1 CH and CH 2 O in Atmospheric Pressure Methane/Air Bunsen Flames Robert J.H. Klein-Douwel, Jorge Luque, Jay B. Jeffries, Gregory P. Smith, and David R. Crosley Molecular Physics Laboratory, SRI International 333 Ravenswood Avenue, Menlo Park, California 94025 Western States Section of the Combustion Institute, Fall 1999, Paper 12F99 Abstract: Laser-induced fluorescence of CH B 2 Σ - v’=1 is excited in atmospheric pressure Bunsen flames using rotational levels above and below the predissociation limit. There is significant rotational, vibrational, and electronic energy transfer even when levels above the predissociation limit are excited. At atmospheric pressure rotational energy transfer produces a variation in the fluorescence quantum yield with rotational level and gas temperature. Laser-induced fluorescence of CH 2 O exciting A-X 4 1 0 allows formaldehyde detection at an excitation wavelength quite near (and often overlapped) with the CH B-X (1,0). The formaldehyde structure is contained inside the CH in the premixed inner cone of the Bunsen flame. Excitation of CH 2 O A-X 4 1 0 provides an LIF strategy that minimizes the variation of the population of the excited level over the 700- 1800 K temperature range important in the flame. 1. Introduction Partially premixed Bunsen type flames form the mode of combustion of nearly all the natural gas consumed in residential appliances and the vast majority burned in small commercial applications, such that usage constitutes over 25% of U.S. natural gas consumption. These flames consist of two stages: a rich premixed flame forms an inner cone and a lean diffusion flame constitutes an outer cone. These partially premixed flames are a key target of present and future pollutant emission regulation. NO x emissions will be the subject of increasingly stringent requirements, and it is likely that CO, aldehydes and aromatics will also come under scrutiny. The consequences of operation under marginal conditions are of particular concern. Understanding the structure of these flames can provide guidelines for burner design efforts to meet the regulatory requirements while still maintaining reasonable efficiency. For example, a simple practical solution to reduction of NO x emissions in some commercially available burners employs metal inserts; these reduce NO x , probably due to cooling of the flame, but the actual mode of operation is unknown and the amount of toxics such as CO and aldehydes is likely increased. A model burner has been designed to study a partially premixed Bunsen flame using laser-induced fluorescence (LIF) diagnostic techniques. The emphasis of the measurements is an understanding of the flame structure and the chemistry leading to pollutant formation. The diagnostics focus on measurements of CH as a marker of the premixed flame front and the key precursor to prompt NO, and on formaldehyde as a