Experimental and numerical investigation on steady and transient flow in the flat-walled micro-diffuser/nozzle

In this paper, experiments and numerical simulations have been conducted to investigate the flow characteristics in the flat-walled micro-diffuser/nozzle on conditions of angle (0°–60°), Reynolds number (100–1500) and excitation frequency (0–5000 Hz). In steady flow, the diffuser efficiency increases with an increase in Reynolds number as θ ≤ 10°, but the optimal diffuser efficiency is found as 20° ≤ θ ≤ 60°, and the internal flow characteristics at the optimal diffuser efficiency are recorded. Based on statistics, it is concluded that the interaction of adverse pressure gradient and diverging flow results in the optimal angle, and the optimal angle decreases with an increase in Reynolds number. While in transient flow, the excitation frequency has great impacts on the optimal angle, the higher frequency results in the larger optimal angle. As the Reynolds numbers are 100 and 1500, the variations of diffuser efficiency with excitation frequency show the opposite trends. At low Reynolds number, the acceleration term caused by the excitation frequency increases the flow loss and reduces the diffuser efficiency. However, at high Reynolds number, the energy of diverging flow is stored in the vortexes and used to prevent the converging flow, this contributes to the great increase in the diffuser efficiency. Therefore, the influence of excitation frequency on the diffuser efficiency is determined by the location and intensity of the vortexes.