| In the gas-liquid two-phase flow, there exist several parameters, such as bubble size, bubble shape, moving path of bubble and interaction between the gas and liquid, which can make an enormerous impact on the behavior of the two-phase flow. Consequently, further investigations regarding to the bubble dynamics could act as guidance for solving many practical engineering problems. In this research, selecting a single bubble in the distilled water, tap water or glycerol aqueous solution as research objects, the high-speed photography combining with digital image processing algorithms was employed to extract related characteristic parameter involving with bubble motion, and then study on bubble dynamics was conducted. Main work and corresponding results can be summaried as follows:(1) A set of test platform, which allows visulazation of the two-phase flow field, was established. An algorithm which is suitable for image processing was proposed. It mainly consists of image preprocessing, image binarization, image filling, image edge detection and extraction of bubble characteristics. Based on the toolbox in the MATLAB, the characteristic parameters of bubbles in the digital image were obtained successfully.(2) The moving path of the bubbles in water is closely related to not only bubble diameter, but also nozzle inner diameter and bubble shape. The bubbles remain spherical and rise up straightly when their diameters are small. Otherwise, the bubbles begin to deformate and proceed in a zig-zag, spiral or random motion. In this case, periodic oscillations occur at the velocity and aspect ratio of the bubbles. Moreover, there exists an inverse function relationship between them. In the glycerol aqueous solution, bubbles pass through a nearly straight path in the first case. Detaching from the nozzle, bubbles reached steady state rapidly with no oscillations.(3) In the water of low viscosity, the bubble shape is mainly dominated by the inertial force and surface tension. The influence cause by the viscous force could be neglected. The impact of gravity should be taken into consideration if the diameter is large enough. In the glycerol aqueous solution, the bubble shape is mainly dominated by the viscous force, surface tension and inertial force.(4) Bubble terminal rising velocity in water is related to the bubble diameter, nozzle diameter and aspect ratio. A greater aspect ratio leads to a smaller terminal rising velocity when the bubble diameter is constant. It shows an S shaped distribution between bubble terminal rising velocity and bubble diameter, then the terminal velocity can be divided to three different regimes:viscous-dominated regime, surface-tension-dominated regime and inertia-dominated regime.(5) The relation between CD and Re, We in the glycerol aqueous solution can be formulated as follows:CD=23.4911Re-0.6668We-0.1531.The relation between CD and Re, Eo, We in the distilled and tap water is: CD=max(14.7825Re-0.6254,min(0.755Eo0.6076,2.063SWe0.0983))(6) The prediction model proposed by Tomiyama revealed the best performance to predict terminal velocity in the water, then followed the one proposed by Ishii and Chawla. However, in contrast to the model of Ishii and Chawla, the first one is not suitable for predicting terminal velocity in the glycerol aqueous solution. |
PDF Full Text Request