Effect of hydraulic diameter and aspect ratio on single phase flow and heat transfer in a rectangular microchannel

The effect of aspect ratio and hydraulic diameter on single phase flow and heat transfer in a single microchannel was investigated numerically and the results are presented in this paper. Previously, many studies in literature investigating the effect of geometrical parameters reached contradictory conclusions leaving some phenomena unexplained. Additionally, most researchers studied the effect of channel geometry by varying the channel height for a constant channel width or varying the width for a constant height. This means that the hydraulic diameter and aspect ratio vary simultaneously, which makes it difficult to identify the relative importance of the aspect ratio and the hydraulic diameter. In the present study, the effect of hydraulic diameter was studied by varying the channel width and depth while keeping the aspect ratio constant. The range of hydraulic diameters was 0.1–1 mm and the aspect ratio was fixed at 1. In the second set of simulations, the aspect ratio ranged from 0.39 to 10 while the hydraulic diameter was kept constant at 0.56 mm. The simulations were performed using the CFD software package ANSYS Fluent 14.5. The geometry investigated in this study includes symmetrical cylindrical inlet and outlet plenums and a microchannel. The fluid entered and left the channel vertically from the top in a direction normal to the channel axis. The dimensions of the inlet/outlet plenums (diameter and height measured from the channel bottom surface) were kept constant while the width and depth of the channel were varied. The simulations were conducted for a range of Reynolds numbers (Re = 100–2000) and water was used as the working fluid. A three dimensional thin wall model was used to avoid conjugate heat transfer effects. A constant heat-flux boundary condition was applied at the bottom and vertical side walls of the channel, while the upper wall was considered adiabatic. The friction factor was found to decrease slightly with aspect ratio up to AR ≈ 2 after which it increased with increasing aspect ratio. The results demonstrated that the slope of the velocity profile at the channel wall changes significantly with aspect ratio for AR > 2. The effect of the aspect ratio and hydraulic diameter on the dimensionless hydrodynamic entry length is not significant. Also, the aspect ratio does not affect the heat transfer coefficient while the dimensionless Nusselt number increases with increasing hydraulic diameter. The friction factor was found to increase with increasing hydraulic diameter