Dual-pump coherent anti-Stokes Raman scattering measurements in a supersonic combustor
SUPERSONIC combustion involves a complex interaction between turbulent mixing and
chemical reaction. Whether efficient supersonic combustion occurs depends strongly upon
the rate and extent to which chemical reactions occur compared to the residence time of the
fluid in the combustor. In addition, no current ground testing facility is capable of perfectly
reproducing the conditions of a real combustor in flight, due to the excess water (and often
also carbon dioxide) in combustion-vitiated facilities, and the vibrational nonequilibrium …
chemical reaction. Whether efficient supersonic combustion occurs depends strongly upon
the rate and extent to which chemical reactions occur compared to the residence time of the
fluid in the combustor. In addition, no current ground testing facility is capable of perfectly
reproducing the conditions of a real combustor in flight, due to the excess water (and often
also carbon dioxide) in combustion-vitiated facilities, and the vibrational nonequilibrium …
SUPERSONIC combustion involves a complex interaction between turbulent mixing and chemical reaction. Whether efficient supersonic combustion occurs depends strongly upon the rate and extent to which chemical reactions occur compared to the residence time of the fluid in the combustor. In addition, no current ground testing facility is capable of perfectly reproducing the conditions of a real combustor in flight, due to the excess water (and often also carbon dioxide) in combustion-vitiated facilities, and the vibrational nonequilibrium, excess nitric oxide and atomic species, and short test times of pulsed facilities. If all these effects are to be accounted for, it is important to have available nonintrusive and spatially resolved methods for determining the location and extent of chemical reaction within an engine. Modeling fuel–air mixing and combustion using computational fluid dynamics (CFD) requires quantitative data from within the flow to evaluate the performance of turbulent mixing models and to develop new models. Although measurements of surface properties such as pressure and heat transfer are commonplace, quantitative or semiquantitative measurements offlowfield properties are difficult to obtain, and hence there are far fewer examples in the literature. Nonintrusive measurements have been performed in supersonic mixing and combustion experiments using coherent anti-Stokes Raman scattering (CARS)[1–7], laser-induced fluorescence (LIF)[8–11], and other techniques. Whereas CARS can obtain pointwise multiparameter measurement of flow properties for major species such as N2 and O2, LIF provides spatially resolved flowfield visualization and measurements of temperature and mole fraction for minority species. Information about OH, CH, and other radical species provided by LIF is particularly useful in understanding combustion interactions. LIF is also easily extended to planar measurements, allowing instantaneous and high-spatial-resolution data to be acquired. However, LIF has not been used to produce quantitative measurements of mole fraction for majority species such as H2, N2, and O2 in combusting flow. Thus, CARS and LIF are complementary measurement techniques. Some forms of the CARS technique [12] are particularly valuable in performing quantitative studies of supersonic combusting flows because they have the potential to measure temperature and the concentrations of multiple important combustion species simultaneously. Other techniques such as Raman scattering can measure the same quantities, but CARS has the advantage of producing a coherent signal beam. This increases the signal-to-background ratio of measurements and permits spatial filtering to reduce the degradation of measurement quality due to interference from flow luminosity. The coherent nature of the CARS signal also means that good signal-to-noise ratio measurements can be made where optical access to the flow is limited, which is typically the case for scramjet combustors. When flow properties are measured simultaneously, correlations between the different species mole fractions and between the temperature and species mole fractions can be measured. These correlations can then be used to evaluate parameters in proposed turbulence models in Reynolds-averaged Navier–Stokes codes, or to determine the effectiveness of large eddy simulation turbulence models.
There are several varieties of the CARS technique, and the choice of a particular method has a significant effect upon the difficulty and results of a given supersonic combustion experiment. Broadband N2 vibrational CARS is one of the most commonly used …
AIAA Aerospace Research Center
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