Vorticity isotropy in high Karlovitz number premixed flames

Abstract

The isotropy of the smallest turbulent scales is investigated in premixed turbulent combustion by analyzing the vorticity vector in a series of high Karlovitz number premixed flame direct numerical simulations. It is found that increasing the Karlovitz number and the ratio of the integral length scale to the flame thickness both reduce the level of anisotropy. By analyzing the vorticity transport equation, it is determined that the vortex stretching term is primarily responsible for the development of any anisotropy. The local dynamics of the vortex stretching term and vorticity resemble that of homogeneous isotropic turbulence to a greater extent at higher Karlovitz numbers. This results in small scale isotropy at sufficiently high Karlovitz numbers and supports a fundamental similarity of the behavior of the smallest turbulent scales throughout the flame and in homogeneous isotropic turbulence. At lower Karlovitz numbers, the vortex stretching term and the vorticity alignment in the strain-rate tensor eigenframe are altered by the flame. The integral length scale has minimal impact on these local dynamics but promotes the effects of the flame to be equal in all directions. The resulting isotropy in vorticity does not reflect a fundamental similarity between the smallest turbulent scales in the flame and in homogeneous isotropic turbulence. Published by AIP Publishing. [http://dx

Enstrophy transport conditional on local flow topologies in different regimes of premixed turbulent combustion

“…A number of recent studies 23 – 29 investigated the Reynolds-averaged enstrophy transport equation in turbulent premixed flames by Reynolds-averaging eq. 4 : …”

“… 30 demonstrated that the characteristic Lewis number significantly affects the vorticity generation within the flame, and a combination of augmented flame normal acceleration and high extent of flame wrinkling for small values of Lewis number may give rise to a significant amount of vorticity generation within the flame brush due to baroclinic torque. Bobbit and coworkers 28 , 29 demonstrated that the enstrophy transport in statistically planar flames propagating in homogeneous isotropic turbulence for large values of Karlovitz number is governed by the relative balance between the vortex-stretching and viscous dissipation similar to its non-reacting counterpart, and also revealed that the choice of chemical reaction mechanism does not affect the qualitative nature of the enstrophy transport. The enstrophy field in turbulent premixed flames using cinema- stereoscopic particle image velocimetry (PIV) measurements of rim-stabilised turbulent premixed flames has been investigated 32 – 34 and confirmed some of the observations based on DNS data.…”

“…The aforementioned analyses 23 – 29 , 31 – 34 provided invaluable insight into the flame-turbulence interaction, but the nature of the enstrophy transport conditional on generic local flow topologies S1–S8 in different premixed combustion regimes is yet to be analysed in detail. Such an analysis is expected to reveal the canonical flow configurations which make dominant contributions to the enstrophy transport for different combustion regimes.…”

Dissipation and dilatation rates in premixed turbulent flames

“…16,[20][21][22] Contrary to the aforementioned studies of the influence of heat release on the dissipation rate in the non-premixed mode of turbulent burning, the present authors are not aware on evaluation of the ratio v in premixed flames (note that such a flame is characterized by a significant spatial gradient of the mean flow velocity in the direction normal to the mean flame surface). Nevertheless, in a recent DNS study 24,25 of highly turbulent premixed flames, the influence of combustion-induced thermal expansion on small-scale turbulence was also attributed to the increase in the mixture viscosity.…”

Enstrophy transport in swirl combustion

“…Recently, direct numerical simulations (DNS) have been used extensively to study enstrophy transport in the canonical configuration of flames embedded in boxes of low Mach number homogeneous isotropic turbulence (HIT) (Bobbitt & Blanquart 2016; Bobbitt, Lapointe & Blanquart 2016; Chakraborty, Konstantinou & Lipatnikov 2016; Dopazo, Cifuentes & Chakraborty 2017; Papapostolou et al. 2017).…”