Experimental investigation of overlapping bubbles based on laser scanning

Abstract

Image analysis technique is a common method to measure the void fraction and bubble size distribution in the bubbly flow. However, the image recognition algorithm will become invalid finally when the the projection overlaps of bubble become more heavily. In this paper, a new approach on three-dimensional (3D) reconstruction and parameters measurement of overlapping bubbles is developed based on laser scanning. The measurement principle of laser scanning is represented firstly, and appropriate scanning parameters are analyzed for 3D reconstruction at certain temporal and spatial resolution. Several image-preprocessing procedures are performed to extract the features effectively from the original slices of bubbly flow. A compensation method for slices offset is investigated to remove the unaligned feature noise of bubbles. An optimization algorithm based on image convolution of second-order differential averaging operator is proposed to remove the image noise caused by multiple exposures. In experiment, the 3D structure of bubbly flow field is reconstructed, and the void fraction and bubble size are measured with the analysis of distribution. The experimental results show that the method is valid for non-intrusive measurement and reconstruction of bubbly flow without the limitation of projection overlap. The optimization algorithms can effectively reduce the reconstruction deformation and improve the reliability, and the measurement error of void fraction is less than 13%, which indicates the relatively high characterization precision for the bubbly flow.