Browsing by Author "Zghal, Malika"

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    Numerical investigation into the impact of operating boundary conditions on NOx formation in hydrogen micromix combustion system
    (American Society of Mechanical Engineers, 2023-09-28) Singh, Gaurav; Zghal, Malika; Sun, Xiaoxiao; Gauthier, Pierre; Sethi, Vishal
    Hydrogen micromix combustion is a promising technology for achieving zero mission-level carbon emissions with ultra-low NOx potential. A reduced-order NOx emissions prediction model is essential for preliminary hydrogen engine cycle design space exploration and optimization studies. Hence, this paper investigates the influence of key operating conditions, including equivalence ratio (ϕ), combustor inlet temperature (T3) and pressure (P3) on NOx emissions in a hydrogen micromix combustion. The assessments were performed using steady Reynolds-Averaged Navier-Stokes (RANS) simulations with thermal NOx model at various power conditions representative of the aircraft mission. The RANS model constants were calibrated against Large Eddy Simulations (LES) conducted previously by the group. The comprehensive numerical database was developed from these assessments to derive a NOx emissions correlation as a function of the operating conditions defined above. The study demonstrates that the LES-calibrated RANS models can predict NOx emissions trends, which agrees with the known physics of NOx formation. When experimental data is not yet available, the resulting correlation can be used at the preliminary stage of the design process to identify low NOx engine cycles that merit (more resource-intensive) higher fidelity numerical simulations or experiments. The methodology is flexible and extensible and may be applied to future low-emissions hydrogen combustion technologies.
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    Validation of a stochastic breakup model for turbulent jets in high-speed crossflow: assessment of turbulent interactions and sensitivity to boundary conditions
    (Begell House, 2023-04-25) Zghal, Malika; Sun, Xiaoxiao; Gauthier, Pierre Q.; Sethi, Vishal
    Improving the mixing of fuel and air by injecting a turbulent liquid fuel jet into a high-speed cross-flowing gas can reduce the emissions of gas turbine applications. To facilitate and hasten the development of such low-emissions technologies, accurate predictions of the spray characteristics are needed. The objective of the present study is to validate the predictive capabilities of a stochastic breakup model for turbulent transverse jets over a wide range of representative pressures and atomization characteristics. The effect of turbulence modeling is also assessed to provide accurate and computationally less expensive Eulerian−Lagrangian transient approaches. To do so, the predictions made with the large eddy simulation (LES) approach for different subgrid-scale (SGS) models and with the synthetic eddy method (SEM) are compared to the ones made using the unsteady Reynolds-averaged Navier-Stokes (RANS) approach with and without a turbulent dispersion model. The sensitivity of the numerical methodology to the upstream velocity profile, pressure, and momentum flux ratio were also assessed. Properly accounting for the upstream gas velocity profile was found to be critical to ensure accurate predictions of the spray characteristics. The unsteady RANS (URANS) turbulent approach coupled with the turbulent dispersion model showed good agreement with experimental data, but the LES approach tends to overpredict the spray penetration and underpredict the Sauter mean diameter (SMD). This could be due to the lower turbulent interactions it predicts, which may lead to lower momentum transfer between the phases.