Chapter IV: Effective Lewis numbers in turbulent non-premixed flames
4.3 Extracting effective Lewis numbers
4.3. Extracting effective Lewis numbers 53 In Eq. (4.4), the “correction terms” include diffusion, correction diffusion, and dif- ferential diffusion contributions. The reader is referred to Ch. 2 for the complete form of these equations.
Equation (4.4) requires user-supplied Lewis numbers, and scalar dissipation rate.
The effective Lewis numbers in Eq. (4.4) are evaluated using Eq. (1.11), where the laminar Lewis numbers are computed from the flamelet equations for each χst at the mixture fraction location of maximum temperature (see Ch.2).
In these flamelet equations, the effect of the flow-field on the flamelet structure, represented by the scalar dissipation, is unknown and must be modeled a priori.
This is typically done by imposing an analytical function for χ, and scaling it by the stoichiometric value, χst,
χ(Z) = χst
f(Z)
f(Zst), (4.5)
where Zst is the stoichiometric mixture fraction. The function f(Z) is evalu- ated from the solution of the mixture fraction equation for counterflow diffusion flames [133,134] and unsteady laminar mixing layers [65], and takes the form
f(Z) =exp
−2f
erfc−1(2Z)g2
. (4.6)
Figure 4.2 shows a comparison between the optimal χ profiles imposed in the FlameMaster code [79] using Eq. (4.6), and the experimentally-measured scalar dissipation rate, referred to as χexpTNF, for flames C, D and E at 7.5d, 15d and 30d downstream of the burner lip (from Karpetis & Barlow [132]), normalized by the stoichiometric value. For the sake of clarity, only the imposed optimal χprofile for flame D is shown. To compare the imposed scalar dissipation profile with χexpTNF, a
‘χTNF’ is computed from χusing
χTNF =2α|∇ZTNF|2= (∂ZTNF/∂Z)2 χ, (4.7) and is shown in Fig. 4.2, normalized by the stoichiometric value. As mentioned in Sec.4.2, ZTNF is computed from the measured species, and is slightly different from Z. This is the reason for the shape difference between the χand χTNFprofiles in Fig.4.2. As can be seen, the shape of the imposed χTNF profile is close to the experimentally-measured profiles.
Finally, the chemical model used in this work is GRIMech3.0 [135] without the nitrogen chemistry, with a total of 36 species. A comparison with Caltech- Mech2.4 [111] is discussed in Sec.4.4.3.
4.3. Extracting effective Lewis numbers 54 4.3.3 Error maps
The flamelet equations are solved for a range of χst and γ values. The values for χstspan the range[2,260], to cover the relevant part of the stable burning branch of the S-shaped curve. The scalar dissipation rate at the extinction point is 260s−1for the laminar Lewis number limit, and 475s−1for the unity Lewis number limit. The Lewis numbers are computed from Eq. (1.11), where γ takes values in the range [0,120], such that the effective Lewis numbers span the range [Lei,1]and[1,Lei], for sub-unity and above-unity laminar Lewis numbers, respectively.
The solutions of the flamelet equations are then compared to the experimental data to obtain two-dimensional error maps, for which a total of 1598 flamelets is used.
The error is defined as L2(χst, γ) =
1 NsNp
X
j,i
Yi(χst, γ,ZTNF,j)−Yiexp(ZTNF,j)
2
"
maxj {Yiexp(ZTNF,j)}
#2
1 2
, (4.8)
where Ns and Np are the number of species and the number of experimental data points, respectively.Yi(χst, γ,ZTNF,j) is theith species mass fraction obtained from the solution of the flamelet equations for given χst and γ, and interpolated onto the experimental mixture fraction point ZTNF,j.Yiexp(ZTNF,j) is theith species mea- sured conditional mean mass fraction at ZTNF,j. The use of different error norms is discussed in Sec.4.4.4.
Theoptimalvalues of χst andγ ,i.e., (χoptst , γopt), are then estimated by solving arg min
χst,γ
L2 χst, γ s.t. χst ∈[2,260], γ ∈[0,120].
(4.9) It should be noted that the approach described in this section provides a simple means of separating two different effects of increasing the jet Reynolds number:
increased scalar dissipation rates [136] and decrease of differential diffusion [69, 71].
In Eq. (4.8), the temperature is not considered. The motivation for this choice is that radiation heat losses are not included in the flamelet model used to extract the optimal flamelet parameters. Doing so would significantly increase the complexity of the present approach. However, while the comparisons between the DNS and
4.3. Extracting effective Lewis numbers 55
(a)Flame B,x/d=15. (b)Flame C,x/d=30. (c)Flame E,x/d=45.
Figure 4.3:Colormaps of L2error by Eq. (4.8) for flames B atx/d=15(left), C atx/d=30 (center), and E at x/d=45(right). The white dashed lines intersect at the optimal flamelet (minimum L2error).
2 10 100 260
5 10 15 20 25 30 35 40 45 50 Extinction limit
χstopt [1/s]
x/d
Flame B Flame C Flame D Flame E
2 10 100 260
5 10 15 20 25 30 35 40 45 50
(a) χoptst vs. x/d.
0.01 0.1 1 10 100
5 10 15 20 25 30 35 40 45 50
γopt
x/d 0.01
0.1 1 10 100
5 10 15 20 25 30 35 40 45 50
(b)γoptvs. x/d.
Figure 4.4: Optimal scalar dissipation rate, χoptst (left), and γopt (right) as a function of the downstream direction x/d for flames B, C, D, and E. In Fig. 4.4a, the shaded region corresponds to values of χstabove the laminar Lewis number extinction limit. In Fig.4.4b the values ofγopt for Flame E andx/d <30are 0, and are not shown.
the measurements for Sandia flame B discussed in Ch.6, suggest that radiation has a non-negligible impact on the global mixing field and on the flame structure, it will be shown in Ch. 7 that the optimal flamelet parameters extracted from DNS data with and without radiation, present only small differences. This suggests that radiation has a secondary effect on the optimal flamelet parameters.
Figure 4.3 shows the error given by Eq. (4.8) for flame B at x/d = 15, flame C at x/d = 30, and flame E at x/d = 45. The first and the third cases were shown in [46] as examples of laminar and turbulent limiting behavior. The white dashed lines intersect at the point of minimum error, i.e., (χoptst , γopt). As expected, for flame B at x/d = 15, γopt is small, such that the effective Lewis number for H2is 0.4, close to its laminar value (LeH2 ≈ 0.3). However, with increasing jet Reynolds
4.3. Extracting effective Lewis numbers 56 number and streamwise distance from the burner exit, LeH2,eff increases to 0.7for flame C atx/d =30, and to1.0in flame E atx/d =45.
Figure4.3also shows that Eq. (1.11) has a high sensitivity onγforγ≈1, but there is low sensitivity (or high error) for lowγ(γ <0.1) and highγ (γ >10).
4.3.4 Optimal flamelet parameters
The optimal χst and γ for all x/d locations for flames B, C, D, and E, are shown in Figs.4.4aand4.4b, respectively. Only two measurement locations are available for flame B. The error maps are computed only up tox/d =45, as the flame length (defined using the iso-surface of stoichiometric mixture fraction at the centerline) is located close to that measurement station for all four flames [72]. The stoichio- metric flame length for flames C, D, and E, can be estimated through interpolation as 45.4, 48.1, and 49.9 diameters, respectively. Furthermore, due to the large un- certainties for the scalars immediately downstream of the burner exit, and the fact that for that regionγopt ≈ 0, only stations starting fromx/d =7.5are shown.
Optimal χst
Figure 4.4a shows that, as expected, χoptst generally decreases with axial distance from the burner exit. The regions of the flame with the highest strain rates are located close to the burner exit, and a progressive reduction of χoptst takes place as the (average) strain rate decreases. Furthermore, as the jet Reynolds number is increased, the χoptst shifts upwards, as greater strain rates are found in the jets.
In fact, χoptst scales almost linearly with the jet Reynolds number, consistent with observed scalings in self-similar regions of isothermal round turbulent jets [136, 137]. One notable exception is flame B, for which the χoptst values are found to be greater than expected. It’s important to consider that flame B is often described as transitional [46].
Figure4.5compares the optimal χTNF,stvalues, with the measurements ofhχ|ZTNFi at the stoichiometric mixture from Karpetis & Barlow [132]. Measurements are available for flames C, D, and E, and at x/d=7.5, 15,and30. The optimal χTNF,st
values are found to be systematically larger than the measuredhχ|ZTNF,sti. While multiple reasons could explain this difference, such as experimental uncertainties, it should be reminded that, in the present approach, a single, one-dimensional, steady- state flamelet is used to represent the conditional mean flame structure. Thus, it is expected that both optimal parameters χst and γopt are also representative of any
4.3. Extracting effective Lewis numbers 57
2 10 100 260
5 10 15 20 25 30 35 40 45 50
opt χ [1/s]TNF,st Exp.
x/d
Flame C Flame D Flame E
2 10 100 260
5 10 15 20 25 30 35 40 45 50
Figure 4.5: Comparison of the optimal scalar dissipation rate, χTNF,stopt with the measured valueshχTNF,sti[132], as a function of the downstream directionx/dfor flames C, D, and E. The vertical bars (shown only for flame C for clarity) represent the reported maximum uncertainty [132].
effect not adequately represented by this approximation. For instance, it is expected that multi-dimensional effects, including convective and diffusive transport along the mixture fraction iso-surfaces, are essentially lumped into the effective χopt. In the work of Sholtisseket al. [138], the analysis of a lifted turbulent hydrogen jet flame showed that multi-dimensional effects can be significant.
Optimalγ
Figure4.4bshows thatγopt generally increases with axial distance from the burner exit, which is consistent with turbulent transport being increasingly dominant over molecular mixing [46, 71]. It is not surprising that the only point which is not increasing is represented by flame C atx/d=45, for which the estimated stoichio- metric flame length is x/d = 45.4, as previously discussed. The values of γopt close to the burner exit are small (in fact, γopt ≈ 0 for x/d < 7.5), as molecu- lar diffusion dominates the mixing of the scalars, consistently with experimental observations [139–145]. Moving further downstream, turbulent transport becomes increasingly dominant, and the effective Lewis numbers become closer to unity.
It should be noted that the extractedγopt values are representative of two effects: as the jet Reynolds number is increased, the Kolmogorov length scale, η, decreases.
At the same time, χoptst increases (in fact the χoptst profiles scale almost withUjet/D, as shown later in Fig. 4.16), and hence the flame thickness decreases, i.e., scalar gradients are increased. These competing effects explain why the profiles shown in