Inside-pipe heat transfer coefficient characterization of a thermosyphon-type heat pipe suitable for the reactor cavity cooling system of the Pebble Bed Modular Reactor
The feasibility of a closed loop thermosyphon for the Reactor Cavity Cooling System of the Pebble Bed Modular Reactor has been the subject of many research projects. One of the difficulties identified by previous studies is the hypothetical inaccuracies of heat transfer coefficient correlations available in literature. This article presents the development of an inside-pipe heat transfer correlation, for both the evaporator and condenser sections, that is specific to the current design of the RCCS. A one-third-height-scale model of the RCCS was designed and manufactured using copper piping and incorporating several strategically placed sight glasses, allowing for the visual identification of two-phase flow regimes and an orifice plate to allow for forward and reverse flow measurement. Twelve experiments, lasting at least 5 hours each, were performed with data logging occurring every ten seconds. The experimental results are used to mathematically determine the experimental inside-pipe heat transfer coefficients for both the evaporator and condenser sections. The experimentally determined heat transfer coefficients are correlated by assuming that the average heat flux can be described by a functional dependence on certain fluid properties, the average heat flux is directly proportional to the heat transfer coefficient and that the heat transfer coefficient is a function of the Nusselt number. The single-phase inside-pipe heat transfer coefficients were correlated to 99% confidence intervals and with less than 30% standard deviation from experimental results. The generated correlations, along with identified and established two-phase heat transfer coefficient correlations, are used in a mathematical model, with experimental mass flow rates and temperatures used as input variables, to generate theoretical heat transfer coefficient profiles. These are compared to the experimentally determined heat transfer coefficients to show that the generated correlations accurately predict the experimentally determined inside-pipe heat transfer coefficients.