The effects of rough surfaces on heat transfer and flow structures for turbulent round jet impingement

Abstract

Heat transfer and flow structures for round jet impingement over rough surfaces have been investigated using the shear stress transport model (SST) with transition model. This model is first evaluated against the experimental data and other numerical results for the smooth surface with Reynolds number Re = 23,000 for the nozzle-plate spacing of 2. Based on the above evaluation, the effects of rough surfaces on the mean velocity, turbulence field, skin friction and heat transfer are investigated with equivalent sand grain roughness heights ks of 50, 100, 250 and 500 μm. Two trends of the effects of roughness on the velocity field are found. First, the mean velocity over rough surfaces becomes steeper than that over the smooth surface in very near the wall. However, for ks ≥ 250 μm, the velocity gradient in the axial direction does not increase with an increase of ks. Second, the decay of the mean velocity is found in the deceleration region, but minimal in the acceleration zone. In addition, it is observed that the increase of roughness height leads to the reduction of the wall shear stress in the stagnation region. On the contrary, downstream, large ks usually contributes to an augmentation of wall shear stress. For heat transfer, the enhancement of heat transfer due to roughness varies from 2.53% to 6.08% compared with the case of smooth surface. However, this enhancement is non-monotonic due to the change of the second peak of heat transfer which is believed to be the key factor.