Spatially resolved absolute concentration and fluorescence-lifetime determination of H2CO in atmospheric-pressure CH4/air flames

Using laser-induced fluorescence (LIF), spatially resolved concentration pr ofiles of formaldehyde (H2CO) were obtained in the preheating zone of atmos pheric-pressure premixed CH4/air flames stabilized on the central slot of a multiple-slot burner similar in construction to domestic boilers. The isol ated pQ(1)(6) rotational line (339.23 nm) in the 2(0)(1)4(0)(1) vibronic co mbination transition in the (A) over tilde (1)A(2)-(x) over tilde (1)A(1) e lectronic band system around 339 nm was excited in the linear LIF intensity regime. For a quantification of quenching effects on the measured LIF sign al intensities, relative fluorescence quantum yields were determined from d irect fluorescence lifetime as a function of height above the slot exit. Ab solute H2CO number densities in the flames were evaluated from a calibratio n of measured LIF signal intensities versus those obtained in a low-pressur e sample with a known H2CO vapor pressure. Peak concentrations in the sligh tly lean and rich flames reached (994 +/- 298) and (174 +/- 52) ppm, respec tively.