About this note
A plain-language summary of the 2 publications listed under this topic on Academic Publications (Topic-wise). Full citations are under References.
Why this matters
Many heat exchangers, and the cores of many nuclear reactors, consist of bundles of closely packed rods with a coolant flowing past them. At practical flow rates that flow is turbulent: irregular, swirling and well mixed. Two quantities matter most to a designer: the pressure drop, which sets the pumping power, and the heat transfer, which sets how hot the rods become. Both depend on rod size, spacing and arrangement in ways simple correlations do not always capture.
What we did
With research scholar L. Wilson and S. P. Venkateshan, we ran pressure-drop experiments with turbulent air flow (Reynolds number, based on the channel, of 2,300 to 6,860) through models of near-compact heat exchangers: bundles of 2, 3 and 4 mm rods in aligned (in-line) arrangements, with surface-to-volume ratios of 100 to 300 m²/m³ (Wilson et al., 2004). Turbulent-flow data for rods this small were scarce. Our approach was to treat each rod bundle as a porous medium. Its pressure drop could then be described by the same two-term relation used for flow through porous materials: a viscous drag that grows with flow speed and a form drag (the resistance from flow separating around the solid shapes) that grows with its square.
With K. Natesan and K. Velusamy of the Indira Gandhi Centre for Atomic Research, Kalpakkam, and T. Sundararajan, we turned to a nuclear application (Natesan et al., 2010). In a liquid-metal-cooled fast breeder reactor, each fuel subassembly holds a tight hexagonal bundle of fuel pins kept apart by a steel wire wound helically around each pin, and liquid sodium flows along them. Using three-dimensional turbulent flow simulations of a 19-pin bundle, we compared three turbulence models (k-ε, k-ω and the Reynolds stress model) against experimental correlations, for Reynolds numbers of 50,000 to 150,000 and helical pitch-to-pin-diameter ratios of 15 to 45.
What we found
Rod bundles behave like porous media, with form drag in charge. In the heat exchanger models, the viscous contribution settled to a fixed value while form drag took over at higher Reynolds numbers. Differences in pressure drop between models were traced to form drag induced by the flow and by geometric parameters, including the rod diameter, rod spacing, number of rows and the clearance between the rods and the channel wall, an effect not considered in earlier correlations. A correlation for the form coefficient predicted the measured pressure drop within ±20%. The data also showed that, inside the bundle, the switch to form-drag-dominated flow happens well before the flow as a whole becomes turbulent, so the two transitions should not be confused (Wilson et al., 2004).
Wire wraps add friction but improve mixing. In the reactor bundle, all three turbulence models gave nearly identical results: friction factors within 6% of the experimental correlation and Nusselt numbers (a dimensionless heat transfer rate) within 1% of each other, though up to 20% below the correlation, pointing to a need for experiments on specific wire-wrap configurations. Tightening the helical pitch from 300 to 100 mm raised the friction factor 1.6 times, yet left the average heat transfer coefficient almost unchanged. What it did change was the cross-flow velocity and turbulence intensity, which improved the thermal mixing of the coolant and reduced temperature variation across each section of the subassembly. Smaller temperature differences mean less differential thermal expansion of the pins, which is expected to help the structural integrity of the fuel subassembly (Natesan et al., 2010).
Related
References
- Natesan, S., Sundararajan, T., Narasimhan, A. and Velusamy, K., (2010), “Turbulent Flow Simulation in a Wire-wrap Rod Bundle of an LMFBR”, Nuclear Engineering and Design, 240(5), 1063-1072. [DOI 10.1016/j.nucengdes.2009.12.025]
- Wilson, L., Narasimhan, A. and Venkateshan, S. P., (2004), “Turbulent Flow Hydrodynamic Experiments in Near-Compact Heat Exchanger Models with Aligned Tubes”, ASME J. Fluids Engineering, 126, 990-996. [DOI 10.1115/1.1845553]