Experimental And Numerical Simulation Investigations On Heat Transfer About Strands In Cross Section Of ITER CICC

The goal of the International Thermonuclear Experimental Reactor(ITER)is to use giant superconducting magnets for constraining the flow of high temperature plasma by generating strong magnetic fields,which could make controlled thermonuclear fusion.And the magnet coil(also known as CICC conductor)is the core component of the ITER device,which directly determines the safety and functionality of the entire system.The CICC conductor is usually composed of thousands of superconducting strands by multistage twist and its cooling through the spiral tube of helium.The electromagnetic force has great influence on the cooling efficiency and thermal stability by the decrease of porosity of CICC conductor.In recent years,the school of mechanics and engineering science of Lanzhou University proposed a method,Discrete Element Method(DEM),which is simplified the section of CICC strands as a particle unit,and equivalent the electromagnetic force as one-way pressure.In this thesis,we use the same method to study the heat transfer characteristics between strands under the uniform pressure.And now due to the traditional method of temperature sensor is difficult working in the particle system in temperature measurement,there are few experimental investigations on the heat transfer of granular system.Therefore,this thesis proposes a method for particle surface temperature distribution fulltime measurement by using fluorescence temperature sensitive paint,combined with the photo-elasticity principle,the heat transfer process under contact force in particles system are studied.And then we try to explore the relationship between heat transfer and vertical external pressure by experiment and numerical simulation of single and multiple particles system,the main results are as follows:(1)The temperature sensitive paints Eu(TTA)3 has good linear relationship between the brightness and temperature in 20-80?,so we prepared and calibrated it appropriately as the basis of our experiment.We built the heat conduction testing system of single particle and multiple particle,and discussed the reliability of the system.Using this system,the heat transfer process of single particle and multiple particle samples under external loading could be measured precisely,and we found that particles heat transfer can be significantly improved by external loading.In particular,for multiple particles system,the heat transfer path is decided by load in different quantity and direction.Finally according to the principle of photo elasticity,we get the conclusion: the heat flow is preferentially transmitted along the direction of the force chain.(2)Using COMSOL simulation software to simulate the heat transfer process of single particle and multiple particle system under external load.The geometric model and boundary conditions in numerical simulation are same with experiment.And the contact heat transfer coefficient between the particles is fitted by the experimental data.The contact area is calculated by the Hertz model.The final simulation results show that the pressure can increase the rate of heat transfer for granular materials.For multiple particle system,the heat transfer path is also consistent with the particle contact force chain.Finally,it is considered that the increase of the contact area by external force,is one of the reasons to improve the heat transfer rate,which could also be the reason of the heat transfer path along the contact force chain of particle system.