Discharge characteristics of small sonic nozzles in the shape of pyramidal convergent and conical divergent

This study combined KOH anisotropic etching with laser machining to fabricate small sonic nozzles in the shape of a pyramidal convergent inlet followed by a conical diffuser with a divergent angle of 5°. The diffuser was made into three different lengths. The throat diameters were around 100 μm. Experiments were performed to obtain discharge coefficients and critical back pressure ratios in Reynolds numbers ranging from 5.8×102 to 4.5×103. The critical back pressure ratio for one of the nozzle types examined reached 0.486 at Re=4.4×103Re=4.4×103. Numerical simulations were also performed to obtain the flow fields at an upstream pressure of 203 kPa. The simulation results revealed that besides flow separation, the first set of oblique shocks appeared in the nozzle jet could also lead to tremendous pressure loss. The weaker the oblique shocks, the higher the critical back pressure ratio obtained.

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► Small sonic nozzles with pyramidal convergent and conical diffuser were studied.
► For small sonic nozzles, a proper diffuser can improve the CBPR.
► The simulation results showed that the longer the diffuser length, the smaller the Cd value was.
► The first set of oblique shocks in the nozzle jet could lead to tremendous pressure loss.