Laser Doppler Velocity Profile Sensor: Technical Advances for the Optical Flow Rate Measurement of Natural Gas

− The precise measurement of the flow rate of natural gas is of vital economical importance since natural gas contributes highly to the total energy mix. In this paper we report about a laser Doppler velocity profile sensor specifically designed to suit the measurement conditions at the optical test facility of the German natural gas flow rate standard. The profile sensor is an extension of the principle of a laser Doppler anemometer (LDA); however, instead of one interference fringe system with (nearly) parallel fringes two fringe systems one with converging and one with diverging fringes – are employed. Thereby both spatial resolution and velocity accuracy are more than one order of magnitude better than for a conventional LDA. The data obtained are compared to measurements with a conventional LDA. As a result the shear layer could be resolved more precisely by the profile sensor which yields a more accurate determination of the flow rate. In addition the mean value of the measured turbulence intensity of the core flow amounted to only 0.14 % for the profile sensor, more than one order of magnitude lower than for the conventional LDA. The minimum measured turbulence intensity was 0.07 %, which is, to our knowledge, the lowest value ever measured by optical methods.