Research On Flow And Heat Transfer And Thermal-Fluid-Structure Coupling Characteristics In Air Cooled Open-Cycle Reactor

The air-cooled open-cycle reactor is with the characteristics of high thermal efficiency,compact structure,and light weight.The reactor is often adopted as the heat source for nuclear engines and has broad application prospects in air transport industry.The reactor operates under high temperature,high and non-uniform power density conditions under the environment with strong parameter coupling effects.The operating environment has a great impact on the Thermal-Fluid-Structure coupling characteristics of air and fuel in the reactor,which will threaten the efficient and safe operation of the nuclear engine.The flow and heat transfer characteristics of air is the basis in the study of Thermal-Fluid-Structure coupling process.At present,although there are many studies on the heat transfer characteristics of air flow,the study on the local heat transfer characteristics of the air in the tube is few.The air flow and heat transfer models are with different forms and the prediction accuracy needs to be improved.In the aspects of Thermal-Fluid-Structure coupling characteristics,the air parameters and heating conditions under multi-factor coupling effects will affect the coupling heat transfer characteristics and the thermal stress distribution characteristics.At present,there is still a lack of systematic research on the mechanism and law of the influence on the related research,which has brought a technical barrier to the thermal safety design of the air-cooled reactor.In view of the above research deficiencies,the research on the flow and heat transfer and Thermal-Fluid-Structure coupling characteristics in the air-cooled reactor is carried out.The main work of present thesis is as follows:Firstly,the experimental method is adopted to study the flow and heat transfer characteristics of air flowing in the tube under direct current heating conditions.The effects of viscous heating and compressibility on the flow and heat transfer characteristics of air are analyzed.The results indicate that the effect of viscous heating effect can be judged jointly by flow velocity and Reynolds number of air.Heating is the dominant factor which will affect compressible flow under low velocity conditions.The effect of physical parameters,Reynolds number and temperature difference between the wall and fluid on heat transfer coefficient are analyzed.The temperature ratio correction coefficient based on the Gnielinski correlation is proposed,which can improve the prediction accuracy of heat transfer capacity under low Nusselt number conditions.The effect of the channel inlet and outlet local parameters and the channel average parameter on the frictional resistance coefficient is obtained.It is found that the change of physical properties and the acceleration process caused by thermal expansion under heating are the main factors which will reduce the accuracy of the frictional coefficient.The correlation can be corrected by using the average film temperature.Secondly,the accuracy of the numerical model was evaluated based on the parameter distribution law and heat transfer correlations obtained in the experiment.The research on the local heat transfer characteristics of air under uniform heat flow was carried out.The results indicate that the Realizable k-ε model can better predict the experimental results.The efficiency and accuracy of numerical results can be improved by using the Coupled algorithm and considering the effect of viscous heating.The effects of variable physical properties,viscous heating and compressibility on air heat transfer characteristics are analyzed.Based on the numerical model,taking the temperature ratio Ts/Tb and the range of Reynolds number into consideration,the applicable range of temperature and pressure of the experimental model is extended.The detailed study on the local heat transfer characteristics under uniform heating conditions is carried out.The distribution characteristic of the local heat transfer coefficient is obtained.The influence mechanism of inlet shape on local parameters and heat transfer coefficients is revealed.What is more,the study on the flow-solid coupled heat transfer characteristics under non-uniform heat flux is carried out based on the verified numerical methods.The influence mechanism of heating form,heating power,inlet air temperature and pressure on the coupled heat transfer characteristics is obtained.The results indicate that the power distribution form has a great influence on the local heat transfer coefficien t.But the effect on the average heat transfer coefficient is less than ±5.5%.The heat transfer coefficient changes smoothly with the increase of power under high heat flux conditions.This is the result of the combined effects of reduced air flow,increa sed velocity and thermal conductivity due to heating.The inlet pressure indirectly affects the heat transfer coefficient by affecting the air flow.The effect of pressure on increasing the heat transfer coefficient is obvious.The heat transfer coefficient increases with the increase of inlet temperature,when the mass flow rate is constant.But the increase rate is small.The distribution characteristics of the parameters at the outlet of the fuel assembly is obtained.The results indicate that the flight altitude and Mach number both affect the core operating pressure and temperature,thereby affecting the air heat transfer capabilities.The heating power and outlet pressure need to be adjusted simultaneously in order to achieve the desired air temperature at the outlet.Finally,based on the fluid-solid coupling model and the analysis results of parameter effect,the thermal-fluid-structure coupling method is used to study the distribution characteristics of thermal stress in the air-cooled reactor.The results indicate that the deformation of the fuel element is large at the position with high temperature,and the thermal stress is large near the position with high heat flux in the axial distribution.Parametric effect analysis shows that the maximum equ ivalent stress under cosine heat flux condition is about 1.4 times large than that of uniform condition,which increases the risky compared to uniform heating conditions.The thermal stress value is directly affected by power level,increasing almost linearly with power.The air inlet temperature has a great impact on the deformation of the fuel pellet,but has little effect on the temperature gradient,therefore,the thermal stress is almost constant and can be ignored.The effect of pressure on thermal stress is more complicated,but the equivalent stress of the fuel pellet decreases slightly when the pressure increases,and the effect of pressure on thermal stress of fuel can be ignored.The research in the present thesis can provide new ideas for the opt imization of heat transfer models of high-speed,compressible,and variable-property gases.The results are helpful for the mechanism analysis of local heat transfer characteristics under non-uniform heat flux.The conclusions can support the research on the coupling heat transfer characteristics and thermal stress distribution characteristics between coolant and fuel under the multi-parameter coupling effect conditions.The data can be provided for the research on the thermal safety characteristics of the open-cycle reactor and the optimization design of the operating scheme.