Natural, mixed and forced convective heat transfer implementation in Flownex® for molten salt internal flow through vertical and horizontal tubes

Natural Convection driven decay heat removal systems are presently used in many of the proposed Gen-IV Reactor designs including IMSR400. The transition from forced to natural circulation flow regime and the thermal-hydraulic transient response modeling is essential to confirm the predicted performance of such systems involving heat removal through natural circulation. The present work discusses the customized script development and implementation of the natural, mixed, and forced convection heat transfer for the horizontal and vertical flow of molten salt inside tubes in Flownex®. Flownex® is a 1-D integrated systems thermal-hydraulic code extensively validated and verified in the nuclear industry and more specifically for the Pebble Bed Modular Reactor (PBMR). Although Flownex® has in-built correlations to calculate the natural convective heat transfer over horizontal and vertical cylinders it does not have the same for flows inside tubes. To overcome this limitation, relevant correlations are identified for the natural convection in internal flows from publicly available literature and methods are implemented based on the combination of Aicher Number (Ai) and Richardson number (Ri) to identify the natural, mixed, or forced convective heat transfer regimes. The enhancing or diminishing effect of the natural convection heat transfer is also determined and polynomial interpolation is used to calculate the Nusselt number (Nu) for the mixed convection regime. Verification of the methodology is carried out using experimental data available in literature for molten salt natural circulation and it shows that the Reynolds number and temperatures during steady state and transients match within 20% of experimental data. (author)