A Study On Flow Resistance And Outlet Jets Of Plate-Type Fuel Assembly Under Flow Fluctuation Condition

Plate-Type Fuel Assembly(PTFA)has been widely used in material test reactor(MTR)and marine-based reactor because of its compact structure,high heat transfer efficiency and mature processing technology.Under the influence of ocean conditions,remarkable flow fluctuation occurs in natural circulation reactor loop.Flow rate fluctuation makes the flow field and resistance characteristics of reactor core in a periodic transient-state,which threatens reactivity control and thermal safety of nuclear reactor.Thus,it has important fundamental research value and engineering application background to study flow resistance characteristics and flow field structure of PTFA under flow rate fluctuations.In the present study,the flow resistance and velocity distribution of narrow-subchannel(with inlet section and outlet jets)and sudden expansion channel,sudden contraction channel under flow fluctuation conditions by theoretical analysis,experimental measurements and numerical simulation.There are pulsating laminar regime,pulsating transitional regime and pulsating turbulence regime in pulsating flows.In pulsating laminar regime,an analytic solution of velocity distribution is deduced by known sinusoidal variation pressure gradient.The influence of relative amplitude and dimensionless frequency of pressure drop on velocity distribution is analyzed.Both theoretical and experimental results show that the flow fluctuation will not cause the transient and time-averaged flow resistance to deviate from the values of steady flow in pulsating laminar regime.In pulsating transitional regime,the laminar-turbulent transition occurs earlier due to the flow fluctuation.By comparing the instantaneous and time-averaged frictional pressure drop with those of steady flow,it is found that the larger the relative amplitude and dimensionless frequency,the smaller laminar-turbulent transition Re.An empirical formula for determining the laminar-turbulent transition point in the range of experimental parameters is proposed.In pulsating transitional regime and turbulence regime,it is found that the pulsating parameters are superimposed on each other.The ratio of the time-averaged pulsating frictional pressure drop to the steady flow values(at the same time-averaged Re)will increases with the increasing relative amplitude and dimensionless frequency,and decreases with theincreasing of time-averaged Re.Based on dimensional analysis,dimensionless acceleration is proposed to represent the comprehensive influence of the pulsating parameters.What’s more,the effect of pulsating flow on frictional pressure drop rapidly decreases as the narrow-side increases from 1 mm to 4 mm.The mechanism is that turbulence can rapidly propagate the entire turbulent Stokes layer before dissipation,thus causing the dissipation of turbulent energy.In narrow sub-channel,the thickness of turbulent Stokes layer is equal to the width of narrow channel,so the influence of pulsating flow is strong.With the increasing of channel size,turbulence can no longer propagate in time and dissipate,thus the effect of fluctuating flow on frictional pressure drop fade rapidly.An analytical solution of 2D velocity development solution is deduced based on the harmonic-sinusoidal-variation inlet velocity is known.The flow resistance at the entrance section of pulsating flow is much larger than that of steady flow and that in full-developed section.PIV results show the collapse of laminar-state and turbulence occurs at the end of the deceleration half-period,and then recover to laminar-state(or called re-laminaration)in acceleration half-period.It is found that the time-averaged frictional pressure drop in the laminar-turbulent transitional regime is much greater than that of steady flow conditions in sudden expansion channel and sudden contraction channel.The larger the relative amplitude and dimensionless frequency,the larger the frictional pressure drop.Acceleration and deceleration lead to periodic increasing and decreasing of the area of recirculation zone.The dissipation of turbulent energy of corner vortex zone and recirculation zone are the main reasons for the increasing of pressure drop in the transitional regime.A modified multiple-jets velocity theory based on Reichardt hypothesis is proposed by introducing the semi-empirical formula of spanwise pressure distribution.The modified theory agrees well with the experimental results.Based on continuous PIV,2D transient velocity vector field of multiple-jets under steady and pulsating flow conditions is measured.Strong flow separation,jet interaction and turbulent mixing were found in potential core region and merging region.The present multiple-jets with low spacing-ratio and narrow nozzle exhibits stronger turbulent anisotropy and jet decay characteristics.Several Reynold Average Navier-Stokes(RANS)turbulence models(k-epsilon,k-omiga and RSM)based on the assumption of turbulence isotropy are quite different from the experimental results.Thedistribution characteristics of turbulent intensity,Reynolds stress,productions of turbulent kinetic energy,turbulent anisotropy,turbulence scales,as well as power spectrum density are analyzed,respectively.PIV results and URANS-based numerical simulations of pulsating multiple-jets show that the pulsating acceleration and deceleration have little effect on the velocity distribution in the range of the present study.In potential core region and merging region,there is little difference between the longitudinal and lateral velocity distributions at the acceleration phase and the corresponding deceleration phase.Acceleration only affects the turbulent kinetic energy in the downstream converging region and flow shear zone.