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A combined OH/acetone planar laser-induced fluorescence (PLIF) imaging technique that provides simultaneous visualizations of regions of unburned fuel and of combustion in a reacting flow is described. OH marks the location of chemical reaction and of combustion products, and acetone vapor, which is seeded into the fuel stream, marks unburned fuel. A single pulse from an ultraviolet laser is used to simultaneously excite both the OH and acetone, and the fluorescence from each is detected on separate cameras. Acetone spectroscopy and chemistry are reviewed to provide a basis for interpreting acetone fluorescence signals in high-temperature combusting environments. The imaging technique is applied to two nonpremixed turbulent reacting flows to assess the utility of the technique for visualizing the instantaneous flow structure and to illustrate the dependence of the interpretation of the acetone PLIF images on the flow conditions. IntroductionInsights into the manner in which fuel and oxidizer mix and burn in nonpremixed turbulent reacting flows can be gained by studying their instantaneous structure. Planar laser imaging techniques (Hanson 1988) are well suited for investigating flow structure because they provide a time-resolved, two-dimensional map of a chosen flow variable (e.g., species number density). In particular, planar laser-induced fluorescence (PLIF) imaging of OH has been used widely because high signals are easily attainable and because OH is naturally present in many of the combusting flows of interest. However, OH measurements only provide information about regions of the flow where combustion has occurred.A better understanding of the mechanisms of mixing and combustion in a reacting flow can be obtained if information about both reacting and nonreacting regions can be obtained simultaneously. Experiments where two detectors were employed to simultaneously visualize different flowfield properties in flames have been reported previously (Long et al. 1985, Dibble et al. 1986, Namazian et al. 1988. While these visualization techniques yielded good results, they may not be easily applicable in larger flames or enclosed combustors. In such facilities, where the signals from Rayleigh scattering, Raman scattering or C2 fluorescence may be too weak relative to background scattering or fluorescence, a more practical diagnostic which produces larger signals is needed.Recently, acetone and acetaldehyde have been demonstrated as fluorescent tracer molecules in nonreacting flows and flames, and strong signals were obtained (Lozano et al. 1992, Arnold et al. 199o, Tait andGreenhalgh 1992). These large molecules exhibit a broadband UV absorption feature which overlaps the (o, o), (1, o) and (3, o) vibrational bands of the A~X system of OH, all of which have been used for OH laser-induced fluorescence (LIF) measurements. Thus, a single UV laser can be used to simultaneously excite fluorescence from the fuel stream tracer molecule and from OH. Since OH fluoresces in the UV and acetone and acetaldehyde f...