Estimated Uncertainties in the Idaho National Laboratory Matched-Index-of-Refraction Lower Plenum Experiment

Abstract

The purpose of the fluid dynamics experiments in the MIR (Matched-Index-of-Refraction) flow system at Idaho National Laboratory (INL) is to develop benchmark databases for the assessment of Computational Fluid Dynamics (CFD) solutions of the momentum equations, scalar mixing, and turbulence models for typical Very High Temperature Reactor (VHTR) plenum geometries in the limiting case of negligible buoyancy and constant fluid properties. The experiments use optical techniques, primarily particle image velocimetry (PIV) in the INL MIR flow system. The benefit of the MIR technique is that it permits optical measurements to determine flow characteristics in passages and around objects to be obtained without locating a disturbing transducer in the flow field and without distortion of the optical paths. The objective of the present report is to develop understanding of the magnitudes of experimental uncertainties in the results to be obtained in such experiments. Unheated MIR experiments are first steps when the geometry is complicated. One does not want to use a computational technique, which will not even handle constant properties properly. This report addresses the general background, requirements for benchmark databases, estimation of experimental uncertainties in mean velocities and turbulence quantities, the MIR experiment, PIV uncertainties, positioning uncertainties, and other contributing measurement uncertainties.The accuracy of the velocity field determination is ultimately limited by the ability of the scattering particles to follow the unsteady motion of the continuous phase. For the particles in the MIR VHTR experiment, the estimated cutoff frequency is about 40 kHz. The frequency of shedding from the posts in the MIR model is estimated to be of the order of 1 Hz and, therefore, the particles should follow the flow.Based on review of the literature, we estimate that the random uncertainty in measured particle displacement for our system and experiment is about 0.3 pixel for a single realization away from the walls. As an example, we estimated the uncertainties in measurements for the maximum velocity in an inlet jet flow operating at about its minimum flow rate. Consequently, this example would provide worst-case estimates in some senses. For the uncertainty in the mean velocity at this location, one could report With the current MIR model and the 3D PIV system, McIlroy and colleagues examined the effects of sample size for a typical set of measurements. Trends were generally as observed by Uzol and Camci. Preliminary conclusions were that (for this view and processing parameters) about 750 images should be collected to reduce the scatter in mean velocity statistics to between 0.4 and 10% v for velocities greater than about 2 m/s. The trends predicted by an uncertainty analysis are consistent with the observations, but, for the most part, the expected values are less than the scatter observed. Preliminary conclusions from the comparisons are that (1) more samples should be collected than predict...