Experimental and Theoretical Study of Oscillatory Two-Phase Flows with Heat and Mass Transfer

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This dissertation presents the experimental and theoretical analysis of oscillatory two-phase flows and heat transfer on the liquid entrainment behavior. In a hypothetical large break loss of coolant accident (LOCA), associated with a break on one of the cold legs, the emergency core cooling system (ECCS) must provide sufficient coolant to the core to remove decay heat and prevent the cladding from exceeding 1477.6 K. During reflood, flow to the core is gravity driven, resulting in an oscillatory delivery of coolant to the core.  These oscillations have been attributed to vapor generation in the core and the dynamic response of the downcomer water level.  Most reflood experiments have been conducted with constant forced reflood rates, and have not considered the effect of oscillations on rod bundle thermal-hydraulics.  The few studies that have been conducted for oscillating flow indicate enhanced entrainment of liquid at the quench front.  While higher entrainment can provide precursory cooling ahead of the quench front, it can also expel more coolant out of the system for the oscillatory reflood.  The amount of liquid entrained can be significant because in an accident scenario, the quench front rate will be slowed, and it can take longer to fully recover the core. At the NRC/PSU Rod Bundle Heat Transfer (RBHT) Test Facility, an electrically heated 7 x 7, 3.66 m (full-length) rod bundle array has the capability to perform both constant and oscillatory forced flooding rate experiments. The facility is heavily instrumented, equipped with seven spacer grids with which to analyze the droplet and heat transfer phenomena. This study includes the analysis of separate effects of the oscillation period (2 to 10 s) and magnitude (0.0254 to 0.0762 m/s), pressure (0.14 to 0.41 MPa), and subcooling (5 to 55 C) on the entrained droplet dynamics and heat transfer under constant and oscillatory flow conditions. 

This dissertation includes the analysis of forced oscillatory reflood experiments, which concentrate on the effects of flow rate (frequency, magnitude, and nominal flooding rate of the flow oscillations), pressure, subcooling, and bundle fouling on heat transfer and entrained droplet dynamics. To the author's knowledge, experiments with the parameters explored in this work, coupled with the rod bundle geometry, have not been explored previously. This dissertation utilizes the unique dataset to perform a stability analysis of oscillatory two-phase flows which incorporates Floquet and viscous-potential flow theories. In addition, a correlation is developed for the liquid entrainment, which uses parameters derived from the stability analysis, to predict the entrainment of oscillatory flows. The entrainment model is validated using RBHT data, as well as other relevant rod bundle data, and the USNRC's TRACE code. 

 

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