to their advantages of good heat and mass transfer rates,wide operating ranges and abilities to handle a large number of particles.Characterization of the two-phase flow behaviours and drying kinetics is essential for the safe,economical and efficient operation of fluidized bed reactors.In view of the fact that a gas-solid fluidized bed is a typical complex system composed of chaotic,nonlinear flow structures and multiscale characteristics of the signals,accurate and reliable measurement of fluids flow and drying characteristics remains challenging.With the development of electrostatic sensing techniques,electrostatic sensors have been applied in fluidized beds to measure fluid characteristics due to their advantages of structural simplicity,cost-effectiveness,and robustness in a harsh environment.Nevertheless,the previous research has mainly focused on obtaining the local or average value of the flow parameters,failing to obtain parameter distributions.In addition,the electrostatic signals are affected by many factors such as particle size,velocity,and humidity.Our knowledge of the electrostatic sensing method for the measurement of flow dynamics and drying kinetics in a fluidized bed is still limited.This thesis describs the novel research on imaging techniques using electrostatic sensor arrays.Theoretical modelling and experimental work are undertaken in order to develop this method.Moreover,it is desirable to conduct in-depth investigations into the flow characteristics and drying kinetics and induction mechanism using electrostatic sensor arrays,and reveal the complex flow and mass transfer phenomena in fluidized beds.The research that has been conducted and its novelties are as follows:(1)In view of the electrostatic characteristics of particles and the dynamic characteristics of gas-solid two-phase flow in a fluidized bed,a two-dimensional electrostatic sensor array with 4×8 electrodes is proposed and implemented for the first time for the measurement of fluid flow characteristics and drying characteristics.The layout design,signal conditioning unit and practical installation of the electrostatic sensor array are studied.Moreover,the mathematical model of the electrostatic sensor array is established through finite element simulation,which provides a reference for its performance analysis and optimal design of the sensor array and also verifies that its sensitivity distribution meets the measurement requirements.(2)An electrostatic imaging method is employed for the experimental investigations into the bubble behaviours in an experimental fluidized bed.The bubble size,shape and rising velocity are measured through signal analysis in the time domain.Moreover,frequency domain analysis is undertaken to measure the generation frequency of the bubble,and the jet formation zone,jet channel zone and bubble eruption zone are distinguished through analysis of the power spectral density function.The influences of different jet velocities in the range of 3.96-7.14 m/s on the bubble characteristics are investigated.An optical imaging system is employed to evaluate the performance of the electrostatic imaging method.Experimental results show that the bubble characteristics measured from the electrostatic sensor array have a good agreement with the results from the optical imaging system.The relative root mean square error between the bubble shapes measured from the electrostatic sensor array and from the optical system is 0.239 with a standard deviation within 4.7%.The experimental results demonstrate that the electrostatic sensor array with the signal conditioning unit is capable of measuring the bubble characteristics in the fluidized bed.(3)An electrostatic sensor array is adopted for the first time to measure the moisture content distribution of biomass in a fluidized bed dryer.The proposed method determines the biomass velocity by the cross correlation of the electrostatic sensor signals and establishes a measurement model that relates the root mean square of the sensor signals,the biomass velocity and the moisture content.Experimental tests were conducted on a bubbling fluidized bed using the proposed method and then the moisture content distribution under different air velocities and temperatures is investigated.A halogen moisture analyzer is used to verify the moisture content measurement of the electrostatic sensor array.The results demonstrate that the proposed method is capable of measuring the moisture content with a relative error within ± 15%when the air velocity is 0.31-0.56 m/s and the air temperature is 45-75℃.(4)A new method is proposed by combining electrostatic sensing and optical imaging techniques to obtain moisture contents,drying models,moisture diffusivities,activation energies and mass transfer coefficients of biomass at different bubble locations.Experimental tests were conducted on a fluidized bed dryer for different air velocities at a range of air temperatures.Five mathematical drying models(Newton model,Page model,Modified Page model,Henderson and Pabis model and Logarithmic model)are evaluated.It is found that the Page model is most suitable for describing the drying process of biomass in the fluidized bed.The results also show that the mass transfer coefficient of biomass at the interior and boundary of the bubble is higher than that at the exterior of the bubble.In addition,although the mass transfer coefficient increases with the air temperature and air velocity,the highest air temperature and highest air velocity are not optimal conditions.