Interference effect between sonic nozzles

Abstract

Calibration of a large capacity gas flow meter may be accomplished with a sonic nozzle bank as a reference system. For the design of a compact movable prover which is made with many nozzles, the nozzles should be installed as close as possible. The interference effect between neighbouring nozzles can be an additional error source for prediction of the mass flow of the assembly from the calibration of individual nozzles. A nozzle bank was assembled from seven toroidal throat nozzles with a nominal throat diameter (d) of 4.3mm in a hexagonal pattern. It was calibrated by a primary gas flow system at stagnation pressures from 0.5 to 2MPa and throat Reynolds number from 0.4 to 1.2×106. Three installation plates of sonic nozzles were fabricated with distances between nozzles of 16–81mm (L/d of 3.7 to 9.5). The average discharge coefficient of the seven nozzles in a nozzle bank was calculated from the discharge coefficients of the individually measured sonic nozzles. The differences from the directly measured results were within ±0.05% for all test cases. Also, the proximity of the wall for the six outer nozzles located at 2.3d from the pipe wall had no influence on the experimental results. The discharge coefficients as a function of throat Reynolds number were in agreement with the recommended equation of ISO 9300.