Symmetric vs asymmetric periodic disturbances at the walls of a heated flow passage

Measurements were made of local and fully developed turbulent heat transfer coefficients for a flat rectangular duct having streamwise-periodic, two-dimensional disturbances symmetrically positioned at both principal walls. Friction factors were also determined. The local transfer coefficient distributions revealed the rapid attainment of the periodic fully developed regime. The fully developed transfer coefficients exceeded (typically by 40%) those for an otherwise identical duct having periodic disturbances at only one of the principal walls (asymmetric case). The symmetric disturbances also yielded significantly increased transfer coefficients (up to a factor of three) compared with a corresponding disturbance-free duct. Large disturbance-related increases in the friction factor were also sustained. An analysis based on fixed duct dimensions and fixed pumping power was made to assess the relative performance of the symmetrically and asymmetrically disturbed ducts and the disturbance-free duct. For a wide range of conditions, the symmetrically disturbed duct yielded the highest fully developed transfer coefficients. Compact correlations in terms of roughness parameters were developed for both the fully developed heat transfer coefficients and friction factors.