Study On Dynamical Behavior Of Deformed Bubbles And Phase Distributions In Laminar Bubbly Flows In Vertical Circular Pipes

Bubbly flows play a significant role in a wide range of nature and industrial processes. It is of great scientific significance and important value for practical application to study on bubbly flows. Both bubble dynamics and phase distribution are the main aspects of bubbly flows. A sequence of theoretical and experimental investigations on deformed bubble dynamics and phase distribution were conducted to unveil characteristics of fully developed incompressible adiabatic dilute laminar bubbly flows without mass transfer in vertical pipes in the present dissertation.A double-shadow-images technique has been developed for measurement of the three-dimensional deformation of a bubble in dilute bubbly flows in order to investigate forces on the deformed bubble and phase distribution. Two projections of the bubble were obtained using image processing techniques and watershed-based segmentation firstly, and the centroid of the bubble shadow was detected using the symmetric lines method which could speed the Hough transform for determination of the elliptical projections of the deformed bubble. Then a pre-processing procedure was applied to adjust the parameters of the projection ellipses of the bubble according to the epipolar geometry constraint. Finally an algorithm was designed for reconstructing the three-dimensional bubble shape from the two ellipsoidal projection contours based on the dual-space theory. The results showed that the bubbles are similar to the oblate ellipsoids which rectilinear rise with an incline angle in fully developed bubbly laminar flows in vertical pipes. And an empirical correlation was presented to estimate the aspect ratio of the deformed bubble.By dividing the velocity field around an oblate ellipsoidal bubble, which was derived by extending the boundary layer theory on a spherical bubble, into the velocity on the bubble surface and its normal decay, an approximate formula was proposed to compute the velocity using the method of undetermined functions. Then the approximate method combined with the coordinate transformation was extended to obtain the velocity field around a rising ellipsoidal bubble with an incline angle. Based on the velocity and the pressure estimated by analyzing the order of magnitude of the various terms in the normal Navier-Stokes equation near the bubble surface, three most important closure relations including the drag force, the liquid Reynolds stress and the difference between liquid interface averaged pressure and liquid averaged pressure were constructed using cell ensemble average method. The lift force on the bubble was obtained by the contrast tests between the bubbly flows and the light suspension particle two phase flows. Based on the above, the effects of some variable parameters, such as aspect ratio, Reynolds number and phase volume fraction, on the interface forces, the stress and the pressure difference were discussed.An image-based method was proposed to measure the radial phase distributions of bubbly flows in vertical circular pipes. The measurement was conducted following the two procedures. Firstly, the projected phase fraction distribution was obtained by counting all pixels of bubble shadow images and taking the overlapping probability of the bubble shadows into consideration. Then an improved Fourier algorithm for Abel inversion calculation was employed to reconstruct the radial phase distribution from the projected phase fraction distribution. Unlike the individual particle labeling and stereo-matching method for measurements of dilute bubbles or particles, the present method can be applied to measure the radial phase distributions in denser dispersed two-phase flows in vertical circular pipes. The method was verified with a series of tests including the standard computer-generated bubbly flows, the denser bubbly flows and the particle-oil two-phase flows. The measured results showed that the method is applicable and has high accuracy.A complete set of two-fluid model for fully developed laminar bubbly flows was established based on the modification of the closure relations derived from the study on the single bubble dynamic considering the effect of the wall and the phase volume fraction. The main influencing factors on the liquid velocity and phase distributions were discussed based on the approximate solutions of the model in upwards laminar bubbly flows in vertical pipes. Finally, a series of bubbly flows were predicted by the two-fluid model. The results are consistent with the experimentally data, which showed that the model is accurate in laminar bubbly flows in vertical pipes.