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The Idaho National Laboratory (INL) matched-index-of-refraction (MIR) flow system.


The Idaho National Laboratory has developed a unique matched-index-of-refraction flow system which is ideal for education and research. The benefit of the MIR technique is that it permits optical measurements for determining flow characteristics in complex passages in and around objects to be obtained without locating a disturbing transducer in the flow field and without distortion of optical paths. The innovation of the INL MIR system is its large size which yields outstanding spatial and temporal resolution. This article discusses the benefits of the MIR technique, characteristics of the system, typical measurements possible for complex flows and information on the studies that have been completed to enhance professional collaboration in engineering education. These experiments usually have provided new fundamental understanding plus benchmark data for assessment and validation of computational fluid dynamic codes.


Unique facilities such as the INL MIR flow system can provide focus for a wide variety of collaborative activities in engineering education. The flow system discussed here evolved from an international advisory committee on experimental thermal science. The system was designed and fabricated by collaboration between faculty, students, and engineers at INL and Universitat Erlangen-Nurnberg. The availability of this unique system has permitted development of many joint proposals for graduate and undergraduate education. Faculty sabbatical leaves, postdoctoral training, doctoral dissertations, and masters theses have all been accomplished with this facility. Undergraduate students have participated in summer internship programs and, as consequences, received deeper understanding of the application and value of their related courses in physics, thermal engineering, and experimental techniques. High quality measurements may be obtained to assess codes and their detailed assumptions in courses on computational thermal fluid dynamics. Conferences, workshops, industrial short courses, and review panels on related topical areas of interest yield further educational opportunities as do collaborative efforts to modify and improve the facilities and design new experiments. Some examples of the employment of these educational approaches will be demonstrated in the later section on "INFORMATION ON PREVIOUS STUDIES CONDUCTED AT THE INL MIR SYSTEM."

Thermal engineering education includes study in thermodynamics, fluid mechanics, heat transfer, mass transfer, and reacting flows. For most flow problems in these disciplines, the limiting case is non-compressed flow with constant thermodynamic and transport properties. This is the situation for which the large INL MIR flow system can provide fundamental insight and high-quality measurements for flows through and around complicated geometries. In addition to thermal engineering, introductory physics courses may be interested in large scale demonstration and application of Snell's Law. Likewise, optical scientists can find interesting practical problems relating to optical fluid measurements and their uncertainties.


The benefit of the INL MIR technique is that it permits optical techniques, such as particle image velocimetry (PIV) and laser Doppler anemometry (LDA), to measure flow characteristics in flow fields and near surfaces, in passages and around objects having complicated geometries without introducing an intrusive probe into the flow field and without distortion of optical paths. One way to eliminate optical interference in these systems is to employ suitable transparent solid materials for the models with fluids that possess the same refractive index as the model itself. In this way, the solid model disappears optically and therefore has no influence on the optical paths for lasers and cameras but maintains its full mechanical influence on the flow. With a transparent model of different refractive index than the working fluid, the light rays of optical measuring instruments can be refracted in such a manner that measurements are either impossible or require extensive, difficult calibrations. Without refractive index matching, LDA beams may not cross to form the measurement control volume at the desired focal length, if they cross at all.

Before the development of the INL MIR flow facility the study of flow past complex geometries requiring high Reynolds numbers and fine spatial resolution at large scales did not exist. Examples of such geometries include heat exchangers, nuclear reactor tube bundles, nuclear reactor coolant channels and boundary layers. A demonstration of the benefits of refractive-index-matching is shown in Figure 1.

Quantification of transitional boundary layers requires measurements very close to the surface for accurate determination of the wall shear stress. LDA measurements usually suffer from optical interference or blockage of the laser beams, especially when two or three component systems are employed. Facility size can help make near-wall velocity measurements by increasing flow-length scales relative to measurement probe size. By using an index-matched boundary, one can measure all three velocity components (u, v, w) and their gradients instantaneously at Iocations extremely close to a surface, e.g., meaningful data to [y.sup.+] < 0.1 [([[y.sup.+] = [[y x [u.sub.[[tau]]]/v]).sup.1] where y is the wall-normal distance, [u.sub.t] is the friction velocity, and v is the kinematic viscosity. Increased size can also improve temporal resolution. Typical nondimensional time increments for sampling may be defined as [DELTA][t.sup.+]= [DELTA]tV/L, so a larger size gives faster, more effective, sampling (here t is time and V and L are characteristic velocity and length scales).


In previous MIR systems, the working fluids have often been either toxic or combustible. Since the volume of the INL MIR system is over 11,000 liters (2,900 gal), light mineral oil was selected as the working fluid due to environmental and safety considerations and because its refractive index matches that of some quartz at specific temperatures and at the wave lengths of the lasers employed. The mineral oil has the same index-of-refraction as fused quartz near laboratory room temperature, is odorless, non-toxic, relatively nonflammable and non-volatile, inexpensive, and very stable.


Figure 2 is a diagram of the INL MIR facility. Flow is clockwise in the figure. The main circulation pump is in the lower right corner (75 hp axial flow pump). From this main circulation pump, the oil passes through a cylindrical section, a round bellows, a round-to-square transition, a five-vane diffuser, and a rectangular bend (flow distributor) before entering the settling chamber. The settling chamber has several screens and a honeycomb for flow quality conditioning. After leaving the settling chamber, the oil passes through a contraction and enters the test section. The refractive index-matching temperature of the fluid is maintained within [+ or -] 0.05 [degrees]C of the prescribed index-matching temperature by an external temperature control system. Table 1 displays the technical specifications of the INL MIR system.


Data are obtained primarily by optical techniques, such as LDA and PIV. Instantaneous velocity components may be obtained by measurements with an existing two- (or one-) component LDA system at fixed positions. In order to obtain good signal quality and high collection rates, the INL MIR system employs the LDA in the forward scattering mode to avoid longer measuring times that would be needed if the back scattering mode were used. Mean velocities along with mean turbulence gradients and statistics are calculated from the LDA algorithms. Typical results include time-resolved, pointwise distributions of the mean velocities (U, V, and W), velocity fluctuations, and their Reynolds stress components.

Instantaneous velocity field measurements are primarily obtained with a stereo PIV system. Two CCD cameras are mounted on a 3-directional traverse system that is controlled by three separate electric stepping motors. The PIV system uses a double-pulsed Nd:YAG laser that is usually mounted below the experiment model and produces a vertical light sheet approximately 1-3 mm thick. The PIV laser is positioned with a linear traverse system that is accurate to [+ or -] 5 [micro]m (0.0002 in). The resulting PIV measurements include instantaneous values of the three velocity components (u, v, and w), their spatial gradients and Reynolds stresses in the plane of the light sheet. Mean statistics are calculated by averaging a sufficient time-series of the instantaneous data.

The three-directional traversing mechanism is mounted on rails parallel to the test section and is used with both the LDA and PIV. This traversing system has a positional accuracy of [+ or -] 2 [micro]m (7.87 x [10.sup.-5] in).



Experimental models have been employed to study external and internal flows as well as coupled external-internal flow situations. External flows are flows over and around a body, internal flows are flows inside of a body and coupled external-internal flows are flows that include the interaction of flows around and inside a body. Internal flows are typically studied inside a quartz model with the main flow and test section windows providing a perpendicular optical interface for the transition from air in the laboratory to the oil in the test section. Examples of coupled internal-external flows include a synthetic jet interacting with an external boundary layer in a study by Professor Douglas R. Smith of the University of Wyoming (2) and a helical nozzle creating a swirling jet by Professor Barton L. Smith of Utah State University and his students (3). With guidelines for verification and validation of CFD (computational fluid dynamics), the models are designed in close collaboration with all participating partners; this coordination is essential to ensure that the measurements and dimensionless parameters from all experiments and computations are comparable.

In addition to the two studies mentioned above, several other studies have been completed in the INL MIR flow system and two additional studies are being prepared. A superb detailed review of all of the previous studies and the current study is available at the INL MIR web site ( Previous studies conducted in the INL MIR flow system include: LDA-Measurements of Transitional Flows Induced by a Square Rib (4), Measurements of Fundamental Fluid Physics of SNF Storage Canisters (5), Physical and Computational Modeling of Airflow Around Buildings (6), Fundamental Thermal Fluid Physics of High Temperature Flows in Advanced Reactor Systems (7), Advanced Computational Thermal Fluid Physics (CTFP) and its Assessment for Light Water Reactors and Supercritical Reactors (8), The Boundary Layer Over Turbine Blade Models with Realistic Rough Surfaces (9), and Measurement of Flow Phenomena in a Lower Plenum Model of a Prismatic Gas-Cooled Reactor (10).

The present experiment in the INL MIR flow system is to develop benchmark databases for the assessment of CFD solutions of the momentum equations, scalar mixing, and turbulence models used for nuclear reactor design and safety analyises. The present study will investigate the flow ratios between coolant channels and bypass gaps in the interstitial regions of typical prismatic standard fuel elements or upper reflector block geometries of typical Very High Temperature Reactors (VHTR) in the limiting case of negligible buoyancy and constant fluid properties. The MIR VHTR Bypass Flow Experiment will measure flow characteristics in the coolant channels and interstitial gaps between typical prismatic block standard fuel elements or upper reflector blocks.

A second project being developed focuses on the application of the second law of thermodynamics to open systems and is designed to measure pointwise entropy generation rates. The point wise entropy generation rate determines the localized contribution to energy losses. Insight into these losses and their locations can improve efficiencies and sustainability. This study will focus on point wise entropy generation in flows undergoing bypass transition where present predictions are not well understood. This information could be important in turbomachinery applications.


The large MIR flow system is an excellent base for interesting collaborations in engineering education. The INL MIR system is a versatile, useful tool for examining flows in complicated situations such as turbulent and transitional flows, flows through porous media and two-phase particulate flows for basic and/ or applied studies. The MIR technique allows measurements which otherwise would be impractical, if not impossible; the large size of the INL system provides better spatial and temporal resolution than comparable facilities. Teaming is a normal mode of operation. Benchmark data for assessing computational fluid dynamics can be acquired for external flows, internal flows and coupled internal/external flows. This paper has demonstrated how the development of INL MIR system can facilitate collaboration between institutions for the benefit of engineering education.


This work was partly supported by the INL Faculty Staff Exchange Program, the Center for Advanced Energy Studies and the U.S. Department of Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. 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. References herein to any specific commercial product, process or service by trade name, trademark, manufacturer or otherwise does not necessarily constitute or imply its endorsement, recommendation or favoring by the U.S. Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the U.S. Government or any agency thereof.

(1) S. Becker, C. M. Stoots, K. G. Condie, F. Durst and D. M. McEligot, "LDA-Measurements of Transitional Flows Induced by a Square Rib," J. Fluids Eng., Vol. 124, 2002, pp. 108-117.

(2) J. M. Shuster, R. J. Pink, D. M. McEligot and D. R. Smith, "The Interaction of a Circular Synthetic Jet with a Cross-flow Boundary Layer," AIAA paper 2005-4749, AIAA Fluid Dynamics Conf., Toronto, June 2005.

(3) Wilson, B.M., Smith, B.L., Spall, R. and McIlroy, H.M. Jr., 2009, "A Non-Symmetrical Swirling Jet as an Example of a Highly Model-able Assessment Experiment," ICONE17-75362, Proceedings of ICONE17 2009, 17th International Conference on Nuclear Engineering, Brussles, Belgium, July 16-19, 2009

(4) Becker, S., Stoots, C.M., Condie, K.G., Durst, F. and McEligot, D.M., 2002, "LDA-Measurements of Transitional Flows Induced by a Square Rib," J. Fluids Eng., Vol. 124, March 2002, pp. 108-117.

(5) Condie, K.G, McCreery, G.E. and McEligot, 2001, "Measurements of Fundamental Fluid Physics of SNF Storage Canisters," INEEL/EXT-01-01269, September 2001.

(6) McEligot, D.M., McCreery, G.E., Pink, R.J, Barringer, C. and Knight, K.J., 2001, "Physical and Computational Modeling for Chemical and Biological Weapons Airflow Applications," INEEL/CON-02-00860, November 2001.

(7) McEligot, D.M., Condie, K.G., Foust, T.D., Jackson, J.D., Kunugi, T., McCreery, G.E., Pink, R.J., Pletcher, R.H., Satake, S.l., Shenoy, A., Stacey, D.E., Vukoslavcevic, P. and Wallace, J.M., 2002, Fundamental Thermal Fluid Physics of High Temperature Flows in Advanced Reactor Systems," INEEL-EXT-2002-1613, December 2002.

(8) McEligot, D.M., Condie, K.G., McCreery, G.E., Hochreiter, L.E., Jackson, J.D., Pletcher, R.H., Wallace, J.M., Yoo, J.Y., Ro, S.T., Lee, J.W.S. and Park, S.Q., 2003, "Advanced Computational Thermal Fluid Physics (CTFP) and its Assessment for Light Water Reactors and Supercritical Reactors," INEEL-EXT-03-01215 Rev 5, December 2003.

(9) McIlroy, H. M. Jr., 2004, "The Boundary Layer Over Turbine Blade Models with Realistic Rough Surfaces," PhD Dissertation, University of Idaho, December 2004.

(10) McIlroy, H. M. Jr., McEligot, D. M., and Pink, R. J., "Measurement of Flow Phenomena in a Lower Plenum Model of a Prismatic Gas-Cooled Reactor," J. of Eng. for Gas Turbines & Power, 132, Feb. 2010, pp. 022901-1--022901-7.

Hugh M. McIlroy, Jr.

Idaho National Laboratory,

Idaho Falls, Idaho 83415-2200

(208) 526-6176

Donald M. McEligot

Center for Advanced Energy Studies

Idaho National Laboratory

Idaho Falls, Idaho 83415-3553

(208) 526-2881/0528

Table 1. Technical Specifications of the INL MIR System.

Characteristic                              Specification

Test Section Cross Section         0.61 m x 0.61 m (24 in x 24 in)

Test Section Length                         2.44 m (8 ft)

Contraction Ratio                                4:01

Working Fluid                        Drakeol #5 Light Mineral Oil

Index-Matching                        Laser Wavelength Dependent
Temperature ([degrees]C)

Mineral Oil Density                 Matching Temperature Dependent

Refractive Index of Mineral Oil     Matching Temperature Dependent
and Fused Quartz

Mineral Oil Kinematic Viscosity     Matching Temperature Dependent

Temperature Control                            External

Maximum Inlet Velocity                    1.9 m/s (6.2 ft/s)

Inlet Turbulence Intensity                     0.5%-15%
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Author:McIlroy, Hugh M., Jr.; McEligot, Donald M.
Publication:Journal of the Idaho Academy of Science
Geographic Code:1USA
Date:Dec 1, 2011
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