  The internal flow in corrugated tubes of different helical pitch, covering
from the laminar to turbulent regime, was studied in order to characterize the
three-dimensional flow and the influence of corrugation geometry on pressure
drop and convective heat transfer. With water as working fluid and an imposed
wall heat flux, ranging from around 4 to 33 kW/m2, a numerical model was
developed with a CFD commercial software, where k-omega SST was used to model
turbulence. Experimental tests were performed covering Reynolds numbers in the
range from around 300 up to 5000, which allowed to identify the transition
region and validate the numerical model. The results show that due to the swirl
induced by the corrugation, the critical Reynolds number for the start of
transition to turbulent flow is reduced. The thermal performance factor, which
quantifies the heat transfer enhancement at the expense of pressure drop, was
used to compare the corrugated tubes against the reference case of smooth tube.
Based on this, all investigated corrugated geometries performed better than the
smooth tube, except for low Reynolds numbers (Re<500) in the laminar regime.
Overall, the corrugated tube with the lowest pitch showed a clearly better
performance for the intermediate range of Reynolds numbers (1000<Re<2300),
being an optimal choice for a wider range of operating conditions in the
transitional and turbulent flow regimes.
