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[en] A research team at University of Wisconsin - Madison designed and constructed a 1/4 scaled experimental facility to study natural circulation cooling in a water based reactor cavity cooling system (WRCCS) for decay heat removal in an advanced high temperature reactor. The facility is capable of natural circulation operation scaled for simulated decay heat removal (up to 45 kW input power, which is equivalent to 1.70 MW at full scale) and pressurized up to 2 bar pressure. The WRCCS facility has been used to study natural circulation flow behavior from transients due to evaporation of the water inventory from decay heat removal. The natural circulation flow is observed to be oscillatory with the mean flow increasing as the pressure head in the system drops due to evaporation of the water inventory during two-phase operation. A first principles model was developed to understand the behavioral limits and trends. Through the use of void fraction measurements and the model, we find that the mass flow rate in the system increases due to small increases in the time-averaged void fraction over the course of a four-hour boil-off experiment. This increase in void fraction causes an increase in the buoyancy head and mass flow. The model also determines the operational limits on the mass flow rate under two-phase operation that can be verified with extended tests in the WRCCS facility. (authors)
[en] Highlights: • Experimental validation of flow transition instabilities. • A method for indicating the presence of the flow transition instability. • Recommendations for the removal of flow transition instabilities in RCCS type facilities via proper design. • Steady-state drift-flux model to characterize the occurrence of a flow pattern instability. - Abstract: Our research analyzed results from a two-phase natural circulation test facility designed to study the performance of reactor cavity cooling systems, and determined the presence of a flow transition instability. Reactor cavity cooling systems are passive safety system’s designed to remove decay heat from high temperature generation IV reactors. Water-based designs achieve this function by boiling-off a set water inventory held in a storage tank. During normal operation under accident conditions, the two-phase natural circulation facility undergoes a transition from stratified to intermittent flow regimes as the water level in the storage tank is lowered due to inventory evaporation. The transition is accompanied with an oscillatory flow rate due to the differing pressure drops between the flow regimes. The instability ceases after fully transiting into the intermittent flow regime. The presence of the transition was determined with a steady-state drift flux model and wire-mesh sensor data.
[en] This project has been focused on the experimental and numerical investigations of the water-cooled and air-cooled Reactor Cavity Cooling System (RCCS) designs. At this aim, we have leveraged an existing experimental facility at the University of Wisconsin-Madison (UW), and we have designed and built a separate effect test facility at the University of Michigan. The experimental facility at UW has underwent several upgrades, including the installation of advanced instrumentation (i.e. wire-mesh sensors) built at the University of Michigan. These provides high resolution time-resolved measurements of the void-fraction distribution in the risers of the water-cooled RCCS facility. A phenomenological model has been developed to assess the water cooled RCCS system stability and determine the root cause behind the oscillatory behavior that occurs under normal two-phase operation. Testing under various perturbations to the water-cooled RCCS facility have resulted in changes in the stability of the integral system. In particular, the effects on stability of inlet orifices, water tank volume have and system pressure been investigated. MELCOR was used as a predictive tool when performing inlet orificing tests and was able to capture the Density Wave Oscillations (DWOs) that occurred upon reaching saturation in the risers. The experimental and numerical results have then been used to provide RCCS design recommendations. The experimental facility built at the University of Michigan was aimed at the investigation of mixing in the upper plenum of the air-cooled RCCS design. The facility has been equipped with state-of-the art high-resolution instrumentation to achieve so-called CFD grade experiments, that can be used for the validation of Computational Fluid Dynanmics (CFD) models, both RANS (Reynold-Averaged) and LES (Large Eddy Simulations). The effect of risers penetration in the upper plenum has been investigated as well.
[en] Highlights: • Void fraction uncertainty via measurement with wire-mesh sensors is less than 11% relative to secondary measurement methods (e.g. radiative, high speed camera…) regardless of flow regimes. • An overview of the algorithms available for the measurement of bubble size, bubble volume, interfacial area, and velocity are presented. • Wire-mesh sensors are found to be applicable to most flow regimes for the measurement of void fraction, but secondary measurement applicability (velocity, bubble size, and interfacial area) are affected by the intrusiveness of the sensor, sensor physical dimensions, and flow parameters. - Abstract: Void fraction has always been an important parameter in the study of multiphase flows and its measurement has proven difficult over the years. This paper is a state of the art review of the application of conductivity based wire-mesh sensors (WMS) for the measurement of void fraction, bubble size, and gas fraction velocity in multiphase flows and their associated uncertainties. At this point in time there is no golden standard for void fraction measurement, so a large bulk of this work is on the uncertainty of the WMSs relative to other void fraction measurement methods, namely radiative methods. It is shown using the available data that the WMS have a void fraction measurement uncertainty of ±10.5% over a variety of flow regimes relative to other measurement methods. However, the accuracy of the instrument is largely based on its applicability to a particular flow. For example, the WMS is an excellent choice when entrapment in the sensor due to surface tension is minimized resulting in best results at higher flow rates compared to radiative methods. An assessment into the uncertainty of velocity and bubble size measurements is also performed: analyzing the current algorithms available and studies on these measurements in comparison with high speed cameras and ultrafast X-ray tomography. The current functioning form of the wire-mesh sensors were developed by Prasser in 1998 as a tomographic technique for the measurement of void fraction using a conductivity approach, as performed by earlier researchers. Later developments with the senors resulted in various techniques that allow for the measurement of velocity and interfacial area concentration.