Coupling regimes of premixed laminar flames with thermal radiation absorption in fresh gases. Application to H2O-/CO2-diluted mixtures

Abstract

In the context of decarbonizing industry and transportation, the combustion of hydrogen and oxycombustion of methane play a pivotal role. Hydrogen combustion can use steam (H${}_2$O) to mitigate pollutant emissions, while methane’s oxycombustion involves recirculating burnt gases (EGR or Exhaust Gas Recirculation process), particularly CO${}_2$. This paper investigates the complex role of thermal radiation in premixed laminar flames, in particular in such H${}_2$ AirH${}_2$O and CH${}_4$ O${}_2$ CO${}_2$ mixtures. It highlights how radiation assumes a significant role in flames diluted with radiative participating gases, through both emission and reabsorption. The main objective is to achieve a comprehensive physical understanding of the coupling between thermal radiation and combustion, examining its effects on flame structure and burning velocity and how it varies with different parameters (equivalence ratio, dilution level, pressure, and domain size). The study employs detailed 1D premixed laminar flame simulations by coupling a fluid and a radiative solver. Both a grey gas approximation for preliminary understanding and realistic radiative gas properties (CK model) are considered. Coupling numbers derived from characteristic time ratios for convection, chemistry, and radiation, are presented. These metrics facilitate the classification of radiation-combustion coupling into three distinct regimes that represent distinct qualitative physical phenomena. The regimes are defined as follows: WeakAbs, where the effects of thermal radiation absorption are minor; RadConv, where thermal radiation competes with convection in the fresh and burnt gases but does not interact directly with chemistry within the flame front; and RadChem, where thermal radiation also competes with chemistry within the flame front. For the investigated conditions in both H${}_2$ and CH${}_4$ flames, thermal radiation also quantitatively alters the flame speed, with an acceleration that can be significant. Furthermore, the paper presents an iterative two-layer model to efficiently estimate the impact of thermal radiation on flames, which shows high accuracy except in the RadChem regime. Lastly, it introduces a predictive model that quickly determines a flame’s coupling regime using only an adiabatic simulation, helping in deciding to neglect, approximate, or fully integrate radiation in premixed diluted flame simulations.