The usage of direct numerical simulations for research of turbulent combustion for space propulsion applications is explored. With this goal in mind, the combustion near injection of a fuel-rich methane-oxygen flame at 20 bar is simulated using a massively parallelized solver. The statistical properties of the relevant physical fields are examined to study the interactions between turbulence and combustion. This analysis is complemented by an investigation and quantification of the error sources in direct numerical simulations of turbulent diffusion flame. A method to estimate the statistical error is derived based on classical inference theory. In addition, critical resolution criteria are discussed using a mesh sensitivity analysis.
The generation of periodic synthetic turbulence through superposition of Fourier modes is investigated. The introduction of directional biases and mismatches in the second-order statistics associated with the enforcement of periodicity is analyzed and quantified. Two strategies for mitigation of these disparities are proposed. The suggested approaches are subsequently validated and compared with the original methodologies using direct numerical simulations. The proposed strategies are capable of neutralizing the disparities in the second-order statistics at the injection region. The development from synthetic to realistic turbulence is evaluated through the resulting flow statistics and spectral analysis. It is found that higher-order statistics and other indicators converge to the expected results with sufficient length.
Direct numerical simulations of a turbulent premixed stoichiometric methane-oxygen flame were conducted. The chosen combustion pressure was 20 bar, to resemble conditions encountered in modern rocket combustors. The chemical reactions followed finite rate detailed mechanism integrated into the EBI-DNS solver within the OpenFOAM framework. Flame geometry was thoroughly investigated to assess its interaction with the transport of turbulent properties. The resulting flame front was remarkably thin, with high density gradients and moderate Karlovitz and Damköhler numbers. At mid-flame positions, the variable-density transport mechanisms dominated, leading to the generation of both vorticity and turbulence. A reversion of this trend towards the products was observed. For intermediate combustion progress, vorticity transport is essentially a competition between the baroclinic torque and vortex dilatation. The growth of turbulent kinetic energy is strongly correlated to this process. A geometrical analysis reveals that the generation of enstrophy and turbulence is restricted to specific topologies. Convergent and divergent flame propagation promote turbulence creation due to pressure fluctuation gradients through different physical processes. The possibility of modeling turbulence transport based on curvature is discussed along with the inherent challenges.
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