An experimental study of momentum and thermal transport in flow through smooth- and rough-wall microchannels

The impact of surface roughness on momentum and thermal transport in microscale flow passages of hydraulic diameter Dh = 600 microm is investigated in the laminar, transitional and turbulent flow regimes using microscopic PIV, two-color LIF thermometry and pressure-drop measurements. In addition to smooth-wall flow, two different rough-wall cases are investigated. The roughness patterns under consideration are unique in that they are reminiscent of surface irregularities one might encounter in practical microchannels due to imperfect fabrication methods. The pressure-drop results reveal the onset of transition above Recr = 1800 for the smooth-wall case, consistent with the onset of transition at the macroscale, along with deviation from laminar behavior at progressively lower Re with increasing roughness. Mean velocity profiles at various Re for each surface condition confirm these trends, meaning Recr is a strong function of roughness. Examination of instantaneous velocity fields indicates that transitional flows at the microscale are composed of a subset of fields illustrating laminar behavior and a subset that capture significant disordered motion, with the latter fraction increasing with Re. This decomposition reveals a clear hastening of the flow toward a turbulent state with increasing roughness due both to the roughness dependence of Rear as well as an enhancement in the growth rate of the non-laminar fraction of the flow in the early stages of transition. From a structural viewpoint, instantaneous velocity fields embodying disordered behavior in the transitional regime are found to contain large-scale motions consistent with hairpin vortex packets irrespective of surface condition. However, single-point velocity statistics and quadrant analysis of the non-laminar ensembles indicate an intensification of the velocity fluctuations by surface roughness.;Local Nusselt number (Nu) for smooth-wall laminar flow in the range 200 ≤ Re ≤ Recr agree well with macroscale predictions in both the thermally-developing and -developed regimes With increasing roughness, while an enhancement in local Nu is noted in the thermally-developing regime, no measurable influence is observed upon attainment of a thermally-developed state. These observations are supported by temperature profiles which suggest that the thermal boundary layer may be regenerated locally by roughness in the thermal entrance region of the flow. In the transitional regime, mean temperature profiles for the smooth- and rough-wall cases show deviation from fully-developed laminar behavior for Re > Recr. Finally, estimates of bulk Nu indicate enhancement in convective heat transfer over the smooth-wall case with increasing surface roughness in the laminar, transitional and turbulent regimes, though the smooth-wall data are in excellent agreement with macroscale predictions for laminar and turbulent flow. While a shift in the transitional pathway of the thermal transport behavior toward lower Re appears partially attributable for this enhancement with increasing roughness, it appears that turbulent convection also contributes significantly once transition is initiated.