Investigations On Effects Of Rolling Motion On Characteristics Of Flow Resistance In Rectangular Duct

Small channel heat transfer technology is one of the effective methods which miniaturizethe heat transfer equipment of ship power systems. Different from the land-based nuclearreactor, the barge-mounted nuclear reactor always works under motion conditions, amongwhich the influence of rolling process of a ship on the thermal-hydraulic behavior is the mostcomplex. The rolling motion induces inertia force acting on the coolant and changes thegravity along flow direction, both of which may influence the operational characteristic ofpower plants.This dissertation investigates flow resistance in mini rectangular duct under rollingcondition experimentally and theoretically. The experiments were conducted under ambienttemperature and pressure, and water and air were used as the test fluids. The5test sectionshad the diameters of2.73~5.99mm, with aspect ratios ranging from0.033to0.075. Theliquid Reynolds number and gas Reynolds number ranges were200~21000and221~8310,respectively, and the rolling period and rolling amplitude were8s~20s and10°~30°,respectively. Different pressure head were acquired by regulating the rotation speed of thecentrifugal pump, in order to compare different influences of rolling motion on single-phaseforced circulation.The flow visualization experiment combining with single-phase flow fluctuation modelindicate that the flow fluctuation of the open loop and closed loop are mainly produced by thegravitational and additional pressure drop, respectively. The flow fluctuation amplitudedecreases as the effective pressure head increases, while increases as the oscillatory pressuredrop increases. Finally, the flow tends to be steady, if the effective pressure head is larger than10~11times of the oscillatory pressure drop. The flow resistance experiments indicate thateffects of rolling motion on single-phase flow resistance wears off as the pressure headincreases, which could be neglected if the pressure head was high enough. For relative lowpressure head, larger rolling amplitude and smaller averaged velocity give rise to largeramplitude of frictional pressure drop, whereas, rolling period have different effects on thefluctuation amplitude because of the inequable flow loops.The momentum conservation equation for single-phase laminar flow was set up, by which the predicted frictional coefficient agrees well with the experiments. The mechanism ofrolling motion on frictional resistance is discussed theoritically. For low pressure head, rollinginduced periodically pulsing flow leads to the periodical variation of the wall shear stress,which results in the fluctuation of frictional resistance. While the pressure head is high,rolling motion just induces the additional pressure drop, nearly having no influnce on the wallshear stress and frictional resistance. Theoretical correlations for frictional coefficient oflaminar flow and transition Reynolds number were obtained for the condition of high pressurehead, and effects of rolling induced flow fluctuation on flow regime transition wereinvestigated by energy gradient method.For two-phase flow, the effect of rolling motion on frictional resistance weakens as theliquid velocity increases. When the liquid velocity is low, the amplitude of relative frictionalpressure gradient decreases as the gas velocity increases, whereas it is nearly independent ofthe rolling parameters. While for relative high liquid velocity, the frictional pressure gradientis nearly invariable. Rolling motion nearly has no influence on time-averaged frictionalresistance. The Chisholm C、Lee&Lee and Mishima&Hibiki correlations work well fortime-averaged frictional resistance, even under rolling condition, but not for the transientfrictional resistance which varies periodically. A correlation for periodical frictional pressuredrop was also achieved by modifying the separate flow model.The rolling motion influences frictional resistance by changing interfacial friction of thetwo phases. The interface distribution is nearly invariable, and the frictional resistance doesnot fluctuate under inclined condition. Different from steady state, the transient force inducesperiodical acceleration of bubbles along flow direction, and leads periodical lateral bubblemotion in transverse direction under rolling condition, which results in periodically frictionalpressure drop. For larger rolling periods, the fluctuation is mainly due to the transversecomponent of the gravity, while for a smaller rolling period, the effect of Coriolis forcecouldn’t be neglected.