Recent Advances in Understanding of Thermal Expansion Effects in Premixed Turbulent Flames

Abstract

When a premixed flame propagates in a turbulent flow, not only does turbulence affect the burning rate (e.g., by wrinkling the flame and increasing its surface area), but also the heat release in the flame perturbs the pressure field, and these pressure perturbations affect the turbulent flow and scalar transport. For instance, the latter effects manifest themselves in the so-called countergradient turbulent scalar flux, which has been documented in various flames and has challenged the combustion community for approximately 35 years. Over the past decade, substantial progress has been made in investigating (a) the influence of thermal expansion in a premixed flame on the turbulent flow and turbulent scalar transport within the flame brush, as well as (b) the feedback influence of countergradient scalar transport on the turbulent burning rate. The present article reviews recent developments in this field and outlines issues to be solved in future research.

Direct numerical simulation study of statistically stationary propagation of a reaction wave in homogeneous turbulence

“…In particular, as shown by Lipatnikov and Chomiak [56], the three most extensive experimental databases obtained from expanding statistically spherical premixed flames [66][67][68] are reasonably well fitted with S T ∝ U Ka −1/3 or S T ∝ U Da 1/4 and, therefore, indicate a less pronounced dependence of S T /U on Ka or Da when compared to the present DNS. Differences between these measured data and the present DNS results could be attributed to a number of factors, such as (i) thermal expansion effects, reviewed elsewhere [69,70], (ii) differences between experimental and numerical flame configurations, (iii) differences between methods adopted to evaluate S T in the experiments and simulations [71], etc.…”

Letter: Does flame-generated vorticity increase turbulent burning velocity?

“…Since that pioneering work, the thermal expansion effects were in the focus of research into turbulent combustion, 3-7 but progress in modeling them has yet been rather moderate, as reviewed elsewhere. 8,9 Nevertheless, the concept 1,2 of combustion acceleration due to flame-generated turbulence has never been disputed, to the best of the present authors' knowledge, at least in the case of weak or moderate turbulence 8 associated with a well-pronounced increase in the burning rate by the rms turbulent velocity u . The present letter aims at putting this widely accepted concept into question by showing that flamegenerated vorticity can smooth wrinkles of the local flame surface, thus, reducing its area and, consequently, decreasing turbulent burning velocity U t .…”

Does Density Ratio Significantly Affect Turbulent Flame Speed?

“…On the contrary, as reviewed elsewhere [19–21], there are a number of well-documented phenomena associated with the influence of combustion-induced density variations on turbulent flow and transport within a premixed flame brush, with these phenomena being argued by many experts to substantially affect S t . For instance, turbulent flame speed was hypothesized to be affected by

flame-generated turbulence highlighted by Karlovitz et al [22] and Scurlock and Grover [23] and investigated in a number of subsequent experimental papers reviewed elsewhere [19–21], e.g.

…”

“…For instance, turbulent flame speed was hypothesized to be affected by flame-generated turbulence highlighted by Karlovitz et al [22] and Scurlock and Grover [23] and investigated in a number of subsequent experimental papers reviewed elsewhere [19–21], e.g. see Direct Numerical Simulation (DNS) study by Poludnenko [24] as a recent example,…”

“…For instance, turbulent flame speed was hypothesized to be affected by

flame-generated turbulence highlighted by Karlovitz et al [22] and Scurlock and Grover [23] and investigated in a number of subsequent experimental papers reviewed elsewhere [19–21], e.g. see Direct Numerical Simulation (DNS) study by Poludnenko [24] as a recent example,

counter-gradient transport (criteria of its appearance [25–27] and various simple models [28–32] consider σ to be an important input parameter, with the influence of the magnitude of the counter-gradient flux on S t being theoretically proved [33, 34]),the local Darrieus-Landau (DL) instability [35] of thin, inherently laminar flame fronts (flamelets), caused by the local density drop (some of models that address influence of the DL instability on premixed turbulent combustion straightforwardly yield an increase in S t by σ [36, 37], whereas other models yield an increase in S t with decreasing M a [38] or a neutral wavelength1 λ n of the DL instability [40]),variations in the mean scalar dissipation rate due to dilatation [41], where D is the molecular diffusivity and c is the combustion progress variable.The reader interested in a deeper discussion of these phenomena and models is referred to recent review papers [20, 21, 42]. Here, we restrict ourselves to pointing out that a substantial effect of the density ratio on turbulent flame speed is widely expected in theoretical and numerical combustion community, but such an effect is not indicated by experimental parameterizations of S t .…”