Study Of Flow Instabilities In A Rectangular Mini-channel Under Rolling Motion Condition

Nuclear propulsion is widely used in marine power systems,such as nuclear warships,merchant ships and ocean platforms,due to its high power density and strong endurance.Because of the space limitation of a marine reactor,the arrangement of compact plate-type fuel assembly and heat exchangers is necessary.Since ocean condition could lead to periodical fluctuation of the coolant,the flow will become more complicated when flow instabilities occur.This work is aimed to experimentally and theoretically(nonlinear analysis)investigate the flow instabilities in a rectangular mini-channel under rolling motion.Experimental investigations of flow instabilities of forced circulation in a rectangular mini-channel under both static and rolling conditions have been carried out by using deionized water as the working fluid.Typical DWO,PDO and Ledinegg instability have been observed under static condition.The predicted OFI correlation and instability boundaries of this work are obtained.Besides,the flow instability mechanisms are compared with that of conventional macro-channel and micro-channel.With decreasing mass flux,single-phase fluctuation,trough-type flow oscillation,coupled flow oscillation and two-phase fluctuation occur in rolling condition.Trough-type flow oscillation is mainly due to the steam generation at the wave trough and it is the typical flow instability induced by rolling motion.Coupled flow oscillation is supposed to be superposition of trough-type flow oscillation and PDO.The fluctuation amplitude,frequency spectrum and flow pattern are used to analyze the evolution characteristics of flow instabilities under rolling condition.Three regions,rolling motion domination region,coupled region and thermal factors domination region,are confirmed.Flow instabilities of natural circulation in a rectangular mini-channel under static and rolling conditions are experimentally investigated.The results show that static flow excursion and DWO occur in static condition.Increasing system pressure and inlet fluid temperature would mitigate the flow excursion.However,the effect of pressure on DWO boundary is unapparent according to Phase-change Number and Subcooling Number.Rolling motion would bring early OFI but lagging coupled flow oscillation under natural circulation.This is mainly because the former is induced by coolant fluctuation,while the latter is owing to DWO occurrence.The rolling parameters have little effect on OFI.However,higher rolling intensity could bring earlier coupled flow oscillation.The spacial variation introduced by rolling motion will lead to early occurrence of flow excursion,which means the decrease of natural circulation ability and heat transport ability.With increasing rolling angles,earlier flow excursion would be found.However,the effect of rolling periods is unapparent.Besides,the backflow induced by rolling motion results in fluctuation of inlet fluid temperature and early OFI,which would reduce the system safety threshold.According to FFT and wavelet analysis,the frequency structure and distribution of the thermal hydraulic system under rolling condition are studied.The results show nonlinear trend to the power of rolling frequency.This is mainly due to the superposition at low heat flux and suppression of high driving head and resistance at high heat flux.By using phase-space reconstruction,attractor structure and calculating results of geometric invariants such as correlation dimension,Kolmogorov entropy and largest Lyapunov exponent,nonlinear analysis has been performed to study flow instabilities under rolling motion.The results show that with increasing heat flux,nonlinearity and complexity of the system gradually increase and tend to be consistent with the static cases.In low heat flux region,the effect of rolling parameters is relatively slight,while in trough-type flow oscillation and coupled flow oscillation region,increasing rolling intensity would decrease the disorder of the system.Furthermore,the influencing mechanisms of rolling motion on flow instabilities have been discussed from the perspective of competition between driving and dissipative forces and system entropy change.