Feasibility of a long-wavelength acoustic flowmeter for measuring smokestack emissions

Abstract

Conventional gas flow measurements in large ducts, such as power plant smokestacks, have uncertainties of 5–20%. As part of its Greenhouse Gas and Climate Science Measurements Program, the National Institute of Standards and Technology (NIST) is testing long-wavelength acoustic flowmeters (LWAFs) as an alternative method to reduce this uncertainty. A LWAF uses the Doppler Effect to determine the speed of sound c and the average flow speed V. Theory predicts that, for plane waves in a duct, corrections due to flow irregularities, such as swirl and turbulence, are proportional to (V/c)2 £ 0.01. To investigate the feasibility of using an LWAF in a smokestack, we constructed a 1:100 scale model (10 cm diameter) test facility that generated flows up to 25 m/s using ambient air. The model LWAF simultaneously determined the speed of sound in air with a standard uncertainty of 0.01%, relative to NIST’s standard reference database, and measured the average flow velocity with a standard uncertainty of ±1% relative to a NIST-calibrated flow standard upstream from the LWAF. Similar results were obtained when the flows were highly distorted by elbows and obstructions, or when water was sprayed into the air, and for flows through larger diameter model LWAFs.