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

Laboratory experiments were conducted to derive the relationship between suspended sediment concentration (SSC) of an industrial ash in water and the signal-to-noise ratio (SNR) measured by a 10 MHz acoustic Doppler velocimeter (ADV). Small portions of ash were added in steps (from 0.006 gl-1 up to 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 and LOW+), transmit length (TL of 0.3 mm and 0.6 mm) and frequencies of data gathering (25 Hz and 100 Hz). Different backscatter responses were detected depending on the ADV settings and SSC. Frequency changes did not produce major changes in SNR values, power intensity variation caused a displacement in SNR as suggested by ADV manufacturer. The major change in SNR vs SSC plot type was noticed when TL was changed, where the linearity of the response was different for both cases. The signal saturation, where the linear evolution starts to decay, occurred for a lower value of SSC for bigger value of TL. Samples were collected for gravimetric analysis of SSC as well as granulometry. On lower SSC samples, only finer ash portion was detected but from SSC greater than 0.15 gl-1 bigger particles were detected. The findings indicate that particle type and size distribution changes with concentration above 0.5 gl-1, and this may lead to errors in the acoustic estimates of SSC in ashes-laden flow.