Diffusive-thermal effect on local chemical structures in premixed hydrogen–air flames

Abstract

The diffusive-thermal effect plays an important role in the intrinsic instability of premixed flames. A two-dimensional direct numerical simulation of the propagation of premixed hydrogen-air flames was performed using a detailed chemical kinetics model. The cellular behavior of a lean hydrogen-air flame was analyzed on the basis of its chemical structure.The primary consequence of the diffusive-thermal effect was found to be a change in the buildup process of reactive intermediates by the chain reaction mechanism at the preheat zone. The resultant chemical structure at the main reaction zone can be explained by the local composition change of the gas flowing into the reaction zone from the preheat zone.

Analysis of premixed flame propagation between two closely-spaced parallel plates

“…In particular, for Le = 1 and ratios of the plate separation h to the thermal flame thickness δ T of the order a ≡ h/δ T = S L h/D Tu ≈ 15, where a can be interpreted as a Peclet number, the authors reported an overall flame speed S T 1 of about 1.4 times that of the adiabatic planar velocity S L . Excluding ST by neglecting the viscous drag due to Poiseuille flow, S T /S L decreased to about 1.25, in agreement with values previously calculated for purely 2D simulations that included DL but not ST effects [33,34,35,36,37]. Experimental studies have investigated and identified the contributions of some of these intrinsic instability mechanisms in Hele-Shaw cells [39,38,40], making use of different mixtures, direction of propagation and distance between the plates.…”

“…Calculations neglecting the transverse confinement [33,34,35,36,37], that is, for purely 2D freely propagating flames without confinement, predict a propagation speed of S T /S L ≈ 1.2. This speed increases when the transverse confinement is included in Hele-Shaw cells, through expansion-induced straining effects, to S T /S L ≈ 1.4 if a = 15 [32] and to S T /S L ≈ 2.2 in the limit of large viscosity effects (a → 0) proposed in the present work.…”