a b s t r a c tLine-imaged measurements of temperature and major species, based on well-established Raman/Rayleigh/CO-LIF techniques, are used to better understand the scalar structure of piloted CH 4 /air jet flames with inhomogeneous inlets. Recent studies using a variant of the Sydney piloted burner have demonstrated that the blowoff velocity of a partially-premixed jet flame of CNG or CH 4 and air can be increased significantly by tailoring the mixture fraction profile at the burner exit. This is done by adding a small, retractable central tube within the main tube of the burner and separately supplying fuel and air for partial premixing. Both tubes are located within the pilot annulus. When the central tube is retracted far upstream of the burner exit, the flame has the same stability as that of the original burner with homogeneous fuel-air composition at the jet exit. However, when the inner tube supplies fuel and is retracted an optimal distance (10-13 times the main tube diameter) the blowoff velocity is increased by nearly 40%. Previous results indicated that combustion very close to the burner exit occurs in a stratified-premixed mode, augmenting the stabilizing effect of the pilot. This is followed by transition to a diffusion-dominated mode of burning within the first ten main tube diameters.The present paper provides a detailed examination of a series of piloted CH 4 /air jet flames with different inlet conditions and with a pilot flame that matches the composition and adiabatic flame temperature of CH 4 /air. It addresses trends in local extinction as well as differences in near-field flame structure. The evolution in the local mode of combustion is traced using instantaneous line-imaged realizations where the change in mixture fraction across a 1000 K interval is used as a conditioning variable. Doubly conditioned means of selected species mass fractions confirm that the flames with inhomogeneous inlet profiles undergo transition from a stratified-premixed mode of combustion to a diffusion-dominated mode of combustion within the first ten jet diameters. Calculations of strained laminar partially-premixed flames are used to gain insight on the effects of strain rate and fuel-side equivalence ratio on flame structure and the rate of heat release. These turbulent jet flames may serve as interesting test cases for models aimed at predicting the performance of practical burners that operate with mixed combustion modes.

The dual pump CARS technique is often used in the study of turbulent flames. Fast and accurate algorithms are needed for fitting dual-pump CARS spectra for temperature and multiple chemical species. This paper describes the development of such an algorithm. The algorithm employs sparse libraries, whose size grows much more slowly with number of species than a conventional library. The method was demonstrated by fitting synthetic "experimental" spectra containing 4 resonant species (N 2, Oz, H2 and CO2), both with noise and without it, and by fitting experimental spectra from a H 2-air flame produced by a Hencken burner. In both studies, wei ghted least squares fitting of signal, as opposed to least squares fitting signal or square-root signal, was shown to produce the least random error and nummize bias error in the fitted parameters.

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Spontaneous Raman scattering measurements of temperature and major species concentration in hydrocarbon-air flames require detailed knowledge of the Raman spectra of the hydrocarbons present when fuels more complex than methane are used. Although hydrocarbon spectra have been extensively studied at room temperature, there are no data available at higher temperatures. Quantum mechanical calculations, when available are not sufficiently accurate for combustion applications. This work presents experimental measurements of spontaneous Stokes-Raman scattering spectra of methane, ethylene, ethane, dimethyl ether, formaldehyde and propane in the temperature range 300-860 K. Raman spectra from heated hydrocarbons jets have been collected with a higher resolution than is generally employed for Raman measurements in combustion applications. A set of synthetic spectra have been generated for each hydrocarbon, providing the basis for extrapolation to higher temperatures. The spectra provided here will enable simultaneous measurements of multiple hydrocarbons in flames. This capability will greatly extend the range of applicability of Raman measurements in combustion applications. In addition, the experimental spectra provide a validation dataset for quantum mechanical models.

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Measurements have been conducted at the University of Virginia Supersonic CombustionFacility in configuration C of the dual-mode scramjet. This is a continuation of previously published works on configuration A. The scramjet is hydrogen fueled and operated at two equivalence ratios, one representative of the "scram" mode and the other of the "ram" mode. Dual-pump CARS was used to acquire the mole fractions of the major species as well as the rotational and vibrational temperatures of N 2 . Developments in methods and uncertainties in fitting CARS spectra for vibrational temperature are discussed. Mean quantities and the standard deviation of the turbulent fluctuations at multiple planes in the flow path are presented. In the "scram" case the combustion of fuel is completed before the end of the measurement domain, while for the ram case the measurement domain extends into the region where the flow is accelerating and combustion is almost completed. Higher vibrational than rotational temperature is observed in those parts of the hot combustion plume where there is substantial H 2 (and hence chemical reaction) present.

Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were performed in the University of Virginia's dual-mode scramjet experiment. The test section was set up in configuration A, which includes a Mach 2 nozzle, combustor, and extender section. Hydrogen fuel was injected through an unswept compression ramp at two different equivalence ratios. Through the translation of the optical system and the use of two separate camera views, the entire optical range of the combustor was accessed. Single-shot, average, and standard deviation images of the OH PLIF signal are presented at several streamwise locations. The results show the development of a highly turbulent flame structure and provide an experimental database to be used for numerical model assessment.

In this paper we describe efforts to obtain canonical data sets to assist computational modelers in their development of models for the prediction of mixing and combustion in scramjet combustors operating in the ramjet-scramjet transition regime. The CARS technique is employed to acquire temporally and spatially resolved measurements of temperature and species mole-fraction at four planes, one upstream of an H 2 fuel injector and three downstream. The technique is described and results are presented for cases with and without chemical reaction. The vibrational energy mode in the heated airstream of the combustor was observed to be frozen at near facility heater conditions and significant nonuniformities in temperature were observed, attributed to non-uniformities of temperature exiting the heater. The measurements downstream of fuel injection show development of mixing and combustion, and are already proving useful to the modelers.