The improvement of combustion models for turbulent non-premixed combustion is severely limited by the lack measurements of mixture fraction and its dissipation in complex reacting systems. Currently, the highest fidelity mixture fraction measurements are inferred from Raman/Rayleigh scattering, but these measurements are difficult to apply in more applied combustion environments where walls, soot or particles may be present. We propose to develop a new technique that holds promise for enabling space- and time-resolved imaging of a conserved scalar in turbulent reacting flows, and which can be applied in a wider range of environments than alternative techniques. This new technique is based on using two-photon laser-induced fluorescence (LIF) of a noble gas (e.g., Kr or Xe), which is seeded into the fuel or oxidizer stream. Noble gases are inert in the presence of combustion and so can be treated as a conserved scalar. The noble-gas LIF measurement gives the mole fraction of the conserved scalar, but a temperature measurement and state relationship are required to infer the mixture fraction (i.e., mass fraction of atoms originating in the fuel stream). Our main objectives will be the following: (i) to improve our understanding of the technique's limitations, particularly regarding the need for an assumed state relationship, (ii) to determine if the technique can be applied with higher-hydrocarbon fuels where laser beam absorption, fluorescence interference and quenching may add additional difficulties, (iii) to combine the technique with particle image velocimetry (PIV) to obtain important and still unique mixture-fraction/velocity correlation data in flames, (iv) to investigate two-photon Xe LIF as an alternative conserved scalar, and (v) explore combining noble-gas LIF with filtered Rayleigh scattering to obtain simultaneous temperature fields when particles/soot/walls are present.
Intellectual Merit: The proposed work has as its objective the further development of a new diagnostic technique, which holds promise for enabling measurements of mixture fraction and its dissipation in non-premixed combustion systems. The main conserved scalar of interest is krypton, although xenon will be investigated. The combined two-photon LIF will enable conserved scalar measurements to be made in more applied flows, such as confined combustors, or in flows with particles. This feature will enable one to combine mixture fraction imaging with PIV, and potentially to make conserved measurements in flows with soot or soot precursors. It also can be effectively used as a technique for studying mixing/combustion where safety concerns preclude the use of toxic-gas markers.
Broader Impacts: This new technique holds promise for enabling measurements of mixture fraction (or a related surrogate conserved scalar quantity) in combustion environments where such measurements were previously not practical, such as gas-turbines, IC engines, fires, and supersonic combustors. Noble-gas LIF is potentially much easier to implement than alternative techniques and it is likely that the technique will see much wider use in academics, government and industry. The data that can be provided by the technique could potentially impact the accuracy of advanced combustion models and therefore impact a wide-range of technologies including those that utilize sustainable fuels. As great strides are made in computational engineering, we must not lose sight of the importance of validating the computations and this will require that we maintain and develop the experimental infrastructure that enables the acquisition of the needed data.
