Distribution Of Void Fraction In Gas-liquid Two-phase Flow In Circular Tubes

In gas-liquid two-phase flow, the flow resistance, the heat and mass transfer characteristics keep varying with the variation of interface structure, and hence the change of local parameters may result in some problems, such as the deterioration of heat transfer in boiling channels. As a fundamental geometrical parameter in the design with respect to thermal hydraulic, void fraction is of great importance in calculating the heat and mass transfer, flow resistance, flow pattern determination, phase distribution and the interfacial transport in two-phase flow. The ships and the aerospace vehicles operated under normal and accident may be in a tilt, rolling or vibration state, and once a two-phase flow process occurs in the system, effect induced by the state variation is inevitably. Therefore, the research of void fraction distribution and its formation mechanism in a circular tube under the conditions for vertical, tilt, rolling has always been one of the focuses in the corresponding research field.Experiments of gas-liquid two-phase flow in tubes with inner diameters of 50mm and 100mm respectively were conducted in vertical, tilt, and rolling states and under ambient temperature pressure, to investigate the distribution of void fraction using of an optical fiber probe.The local void fraction shows a "core" or "wall" peak distribution along the radius with different gas-liquid flow rates and inclined angles; with increasing inclined angle, the radial distribution of void fraction gradually shifts from the "core" peak or "wall" peak to the single "wall" peak distribution; with increasing rolling amplitude,the local void fraction distribution gradually shifts from the "core" peak to the "wall" peak distribution at low gas flow rates; compared to the vertical state, the area-averaged void fraction under rolling amplitude 5°and 100° reduces 20.7% and 24.1%, respectively.A balance equation of forces on a single bubble in radius was set up for the tube of 50 mm in diameter, by which the correlation of wall force coefficient under inclination condition was obtained. Combining the modified wall force, lift force and buoyancy force, a new force balance equation in radius was re-established, with an averaged error of 15.6% against the experimental data in the prediction covering the range of 0-21 mm inclined angles of 15° and 30°.Through the analysis of the lift force, turbulent diffusion force,wall force acting on a single bubble,the reason of local void fraction showing "core" or "wall" peak distribution in radius was preliminary obtained in the vertical, inclined, rolling states. In vertical state, the lift force and wall force are of great importance in leading to a "core" or "wall" peak distribution.While under inclined conditions, the combined effects of the component of buoyancy in radius, lateral lift force and wall force lead to the bubbles gathering around the region of "15 mm < r< 22mm",which results in the 'wall peak' distribution. Under the rolling condition,turbulent diffusion force and buoyancy force, wall force are of great importance in leading to the gradual shift from the "core" peak to "wall" peak distribution with the increase in rolling amplitude.