Effects of inlet shapes of critical venturi nozzles on discharge coefficients

Abstract

The discharge coefficients of critical venturi nozzles were obtained in a high-pressure gas flow standard system, which was a gravitational weighing system. The calibration uncertainty of a critical venturi nozzle of about 8 mm throat diameter was ±0.08% and ±0.14% at upstream stagnation pressures of 4 MPa and 1 MPa, respectively. The discharge coefficients of critical venturi nozzles fabricated according to ISO specifications are in good agreement with ISO correlation. The discharge coefficients for small inlet radius decrease significantly as the inlet length becomes short, owing to separation at the sharp inlet. For nozzles having long inlet radius, the effects of inlet length on the discharge coefficients were relatively small, but the effects become significant at the short inlet length. The effect of separation at the inlet is stronger than that of the boundary layer growth. The experimental results provide excellent support for the shape of critical venturi nozzles suggested by ISO specifications.