Simutaneous Measurement Of Size And Velocity Of Burning Particles In Metal Fuels Based On Light Field Imaging

Aluminum powder is widely used in various solid propellants as metal fuel.However,large numbers of agglomerates occur because of the agglomeration behavior of aluminum powder during combustion,which hinders the further combustion of internal aluminum and leads to the energy loss of flow.This finally affects the release and transfer of power in aluminum fuel.The size and velocity of aluminum particles and agglomerates are the key parameters that affect the power releasing and transferring efficiency of propellants.Accurate characterization of dynamic metal fuel combustion is of great significance to effectively organize particulate matter combustion and improve combustion characteristics of propellant.Therefore,a simultaneous measurement method for the size and velocity of aluminum particles based on light filed imaging is proposed in this paper.A light field imaging system is established and experiments of burning metal fuel are finally carried out to evaluate the measurement method for the size and velocity.Firstly,an automatic recognition procedure of burning aluminum particles based on digital image processing technology is explored.The basic concepts of parametric characterization,sampling,and refocusing of light field imaging are introduced.The advantages of light field imaging in three-dimensional measurement compared with traditional optical imaging are discussed.From the raw light field images of particles,the refocused images of particles in different spatial depths are obtained through refocusing algorithm.After the preprocessing of graying,filtering,and denoising,a focus-stack image containing all the particles at different depths is generated according to the image grayscale and gradient.Then the burning particles are separated from the background through the binary segmentation and watershed algorithm.Secondly,the three-dimensional(3D)reconstruction model of the burning particles is established.The equivalent diameter of the particle’s projection area is taken as the equivalent diameter of the reconstructed particle sphere for particle identification.Based on the refocusing characteristics of light field imaging,a three-dimensional particle locating method adopting optical ’Depth From Focus’ is proposed.The depth of particles is related to their focal plane,so they can be determined by finding the focus position of each particle through the sharpness evaluation for refocused images.Furthermore,the velocity measurement method based on particle tracking is studied.The matching effects of Polar Coordinate System Similarity method(PCSS)and the iterative relaxation method are compared and discussed.Finally,experiments are carried out to verify the reliability of the proposed methods.A special test rig of solid propellants combustion is employed,and a light field imaging system is established.The software for processing light field imaging and particle parameters decoding is designed and programed based on MATLAB.The calibrations of the light field system are carried out to determine the relationship between the spatial depths and the refocusing parameters.Performance of the imaging system for particle size measurement was evaluated by dot calibration board.Results show that the average relative error of particle size measurement is less than 2%.Experimental measurement for the size,position and velocity of aluminum particles was carried out in the aluminum propellant strip combustion device.The results show that the particle size distribution of aluminum particles and agglomerates is in the range of 50～500μm,and the velocity distribution is in the range of 0～6m/s.Therefore,the proposed light field measurement method is capable of measuring 3D parameters of burning particles instantly,tracking the motor process effectively,and characterizing dynamic particle behavior accurately.