Heat transfer and pressure drop in a square spiral channel roughened by in-line skew ribs

This experimental study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients (f) and thermal performance factors (TPF) for a square spiral channel with two opposite endwalls roughened by in-line 45° ribs. Detailed Nu distributions over the rib floor with forward and backward flows are measured using the steady-state infrared thermo-graphic method at Reynolds numbers (Re) of 900–30,000. With the increased f from the smooth-walled straight tube references (f∞), the rib-induced sectional vortical flows interact with the centrifugal-force driven Dean vortices along the entire spiral passage to generate the considerable heat transfer enhancement (HTE) impacts that raise the area-averaged turbulent Nusselt numbers (Nu¯) to 8.41–3.66 and 5.05–2.78 times of the Dittus–Boelter Nusselt numbers (Nu∞) for forward and backward flows at 2700 ⩽ Re ⩽ 30,000, respectively. Comparisons of Nu¯/Nu∞,f/f∞ and TPF obtained from the ribbed spiral channel with other types of HTE devices reveal the thermal performances of the present HTE measure. Two sets of Nu¯ and f correlations for the square spiral ribbed duct with forward and backward flows are generated to assist the design applications using this HTE measure.