EXP 1-2 Measurement Quantities and Techniques
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Measurement techniques
When measuring the turbulent pollution fluxes at the openings of the street canyons, two velocity components were measured simultaneously and point by point. For the upper openings, the longitudinal and vertical components were measured, and for the lateral openings, the longitudinal and lateral components were measured. This measurement was made with a combination of LDA (Laser Doppler Anemometry) and FFID (Fast-Response Flame Ionisation Detector, HFR400, Cambustion Ltd.).
To ensure proximity between the LDA measurement volume and the inlet of the FFID sampling tube, the LDA and FFID probes were mounted together on a 3D traverse system. The position of the inlet of the FFID sampling tube was carefully adjusted to be 1 mm above, 1 mm behind and 1 mm beside the centre of the LDA measuring volume (Fig. 3). Through various test measurements with different probe positions, we confirmed that the influence of the FFID sampling tube on the LDA measurement was negligible.
During the measurement campaign, the LDA sampling frequency was kept between 0.5 and 1 kHz, depending on the flow range investigated. The FFID sampling frequency was set to 0.5 kHz to achieve the desired response time of 2 ms. However, due to the physical characteristics of the FFID sampling tube (length of 200 mm and diameter of 1.2 mm), an average time delay of about 12 ms was obtained in contrast to the LDA. This time delay varied depending on the air density and the dynamic pressure at the sampling point. To obtain a more accurate individual FFID time delay, we used the maximum correlation coefficient between the time series of the velocity component and the concentration. This resulted in an adjusted time delay between 10 and 13 ms.
To account for the influence of the LDA seed particles (with a diameter of about 1 μm) on the FFID concentration measurements, a correction was applied during the FFID calibration process. The measurements of the background concentration of the LDA seed particles were performed separately and subtracted from the measured calibration gas without activating the line source.
Contributed by: Štěpán Nosek — Institute of Thermomechanics of the CAS, v. v. i.
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