Turbulent flame speed and reaction layer thickening in premixed jet flames at constant Karlovitz and increasing Reynolds numbers

Abstract

A series of Direct Numerical Simulations (DNS) of lean methane/air flames was conducted in order to investigate the enhancement of the turbulent flame speed and modifications to the reaction layer structure associated with the systematic increase of the integral scale of turbulence l while the Karlovitz number and the Kolmogorov scale are kept constant. Four turbulent slot jet flames are simulated at increasing Reynolds number and up to Re ≈ 22000, defined based on the bulk velocity, slot width, and the reactants' properties. The turbulent flame speed S T is evaluated locally at select streamwise locations and it is observed to increase both in the streamwise direction for each flame and across flames for increasing Reynolds number, in line with a corresponding increase of the turbulent integral scale. In particular, the turbulent flame speed S T increases exponentially with the integral scale for l up to about 6 laminar flame thicknesses, while the scaling becomes a power-law for larger values of l. These trends cannot be ascribed completely to the increase in the flame surface, since the turbulent flame speed looses its proportionality to the flame area as the integral scale increases; in particular, it is found that the ratio of turbulent flame speed to area attains a power-law scaling l 0.2 . This is caused by an overall broadening of the reaction layer for increasing integral scale, which is not associated with a corresponding decrease of the reaction rate, causing a net enhancement of the overall burning rate. This observation is significant since it suggests that a continuous increase in the size of the largest scales of turbulence might be responsible for progressively stronger modifications of the flame's inner layers even if the smallest scales, i.e., the Karlovitz number, are kept constant.

Statistics of local and global flame speed and structure for highly turbulent H2/air premixed flames

“…In contrast to a large number of studies in the literature, where S T /S L has been sought to correlate with either u /S L or non-dimensional numbers including Re, Da, or Ka, the current result indicates that S T /S L and A T /A L strongly correlate and proportionally increase with the integral length scale. This is consistent with the previous work by Luca et al [20] and Attili et al [21] for a slot jet flame configuration in that S T /S L is proportional to l T /δ L . In fact, Luca et al [20] reported that the stretch factor was nearly unity in a global sense, while from a local perspective, Attili et al [21] suggested that the slight increase of the local I 0 with local l T /δ L involves an extra factor in enhancing the turbulent flame speed, which is the inner layer thickening.…”

“…This is consistent with the previous work by Luca et al [20] and Attili et al [21] for a slot jet flame configuration in that S T /S L is proportional to l T /δ L . In fact, Luca et al [20] reported that the stretch factor was nearly unity in a global sense, while from a local perspective, Attili et al [21] suggested that the slight increase of the local I 0 with local l T /δ L involves an extra factor in enhancing the turbulent flame speed, which is the inner layer thickening. A stretch factor of nearly unity was also reported by Lapointe [42] based on DNS data of a turbulence-in-a-box configuration and by Kulkarni et al [41] for spherically expanding flames with different integral length scales.…”

“…To gain fundamental understanding, laboratory-scale flames at Ka > 100 conditions have been studied experimentally and numerically. Experiments were conducted in swirl [6][7][8][9][10] and pilot [11][12][13][14][15] configurations, while direct numerical simulation (DNS) studies are largely limited to the turbulence-in-a-box configuration, except for a few studies of laboratory-scale combustors [10,[16][17][18][19][20][21], due to high computational costs. A number of review papers provided a detailed account of recent progress on turbulent premixed combustion research [22][23][24][25].…”

“…The effects of l T on S T have so far remained largely unexplored and have been partly investigated in turbulent jet flames [20,21], spherically expanding flames [41], and turbulencein-a-box flames [42]. Although these studies commonly reported that S T has a proportional relation with l T , the dependence of the stretch factor (defined by (S T /S L )/(A T /A L ), with…”

“…A denoting flame surface area and the subscripts T and L corresponding to the turbulent and laminar counterparts) on the integral length scale was reported to increase in [21] but remained constant in [41,42]. Moreover, discussion on flame structure over a wide range of parametric spaces is insufficient in the literature despite its importance to turbulent combustion modeling.…”