Детальная информация

Twisted elliptical tubes have been around long enough that engineers generally trust them to boost heat transfer but almost everything we know about them comes from turbulent, high-speed flows. What happens when the flow is slow and gentle, the kind you find in compact heat exchangers and low-flow systems? That question has largely been left unanswered, and that is exactly what this thesis tries to address. To investigate, a series of numerical simulations were run on a double-pipe heat exchanger across Reynolds numbers from 100 to 1000 firmly in the laminar range. Six tube geometries were tested in total, a straight oval tube for reference, three tubes with progressively tighter twists (P/L = 1, 0.5, and 0.25), and two tubes with different aspect ratios (A/B = 1.180 and 1.845). All simulations used water as the fluid, were carried out in ANSYS Fluent, and were checked against existing published data before any conclusions were drawn. The findings are pretty striking. Simply twisting the tube makes a real difference tightening the twist to P/L = 0.25 pushed the Nusselt number up by as much as 47% compared to the loosest twist, while the pressure drop penalty stayed relatively modest at 12–23%. Making the tube flatter turned out to be just as important, the most flattened geometry nearly doubled the heat transfer of a straight tube under the same conditions. Looking at the flow patterns makes clear why tighter twists and flatter shapes stir the fluid more aggressively and even out the temperature distribution across the annulus. Interestingly, the same trends appear in high Reynolds number studies too, which suggests these effects are not a quirk of slow flow but something more fundamental. The bottom line is that twisted elliptical tubes work well even at low flow rates, and the best results came from combining a tight twist with a high aspect ratio. For engineers designing heat exchangers for solar thermal systems, oil coolers, or heat recovery units, these results offer some concrete numbers to work with. The logical next steps are testing at higher Reynolds numbers, trying different fluids, and eventually building a physical prototype to confirm what the simulations are showin.