Effects of macro and micro roughness in forced convective heat transfer

The article reports a comparative study of macro and micro type artificial roughness in convective heat transfer performance under laminar and low turbulent regime. Circular ribs with different rib height to channel height ratios (e/H = 0.05, 0.1, 0.15) fabricated on copper substrate are introduced as macro type roughness whereas copper (Cu) nano-porous layer (avg. thickness about 5 μm) is considered as micro roughness. Surface heat transfer and friction characteristics are investigated under different scale roughnesses on one principle wall of a rectangular channel having an aspect ratio (AR) of 7.5. Result shows that the average turbulence intensity between two ribs decreases with decreasing roughness height. On the other hand, nano-porous layer provides significant heat transfer efficiency (about maximum 42% more than bare copper plate) under laminar and low turbulent region without inducing significant turbulence into the channel. Nano-porous layer less than 5 μm is found to increase heat transfer surface area significantly that influences the dynamic behaviors of working fluids in the vicinity of heat transfer wall.