Piyush Sabharwall, Donald M. McEligot, and Carl M. Stoots*
Idaho National Laboratory, Idaho Falls, Idaho 83415‑3730
The Matched‑Index‑of‑Refraction Flow Facility at the Idaho National Laboratory has a unique capability to contribute to the development of validated computational fluid dynamics codes through the use of state-of-the-art optical measurement techniques, such as laser Doppler velocimetry, particle tracking velocimetry, and particle image velocimetry. The fluid/solid refractive index matching technique allows optical access in and around geometries that would otherwise be impossible, while the large test section of the Idaho National Laboratory system provides better spatial and temporal resolution than other matched‑index‑of‑refraction facilities. Benchmark data for assessing computational fluid dynamics can be acquired for external flows, internal flows, and coupled internal/external flows for better understanding of physical phenomena of interest.
The matched‑index‑of‑refraction (MIR) flow system was modeled after a typical wind tunnel and was designed and fabricated by collaboration between scientists and engineers at Idaho National Laboratory (INL) and Universität Erlangen‑Nürnberg. The facility's key objectives are to understand fundamental physical phenomena better and to provide experimental data for assessment and validation of computational fluid dynamics and nuclear reactor system safety codes. The MIR facility permits non‑intrusive velocity measurement (state‑of‑the‑art) techniques—such as particle image velocimetry (PIV), particle tracking velocimetry (PTV), and laser Doppler velocimetry—through complex models/geometries without requiring probes and other instrumentation that could disturb the flow.[1,2] The large MIR system enables high spatial and temporal resolution. Optical refraction is a common phenomenon, often leading to distorted views. In this facility, the flow models are constructed from fused quartz and submersed in temperature‑controlled mineral oil (the working fluid). Quartz and mineral oil have similar refractive indices (near room temperature); thus, the model optically disappears, making high‑resolution optical measurement possible. Figure 1 demonstrates how refractive index‑matching causes the quartz to disappear in the mineral oil.
Figure 1. Refractive index‑matching of quartz and mineral oil: (a) quartz outside the oil and (b) quartz while submersed in the oil.
Description of the Facility
The MIR Facility was designed with an isothermal capability of sustaining a precise temperature in its test section. The facility consists of several components, including a settling chamber, square contraction, test section, and a separate auxiliary loop to provide independently controllable internal flow to installed models, as shown in Figure 2. The settling chamber is comprised of a single, stainless‑steel honeycomb structure and several screens that straighten the flow, reduce turbulence, and remove non‑uniformities. The 4:1 square contraction is attached downstream of the settling chamber and produces nearly uniform flow at the entrance of the test section. The test section is made from a polycarbonate material and contains large, optical glass windows to permit PIV and laser Doppler velocimetry measurements of the flow. The large size of the test section provides high spatial resolution that makes it easier to obtain accurate data near the wall, which is not easy with smaller‑scaled facilities.
The refractive index‑matching temperature of the fluid is maintained within ±0.05°C of the prescribed index‑matching temperature by an external control system. Table 1 displays the technical specifications of the MIR system.
Table 1. Technical specifications of the INL MIR system.
Measurement Technique and Uncertainty
Instantaneous velocity field measurements are primarily obtained with a stereo PIV system. Two charge‑coupled device cameras are mounted on a three‑directional traverse system that is controlled by three separate electric stepping motors. The PIV system uses double‑pulsed, neodymium‑doped yttrium aluminum garnet lasers that are usually mounted below the experiment model and produce vertical light sheets approximately 1–3 mm thick. The three‑directional traversing mechanism is mounted on rails parallel to the test section and is shown in Figure 3. This traversing system has a positional accuracy of ±2 µm (7.87 × 10-5 in).[2,3] Figure 3 shows the MIR flow system with the 3‑D PIV system mounted on the three‑directional traverse.
Figure 3. MIR flow system used to study fluid physics phenomena with a 3‑D PIV system mounted on a three‑directional traverse.
MIR results are typically in the form of three‑component instantaneous and/or time‑averaged velocities and Reynolds stresses. Two independent methods for computation of uncertainty from two‑component PIV measurements are still being developed to quantify uncertainty and obtain accurate and reliable experimental data. These methods are the uncertainty surface method and the cross‑correlation method (signal‑to‑noise ratio method). In the uncertainty surface method, an algorithm is tested to determine its response to various uncertainty contributors; in the cross‑correlation method, the magnitude of the correlation peak is quantified to determine the uncertainty (INL/EXT-12-27728, INL report).
Previous experiments performed in the MIR Facility focused on external flows (fluid flow over and around a body), internal flows (fluid flow inside of a body), and coupled internal/external flows, leading to the generation of experimental data for validation purposes and to better understanding of the physical phenomena of interest. Review of all of the previous studies and the current ongoing study is available at the MIR website (https://mir.inl.gov). Previous studies conducted in the MIR flow system include: "Measurements of Fundamental Fluid Physics of Spent Nuclear Fuel Storage Canisters," "Transition Induced by a Square Rib," "Convective Processes in Spent Nuclear Fuel Canisters," "Fundamental Thermal Fluid Physics of High Temperature Flows in Advanced Reactor Systems," "Advanced Computational Thermal Fluid Physics (CTFP) and its Assessment for Light Water Reactors and Supercritical Reactors,"[9,10] "The Boundary Layer Over Turbine Blade Models with Realistic Rough Surfaces," "Particle Image Velocimetry Measurements in a Representative Gas Cooled Prismatic Reactor Core Model For the Estimation of Bypass Flow," and "Criteria for Boundary Layer Transition."
In one of the studies carried out in the MIR system that was focused on flows over the turbine blades, McIlroy and Budwig made extensive boundary layer measurements over a flat, smooth plate model with non-dimensional parameters as on the front one‑third of a first stage turbine blade from a high‑pressure gas turbine engine and over the same model with an embedded strip of a realistic rough surface, as shown in Figure 4. The realistic rough surface was developed by scaling
actual turbine blade surface data provided by the U.S. Air Force Research Laboratory by a factor of about 100, as shown in Figure 5. The boundary layer remained unstable (transitional) throughout the entire length of the test plate, and the rough patch caused the Reynolds shear stresses to increase in the region close the plate surface.
Figure 4. Quartz plate model with the rough patch shows the quartz is barely visible to the naked eye.
Figure 5. Displacement (d*) and momentum thicknesses (q) increase over the rough patch (repesentative of turbine blade) with rough patch profile (right axis).
As another example, a very high temperature nuclear reactor lower plenum study provided an extensive benchmark (~2Tb of data) for computational fluid dynamics code validation. A section of the lower plenum, which included four inlet jets from the simulated reactor above, was modeled of machined quartz, as shown in Figure 6. Two cameras slightly off angle from one another captured 3‑D PIV data of various flow measurements at two Rejet numbers (Re= 4,300 and Re = 12,400), as shown in Figure 6. The measured quantities included: streamwise‑normal velocity, spanwise velocity, streamwise velocity (as shown in Figure 7), average turbulence intensity, and average turbulence kinetic energy. An example is shown in Figure 7; the quartz‑to‑mineral oil refraction index matching allowed for entire vector maps to be studied and recorded.
Figure 6. Quartz model of a very high temperature nuclear reactor lower plenum with inlet jets at the upper right.
Figure 7. Mean streamwise velocity of a very high temperature nuclear reactor lower plenum at a horizontal slice at x = 160 mm from the top surface of the lower plenum (as shown on lower right).
Recent research conducted measurements of bypass transition with and without streamwise pressure gradients in the MIR test section. The research examined three cases: (1) negligible pressure gradient with low freestream turbulence, (2) negligible pressure gradient with high freestream turbulence, and (3) adverse pressure gradient with high freestream turbulence (Figure 8). The end goal was to deduce entropy generation rates from PTV and PIV measurements. Employing a small field of view (FoV) with a telescoping camera provided instantaneous velocities by PTV computations in the near‑wall region where entropy generation is concentrated. Further, this research developed data analysis procedures to deduce distributions of entropy generation rates from the instantaneous PTV results.
[14,15] The boundary layer away from the wall used a larger FoV and reduced data with PIV software. Figure 9 demonstrates the high quality data obtained and its excellent spatial resolution in a resulting turbulent boundary layer downstream in the case with an adverse pressure gradient.
Figure 8. Schematic diagram of an experimental model within the MIR tunnel for adverse pressure gradients (flow is from right to left). 13
Figure 9. Streamwise mean velocity measurements by PTV and PIV in a developed turbulent boundary layer; circles = PTV from small FoV, squares = PIV from large FoV.
National and International Collaborations
Most studies in the MIR flow system have been done with international and/or national partners, and supported by a variety of sponsors. Examples include:
Availability of the MIR flow system has formed the basis for projects with international partners including Universität Erlangen, Universität Stuttgart, Universität der Bundeswehr München, Imperial College, Oxford University, University of Manchester, University of Montenegro, Kungliga Tekniska högskolan Stockholm, Kyoto University, University of Toyama, Seoul National University, and the Korea Advanced Institute of Science and Technology. These international projects have been summarized by McIlroy, Becker, and McEligot. Most U.S. partners have been faculty and graduate students from universities including the University of Idaho, Boise State University, Utah State University, the University of Wyoming, Iowa State University, Johns Hopkins University, the University of Maryland, Ohio State University, and Pennsylvania State University. Some of these investigations are summarized by McIlroy and McEligot. General Atomics, Bechtel, and Clarksean Associates have been industrial participants.
The MIR Facility enables optical measurements for determining flow characteristics in complex passages/geometries (such as turbomachinery passages, nuclear reactor tube bundles, nuclear reactor coolant channels, and boundary layers) in and around objects without distortion of optical paths. The MIR Facility is used as an informal user facility for basic and applied research by government, industry, and academia where accurate and reliable uncertainty quantification is essential for producing high‑quality data for computer code validation, truly an international asset.
The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.
* Corresponding author: Carl Stoots,
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