A Study Of Flow Stability And Phase Transition Of Dense Granular Target

Nuclear energy has the greatest potential to meet the world’s growing demand for energy.However,the biggest problem facing nuclear energy development today is the disposal of nuclear waste from nuclear reactor.The Accelerator Driven Subcritical System(ADS)is recognized as one of the most effective ways to dispose of nuclear waste,transmuting long-lived nuclear waste into short-lived nuclides and reducing the impact of nuclear waste on humans and the environment.ADS uses a proton beam from a high-energy accelerator to bombard a spallation target,which generates a large number of high-energy forward neutrons through the spallation reaction to drive a subcritical nuclear reactor to complete the transmutation of spent fuel and generate nuclear energy.The spallation target is one of the three key components of the ADS.The material and structure of the spallation target determines the neutron yield of the spallation reaction and determines the power of the subcritical nuclear reactor.The power of solid spallation target is limited by the thermal conductivity of the material.Meanwhile,the flow velocity and pressure waves in the liquid spallation target have certain damage to the beam window,which limit the power of the liquid spallation target.China Initiative Accelerator Driven System(Ci ADS)proposes to build hundreds of megawatts of spallation targets to complete the transmutation of subcritical nuclear reactors.To achieve this design goal,Institute of Modern Physics innovatively proposed the conceptual design of a Dense Granular Target(DGT).DGT consists of a large number of discrete tungsten granules and features both solid and liquid targets,which can greatly increase the target power while enabling off-line removal of deposited thermal power.Based on this feature,the ADS team proposed the conceptual design of the granular waterfall target and the chute target in the Mu On-decay MEdium baseline Neu Trino beam experiment(MOMENT)and the Compact Material Irradiation Facility(CMIF),respectively.The flow stability and flow phase transition in the dense granular target are very important for the safe operation of the spallation target.In the granular waterfall target and chute target,the control of flow rate is achieved by adjusting the width of the funnel outlet control plates.When the width of the control plates is small,the funnel outlet has a slit shape.At the same time,this slit-shaped exit is also present at the entrance of the quasi-two-dimensional funnel.In applications,the flow instability occurs when the granular pass through the slit exit.In this paper,simulations and experiments are conducted for the flow phase transition behavior of granular through the slit exit,and the critical exit size for slit funnel clogging is obtained by the statistics of flow rate and clogging probability.When the funnel outlet width is less than 1.9 times the granular diameter,the funnel will be clogged.In addition,the effects of funnel half-angle,granular density,granular diameter distribution and material friction coefficient on flow rate and clogging behavior were analyzed.In order to solve the clogging problem of granules in the slit funnel,a novel funnel structure,the slit baffle funnel,is designed in this paper.The effects of funnel outlet width,funnel half-angle,and baffle height on the granular flow pattern were analyzed by a combination of simulation and experiment.This new funnel structure was found to be effective in alleviating the clogging in a slit funnel,providing a stable and continuous flow rate even with a small outlet width,and a greater flow rate with a smaller funnel half-angle or a higher baffle height.The granular velocity field inside the funnel is analyzed,and the granular flow pattern inside the funnel can be classified into three regimes: quasi-static flow,surface flow and funnel flow.When the outlet width is small,the granular flow at the edge of the funnel basically stops,similar to quasi-static flow,and this part of the funnel is clogged.The granules flow out from the bottom of the baffle and form a stable surface flow on the right side of the baffle,while causing disturbance to the granules at the lower of the baffle and forming a funnel flow.The flow of a slit baffle funnel consists of both surface flow and funnel flow,which makes it difficult to be clogged.This funnel structure can be adopted in the dense granular target to improve the safety and stability of spallation target.In addition,this funnel structure can be extended to the mining,agricultural production,and pharmaceutical industries to solve the clogging problem of granular matter in the slit geometry.Meanwhile,in the conceptual design of the dense granular target of Ci ADS,the proton beam is introduced into the granular target through the beam pipe.However,the insertion of the beam pipe has a serious impact on the granular flow pattern inside the funnel.In Chapter 5,we initially divide the granular flow region below the beam pipe into four regions: the free surface region,the turbulence-like region,the stable dense flow region,and the surrounding stationary region.The granules flow out from the side of the beam pipe and form a free surface under the combined action of gravity and the interaction forces between the granules.And then,the granules converge at the lowest of the free surface,and some of them hang briefly to form a turbulence-like zone.Near the funnel exit,the granular flow is stable,while the velocity of the surrounding granules is close to zero and the granular flow stops.The effects of beam pipe size,shape and insertion height on the granular flow pattern in the funnel were analyzed by coupling experiments and simulations.When the insertion height of the beam pipe is greater than 55 times of the granular diameter,the effect of the insertion height on the granular flow pattern is almost negligible.The size of the beam pipe has basically no effect on the flow pattern,while the shape of the beam pipe has a greater effect on the flow pattern.The diamond-shaped pipe exit can restrict the flow of particles,so that particles cannot form a turbulence-like region,improving the safety of the spallation target.In this paper,the study of the granular flow stability and phase transition in the structure related to dense granular target was completed by combining physical experiments and DEM simulations.And this study will provide a technical reference for the structural design of dense granular target.