Study On Flow Pattern And Resistance Characteristics Of Air-Water Two-Phase Flow In Rolling Motion

Two-phase flow occurs widely in heat exchangers of power systems, however some power equipments often operating under oceanic conditions, are influenced by sea wave oscillations, such as rolling, pitching and heaving. Thus equipments as well as connection loops change with its position with respect to the axis of the rolling motion, which may induce the flow instability and have an impact on the system performance. Hence it is important to study the two-phase flow in different pipe arrangement in rolling motion. In the paper, flow pattern and pressure drop for air-water two-phase flow in rolling horizontal and vertical pipes were studied experimentally at room temperature condition.Single-phase flow characteristic through vertical and horizontal pipe was investigated experimentally using water as working fluid in rolling motion. The influences of factors (for example, pipe diameter, Reynolds number, rolling amplitude and rolling period) on friction factor were analyzed. The results show that instantaneous friction coefficient varies periodically in the rolling straight pipe, which is different greatly from that in a steady pipe flow. By the dimensional analysis, correlations for calculating the friction factor of single-phase flow were regressed from the experimental data.Experimental investigations of air-water two-phase flow patterns in horizontal pipe under rolling motion were also performed. By combination of visual observation and pressure tracers, flow patterns were defined by possible flow characteristics. By comparing with the data in the steady condition, it is showed that rolling motion affect flow patterns considerably:at middle low flow rates, flow patterns take great change with the inclination caused by the rolling motion, and tend to develop towards steady-state flow patterns in upward inclined or downward inclined flow; only at high liquid rates or at high gas flow rates some typical flow pattern occurred such as bubble flow, intermittent (slug flow, pseudo-slug flow), annular flow in rolling horizontal pipe, which is similar to steady-state two-phase flow patterns, but the region existed in the flow maps reduces. By analyzing the force on the bubble under rolling motion and introducing the effective gravity acceleration, the transition from bubble flow and from annular flow was provided which adapt to the rolling condition. When compared with the steady transition model, the existing model is not able to well predict air-water flow. The effects of rolling period and rolling amplitude on the transition boundary of flow pattern were discussed.The flow structure along a vertical pipe was investigated from visual observation of two-phase flow, and it is found that four flow regimes, bubble, slug, churn and annular, exist under rolling condition, which is similar to the flow patterns in the steady condition. Flow pattern map shows the distribution of flow patterns in terms of the liquid and gas superficial velocity. The effect of pipe diameter, rolling periods and rolling amplitude on flow transition was analyzed. At the same time, in order to conveniently clarify the effect of rolling motion, two-phase flow in a vertical pipe was experimentally investigated under steady conditions, and difference in flow transition between rolling condition and steady condition was analyzed and compared.Finally, frictional pressure drop of annular flow in horizontal and vertical pipe was investigated under rolling condition. By considering the effect of rolling factor on flow structure, and the predicting method was proposed based on Chisholm model. The new method is good to predict the pressure drop by comparing the experiment data from the present work.