2020 Volume 7 Issue 3 Pages 19-00578
Two-phase thermosyphons, which use sodium at 960 K as a refrigerant for heat transportation in small nuclear reactors, are promising for manned exploration to Mars. This is because the concentric-tube type thermosyphon may not have a flooding limit, and so the heat transfer performance per unit volume is comparatively large. Moreover, since its external form is a single tube by inserting a pipe through which the liquid flows into a pipe through which the gas flows, the reactor core can be made smaller. Experimental investigations of heat transport characteristics using water, R113, ethanol, and nitrogen as refrigerants and development of prediction formulas are progressing for establishing the design of the concentric-tube two-phase thermosyphon. However, it is necessary to use sodium at 960 K for application to heat transportation of a reactor, and further elucidation of the flow phenomenon is necessary to establish its performance prediction model. We have proposed a model based on bubble pump theory in order to take into consideration the rise of liquid level of the heating section. We conducted an experimental study on the flow inside a thermosyphon made of transparent material, and evaluated the maximum heat transfer rate using a low boiling point refrigerant, HFE-7100. As a result, even when the heat transfer rate was close to the maximum, it was shown that the flow regime in the outer tube of the adiabatic section of the concentric tube is two-phase flow. However, the experimental value of the maximum heat transfer rate was found to be about 17 % smaller than the calculated value. We therefore investigated the cause of this difference by making pressure measurements and flow observations, and found that bubble entrainment in the inner tube of the concentric tube of the adiabatic section greatly influences the pressure distribution. It is thought that taking mixing into consideration would help improve the model.