Flow characteristics of pyramidal shaped small sonic nozzles

Four types of pyramidal sonic nozzles made of silicon crystal were studied experimentally. The throat sizes varied from 38 to 140 μm for type A and D nozzles and from 75 to 188 μm for type B and C nozzles. For each of the nozzle types, the results show that the discharge coefficient is proportional to the throat size, and the critical back pressure ratio for choking is insensitive to Reynolds’ number. In parallel, the flow field of a type B nozzle was investigated by numerical simulation. The effect of heat flux coming from the nozzle body was examined and the flow patterns obtained from Spalart–Allmaras and standard k−ωk−ω turbulence models were compared. The simulation results indicate the heat flux does not noticeably change the velocity field and discharge coefficient. Also, the flow downstream of the nozzle throat develops into an under-expanded supersonic jet in which expansion and oblique shock waves appear alternately.

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► Size effects for pyramidal sonic nozzles with dd smaller 200 μm were investigated.
► Vena contracta phenomena and viscous boundary layer affect the Cd curve.
► The throat size has little effect on the CBPR for the pyramidal sonic nozzles.
► The pyramidal configuration reduces heat transfer from the wall to the sonic flow.
► The jet flow from the throat is under-expanded.