Study Of Three-Dimensional Measurement Of Bubble In Gas-Liquid Flow By Lightfield Photography

Gas-liquid two-phase flow,one of classical multiphase flows,is widely applied in energy and chemical industries,and environment field.The measurement technology of gas-liquid two-phase flow is of great significance for the study of gas-liquid flow and mass transfer.As bubble is one of features of gas-liquid two-phase flow,the measurement of its parameters is particularly significant and yet challenging.Some problems such as system complexity,poor real-time performance,and two-dimensional measurement are desirable to be solved in bubble measurement.This thesis aims to propose a three-dimensional measurement method for gas bubbles in gas-liquid two-phase flow based on light field photography,and theoretical and experimental researches on this method are carried out.Firstly,the concept and characterization of light field are studied.The theory of capturing light field information is introduced,and the advantages of light field photography in comparison with conventional imaging are presented.The imaging principle of the light field camera are discussed and the digital refocusing algorithm is studied.According to the principle of geometric optics and pinhole,the angular resolution,spatial resolution and depth resolution of the light field camera along with their influence factors are discussed.Secondly,a three-dimensional reconstruction model of bubbles is established,which solves the problem that conventional imaging can only perform two-dimensional measurement.In order to obtain the two-dimensional information of the bubble,a series of digital image processing is performed on the total focus image,and the overlapping bubbles are segmented by the improved watershed algorithm.In order to derive the depth of bubbles,in combination with the point sharpness evaluation function,the DFF algorithm is applied to process the refocused image sequence.With the focus position of the bubble image determined,the bubble centroid depth is defined as the average depth of the bubble edge.Combined with the depth and magnification calibration,the three-dimensional reconstruction of the bubble can be achieved,and then the parameters such as bubble size distribution,spatial position and void fraction are calculated.The three-dimensional cross-correlation algorithm is further used to calculate the bubble velocity from three-dimensional reconstructions of the bubbles.Then,the light field imaging calibrations are performed,and the refocused image depth and magnification are calibrated.Using the sharpness evaluation function,the refocused image is calibrated to the actual position in space,and the corresponding relationship between the object space depth of focus and the refocusing parameter is fitted with the calibration error less than 1 mm.The black and white checkerboard is used to calibrate the magnifications and the curves of the magnification and depth positions are established according.Through the calibration of depth and magnification,the relationship between the actual parameters of the bubble and the image parameters is established.In addition,based on the measurement results in a thin flat bubble bed,the error of bubble reconstruction depth is studied and analyzed.The average error of bubble reconstruction depth is 0.58 mm,which can meet the needs of industrial macro-bubble measurement accuracy.Finally,experiments are carried out to measure the bubbles in a flat bubble bed.The three-dimensional vortex field in the lower right region of the bed are reconstructed at the gas flow rate of q=2.5·L.min^-1.The parameters of bubble including diameter,depth distribution and void fraction in the cross section are further discussed.The measured bubble velocity field is in good agreement with the shape of vortex flow.The three-dimensional reconstruction of bubbles in the lower right region of the bed and the variation of velocity field with gas flow rate are studied.The results show that with the increase of gas flow rate,the number of bubbles increases proportionally,and the depth distribution of bubbles increases gradually.The gas flow rate has obvious influence on the flow of bubbles.The void fraction,bubble diameter and bubble velocity decrease during the formation of vortex.In the end,the variation of void fraction and velocity field with gas flow rate in six different regions of the flat bubble bed are further studied.The results show that when the gas flow rate q is 2.5 L·min^-1,the two-sided vortex is formed due to the change of initial bubble velocity,and the area of vortex increases with the gas flow rate,which results in the obvious difference of the void fraction in different areas of the flat bubble bed.These results further validate the feasibility of the three-dimensional measurement method for gas bubbles in gas-liquid two-phase flow based on light field photography.