Flow and heat transfer in a closed loop thermosyphon. Part I – theoretical simulation

A natural circulation, closed loop thermosyphon can transfer heat over relatively large distances without any moving parts such as pumps and active controls. Such loops are thus considered suitable for nuclear reactor cooling applications where safety and high reliability are of paramount importance. A theoretical basis from which to predict the flow and heat transfer performance of such a loop is present-ed. A literature survey of the background theory is undertaken and the theoretical equations describing the single and two-phase flow as well as heat trans-fer behaviour are given. The major assumptions made in deriving these equations are that the work-ing fluid flow is quasi-static and that its single, two-phase flow and heat transfer behaviour may be cap-tured by dividing the working fluid in the loop into a number of one dimensional control volumes and then applying the equations of change to each of these control volumes. Theoretical simulations are conducted for single phase, single and two-phase and heat pipe operating modes, and a sensitivity analysis of the various variables is undertaken. It is seen that the theoretical results capture the single and two-phase flow operating modes well for a loop that includes an expansion tank, but not for the heat pipe operating mode. It is concluded that the theo-retical model may be used to study transient and dynamic non-linear effects for single and two-phase modes of operation. To more accurately predict the heat transfer rate of the loop however, loop specific heat transfer coefficients need to be determined experimentally and incorporated into the theoretical model