Measurements of suspended ashes concentration in turbulent flow with acoustic doppler velocimeter

Abstract

Measuring and estimating dispersion coefficients for ash particles released in watercourses is required for a broad understanding of their transport and behaviour in aquatic ecosystems. Calibration of a high-frequency acoustic backscattering system and estimates of its sensitivity to particulates of known size distribution and concentration are presented. Laboratory experiments were conducted to derive the relationship between suspended sediment concentration (SSC) of biomass ashes and the signal-to-noise ratio (SNR) measured by a 10 MHz ADV. Small portions of ashes were added in steps (0.006–20 gl−1) into a stirred water tank while ADV was recording changes in SNR, with various combinations of ADV settings, which included power (LOW, LOW+), transmit length (TL = 0.3 mm, 0.6 mm) and frequencies of data acquisition (25 Hz, 100 Hz). Samples were collected for gravimetric and particle size distribution (PSD) analysis. As expected, the salinity and pH of the collected samples increased with SSC. Different backscatter responses were detected depending on the ADV settings and SSC. To maximize the calibration range, the ADV settings power LOW+, TL = 0.3 mm and sample frequency 25 Hz were combined, resulting in a calibrated SSC range of 10–500 mgl−1, where a linear relationship was obtained. The results suggest that it may be possible to apply a single-frequency acoustic technique to the detection of suspended ashes. The particle type and size distribution changed with a concentration above 0.5 gl−1, which may lead to errors in the acoustic estimates of SSC in ash-laden flows.