Special Instrumentation for Two-Phase Flow

The design of engineering systems, involving two-phase flow, such as nuclear water reactors, requires the ability to model and predict the detailed behavior of those flows and the phenomena that they manifest, with a required degree of accuracy. In the past a significant amount of effort has been addressed to the development of intrusive and nonintrusive measurement techniques of two-phase flows, with special application to the determination of mass flow rates. Many extensive experiments are being performed to investigate in detail loss of coolant accidents (LOCA). During these experiments the coolant is released as a two-phase mixture through a simulated break of a coolant pipe, and the measurement of the mass flow rate of the two phases was required to analyze the accident evolution and consequences. In this context, most flow meters have been designed to measure the single-phase flow of a Newtonian fluid, and then used to measure quantities in more complex fluids. The key to fundamental understanding of two-phase flow is still the careful development of specialized instrumentation, in particular for special and complex geometrical applications. Within the framework of an Italian R&D program on Nuclear Fission, supported by the Ministry of Economic Development, the SPES3 experimental facility, able to simulate the innovative small and medium size PWR nuclear reactors, is being built and will be operated at SIET Company laboratories. In such facility some design and beyond design basis accidents, like LOCAs, with and without the emergency heat removal systems, will be simulated. In most accident simulations, a two-phase flow mixture will occur in the lines, during the transient evolution, due to the simulated strong depressurization of the system. An accurate accident analysis requires the measurement of the mixture mass flow rate and for this reason, instruments and methodologies to evaluate different two-phase flow parameters need to be developed. Typically a set of instruments (Spool Piece - SP) must be installed in order to evaluate the mass flow rate of the phases in a large range of flow patterns, pressures and temperatures. An ideal SP is a control volume constituted by different measurement instruments, fed with a two-phase flow. In single-phase flow, each instrument is able to measure a well defined flow parameter, while the instrument signal interpretation, in two-phase flow, is not easy due to the different flow patterns and the to the large number of parameters that influence the flow, so that a model of the SP, depending on the geometry and on the SP orientation, is required. Moreover the selection of the instruments strongly depends on the experimental conditions: pressure, temperature and phases velocities. The thesis work consists in the development of special instrumentation and in the development of models, based on the analysis of experimental data, that are able to interpret the measurement signals for many possible two-phase conditions. The two different measurement fields, internal flow structure investigation and instrument modeling for phases mass flow rate reconstruction purposes, have been analyzed. In the first field the instrumentation must be able to characterize momentum, mass, energy balance with a resolution sufficient to investigate local phenomena and characteristic structure (interface evolution, void profiles, liquid film level, characteristic frequencies, etc..). The investigation of an horizontal two-phase flow has been performed by means of a Wire Mesh Sensor. Local, chordal, cross-section void fraction values are derived from the sensor data in a wide range of phases superficial velocities, and a new signal methodology, able to characterize the flow in terms of phases distribution (flow patterns) and time evolution, has been developed. Moreover the methodology allows the extraction of important flow information, such as the local and time average void fraction, the interface evolution, and characteristic frequencies. The evolution of the void fraction profiles has been related to the superficial velocity of the two-phases (Jg and Jl) and the flow evolution in time and space has been analyzed and discussed, showing that such methodology is useful to identify and characterize in detail the two-phase flow. Concerning the second measurement field, the analysis of the instruments used for two-phase flow measurement applications has been described. This bibliographic research allowed the definition of the candidate instruments suitable to be installed in a nuclear safety experimental facility, and their measurement characteristics. The selection of the candidate instruments has been made defining some fundamental criteria that should be satisfied: range of measurement, dynamic response, installation requirements, materials/electrical compatibility with pressure and temperature conditions, flow velocity compatibility. The selected instruments have been experimentally studied in different pipe configurations, and different models have been developed for each one. Different instrument combinations have been tested, and the performance of each one has been analyzed in terms of estimation of the mass flow rate of the two phases. The performed research allows the identification of the advantage and drawbacks of the different instrument combinations, and the identification of the phases mass flow rate measurement accuracy achievable for each SP configuration.