Some Fundamental Acoustic Observations in Combusting Turbulent Jets

Abstract

An experimental study has been made of the acoustic emissions from turbulent combustion zones with the specific intention of inferring the combustion details from the acoustic field. Included are data from a laboratory burner, a commercial burner tested in the laboratory, and a seven-head system of burners installed commercially on a rotary kiln. The fuels used include methane, ethane, propane, and natural gas. Based on an order-of-magnitude analysis of the turbulent combustion zone, specific frequency bands of the acoustic emissions are examined. It is found that any change in the combustion zone (air/fuel ratio) is invariably reflected in the acoustic amplitudes in three bands. Product gas analyses show that the nonefficient combustion correlates with these perceptible changes in the acoustic signatures. The theoretical reasoning and the experimental results show the feasibility of acoustically monitoring the combustion zones to infer such details as efficiency and pollutant production. The acoustic spectrum of a combustion zone reveals many details which can be deduced to be geometry dependent, flow dependent, turbulence dependent, or combustion dependent. The close interplay of these influences makes it difficult, in general, to isolate the individual effects. The simplified analysis here attempts to deduce the geometry and turbulence effects. It is consistent with the observed behavior for both premixed and nonpremixed flames. Also, this simplified analysis identifies the high-frequency zone as a possible region of acoustic emission directly influenced by combustion.Nomenclature a = sonic velocity in the ambient atmosphere, m/s 3* = distance from burner exit to microphone, m D = burner exit diameter, m