A review is presented of measurement techniques to characterise dispersed multiphase flows, which are not accessible by means of conventional optical techniques. The main issues that limit the accuracy and effectiveness of optical techniques are briefly discussed: cross-talk, a reduced signal-to-noise ratio, and (biased) data drop-out. Extensions to the standard optical techniques include the use of fluorescent tracers, refractive index matching, ballistic imaging, structured illumination, and optical coherence tomography. As the first non-optical technique, a brief discussion of electrical capacitance tomography is given. While truly non-invasive, it suffers from a low resolving power. Ultrasound-based techniques have rapidly evolved from Doppler-based profiling to recent 2D approaches using feature tracking. The latter is also suitable for time-resolved flow studies. Magnetic resonance velocimetry can provide time-averaged velocity fields in 3D for the continuous phase. Finally, X-ray imaging is demonstrated to be an important tool to quantify local gas fractions. While potentially very powerful, the impact of the techniques will depend on the development of acquisition and measurement protocols for fluid mechanics, rather than for clinical imaging. This requires systematic development, aided by careful validation experiments. As theoretical predictions for multiphase flows are sparse, it is important to formulate standardised ‘benchmark’ flows to enable this validation.

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Measurement in opaque flows: a review of measurement techniques for dispersed multiphase flows

The study of flow and heat transfer through a bundle of rods is important, as this type of configuration is widely used in many engineering applications, for example, in heat exchangers, clusters of support structures, steam generators, and nuclear power reactors. This work experimentally investigated the flow-field characteristics of rod bundles with helically wrapped wires (wire-wrapped rod assemblies) proposed for advanced burner reactors. Subchannel blockage is one of the most interesting accident scenarios for liquid metal fast reactors as different mechanisms can lead to a partial or total subchannel blockage. It is of paramount important to understand the thermal-hydraulic behavior of the flow in the vicinity of such blockages. The flow mixing characteristics in two interior subchannels located near the core of the bundle, one without and one with the presence of a blockage, were experimentally investigated. The velocity fields within the spatial gaps of two subchannels were obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) technique for Reynolds numbers of 4000 and 17,000. The first- and second-order statistics, including the mean velocity, root-mean-square fluctuating velocity, and Reynolds stress, were computed from the obtained TR-PIV velocity vector fields. In the blocked sub-channel, the TR-PIV results revealed a large recirculation flow region in the vicinity of the blockage that was created by the interaction between inflows from neighboring subchannels. Spectral analysis was performed and the vortex shedding frequency indicated by the Strouhal number was found to be $$St=0.16$$
 for the two Reynolds numbers investigated. The flow characteristics of the interior subchannels were investigated via the two-point cross-correlation of fluctuating velocities. In addition, proper orthogonal decomposition (POD) analysis was applied to the instantaneous vorticity fields to extract the statistically dominant flow structures. Finally, the statistical characteristics of the vortex, such as the vortex populations, spatial distributions, and vortex strengths, were acquired by combining POD analysis and the vortex identification algorithm.

Turbulent flow and vortex characteristics in a blocked subchannel of a helically wrapped rod bundle

Review of sub-channel code development for pressurized water reactor and introduction of CORTH-V2.0 sub-channel code

\n The thermal-hydraulic behavior of the flow in rod bundles has motivated numerous experimental and computational investigations. Previous studies have identified potential for accumulation of debris within the small subchannels of typical wire-wrapped assemblies with subsequent total or partial blockage of subchannel coolant flow. A test campaign was conducted to study the effects of localized blockages on the bundle averaged friction factor of a tightly packed wire-wrapped rod bundle. Blockages were installed within the bundle, and fluid pressure drop was measured across one wire pitch for a Reynolds number range of 500–17,200. The Darcy–Weisbach friction factor of the perturbed rod bundle geometry was compared with that of the unblocked bundle, as well as with the predictions of a well-established friction factor correlation. Differing effects based on blockage size and location for various flow regimes were studied. A number of conclusions can be made about the effects of the blockages on the friction factor, such as an increasing effect of the blockage on friction factor with an increase in Reynolds number, a change in flow behavior in the turbulent transition flow regime near Reynolds number 3000, differences in effect on friction factor for different types of subchannel blockage, and a nonlinear trend in friction factor variation with flow area impeded for edge subchannels. To this end, all data and quantified uncertainty produced in this study are made available for comparison and validation of advanced computational tools.

B. Tensuda

Papers

Fluid flow investigations within a 37 element CANDU fuel bundle supported by magnetic resonance velocimetry and computational fluid dynamics

Fluid flow in a diametrally expanded CANDU fuel channel – Part 2: Computational study