S. Lomperski

Proposed experiment for validation of CFD methods for advanced SFR design: Upper plenum thermal striping and stratification

In response to the goals outlined by the U.S. Department of Energy’s Advanced Fuel Cycle Initiative, an effort is underway to develop an integrated multi-physics, multi-resolution thermal-hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, initial guidance has been proposed for the development of experiments to supply validation data sets for the CFD-based thermo-fluid simulation capability. To demonstrate that the proposed data requirements can be achieved using current generation measurement methods and to refine correlation and data comparison methods suitable for very large data sets, an initial experiment focused on turbulent mixing in the upper plenum of an advanced sodium fast reactor has been proposed. Prior validation efforts to support the use of one-dimensional lumped parameter models in the analysis of reactor safety performance relied primarily on data from carefully scaled integral system experiments to validate and tune correlations used to represent the physics associated with a particular transient in a particular reactor design. Unlike the correlation-based lumped parameter codes, computational fluid dynamics simulations reduce the reliance on experimentally derived correlations to the prediction of local turbulence effects rather the prediction of integral quantities like pressure drop and heat transfer coefficients. As a consequence, simpler separate effects experiments, which capture the turbulence effects but not necessarily the integral effects within a specific component of a system, can be utilized as the primary validation basis for the CFD codes. However, while the need for large carefully scaled integral experiments is reduced, the high spatial and temporal resolution of these codes requires that experimental data be collected at fine spatial and temporal resolutions. An initial series of simulations has been completed to support the development of the proposed experimental facility using air as a surrogate for the sodium coolant. Design options considered in RANS simulations using the commercial CFD code Star-CCM+ include mixing facility dimensions, the number of inlet jets to be included and outlet position. The use of RANS simulations is supported by an initial benchmarking comparison with predictions from the spectral element large eddy simulation code Nek5000 for the nominal experimental geometry.© 2009 ASME

Fiber Optic Distributed Sensors for High-resolution Temperature Field Mapping

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists chiefly of velocity and temperature readings, both ideally having high spatial and temporal resolution to facilitate rigorous code validation. While high resolution velocity data is readily obtained through optical measurement techniques such as particle image velocimetry, it has proven difficult to obtain temperature data with similar resolution. Traditional sensors such as thermocouples cannot fill this role, but the recent development of distributed sensing based on Rayleigh scattering and swept-wave interferometry offers resolution suitable for CFD code validation work. Thousands of temperature measurements can be generated along a single thin optical fiber at hundreds of Hertz. Sensors function over large temperature ranges and within opaque fluids where optical techniques are unsuitable. But this type of sensor is sensitive to strain and humidity as well as temperature and so accuracy is affected by handling, vibration, and shifts in relative humidity. Such behavior is quite unlike traditional sensors and so unconventional installation and operating procedures are necessary to ensure accurate measurements. This paper demonstrates implementation of a Rayleigh scattering-type distributed temperature sensor in a thermal mixing experiment involving two air jets at 25 and 45 °C. We present criteria to guide selection of optical fiber for the sensor and describe installation setup for a jet mixing experiment. We illustrate sensor baselining, which links readings to an absolute temperature standard, and discuss practical issues such as errors due to flow-induced vibration. This material can aid those interested in temperature measurements having high data density and bandwidth for fluid dynamics experiments and similar applications. We highlight pitfalls specific to these sensors for consideration in experiment design and operation.