This experimental study examines the detailed Nusselt number (Nu) distributions, pressure drop coefficients Wand thermal performance factors (TPF) for a square spiral channel with two opposite endwalls roughened by in-line 45 degrees 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(infinity)), 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) over bar) to 8.41-3.66 and 5.05-2.78 times of the Dittus-Boelter Nusselt numbers (Nu(infinity)) for forward and backward flows at 2700 <= Re <= 30,000, respectively. Comparisons of (Nu) over bar /Nu(infinity), f/f(infinity) 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) over bar 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. (C) 2011 Elsevier Ltd. All rights reserved.