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Interplay of hydrodynamic instabilities and turbulence in premixed flames
No full textThis thesis is devoted to the numerical investigation of premixed Flames subject to intrinsic hydrodynamic instabilities as well as turbulence. Laminar as well as turbulent premixed combustion can be largely influenced by the onset of hydrodynamic instabilities which can cause a significant increase in flame corrugation and, as a result, in the turbulent flame speed, especially when low turbulence intensity level are present. Indeed, such instability is responsible for the formation of sharp folds and creases in the flame front and for the wrinkling observed, undergo certain conditions, over the surface of expanding flames. Hydrodynamic instability is a result of thermal expansion across the flame and its role is particularly dominant in large-scale flames, when flames are constrained by domains larger than several hundred times the flame thickness. The understanding of these phenomena and their interaction with turbulence can play a potentially significant role in practical combustion systems such as gas turbines and, more generally, in industrial and domestic burners. The aim of this thesis is to develop a numerical tool capable of simulating the propagation of turbulent premixed flames under the influence hydrodynamic instabilities and gather qualitative and quantitative data on flame properties such as morphology and global propagation
Large scale effects in weakly turbulent premixed flames
Get PDFIn this study we numerically investigate large scale premixed flames in weakly turbulent flow fields. A large scale flame is classified as such based on a reference hydrodynamic lengthscale being larger than a neutral (cutoff) lengthscale for which the hydrodynamic or Darrieus-Landau (DL) instability is balanced by stabilizing diffusive effects. As a result, DL instability can develop for large scale flames and is inhibited otherwise. Direct numerical simulations of both large scale and small scale three-dimensional, weakly turbulent flames are performed at constant Karlovitz and turbulent Reynolds number, using two paradigmatic configurations, namely a statistically planar flame and a slot Bunsen flame. As expected from linear stability analysis, DL instability induces its characteristic cusp-like corrugation only on large scale flames. We therefore observe significant morphological and topological differences as well as DL-enhanced turbulent flame speeds in large scale flames. Furthermore, we investigate issues related to reaction rate modeling in the context of flame surface density closure. Thicker flame brushes are observed for large scale flames resulting in smaller flame surface densities and overall larger wrinkling factors
Darrieus-Landau induced regime of propagation of turbulent premixed flames in Bunsen configurations
No full textInterplay of Darrieus-Landau instability and weak turbulence in premixed flame propagation
No full textIn this study we investigate, both numerically and experimentally, the interplay between the intrinsic Darrieus-
Landau (DL) or hydrodynamic instability of a premixed flame and the moderately turbulent flow field in which
the flame propagates. The objective is threefold: to establish, unambiguously, through a suitably defined marker,
the presence or absence of DL-induced effects on the turbulent flame, to quantify the DL effects on the flame
propagation and morphology and, finally, to asses whether such effects are mitigated or suppressed as the
turbulence intensity is increased. The numerical simulations are based on a deficient reactant model which lends
itself to a wealth of results from asymptotic theory, such as the determination of stability limits. The skewness
of the flame curvature probability density function is identified as an unambiguous morphological marker for
the presence or absence of DL effects in a turbulent environment. In addition, the turbulent propagation speed is
shown to exhibit a distinct dual behavior whereby it is noticeably enhanced in the presence of DL instability while
it is unchanged otherwise. Furthermore, increasing the turbulence intensity is found to be mitigating with respect
to DL-induced effects such as the mentioned dual behavior which disappears at higher intensities. Experimental
propane and/or air Bunsen flames are also investigated, utilizing two distinct diameters, respectively, above and
below the estimated DL cutoff wavelength. Curvature skewness is still clearly observed to act as a marker for DL
instability while the turbulent propagation speed is concurrently enhanced in the presence of the instability
Strain rates, flow patterns and flame surface densities in hydrodynamically unstable, weakly turbulent premixed flames
No full textRecent numerical and experimental studies have unveiled a potentially marked difference between the laminar as well as turbulent propagation of premixed flames exhibiting Darrieus-Landau (DL) (or hydrodynamic) instabilities from flames for which instabilities are inhibited. In this study we utilize two-dimensional numerical simulations of slot burner flames as well as experimental Propane-Air Bunsen flames to analyse differences in turbulent propagation, strain rate and induced flow patterns of hydrodynamically stable and unstable flames. We also investigate the effects of hydrodynamic instability on quantities which are directly related to reaction rate closure models, such as flame surface density and stretch factor. A clear enhancement of turbulent flame speed can be observed for unstable flames, generally mitigated at higher turbulence intensity, which is attributed to a flame area increase induced by the characteristic cusp-like DL-induced corrugation, absent in stable flames, which occurs concurrently and in synergy with turbulent wrinkling. Unstable flames also exhibit, both numerically and experimentally, a different correlation between strain rate and flame curvature and are observed to give rise to a channeling of the induced flow in the fresh mixture. Conditionally averaged flame surface density is also observed to attain smaller values in unstable flames, as a result of the thicker turbulent flame brush, indicating that closure models should incorporate instability-related parameters in addition to turbulence-related parameters
Flame induced flow features in the presence of darrieus-landau instability
No full textThe onset of hydrodynamic or Darrieus-Landau (DL) instability can largely impact on premixed flame morphology, turbulent flame speed and induced flow field. In this work, we focus on the latter induced flow by means of two dimensional direct numerical simulations (DNS) of slot burner flames performed in a parametric fashion. Results from linear stability analysis are used to select the adequate parameter range to be investigated. The presence of DL instability is initially assessed using a recently proposed statistical marker related to flame morphology. The differences between stable and unstable flames are then statistically investigated, utilizing a single, laminar, DL-induced corrugation as a reference state. Such DL-induced effects are investigated at various turbulence intensities, in terms of local propagation, induced strain rate patterns and flow field as well as vorticity production and transformation. Using displacement speed as a measure of local propagation, no noticeable statistical difference is observed between stable and unstable flames while strain rate and vorticity patterns are shown to be largely influenced by the DL induced morphology. From the modeling point view, an enhancement of counter gradient type transport for turbulent scalar fluxes is observed for hydrodynamically unstable flames
Low-mach number simulations of transcritical flows
No full textA numerical framework for the direct simulation, in the low-Mach number limit, of reacting and non-reacting transcritical flows is presented. The key feature are an efficient and detailed representation of the real fluid properties and an high-order spatial discretization. The latter is of fundamental importance to correctly resolve the largely non-linear behavior of the fluid in the proximity of the pseudo-boiling. The validity of the low-Mach number assumptions is assessed for a previously developed non-reacting DNS database of transcritical and supercritical mixing. Fully resolved DNS data employing high-fidelity thermodynamical models are also used to investigate the spectral characteristic as well as the differences between transcritical and supercritical jets
Effects of three-dimensional slit geometry on flashback of premixed hydrogen flames in perforated burners
No full textAddressing flashback represents a pivotal challenge in the advancement of innovative perforated burners intended to substitute natural gas with hydrogen in household appliances. Current numerical models, employing 2D configurations to estimate flashback velocities in the slits of such burners, offer valuable insights with reasonable computational costs. However, the inherent complexity of the phenomena suggests that a 2D model may inadequately capture flashback dynamics, resulting in inaccurate estimations of flashback limits. In this study, 3D simulations are employed for the first time to explore the impact of the three-dimensional shape of slits on the flashback limits of hydrogen-premixed flames. Steady-state simulations are conducted to compute flashback limits for different equivalence ratios, investigating slits with fixed width and varying lengths up to 8 mm. Additionally, transient simulations are performed to investigate the flashback dynamics. The results are compared with those from 2D configurations to assess the reliability of the infinite slit approximation. Notably, 2D simulations significantly underestimate flashback limits as the critical initiation region is consistently located at the slit extremities, which are neglected in 2D configurations. For the same reason, the flashback velocity exhibits weak dependence on slit length, since the flashback is consistently initiated at the slit far ends regardless of length. The physical mechanisms driving the initiation of flashback in that zone are identified as preferential diffusion effects, which cause the enrichment of the mixture at the slit extremities, and enhanced heat transfer promoted by the enclosed geometry, which increases the pre-heating of the fresh gases in that regions
On the impact of CFD turbulence models for premixed NH3/H2 combustion on emissions and flame characteristics in a swirl-stabilized burner
Get PDFAmmonia combustion is gaining interest as a feasible alternative to traditional fossil fuels because of to the low environmental impact and as hydrogen and energy carrier. This study used Computational Fluid Dynamics (CFD) simulations to compare various turbulence models for premixed ammonia/hydrogen combustion in a swirl-stabilized burner. The primary aim was to identify the best turbulence model for accurately predicting the flow dynamics, combustion behaviour, and emissions profiles of ammonia/hydrogen fuel blends. The turbulence models evaluated were Large Eddy Simulation (LES), Realizable k-
, Renormalization Group (RNG) k-
, k-
SST, and Reynolds Stress Model (RSM). On the LES side, a further comparison of two subgrid models (Smagorinsky-Lilly and WALE) was investigated. The Flamelet Generated Manifold (FGM) method was utilized with a detailed chemistry scheme taking into consideration all
reactions. To improve the prediction of
emissions, additional scalar transport equations for NO and
were included. This methodology aimed to be a balance between computational efficiency and the accuracy expected of detailed chemistry models. Validation was done with a swirl burner from Cardiff University’s Gas Turbine Research Centre. Results showed that all turbulence models accurately captured flame characteristics in terms of exhaust temperature and axial velocity with minor differences in the recirculation zones, where only the RSM model can predict the velocity trend as the LES simulation while other RANS models differ by at least 7 m/s. The temperature reached by the LES resulted 100 K higher than the other models in the flame zone. LES simulation can predict the emission value with an error of less than 10
. Moreover, the error related to emissions derived from the RANS simulations was not negligible, underestimating
emissions by about 35
. However, RSM model produced results that were closer to those derived from the high-fidelity LES when compared to the others RANS models, particularly in terms of flame thickness and emissions. It was concluded that it is mandatory to perform an unsteady analysis to reach reasonable results
The importance of Soret effect, preferential diffusion, and conjugate heat transfer for flashback limits of hydrogen-fueled perforated burners
No full textAvoiding flashback is a primary challenge in the development of modern burners, which should be capable of substituting natural gas with hydrogen in domestic end-user devices. The heat exchange between the burner plate and the burned and unburned gases, as well as the effects of preferential diffusion, significantly impact the flashback of such burners fueled with hydrogen. For these effects, the design of the burner plate plays a pivotal role. In this study, three-dimensional simulations with detailed chemistry have been performed to investigate the effect of three competing physical mechanisms, namely, preheating of fresh gases, preferential diffusion, and Soret effect, which drive the flame flashback dependence on the holes/slits size. Two different geometries are considered: circular holes with varying diameters and slits with fixed lengths but different widths. Steady-state simulations with decreasing inlet velocities are employed to estimate the critical inlet velocity for flashback. Conjugate heat transfer (CHT) is considered for the heat exchange between the burner plate and the gases. For circular holes, the enclosed geometry promotes more effective heat transfer, leading to a higher influence of preheating effects for small diameters. This results in a non-monotonic dependence on hole size, with a non-trivial optimum diameter to avoid flashback. This behavior is specific to circular holes and differs from that observed in previously studied infinitely long slits, where a linear dependence on the slit width was found. Additionally, the individual influence of non-unity Lewis numbers and Soret diffusion is analyzed. Notably, the Soret effect, in combination with CHT, is found to instaurate a strong, non-linear, self-accelerating mechanism that has a leading-order effect on the flashback propensity of larger holes. This finding underscores the necessity of including both effects in numerical simulations for accurate estimations of the flashback limits in domestic burners.</p
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