| 1. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
A direct numerical simulation study of the influence of flame-generated vorticity on reaction-zone-surface area in weakly turbulent premixed combustion
- 2019
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 31:5
-
Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios sigma= 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of sigma= 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., sigma= 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.
|
|
| 2. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames
- 2018
-
Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 22:1, s. 131-155
-
Tidskriftsartikel (refereegranskat)abstract
- Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing largescale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.
|
|
| 3. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
A priori DNS study of applicability of flamelet concept to predicting mean concentrations of species in turbulent premixed flames at various Karlovitz numbers
- 2020
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 222, s. 370-382
-
Tidskriftsartikel (refereegranskat)abstract
- Complex-chemistry direct numerical simulation (DNS) data obtained earlier from lean hydrogen-air flames associated with corrugated flame (case A), thin reaction zone (case B), and broken reaction zone (case C) regimes of turbulent burning are analysed to directly assess capabilities of the flamelet approach to predict mean concentrations of species in a premixed turbulent flame. The approach consists in averaging dependencies of mole fractions, reaction rates, temperature, and density on a single combustion progress variable c, which are all obtained from the unperturbed laminar flame. For this purpose, four alternative definitions of c are probed and two probability density functions (PDFs) are adopted, i.e. either an actual PDF extracted directly from the DNS data or a presumed β-function PDF obtained using the DNS data on the first two moments of the c(x,t)-field. Results show that the mean density and mean mole fractions of H2, O2, and H2O are well predicted using both PDFs for each c, although the predictive capabilities are little worse in case C. In cases A and B, the use of the actual PDF and the fuel-based c also offers an opportunity to well predict mean mole fractions of O and H, whereas the mean mole fraction of OH is slightly underestimated. In the highly turbulent case C, the same approach performs worse, but still appears to be acceptable for evaluating the mean radical concentrations. The use of the β-function PDFs or another combustion progress variable yields substantially worse results for these radicals. When compared to the mean mole fractions, the mean rate of product creation, i.e. the source term in the transport equation for the mean combustion progress variable, is worse predicted even for a quantity (species concentration or temperature) adopted to define c and using the actual PDF. Consequently, turbulent burning velocity is not predicted either.
|
|
| 4. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
An extended flamelet-based presumed probability density function for predicting mean concentrations of various species in premixed turbulent flames
- 2020
-
Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 45:55, s. 31162-31178
-
Tidskriftsartikel (refereegranskat)abstract
- Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed to perform a priori assessment of predictive capabilities of the flamelet approach for evaluating mean species concentrations. For this purpose, dependencies of mole fractions and rates of production of various species on a combustion progress variable c, obtained from the laminar flame, are averaged adopting either the actual Probability Density Function (PDF) P(c) extracted from the DNS data or a common presumed β-function PDF. On the one hand, the results quantitatively validate the flamelet approach for the mean mole fractions of all species, including radicals, but only if the actual PDF P(c) is adopted. The use of the β-function PDF yields substantially worse results for the radicals’ concentrations. These findings put modeling the PDF P(c) on the forefront of the research agenda. On the other hand, the mean rate of product creation and turbulent burning velocity are poorly predicted even adopting the actual PDF. These results imply that, in order to evaluate the mean species concentrations, the flamelet approach could be coupled with another model that predicts the mean rate and turbulent burning velocity better. Accordingly, the flamelet approach could be implemented as post-processing of numerical data yielded by that model. Based on the aforementioned findings and implications, a new approach to building a presumed PDF is developed. The key features of the approach consist in (i) adopting a re-normalized flamelet PDF for intermediate values of c and (ii) directly using the mean rate of product creation to calibrate the presumed PDF. Capabilities of the newly developed PDF for predicting mean species concentrations are quantitively validated for all species, including radicals.
|
|
| 5. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
Assessment of a flamelet approach to evaluating mean species mass fractions in moderately and highly turbulent premixed flames
- 2021
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:4
-
Tidskriftsartikel (refereegranskat)abstract
- Complex-chemistry Direct Numerical Simulation (DNS) data obtained from lean methane-air turbulent flames are analysed to perform a priori assessment of predictive capabilities of the flamelet approach to evaluating mean concentrations of various species in turbulent flames characterized by Karlovitz numbers Ka=6.0, 74.0, and 540. Six definitions of a combustion progress variable c are probed and two types of Probability Density Functions (PDFs) are adapted: (i) actual PDFs extracted directly from the DNS data or (ii) presumed β-function PDFs obtained using the DNS data on the first two moments of the c-field. Results show that the mean density, the mean temperature, and the mean mass fractions of CH4, O2, H2O, CO2, CO, CH2O, CH3, and HCO are very well predicted using the temperature-based combustion progress variable c_ and the actual PDF. For other considered species, the quantitative predictions are worse, but still appear to be encouraging (with the exception of CH3O at Ka=540). The use of the flamelet library obtained from the equidiffusive laminar flame improves results for H2, HO2, and H2O2 at the highest Karlovitz number. Alternative definitions of the combustion progress variable perform worse and the reasons for this are explored. The use of the β-function PDF yields worse results fo r intermediate species such as OH, O, H, CH3, and HCO, with this PDF being significantly different from the actual PDF. Application of the flamelet approach to rates of production/consumption of various species is also addressed and implications of obtained results for modeling are discussed.
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
Does flame-generated vorticity increase turbulent burning velocity?
- 2018
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 30:8
-
Tidskriftsartikel (refereegranskat)abstract
- Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
|
|
| 9. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
-
Flame folding and conditioned concentration profiles in moderately intense turbulence
- 2022
-
Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 34:6
-
Tidskriftsartikel (refereegranskat)abstract
- While the flamelet paradigm offers the opportunity to simplify computations of mean species concentrations in turbulent flames, a widely accepted criterion of the validity of this paradigm has not yet been elaborated. In this regard, different physical mechanisms are discussed, and flame folding is one of them. The present work aims at exploring the eventual influence of flame folding on the local flame structure in a turbulent flow. For this purpose, a new diagnostic technique was applied to processing complex-chemistry direct numerical simulation data obtained earlier from a lean hydrogen-air turbulent flame [Dave and Chaudhuri, J. Fluid Mech. 884, A46 (2020)]. The technique consists of counting crossing numbers Nf for a cold boundary of the local reaction zone and a ray normal to the mean flame brush, followed by analyzing statistics sampled from rays characterized by Nf>2. More specifically, profiles of species mole fractions, temperature, heat release rate, and species production rates, conditioned to combustion progress variable and either Nf or axial distance dx between two neighboring reaction zones, are sampled and compared with the counterpart profiles obtained from the laminar flame. Results show that these doubly conditioned profiles are close to each other for various crossing numbers or for various axial distances even if the distance is as small as half laminar flame thickness. The lack of a substantial effect of the crossing number or the axial distance on the doubly conditioned profiles implies that small-scale flame folding does not limit the validity of the flamelet paradigm.
|
|
| 10. |
|
|
