Computational and experimental assessment of supercritical natural circulation loop : Steady-state thermalhydraulics and stability aspects

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Despite the mathematical intricacy, the natural circulation loop (NCL) proposes a convenient route of energy and species transport from a high-temperature source to a low-temperature sink, without them in direct contact. The buoyancy force originating from the density gradient is the prime driving force of any natural circulation system. Operating regime of single-phase NCLs is limited by the constraints of saturation temperature and low flow rate, whereas the possibility of dry-out and appearance of different flow regimes with contrasting heat transfer behavior are of great concern in two-phase loops. Supercritical fluid offers a potent alternative due to its good heat transport capability and large volumetric expansion, thereby coupling the advantages of single- and two-phase versions. Accordingly the concept of supercritical natural circulation loop (SCNCL) has evolved in the present millennium as one of the most important initiatives under generation-IV nuclear reactors.Both numerical and experimental appraisal of SCNCL is presented in the current thesis. Thorough numerical investigation have been performed to explore the steady-state, as well as transient, behaviour of SCNCL. The mode of heating is a critical factor on steady-state thermalhydraulics of SCNCL. Both Dirichlet and Neuman type modes are of equal importance, according to the design of system. Thus, several subsequent steady-state studies have been performed, where, heating is envisaged in both constant temperature and constant heat flux modes and cooling is always through a constant temperature sink. To find out a complete observation on the thermalhydraulic of SCNCL, influence of various operating parameters, like system pressure, source and sink temperature, working fluids, inclination angle, heating power, and various geometric dimensions have been studied meticulously. Steady-state analysis of SCNCL begins with the study of thermalhydraulic comparison of water, CO2 and R134a as the working fluids under identical set of operating conditions. Effort is made to identify the best working fluid from heat transfer point of view.
Supervisor: Dipankar Narayan Basu