Influence of roughness density and Prandtl number on convective heat transfer over artificial additive manufactured roughness

• Heat transfer nearly doubled with a lesser friction drag increase • A high Reynolds Analogy efficiency, up to 2.5, was obtained. • Nusselt number scaled with the Péclet number to the power of 0.8. • Heat transfer and flow friction increased with roughness density. • Heat transfer seems to scale with roughness’ thermal conductivity. The flexibility of additive manufacturing (AM), especially Laser-based Powder Bed Fusion (L-PBF), has made it a key method for fabricating enhanced heat transfer devices. The inherent nature of the L-PBF process results in surface roughness with irregular topologies, offering potential for heat transfer enhancement. As a result, extensive research has focused on optimising L-PBF process parameters to control roughness topology and evaluate its impact on convective heat transfer characteristics. This study takes a step forward by utilising L-PBF technology to fabricate aluminium alloy test surfaces with tailored irregular artificial roughness of varying roughness densities for heat transfer enhancement applications. These samples were then tested experimentally in a convective heat transfer rig at Reynolds numbers ranging from 500 to 18,000. The inlet temperature was varied to obtain three Prandtl numbers ( Pr = 4.18 , 5.25 , a n d 6.82 ). Flow friction and heat transfer were analysed by examining the variation of the Darcy friction factor and Nusselt number, respectively. The study evaluated how the heat transfer scales with Reynolds and Prandtl numbers and assessed the surfaces’ heat transfer enhancement using Reynolds Analogy (RA) efficiency. Subsequent to the experiments, a numerical study was performed using roughness-resolving approach coupled with RANS to gain better understanding of the heat transfer mechanisms. The results showed significant heat transfer enhancement (double to nearly 2.3-fold) with a lesser friction drag increase (7.6 %–43 %), achieving high Reynolds Analogy efficiency (up to 2.5). Heat transfer showed a power-law scaling of Péclet number to the power of 0.8 (i.e. Nu ∝ P e 0.8 = Re 0.8 Pr 0.8 ), driven by enhanced energy transfer from roughness peaks. Heat transfer and flow friction are seen to increase with roughness density (slope) until reaching an upper limit or saturation across all Prandtl values. For the range studied, the thermal performance strongly correlated with roughness density at a constant height, thermal conductivity, and Prandtl number, with peak efficiency near the beginning of the transitionally rough regime.