NUMERICAL ANALYSIS OF THE TRANSITION FROM LAMINAR TO TURBULENT FLOW IN A NOZZLE/DIFFUSER CHANNEL

The nozzle/diffuser is a small-angle flow channel, often with a rounded inlet and a sharp outlet, commonly used in microfluidics. In particular, its use is widespread in the design of micropumps since the asymmetry of the shape promotes a preferential flow direction when the diffuser/nozzle is excited by pulsatile overpressure. A critical research topic in this field is the estimation of energy losses, which have a noticeable effect on the pump efficiency and are strictly related to the flow regime - laminar or turbulent - in the diffuser/nozzle channel. Recent studies highlighted a possible link between the Reynolds number characterizing the regime transition and the Womersley number, a dimensionless expression of the pulsatile flow frequency in relation to viscous effects. Here, to gain further insight into this relation, we analysed by numerical simulations multiple cases of study with different applied combinations of the amplitude and frequency of the external pulsatile pressure. To estimate the critical Reynolds number that triggers the regime transition, we implemented for each case two numerical models that solve the standard Navier-Stokes and the RANS equations of the flow, respectively. The grid resolution adopted in the first model does not allow for a proper solution of the flow field in the turbulent regime. Accordingly, the two schemes show the similar results in the laminar regime until the Reynolds number does not exceed the critical value, which so was easily evaluated. Results suggest that the transition from laminar to turbulent flow occurs at values of the Reynolds number that increase with the nozzle/diffuser Womersley number. Moreover, an analysis of the global energy loss across the pipe for laminar flow conditions is presented. Both the effects of viscous and dispersive stresses have been studied. The analysis shows that the highest energy dissipation occurs at the channel’s throat and the losses are overall more prominent when the pump is working in the nozzle configuration.