Y11 r 1 p4 r 2Y

where Qscfm is the volumetric flow rate in scfm, P0, P, and T0 are the stagnation pressure, static pressure, and stagnation temperature at the venturi in psia and °R. The parameter Y is the adiabatic expansion coefficient, with r=P/P0, venturi diameter ratio p=0.41, and specific heat ratio y=1.40.

Stagnation and static pressure taps from the venturi are connected via pneumatic tubing to a Scanivalve pressure transducer. Stagnation temperature at the venturi is assumed to be equal to the measurement made at Station 4 downstream of the last turbine stage since convection time is only approximately 0.3 sec from that point to the flowmeter. With the volumetric flow rate known, the area average axial velocity Uxi at station i is found from where the annular area of the turbine A=0.90972 ft2, static temperature and static pressure are in °R and psia, yielding Uxi in ft/sec.

Rotational speed measurements of the turbine rotor are made with a one-per-revolution optical trigger mounted on the turbine. The photodiode is connected to a Schmidt Trigger Integrated circuit. The resulting TTL output waveform has a fundamental harmonic at the rotor rotation rate. A National Instruments NB-MIO-16XL Pc-based data acquisition board samples this frequency signal and measures the time between pulses, which is the period of one rotor revolution. This waveform from the trigger is also routed through a frequency to voltage converter to the Honeywell UDc controller which protects the system from rotor over-speed.

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