Simulation study on enhanced heat transfer of annular inner rib tube in geothermal coaxial heat transfer wellbore

Abstract

As a clean and renewable energy source, geothermal energy offers several advantages, including high reserves, safety, and stability. To enhance the heat extraction capacity of wellbores, this study establishes numerical models for triangular, rectangular, and fan-shaped internally ribbed tubes. It further analyzes their differences in heat transfer and resistance performance compared to smooth circular tubes. The results indicate that the internally ribbed tube significantly improves heat transfer efficiency by disrupting the boundary layer and promoting fluid mixing. Among the configurations studied, the rectangular internally ribbed tube exhibits the highest level of heat transfer enhancement and turbulence generation. The Nusselt number (Nu) recorded for the rectangular inner ribbed tube is 113.8 %–200.9 % greater than that of the smooth circular tube; concurrently, the friction factor (f) increases by 248.4 %–909.3 %. Key parameters such as rib spacing, rib height, and rib width substantially influence heat transfer performance. Specifically, it was observed that as rib spacing increases or as rib height rises, Nu decreases; conversely, an increase in rib width leads to an increase in Nu. In terms of resistance characteristics: f decreases with increasing rib spacing but increases with both rising rib height and width. The comprehensive performance evaluation factor (η) reveals that while the fan-shaped inner ribbed tube performs optimally at a rib height of 12 mm, η declines linearly with increasing rib width. Therefore, it can be concluded that while rectangular internally ribbed tubes are suitable for applications requiring high levels of heat transfer efficiency, fan-shaped internally ribbed tubes are more appropriate for scenarios demanding superior overall performance.