Numerical simulation of flow transition in a rectangular microchannel

The behavior of flow transition in a rectangular microchannel was numerically investigated. In the simulation, three flow models were adopted, namely, a γ-Re_θt transition model, a laminar model and a shear stress transport (SST) model. The simulation was conducted using ANSYS CFX, and the results were compared with experimental data, then the effect of length-to-diameter ratio (L/D) on the critical Reynolds number was studied. Results indicate that the laminar model and the γ-Re_θt transition model produce similar pressure drops when the mass flow rate is smaller. When the mass flow rate increases, values predicted by the γ-Re_θt transition model matches best with the experimental data. The laminar model and the SST model are incapable of predicting transition, while the γ-Re_θt transition model forecasts the critical Reynolds number well and the predicted values match well with the experimental data. In the present study, the transition is accurately simulated and the flow mechanism is revealed. The results also show that local turbulent regions appear at the rear of the microchannel before the Reynolds number reaches the critical value and the turbulent regions expand with the increase of the Reynolds number. For microchannels with L/D≥100, the transition from laminar to turbulent regime occurs for a Reynolds number in the range 2000-2500, and the length-to-diameter ratio has no significant effect on the critical Reynolds number.