Comparative study of turbulent heat transfer in smooth and scale-rib roughened reciprocating blind ducts with laterally tangent entry jet

Abstract

This experimental study investigated the performances of heat transfer enhancement (HTE), pressured drop coefficient ( C f ) and relative thermal performance factor ( η th) generated by a lateral entry jet for two reciprocating square blind ducts with smooth and scale-rib roughened walls for piston cooling applications. At each test reciprocating frequency ( f) in the range of 0–2 Hz, the lateral and longitudinal Nusselt number ( Nu) distributions over two opposite duct walls, namely the jet wall and the back wall, are measured at duct (jet) Reynolds numbers Re( Re j ) of 5000(31,850), 10,000(63,690), 15,000(95,540) and 20,000(127,400). The jet-induced flow phenomena in each blind duct interact with the reciprocating and buoyancy forces to exhibit synergistic impacts on HTE performances. This is demonstrated by examining the Nu variations over the reciprocating jet and back walls in response to the changes of Re, pulsating ( Pu) and reciprocating Grashof ( Gr p ) numbers for each blind duct. The individual and interdependent Re, Pu and Gr p effects on heat transfer are comparatively examined to reveal the influences of the scale-rib roughness on HTE performances. Within the parametric conditions tested, the ratios of the averaged Nusselt number ( N u ¯ ) between those measured from the reciprocating and static jet and back walls fall respectively in the ranges of 0.68–0.9 and 0.69–0.89 for the smooth blind duct; and 0.63–0.97 and 0.62–0.99 for the roughened blind duct. Due to the presence of the scale-rib roughness, the ( N u ¯ ) over the roughened reciprocating jet and back walls are respectively raised to 1.04–1.4 and 1.01–1.14 times of the smooth-walled references; while the pressure drop coefficients are raised to about 2–4.2 times of the smooth-walled duct levels. Two sets of N u ¯ correlations over the jet and back walls for the reciprocating blind ducts with smooth and scale-rib roughened walls are generated. Justified by the relative thermal performance factor results, the present wall roughness in the reciprocating blind duct with a lateral entry jet can provide both improvements in HTE effectiveness and the thermal efficiency at Re = 20000.