The "sonic nozzle principle"
The adjustment of a constant volume flow at the inlet side of the nozzle results from the physical effect, that speed of sound occurs in the nozzle throat when the critical pressure ratio between inlet and outlet of the nozzle is reached.
Speed of sound is reached in the throat of a nozzle, if the absolute inlet pressure is at least twice as high as the outlet pressure. It cannot be exceeded in the constriction. The volume flow on the inlet side cannot increase then. However, the mass and standard volume flow increases with gas density, which depends on pressure and temperature.
Test section design and flow evaluation with critical nozzles:
For flow measurement with critical nozzles an absolute, temperature and humidity sensor, if required, must be provided in the test section design.
The current density and viscosity of the medium is calculated by the sensor data. The current flow is determined by the characteristics and calibration data of the critical nozzle then.
The recording of the sensor data and the calculation of the flow values are carried out the easiest way by our efficient flow computer.
Suction operating mode is the preferable operating mode for these nozzles, since the atmospheric density on the inlet is very constant and disturbances on the outlet side cannot pass the constriction.
In order to ensure that an critical nozzle works optimally, geometric conditions have to be met. Free and undisturbed incoming flow is observed at high-capacity conditions at the inlet section of the nozzle: no barrier is allowed to get closer to the nozzle axis and the inlet level than 5 times the nozzle diameter. In general, the inlet diameter should be at least four times wider than the throat diameter of the nozzle, so that the flow rate in front of the nozzle remains low.