Investigations On Effects Of Rolling Motion On Flow Pattern Of Two-Phase Flow In Rod Bundle

Rod bundle channel is a typical coolant passageway in the core of nuclear reactor.In some severe accidents,two-phase flow may occur at the reactor core.With the variation of the gas-liquid flow rate,different flow patterns will appear with distinct thermal-hydraulic characteristics,which are related to the safety of the reactor.Under ocean conditions,there will be some distinguishes in flow pattern characteristics by the effects of inertial force.The investgations of the flow pattern in rod bundles under rolling condition can give a meaningful reference for the design of the nuclear reactor systems.In this paper,the investigations of flow pattern in 3×3 rod bundles have been carried out by high-speed camera visual experimental study as well as theoretical analysis under vertical,inclined,rolling conditions.Deionized water and air were used as test fluid.The experimental loop was forced circulation.The experiments were conducted under ambient temperature and pressure.The gas superficial velocity ranged from 0.01 to 7.61m/s,the liquid superficial velocity ranged from 0 to 2.0m/s,the angle of inclination ranged from 5°to 15°,the rolling period ranged from 8s to 16s,the rolling amplitude ranged from 5°to 15°.Under steady-state condition,the typital flow patterns in the rod bundle channel were classified and defined,based on the distinct characteristics,which were divided into bubble flow,cap-bubble flow,slug flow,churn flow and annular flow.The liquid Reynolds number,gas superficial velocity and angle of inclination were chosen as parameters to analyse their influence on the features,regions and transformational boundaries of all the flow parrerns.It shows that the shifting gas velocity will increase with the liquid turbulence intensity linearly.The shifting gas superficial velocity is lower,when flow pattern chang from bubble flow to cap-bubble and cap-bubble flow to slug flow,but slug flow to churn flow is opposite.The flow pattern map was drawn for vetical and inclined conditions.Compared with Venkateswararao flow pattern transition model,the criterions were modified to obtain a better prediction.Using the pressure data,pressure fluctuating curves were made for each flow pattern to indentify them.Under rolling condition,there were three unique phenomenones:gas phase distribute periodically,flow pattern change periodically and gas phase move transversly.Chosing dimensionless number Fr as parameter,analyzed the impact of rolling amplitude and rolling period.Pressure curves were made under rolling conditions.The pressure curves fluctuated periodically under some certain rolling conditions.The flow map on rolling conditions was made to be compared with vetical and inclined conditions.Under liquid static condition,the void fraction in rod bundles was obtained under different angles of inclination.By using the drift-flow model,velocity distribution parameter C₀ and two-phase drift velocity V_gj were obtained,which were applied to calculate the gas rising velocity.The accuracy has been demensrated by the images captured by high-speed camera.The existing empirical formulas of drift-flow models were evaluated.The result shows that the formulas of pipe is not suitable for prediction,whereas the rod bundle formulas can get a good result,average predicting error is about 25%.Images processed by high-speed camera were utillized to investigate the slug velocity,length and nose position.The dimensionless number St was used to evaluate the slug frequency.The empirical formulas built by Fossa were modified under lower value of Reynolds number,achieving a perfect fitness.