Measurements of the local equivalence ratio and its impact on the thermochemical state in laminar partially premixed boundary layer flames

Local fuel–air equivalence ratios, gas phase temperature and CO₂ mole fractions were measured by a combination of laser-induced fluorescence of nitric oxide used as a tracer and dual-pump coherent anti-Stokes Raman spectroscopy in a vertically oriented partially premixed boundary layer flame under laminar flow conditions. By embedding a secondary effusive fuel inlet into the temperature-controlled wall of a sidewall-quenching configuration, different pyrolysis rates of a wall-embedded polymer are mimicked with reduced chemical complexity and well-controlled boundary conditions. The resulting boundary layer flames were investigated experimentally and numerically. The simulation results and measurements show a very good agreement on the complex interplay between local mixing and heat losses, even though inhomogeneities of the wall inlet complicate the comparison of data. Local equivalence ratios upstream of the reaction zone reach values of up to Φ=2. Under these conditions, a clear quenching location could not be identified based on the experimental data. A significant trend toward lower temperatures and CO₂ mole fractions with increasing amount of secondary fuel was found in the thermochemical state close to the temperature-controlled wall, downstream of the effusive inlet

Time-resolved temperature profile measurements in the exhaust of a single sector gas turbine combustor at realistic operating conditions

Records of the time-varying temperature profile at flight relevant operating conditions are acquired at the exit of a combustion chamber fitted with a staged, lean-burn fuel injector using high-speed laser induced fluorescence (LIF) at a sample rate of 10 kHz. Temperatures are estimated from the concentration dependent fluorescence of the hydroxyl (OH) radical under the assumption of local equilibrium. Beyond the time-series analysis, the acquired data is correlated with simultaneously acquired OH chemiluminescence sampled in the primary zone near the fuel injector. These analyses reveal a strong influence from the precessing vortex core, originating in the primary zone, on oscillations in the temperature profiles measured at the exit of the combustor

CARS.jl: Efficient fitting of dual‐pump multi‐species CARS spectra using compressed libraries

In this publication, we demonstrate the extension of our loss‐less library approach previously limited to fitting ro‐vibrational N₂CARS spectra for thermometry purposes to accurate and efficient fitting of dual‐pump CARS spectra with multiple species. The fundamental reasoning behind the compression approach is to make use of finite resolution effects arising from the laser linewidths, which are typically larger than the transitional linewidths. For accurate reconstruction at runtime of the fit, in addition to the squared modulus and real component, also the imaginary component as well as intra‐region species cross terms have to be compressed and tabulated for dual‐pump multi‐species spectra. The resulting libraries do not require to tabulate combinations of mole fractions, leading to a near linear growth of the library with respect to the number of species. The accuracy and computational efficiency of the method is benchmarked using a reference implementation provided alongside this publication. Library generation times, mainly depending on the underlying spectral model, are typically in the order of seconds to a few minutes. The simulation of a single dual‐pump spectrum containing three species is sped up by a factor of >5000 without significantly affecting the accuracy of reconstruction. In a simulated experiment with Stokes‐noise imposed data, the time required to it temperature, mole fraction of three resonant species and wavenumber shift on a single spectrum required ≈90 ms

CARS.jl: efficient fitting of dual‐pump multi‐species CARS spectra using compressed libraries

In this publication, we demonstrate the extension of our loss‐less library approach previously limited to fitting ro‐vibrational N₂CARS spectra for thermometry purposes to accurate and efficient fitting of dual‐pump CARS spectra with multiple species. The fundamental reasoning behind the compression approach is to make use of finite resolution effects arising from the laser linewidths, which are typically larger than the transitional linewidths. For accurate reconstruction at runtime of the fit, in addition to the squared modulus and real component, also the imaginary component as well as intra‐region species cross terms have to be compressed and tabulated for dual‐pump multi‐species spectra. The resulting libraries do not require to tabulate combinations of mole fractions, leading to a near linear growth of the library with respect to the number of species. The accuracy and computational efficiency of the method is benchmarked using a reference implementation provided alongside this publication. Library generation times, mainly depending on the underlying spectral model, are typically in the order of seconds to a few minutes. The simulation of a single dual‐pump spectrum containing three species is sped up by a factor of >5000 without significantly affecting the accuracy of reconstruction. In a simulated experiment with Stokes‐noise imposed data, the time required to it temperature, mole fraction of three resonant species and wavenumber shift on a single spectrum required ≈90 ms

MARSFT: Efficient fitting of CARS spectra using a library-based genetic algorithm

Abstract A loss-less compressed library scheme is presented in this publication that allows for computationally efficient fitting of coherent anti-Stokes Raman spectra with no restriction to the number of degrees of freedom for the spectral fit. The compression is achieved by convolving the squared modulus and the real part of the complex susceptibility with a Gaussian kernel narrower than the experimental apparatus function. This effectively reduces library size while allowing to convolve to the final experimental linewidth during the fit. For the optimization procedure, a gradient-free mixed-integer genetic algorithm was implemented due to its ability to extract library spectra without interpolation. We demonstrate the ability of the code by comparing it to CARSFT in terms of dependency on starting solution, computational cost and accuracy using simulated spectra with varying noise contribution