Modelling of a Coupled Radiation-conduction Heat Transfer through a Heat Shield in Vacuum Thermal Isolation Applications

ITER (International Thermonuclear Experimental Reactor) is currently in design phase of the nuclear fusion research project, which is the world's largest experimental tokamak reactor. Process of plasma generation in the Vacuum Vessel requires an ultra-high quality of vacuum. To meet those requirements the Vacuum Vessel and adjacent diagnostic systems are exposed to elevated temperature after its assembly to outgas all volatile surface contaminants. The baking temperature peaks at 240°C for up to 24 hours. On the other side most of diagnostic instruments and detectors adjacent to the Vacuum Vessel, which are exposed to the same conditions, require much lower temperature during measurement and slightly higher temperature in a stand-by mode. The strictest conditions are defined for liquid scintillators, which require the working temperature around 20°C and the temperature of destruction several degrees higher. In order to keep diagnostic instruments within required temperature range, a heat shield with a heat removal is necessary. The temperature range is often ± 2°C or even more narrow in time and space allotted for detectors. Due to space limitations only a thin multilayer solution, including vacuum layers, can be considered. Preliminary calculations indicate that to meet the above defined requirements, only a water jacket as the heat shield can be efficient enough to keep detectors at the required temperature level and within defined range. An additional multilayer vacuum insulation can significantly decrease the cooling demand for the water jacket cooling system. The aim of proposed paper is to find an optimal structure of the insulation thermal shield, based on the water jacket solution by using the CFD simulation in ANSYS Fluent software. Moreover an extended Parametric Analysis is carried out and discussed. The Surface-to-Surface (S2S) model is applied to simulate heat transfer by radiation. In addition the paper contains a discussion about multilayer solutions, characterized by a high thermal resistance, and dedicated particularly for vacuum transfer lines with high temperature difference.