| For the reason that the heat transfer coefficient in the rectangular narrow channel is much higher than that in the conventional channel,the enhanced heat transfer technology based on the rectangular narrow channel is widely applied to fields,such as the cooling of core components in nuclear reactor.The flow resistance and heat transfer characteristics of rectangular narrow channels are different from those of circular narrow channels.Under the accident conditions of the reactor with plate fuel elements,such as loss of flow accident,blockage accident and small fracture accident,the coolant flow in the rectangular narrow channel decreases,and there is a transition flow stage in the process of fluid transition from turbulent flow to laminar flow.At the present stage,due to the lack of relevant researches,linear interpolation method is adopted to deal with the transition flow in the rectangular narrow channel,which will introduce great uncertainty to the calculation of flow and heat transfer in the rectangular narrow channel,resulting in limited accuracy of the system design.In order to study the flow and heat transfer characteristics of laminar-turbulent transition flows in rectangular narrow slit channels,this paper simulates the rectangular channels with different narrow slit widths,and considers the influence of inlet conditions and geometric conditions on the transition flows.Based on the simulated data,the laminar flow-transition flow-turbulence flow relation in the rectangular channel was fitted,and the RELAP5 program source code was modified by using the fitting relation,which was applied to the transient calculation in the IP200 reactor accident condition.In order to simulate the transition flow in the rectangular narrow channel,the best model for predicting the internal transition flow should be selected from the existing transition flow models.In this paper,ANSYS-Fluentl9.0 software for fluid mechanics calculation was used to simulate the flow of a round pipe with a diameter of 0.004 m and a pipe length of 1.6 m,and the Reynolds number was between 1200 and 8000.To evaluate the applicability of each model and select the best model,the results predicted by the model were compared with the results of experimental data and theoretical formula calculation.Through comparative analysis,it can be seen that among the existing transition flow models,the original transition SST model can best meet the calculation needs of the transition flow in the pipe while meeting the calculation accuracy.In order to study the flow characteristics of the internal transition flow,the original transition SST model was used to further simulate the internal flow of the circular tube at a Reynolds number of 1200-8000,and to analyze the friction resistance coefficient of the fluid along the flow direction and the turbulence intensity distribution and velocity distribution on different sections.The simulation results show that the velocity distribution and turbulence intensity distribution in the transition flow have different characteristics compared with the laminar flow and turbulence.On the basis of the above work,the original transition SST model is used to simulate the rectangular narrow channel.In order to study the influence of different inlet conditions and narrow slit widths on the flow and heat transfer of the fluid in the development section containing the inlet area,under the condition of Reynolds number 1670-5970,the inlet turbulence was selected as 1%or 5%,and the rectangular channel with narrow slit width of 0.001m to 0.002m was simulated.The results show that:(1)starting from the entrance of the rectangular narrow channel,the local nusselt number of the fluid drops sharply until it reaches a minimum value;(2)at the transition flow,the larger the Reynolds number,the higher the heat transfer coefficient of the fluid and the smaller the length of the inlet section needed to reach full development;(3)the lower the inlet turbulence intensity,the lower the local nusselt number along the flow direction under the same Reynolds number condition,and the higher the laminar critical Reynolds number of the fluid;(4)the smaller the narrow slit width is,the shorter the relative inlet length required for full development is,and the higher the nusselt number is for full development of non-laminar flow zone.Compared with a rectangular slit channel simulation results and empirical formula calculation results found that the current is used to calculate the relation between the friction resistance coefficient and the interpolation of rectangular slit channel of internal laminar flow and turbulent transition flow calculation error in the acceptable range,while empirical formulae used to calculate the heat transfer coefficient calculation error is bigger,so need in a rectangular slit channel with laminar flow,transition flow and turbulent heat transfer equation for fitting.After fitting the laminar flow-transition flow-turbulent flow heat transfer relation in the rectangular channel,the fitting relation and Blasius relation were used to modify the source code of the thermal hydraulic module of RELAP5 program,and the reactor IP200,a plate fuel element,was taken as the object to compare the calculation results before and after source code modification.Through comparative analysis,it is found that the simulated data of IP200 reactor operating under the accident condition are quite different before and after the modification of the program source code.This paper gives some suggestions on the model selection of flow simulation for internal transient flow.The different characteristics of transition flow compared with laminar flow and turbulent flow are explained.The effects of different inlet velocities,narrow slot widths of rectangular channels and inlet turbulence intensity on the flow and heat transfer characteristics of the transition flow in rectangular narrow slot channels were analyzed.The laminar-transitional-turbulent relation of rectangular channel was fitted,and the source code of RELAP5 program was modified by using the fitting relation.The simulation results of IP200 reactor by RELAP5 program before and after the modification of thermal hydraulic model in the source code were compared.This paper is of great significance to the selection of relevant models for the simulation of internal transition flow,the understanding of the influence trend of different factors on the flow and heat transfer characteristics of transition flow,and the development of the laminar-transition-turbulent heat transfer model for rectangular channels. |
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