06.10.2011
It was possible to successfully conclude the research work on the optical flow rate standard with measurements of the boundary layers for flow rates up to 1600 m³/h. The examination has shown the same dependence on the Reynolds number for both the large and the medium nozzle and their consistent characteristic. Thus with the optical standard, an independent method has become available for the first time. It provides calibration values for the larger working standards at the High-Precision Test Facility for High-Pressure Gas Meters pigsarTM without intermediate steps, and strongly assures the reliability of the previous trace of flow rate to the volumetric primary standard, the so-called high-pressure piston prover.
The optical standard complements the high-pressure piston prover (HPPP) used as primary standard to flow rates up to 1600 m³/h with one run. In the nozzle module with windows for the optical access, the flow is measured. The boundary layers have been gauged using a cross-beam laser Doppler anemometer with an adaption of a reference beam technique for three components. The precise location of the velocity signal is obtained from the phase difference of the components, and the velocity itself from the frequencies. The measured boundary layer profiles are then reduced by an orthogonal regression and a least median of squares method and then are finally fitted with a modified hyperbolic tangent function. The reduced profiles obtained allow the determination of the displacement thickness as well as the discharge coefficient cD of the nozzle. If the discharge coefficient’s dependency on the Reynolds number is known, then the flow rate can be determined contact-free with an optical one-point measurement in the middle of the nozzle flow.
The figure shows the extensive consistency of the cD values via the Reynolds number for both nozzles.
Bild 1: Comparison of the cD values of the medium sized and the large sized nozzle
Contact person:
Volker Strunck, Dept. 1.4, WG 1.43, E-mail: volker.strunck@ptb.de