Multi-fidelity Bayesian optimization framework for CFD-based non-premixed burner design

We propose a multi-fidelity Bayesian optimization (MF-BO) framework that integrates computational fluid dynamics (CFD) evaluations with Gaussian-process surrogates to efficiently navigate the accuracy-cost trade-off induced by mesh resolution. The design vector x = [h, l, s] (height, length, and mesh element size) defines a continuous fidelity index Z(h, l, s), enabling the optimizer to adaptively combine low- and high-resolution simulations. This framework is applied to a non-premixed burner configuration targeting improved thermal efficiency under hydrogen-enriched fuels. A calibrated runtime model t_hat(h, l, s) penalizes computationally expensive queries, while a constrained noisy expected improvement (qNEI) guides sampling under an emissions cap of 2e-6 for NOx. Surrogates trained on CFD data exhibit stable hyperparameters and physically consistent sensitivities: mean temperature increases with reactor length and fidelity and is weakly negative with height; NOx grows with temperature yet tends to decrease with length. The best design achieves T_bar approx 2.0e3 K while satisfying the NOx limit. Relative to a hypothetical single-fidelity campaign (Z = 1), the MF-BO achieves comparable convergence with about 57 percent lower total wall time by learning the design landscape through fast low-Z evaluations and reserving high-Z CFD for promising candidates. Overall, the methodology offers a generalizable and computationally affordable path for optimizing reacting-flow systems in which mesh-driven fidelity inherently couples accuracy, cost, and emissions. This highlights its potential to accelerate design cycles and reduce resource requirements in industrial burner development and other high-cost CFD-driven applications.