The Role of Flame–flow Interactions on Lean Premixed Lifted Flame Stabilization in a Low Swirl Flow

ABSTRACT

Swirling flows have been widely used to stabilize lean premixed combustion in various gas turbines and furnaces. In such flows, understanding and characterizing the flame stabilization are of both practical and fundamental interests. It is known that the swirling motion decreases the flow velocity at the burner outlet, which contributes to flame stabilization. In low swirl flows, such a deceleration stabilizes a premixed flame aerodynamically. The present investigations, using large eddy simulations, study flame–flow interactions in lean premixed lifted flame stabilized in a low swirl flow. The results show that in addition to the swirling motion, combustion heat release reduces axial velocity in the reactant stream by reducing dilatation, vortex stretching, and baroclinic vorticity production terms. The analyses of vorticity production source terms show that besides the flow deceleration induced by the swirling motion, the dominant mechanism for the flow deceleration upstream of the low swirl lifted flame is the baroclinic torque. However, unlike the dilatation term, the effects of the vortex stretching and baroclinic diminish further upstream of the flame front.

KEYWORDS:

• Flame stabilization
• Large eddy simulation
• Lean premixed flame
• Low swirl lifted flame
• Vorticity production

Nomenclature

$\dot{\omega }=0.01\dot{\omega }max$$\rho$=density

$U$=velocity

$P$=pressure

${\tau }_{ij}$=stress tensor

$\mathrm{Y}$=mass fraction

$h_s$=sensible enthalpy

$Sc$=Schmidt number

$Pr$=Prandtl number

$S_{ij}$=strain rate tensor

$\mu$=viscosity

$k$=kinetic energy

$\delta$=Kronecker delta

$C_k$, $\beta$, $C_{ms}$,$\mathrm{a}$=Costants

$\mathrm{\Delta }$=grid size

${\dot{\omega }}_h$=source term in conservation of sensible enthalpy equation

${\dot{\omega }}_k$=source term in conservation of mass equation

$S_L^{\hspace{0.17em}}$=laminar flame speed

${\delta }_L^{\hspace{0.17em}}$=laminar flame thickness

$D_m$=molecular diffusivity

${\dot{\omega }}_{\hspace{0.17em}}$=reaction rate

$F$=thickening factor

$E$=efficiency function

$u^{\prime }$=velocity fluctuation

$Re$=Reynolds number

$RR$=heat release rate

$A$=pre-exponential factor

$T$=temperature

${\dot{q}}_s$=spark source term

$E_s$=total spark energy

${\sigma }_s$=size of the spark kernel

${\sigma }_t$=duration of spark discharge

$\mathrm{t}$=time

$\left(r,\hspace{0.17em}\theta ,Z\right)$=cylindrical coordinate

${\mathrm{\Delta }}_s$=spatial characteristic length of the spark

${\mathrm{\Delta }}_t$=temporal characteristic length of the spark

$C_p$=heat capacity at constant pressure

$\mathrm{D}$=burner diameter

φ=root-mean-square velocity fluctuation

$u$=equivalence ratio

$\omega$=vorticity

Subscripts/Superscripts

Subscripts

$T$=turbulent

$sgs$=Subgrid-scale

$max$=Maximum

$is$=instant sparking

$un$=unburnt mixture

rms=root-mean-square fluctuation

Superscripts

$sgs$=Subgrid-scale

$0$=non-thickened flame

1=thickened flame

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.