Three-component Laser Doppler Anemometer for Gas Flowrate Measurements up to 5 500 m³/h

Abstract

In the Physikalisch-Technische Bundesanstalt (PTB) the primary standard for on-line flowrate measurements using the laser Doppler anemometer (LDA) technique has been extended to a three-component LDA to improve velocity profile measurements in the boundary layers of a nozzle flow. The LDA flowrate measuring facility now consists of a two-colour argon ion LDA and a wavelength-stabilized GaAlAs diode laser LDA. The gas flowrate is obtained by numerical integration of the measured velocity profiles across the exit plane of the nozzle. High local resolution of the velocity profile measurements is achieved by perpendicular orientation of the measurement volumes of the two-component gas laser LDA and the semiconductor diode laser LDA (LD-LDA). This allows the resolution in the boundary layer to be improved significantly to velocity gradients. The present work presents the LD-LDA system for precise velocity profile measurements at flow velocities of up to 120 m/s; selected profile measurements are described in detail to demonstrate the high resolution and the symmetry of the flow profile. For the first time a wavelength-stabilized miniaturized diode laser LDA has been successfully applied in precise velocity measurements, and comparisons with well-established gas laser LDAs have been made. The uncertainty of the flowrate measurement up to 5 500 m3/h is 0,l YO for air at atmospheric pressure. A turbine gas meter, type Elster G2500, was calibrated with the LDA and used as a transfer standard for an intercomparison with the Nederlands Meetinstituut (NMI) in the flowrate range up to 5 500 m3/h with and without the installation of perforated plates to condition the flow in the inlet section of the gas meter. The results of the comparison experiment clearly show the reliability and accuracy of the online flowrate measurement of gases and underline the necessity for a detailed research programme to investigate the relationship between installation effects, upstream flow conditions and the measurement uncertainty of gas meters. The design of a test rig now under construction at the PTB is shown. This will allow the diode laser LDA technique to be applied to the measurement of installation effects according to OIML Recommendation R-32.

Gas flow standards and their uncertainty

“…The speed the disk's edge is easily made traceable to length and time standards. Dopheide et al [106] designed a nozzle to produce a jet with a nearly symmetric 'top-hat shaped' (uniform) velocity profile (figure 35). They used a LDA to profile the jet and integrated the profile to determine the mean gas velocity in the cross section immediately downstream from the nozzle outlet.…”

Measurement of velocity gradients in boundary layers by a spatially resolving laser Doppler sensor

“…Methods (i) and (ii) require a small detection volume for high spatial resolution. This prerequisite has been fulfilled by a small cross section of the two laser beams [4], reduction of the measurement volume's extent by beam stops [5], the use of spatially low-coherent light [6] or coincident measurements of multi-component LDA systems [7]. The mechanical scanning of the detection volume necessary for method (i), does not allow real-time velocity measurements.…”

Laser Doppler velocity profile sensor using a chromatic coding

“…Methods (i) and (ii) require a small detection volume for high spatial resolution. This pre-requisite has been fulfilled by a small cross section of the two laser beams [4], reduction of the measuring volume's extent by beam stops [5], the use of spatially low-coherent light [6] or coincident measurements of multi-component LDV systems [7]. The mechanical scanning of the detection volume necessary for method (i) does not allow real-time velocity measurements.…”