Numerical analysis of the stability and transient behavior of natural convection loops

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A general numerical model is developed for analyzing the transient thermal-hydraulic behavior of a single-phase, toroidal, natural convection loop (thermosyphon) connected to a pressurizer. The thermosyphon torus is positioned vertically, with heat sources and sinks distributed over its lower and upper sections, respectively. A set of three basic simultaneous time-dependent conservation equations for 1-dimensional flow is solved, coupled with a lateral momentum equation in one of the volume cells interacting with an external pressurizer. The unknown dependent variables are the mass velocity, enthalpy, pressure and the lateral mass velocity into/from the pressurizer. Other fluid thermodynamic and transport properties are accurately determined using tabulated steam/water table module incorporated in the model. A parametric study is performed over a wide range of operational parameters and angular displacements of the heated and cooled sections, which was impracticable in previous studies. Steady-state, transient behavior, and instability characteristics are studied and discussed. The main conclusions obtained are; (i) The prediction of dynamic instability largely depends on the assumptions related to the heat transfer coefficient and friction factor and their transient dependence on local and instantaneous thermodynamic and transport properties, (ii) In the turbulent flow regime the instability threshold qas found to be sensitive to cooling section wall temperature, (iii) Rotation of the heated/cooled sections in the vertical plane by angles of θ 0>20° about the horizontal axis θ 0= 0 , stabilizes the system's operation for all heat fluxes and wall temperatures. Good agreement is obtained between the present predictions and experimental results available in the literature.