| 1. |
- Lee, Hsu Chew, et al.
(författare)
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A DNS study of extreme and leading points in lean hydrogen-air turbulent flames – Part I: Local thermochemical structure and reaction rates
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
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Tidskriftsartikel (refereegranskat)abstract
- A Direct Numerical Simulation (DNS) study of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by a low Lewis number Le and three different Karlovitz numbersKa ranging from 3 to 33 is performed, with the same complex-chemistry flames being also simulated by setting molecular diffusivities of all species equal to the heat diffusivity of the mixture. The simulations predict a significant increase in a ratio of turbulent burning velocity to the laminar flame speed in the former (Le<1) flames when compared to the latter (equidiffusive) flames. Extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) are found at each instant. In the equidiffusive flames, such extreme FCR and HRR are close to their peak values in the unperturbed laminar flame. If Le is low, the former rates are significantly higher than the latter ones due to an increase in the local temperature, equivalence ratio, and radical mass fractions, caused by diffusive-thermal effects. While the studied extreme points may appear sufficiently far from the leading edge of the instantaneous flame brush, leading points characterized by a lower, but still high (Le<1) FCR or HRR are observed close to the leading edge at each instant. Various local characteristics (temperature, equivalence ratio, species mass fractions and their gradients, reaction rates, etc.) of the extreme and leading points are explored and significant differences between zones characterized by high FCR or HRR are revealed. For instance, in the latter zones, major chemical pathways are changed. Moreover, while the extreme HRRs strongly fluctuate in time, with their mean and rms values being significantly increased by Ka, the extreme FCRs fluctuate weakly and are close at different Ka, thus, implying that almost the same extreme FCR can be reached in substantially different local burning structures.
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| 2. |
- Lee, Hsu Chew, et al.
(författare)
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A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
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Tidskriftsartikel (refereegranskat)abstract
- Data obtained in recent direct numerical simulations (Lee et al.) of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by three different Karlovitz numbers Ka ranging from 3 to 33 are further analyzed in order to explore local characteristics and structure of (i) extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) and (ii) leading points that are also characterized by a high FCR or HRR, but advance furthest into unburned reactants. Results show that, on the one hand, common characteristics of flame perturbations (curvature, strain and stretch rates, displacement speed) fluctuate significantly in the extreme or leading, FCR or HRR points and are different in different flames. Moreover, other two-point local quantities such as the local gradients of combustion progress variables or species (e.g., the radical H) mass fractions are different in different flames. Therefore, a common simple configuration of a perturbed laminar flame cannot be used as a catchall model of the entire local structure of zones surrounding the discussed points at various Ka. On the other hand, single-point local characteristics (temperature, species mass fractions, rates of their production) of the FCR extreme points are comparable in all three turbulent flames and in the critically strained planar laminar flame. In particular, the FCRs in the extreme points fluctuate weakly and are approximately equal to each other and to the peak FCR in the critically strained laminar flame. The latter finding implies that (i) the maximum FCR evaluated in the critically strained laminar flame could be used to characterize, in a first approximation, the local FCR in the extreme or leading points in turbulent flames, thus, supporting the leading point concept, and (ii) almost the same extreme FCR can be reached in substantially different local burning structures.
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| 3. |
- Lee, Hsu Chew, et al.
(författare)
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A numerical support of leading point concept
- 2022
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 47:55, s. 23444-23461
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Tidskriftsartikel (refereegranskat)abstract
- Unsteady three-dimensional Direct Numerical Simulation (DNS) data obtained from 16 statistically planar and one-dimensional, complex-chemistry, lean (equivalence ratio is equal to 0.50 or 0.35) hydrogen-air flames propagating in forced, intense, small-scale turbulence (Karlovitz number up to 565) are reported. The data are analyzed to compare roles played by leading and trailing edges of a premixed turbulent flame brush in its propagation. The comparison is based on the following considerations: (i) positively (negatively) curved reaction zones predominate at the leading (trailing, respectively) edge of a premixed turbulent flame brush and (ii) preferential diffusion of molecular or atomic hydrogen results in increasing the local fuel consumption and heat release rates in positively or negatively, respectively, curved reaction zones. Therefore, turbulent burning velocities computed by deactivating differential diffusion effects for all species with the exception of either H2 or H are compared for assessing roles played by leading and trailing edges of a premixed turbulent flame brush in its propagation. By analyzing the DNS data, a significant increase in the local fuel consumption and heat release rates due to preferential diffusion of H2 or H is documented close to the leading or trailing, respectively, edges of the studied flame brushes. Nevertheless, turbulent burning velocities computed by activating preferential diffusion solely for H2 are significantly higher than turbulent burning velocities computed by activating preferential diffusion solely for H. This result indicates an important role played by the leading edge in the propagation of the explored turbulent flame brushes
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| 4. |
- Lee, HsuChew, et al.
(författare)
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Displacement speed, flame surface density, and burning rate in highly turbulent premixed flames characterized by low Lewis numbers
- 2023
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Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 961
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Tidskriftsartikel (refereegranskat)abstract
- Direct Numerical Simulation data obtained earlier from four pairs of moderately or highly turbulent, lean hydrogen-air, complex-chemistry flames are analyzed to explore influence of molecular diffusion on differently averaged Flame Surface Densities (FSD), displacement speeds Sd, and various terms in the FSD transport equation. For this purpose, each pair involves (i) a flame where mixture averaged molecular diffusivities are adopted and Lewis number Le is significantly less than unity and (ii) an equidiffusive flame where all molecular diffusivities are set equal to molecular heat diffusivity of the mixture and Le=1, with other things being equal in the two flames. Reported results show, in particular, that significantly higher turbulent burning rates simulated in the former flames result mainly from an increase in the local fuel consumption rate, whereas an increase in flame surface area plays a secondary role, especially in more intense turbulence. The rate increase stems from (i) an increase in the peak local fuel consumption rate and (ii) an increase in a width of a zone where the rate is significant. The latter phenomenon is of more importance in richer flames and both phenomena are most pronounced in the vicinity of the flame leading edges, thus, further supporting a crucial role played by the leading edge of a premixed turbulent flame in its propagation. Moreover, mean displacement speed differs significantly from the laminar flame speed even in the equidiffusive flames, vary substantially across flame brush, and may be negative at the leading edges of highly turbulent flames.
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| 5. |
- Lee, HsuChew, et al.
(författare)
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Influence of equivalence ratio on turbulent burning velocity and extreme fuel consumption rate in lean hydrogen-air turbulent flames
- 2022
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 327
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Tidskriftsartikel (refereegranskat)abstract
- Unsteady three-dimensional Direct Numerical Simulations of seven statistically one-dimensional, planar, highly turbulent, complex-chemistry, lean H2-air flames are performed using either mixture averaged or equidiffusive model of molecular transport. The equivalence ratio is varied from 0.35 to 0.70 and the Karlovitz number Ka is varied from 3 to 565. Normalized turbulent burning velocities UT/SL are strongly increased when using the mixture-averaged model, with an increase by a factor of 4.1 being documented even at Ka as high as 565. Here, SL is the laminar flame speed. Moreover, the increase in UT/SL is significantly more pronounced in leaner flames, which are characterized by a thinner reaction zone and a larger Zel’dovich number. Furthermore, UT/SL is increased by the turbulence length scale. The extreme (maximum over the computational domain at a single instant) local values of fuel consumption rate (FCR) exhibit a high degree of universality, i.e., in all studied cases and at all instants, these rates are close to the peak values of FCR obtained from the counterpart critically strained, twin, counter-flow laminar premixed flames. This finding appears to directly support a corner-stone hypothesis of the leading point concept of premixed turbulent burning, thus, suggesting the use of characteris tics of the critically strained laminar premixed flames as input parameters for models of turbulent combustion of lean H2/air mixtures.
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| 6. |
- Lee, HsuChew, et al.
(författare)
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Influence of molecular transport on burning rate and conditioned species concentrations in highly turbulent premixed flames
- 2021
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Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 928
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Tidskriftsartikel (refereegranskat)abstract
- Apparent inconsistency between (i) experimental and Direct Numerical Simulation (DNS) data that show the significant influence of differential diffusion on turbulent burning rate and (ii) recent complex-chemistry DNS data that indicate mitigation of the influence of differential diffusion on conditioned profiles of various local flame characteristics at high Karlovitz numbers is explored by analyzing new DNS data obtained from lean hydrogen-air turbulent flames. Both aforementioned effects are observed by analyzing the same DNS data provided that the conditioned profiles are sampled from the entire computational domain. On the contrary, the conditioned profiles sampled at the leading edge of the mean flame brush do not indicate the mitigation, but are significantly affected by differential diffusion phenomena,e.g., because reaction zones are highly curved at the leading edge. This observation is consistent with a significant increase in the computed turbulent burning velocity with decreasing Lewis number, with all the results considered jointly being consonant with the leading point concept of premixed turbulent combustion. The concept is further supported by comparing DNS data obtained by allowing for preferential diffusion solely for a single species, either atomic or molecular hydrogen.
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| 7. |
- Lee, HsuChew, et al.
(författare)
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Lewis number and preferential diffusion effects in lean hydrogen–air highly turbulent flames
- 2022
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:3
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Tidskriftsartikel (refereegranskat)abstract
- Unsteady three-dimensional direct numerical simulations of highly turbulent, complex-chemistry, lean hydrogen-air flames were performed by changing the equivalence ratio F, root mean square velocity u' , and turbulence length scale L. For each set of {F, u', L}, to explore the influence of molecular transport coefficients on the turbulent burning velocity UT , four cases were designed: (i) mixture-averaged diffusivities; (ii) diffusivities equal to the heat diffusivity a of the mixture for all species; (iii) mixture-averaged diffusivities for all species with the exception of O2 , whose diffusivity was equal to the diffusivity DH2 of H2 to suppress preferential diffusion effects; and (iv) mixture-averaged diffusivities multiplied with a/DH2 to suppress Lewis number effects but retain preferential diffusion effects. The computed results show a significant increase in UT due to differences in molecular transport coefficients even at Karlovitz number Ka as large as 565. The increase is documented in cases (i) and (iii) but is not observed in case (iv) —indicating that this phenomenon is controlled by Lewis number effects, whereas preferential diffusion effects play a minor role. The phenomenon is more pronounced in leaner flames, with all other things being equal. While the temperature profiles (c) conditionally averaged at the local value of the combustion progress variable c and sampled from the entire flame brushes are not sensitive to variations in molecular transport coefficients at high Ka, the (c)-profiles sampled from the leading edges of the same flame brushes show significant increase in the local temperature in cases (i) and (iii) characterized by a low Lewis number.
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| 8. |
- Lee, HsuChew, et al.
(författare)
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Turbulent burning velocity and thermodiffusive instability of premixed flames
- 2023
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 108:3
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Tidskriftsartikel (refereegranskat)abstract
- Reported in the paper are results of unsteady three-dimensional direct numerical simulations of laminar and turbulent, lean hydrogen-air, complex-chemistry flames propagating in forced turbulence in a box. To explore the eventual influence of thermodiffusive instability of laminar flames on turbulent burning velocity, (i) a critical length scale $\Lambda_{n}$ that bounds regimes of unstable and stable laminar combustion is numerically determined by gradually decreasing the width $\Lambda$ of computational domain until a stable laminar flame is obtained and (ii) simulations of turbulent flames are performed by varying the width from $\Lambda<\Lambda_{n}$ (in this case, the instability is suppressed) to $\Lambda>\Lambda_{n}$ (in this case, the instability may grow). Moreover, simulations are performed either using mixture-averaged transport properties (low Lewis number flames) or setting diffusivities of all species equal to heat diffusivity of the mixture (equidiffusive flames), with all other things being equal. Obtained results show a significant increase in turbulent burning velocity $U_T$ when the boundary $\Lambda=\Lambda_{n}$ is crossed in weak turbulence, but almost equal values of $U_T$ are computed at $\Lambda<\Lambda_{n}$ and $\Lambda>\Lambda_{n}$ in moderately turbulent flames characterized by Karlovitz number equal to 3.4 or larger. These results imply that thermo-diffusive instability of laminar premixed flames substantially affects burning velocity in weak turbulence only, in line with a simple criterion proposed by Chomiak and Lipatnikov (Phys. Rev. E 107, 015102, 2023).
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| 9. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Transition from turbulence-dominated to instability-dominated combustion regime in lean hydrogen-air flames
- 2024
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Ingår i: Combustion and Flame. - 1556-2921 .- 0010-2180. ; 259
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Tidskriftsartikel (refereegranskat)abstract
- To explore the importance of thermodiffusive and hydrodynamic instabilities of laminar flames in turbulent flows, previously generated direct numerical simulations of statistically one-dimensional complex-chemistry lean hydrogen-air flames in forced turbulence were continued by switching-off turbulence forcing. Three sets of flames characterized by different ratios of initial root-mean-square velocity to laminar flame speed, i.e., (A) u′/SL=2.2, (B) u′/SL=4.0, and (C) u′/SL=8.3, were addressed. Moreover, new complementary simulations of unstable laminar flames were performed. Analyses from the obtained numerical results indicate (but do not prove) that laminar flame instabilities play a minor role at sufficiently high Karlovitz numbers Ka. This supposition is supported, first, in cases B and C, where the initial value of turbulent burning velocity UT is significantly higher than burning velocity evaluated during non-linear stage of laminar flame instability development in the same computational domain. Second, regular large-scale perturbations of instantaneous flame surface are prominent in the unstable laminar flame but are not observed at high Ka. Third, Karlovitz numbers associated with appearance of such perturbations as the turbulence decays are scattered between 4 and 10 and are consistent within an order of magnitude to a recently proposed criterion (Chomiak and Lipatnikov, Phys. Rev. E 107: 015102, 2023) of importance for laminar flame instabilities in turbulent flows. Fourth, at such transition instants, a ratio of potential and solenoidal turbulent kinetic energies, averaged over the flame-brush leading edge, is close to 2.0 in 11 studied cases and a ratio of UT/SL varies between 3.0 and 3.5. Fifth, the maximum fuel consumption rate (over the computational domain) decreases as the turbulence decays. Thus, the initial maximum rates are significantly higher than the counterpart rate in the unstable laminar flame. Together these results show that the hypothesis that laminar flame instabilities play only a minor role at high Ka deserves further study.
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