DEVELOPMENT OF A LONG WAVELENGTH ACOUSTIC FLOWMETER FOR MEASURING POWER PLANT EXHAUST FLOWS

Lee J. Gorny and Keith A. Gillis

As a part of NISTís program to standardize measurements of greenhouse gas (GHG) emissions, we are developing a long-wavelength acoustic flowmeter (LWAF) for accurate, economical measurements of exhaust flows from coal-burning power plants.† Measurements of the flue gas mass flow combined with the gasís composition are used to determine the emitted CO2 and other by-products of combustion.† Today, best measurements of mass flow are made with uncertainties of 5 % or more due to the large size of the flue-gas stacks and the high Reynolds number (Re8) flowís non-uniform, unsteady, swirling profile and no standard exists to calibrate these meters.† A platform of technologies capable of measuring these flows will enhance the regulatory effort of the EPA and other such organizations worldwide and will lead to improved control and higher efficiencies for operating power plants.

In its simplest embodiment, a LWAF is a device that measures flowrate by tracking propagation of acoustic plane waves within a duct.† A LWAF is effective in complex, changing flows, largely because spatial non-uniformities of the axial velocity are averaged over the entire cross section in a manner that is insensitive to flow distortions.† In contrast, conventional techniques measure gas flow only at isolated points or are averaged along a chord across the stack using methods that are not robust in distorted flow.† †††

We constructed a 1:100 scale, calibrated flow facility for the initial development of a LWAF which is currently located in Building 221, Room A108. †In recent research, several LWAF concepts have been developed and characterized. †Long wavelength sound propagations are measured and analyzed in both the time and frequency domains to obtain the speed of sound (c) within the fluid as well as its flowrate (V).† Frequency domain based approaches use a swept sine acoustic source or the existing sound field within the duct.† Wavelength measurement is used to determine c due to its flow independence, while phase difference measurements between axially spaced microphones are proportional with V.† Time based method measures the time of flight of a propagating pressure wave generated by a pulsed source, by tracking the signalís delay between axially spaced transducers.† With refined signal processing techniques, the test LWAF can measure the c in the fluid to a standard uncertainty of .01 % with respect to values calculated from NISTís REFPROP database using measured fluid parameters.† Flowrate can be measured with an uncertainty of 1 % over a range of flows of 2 m/s to 5 m/s in the presence of swirl indicting a potential of these methods to be used with flow distortions. †

 

In this poster presentation, we present the principle behind the LWAF, our design considerations, and the current performance of time and frequency-based approaches to metering.† We will also describe technical challenges that must be overcome to use the current LWAF technique at a full-scale power plant.