Numerical Study On Thermal Characteristics Of Assembly With Wrapped-wire

Wire wrapped fuel assemblies are widely used in fourth-generation nuclear reactors.As the core component of the reactor,the existence of wrapped-wire not only plays a role in fixing and supporting fuel rods,but also complicates the flow field distribution.The analysis of its thermal and hydraulic characteristics is crucial to the safety and economy of the reactor.Due to the complexity of its structure,some difficulties to thermal hydraulic analysis are brought about.In this paper,the SST k-ω turbulence model and commercial CFD software are used to perform thermal and hydraulic analysis and unidirectional fluid-structure analysis of fuel assemblies with different numbers of wires.The commercial software ICEM is used to divide the entire model’s fluid domain and solid domain using a structured hexahedral mesh,and the model is solved by CFX.In order to obtain accurate and reliable calculation results,the grid independence verification of the model is performed to obtain grid independent solutions firstly.The calculated results are compared with the benchmark model of ANL.After meeting the basic requirements of the model,fuel assembly models with different numbers of wires are designed.Wrapped wires of four models have the same diameter,pitch and period of rotation,which controls a single variable for easy observation.Through numerical simulation,the pressure drop,axial flow,cross flow,vortex flow of each channel,temperature distribution in sub-channels,temperature hot spots and equivalent thermal stresses in the axial direction of the fuel cladding are mainly studied.Unlike most studies in the country and abroad,this article focuses on the effects of different numbers of wrapped wire on the flow and heat transfer of fuel assembly components.The results show that with the increase of the number of wires,the fluid velocity fluctuation range increases,the direction is the same as the wrapped-wire direction.At the same time,a strong separation flow occurs behind the wire.According to the difference between the adjacent rod bundle and the cladding wall surface,it can be divided into three channels:edge subchannel,corner subchannel and interior subchannel.The velocity fluctuation range of the fluid increases,and the lateral flow becomes more intense.Four-wire model is the largest,which is about twice the value of the lateral flow of the single-wire model.The fluid is divided into two parts by the wire-wrapped.The fluid flowing in the wire-wrapped direction is compressed and the pressure drop increases.The pressure drop loss abruptly decreases on the back of the wire.Uneven pressure on two sides of the wire is responsible for the lateral flow.The vortex is generated after the fluid flows around the wire.Position of the wire corresponds to the size and number of different vortices.In the dense wire rod bundle,the vortex phenomenon is more obvious,the number of vortices in the interior channel is increasing,and the edge channel is easy to form large-scale vortices.With the number of wires increasing,on the one hand,the lateral flow velocity is improved,the distribution is more uniform and thermal stress is reduced.Fluid-structure interaction results also verify this point.On the other hand,it also increases the pressure drop and the friction coefficient,additionally,it reduces the core thermal efficiency.Thus the number of wrapped-wire around the fuel rod should not be excessive.