Bayesian inference and uncertainty quantification of reduced chemical schemes

This work concerns the uncertainties arising from the derivation of global chemistry models and their impact on the predictions using modern combustion simulations. We perform the inference of the parameters of a two-step reaction mechanism for CH 4 , using synthetic observations of one-dimensional laminar flames generated using detailed mechanism simulations. Introduction of Principal Component Analysis (PCA) and the Polynomial Chaos (PC) expansion, to approximate the global model predictions, enables a full assessment of the inferred global model's posterior. In particular, we employ the Bayesian posteriors' extensive sampling to estimate mean, Maximum a Posteriori, and confidence intervals of the inferred global model's predictions. We contrast the posterior distributions of global quantities of the flame, namely the laminar flame speed, the thermal flame thickness, and the reaction zone thickness, depending on the inference's observations. Finally, we propagate the global chemistry model's posterior distribution through two-dimensional direct numerical simulations (DNS) of a flame-vortex interaction problem. This study highlights the importance of quantifying posterior uncertainties to fully appreciate the impact of using a global model in real-world reactive simulations

Physical study of the non-equilibrium development of a turbulent thermal boundary layer

The direct numerical simulation of a non-equilibrium turbulent heat transfer case is performed in a channel flow, where non-equilibrium is induced by a step change in surface temperature. The domain is thus made of two parts in the streamwise direction. Upstream, the flow is turbulent, homogeneous in temperature and the channel walls are adiabatic. The inflow conditions are extracted from a recycling plane located further downstream so that a fully developed turbulent adiabatic flow reaches the second part. In the domain located downstream, isothermal boundary conditions are prescribed at the walls. The boundary layer, initially at equilibrium, is perturbed by the abrupt change of boundary conditions and a non-equilibrium transient phase is observed until, further downstream, the flow reaches a new equilibrium state presenting a fully developed thermal boundary layer. The study focuses on the spatial transient phase, identifies the main non-equilibrium effects and contrasts these results with usual assumptions of equilibrium turbulent heat transfer. Mean and root-mean-square profiles of temperature and velocity, as well as the respective energy and momentum balances, are presented and discussed along with budgets of second-order moment balance equations for the enthalpy variance and the wall-normal heat flux. For several quantities, an equilibrium near-wall region is identified even near the leading edge while the boundary layer is still developing. Finally, the evolution of the turbulent Prandtl number along the channel flow is investigated and shows that it reaches equilibrium only further downstream

Simulation of the ignition process in an annular multiple-injector combustor and comparison with experiments.

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