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On flame speed enhancement in turbulent premixed hydrogen-air flames during local flame-flame interaction(arXiv, 2023-03-02) [Preprint]Given the need to develop zero-carbon combustors for power and aircraft engine applications, Sd of a turbulent premixed flame, especially for H2-air, is of immediate interest. The present study investigates 3D DNS cases of premixed H2-air turbulent flames at varied pressures for different Ret and Ka with detailed chemistry to theoretically model Sd at negative curvatures. Prior studies at atmospheric pressure showed Sd˜ to be enhanced significantly over SL at large negative κ due to flame-flame interactions. 1D simulations of an imploding cylindrical H2-air laminar premixed flame used to represent the local flame surfaces undergoing flame-flame interaction in a turbulent flame at the corresponding pressure conditions are performed to understand the interaction dynamics. These simulations emphasized the transient nature of the flame structure during flame-flame interactions and enabled analytical modeling of Sd˜ at these regions of extreme negative κ of the 3D DNS. The JPDF of Sd˜ and κ and the corresponding conditional averages from 3D DNS showed a negative correlation between Sd˜ and κ. The model successfully predicts the variation of ⟨Sd˜|κ⟩ with κ for the regions on the flame surface with κδL≪−1 at all pressures, with good accuracy. This shows the aforementioned configuration to be fruitful in representing local flame-flame interaction in 3D turbulent flames. Moreover, at κ=0, on average Sd˜ can deviate from SL, manifested by the internal flame structure, controlled by turbulence transport in the large Ka regime. Thus, the correlation of ⟨Sd˜⟩/SL with ⟨|∇cˆ|c0⟩ at κ=0 is explored.
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Role of the outer-edge flame on flame extinction in nitrogen-diluted non-premixed counterflow flames with finite burner diameters(Fuel, Elsevier BV, 2013-03) [Article]This study of nitrogen-diluted non-premixed counterflow flames with finite burner diameters investigates the important role of the outer-edge flame on flame extinction through experimental and numerical analyses. It explores flame stability diagrams mapping the flame extinction response of nitrogen-diluted non-premixed counterflow flames to varying global strain rates in terms of burner diameter, burner gap, and velocity ratio. A critical nitrogen mole fraction exists beyond which the flame cannot be sustained; the critical nitrogen mole fraction versus global strain rate curves have C-shapes for various burner diameters, burner gaps, and velocity ratios. At sufficiently high strain-rate flames, these curves collapse into one curve; therefore, the flames follow the one-dimensional flame response of a typical diffusion flame. Low strain-rate flames are significantly affected by radial conductive heat loss, and therefore flame length. Three flame extinction modes are identified: flame extinction through shrinkage of the outer-edge flame with or without oscillations at the outer-edge flame prior to the extinction, and flame extinction through a flame hole at the flame center. The extinction modes are significantly affected by the behavior of the outer-edge flame. Detailed explanations are provided based on the measured flame-surface temperature and numerical evaluation of the fractional contribution of each term in the energy equation. Radial conductive heat loss at the flame edge to ambience is the main mechanism of extinction through shrinkage of the outer-edge flame in low strain-rate flames. Reduction of the burner diameter can extend the flame extinction mode by shrinking the outer-edge flame in higher strain-rate flames. © 2012 Elsevier Ltd. All rights reserved.
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Impact of the bluff-body material on the flame leading edge structure and flame-flow interaction of premixed CH4/air flames(COMBUSTION AND FLAME, ELSEVIER SCIENCE INC, 2016-07-25) [Article]In this paper we investigate the interaction between the flame structure, the flow field and the coupled heat transfer with the flame holder of a laminar lean premixed CH4/air flame stabilized on a heat conducting bluff body in a channel. The study is conducted with a 2-D direct numerical simulation with detailed chemistry and species transport and with no artificial flame anchoring boundary conditions. Capturing the multiple time scales, length scales and flame-wall thermal interaction was done using a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid body. The flame structure displays profiles of the main species and atomic ratios similar to previously published experimental measurements on an annular bluff body configuration for both laminar and turbulent flow, demonstrating generality of the resolved flame leading edge structure for flames that stabilize on a sudden expansion. The flame structure near the bluff body and further downstream shows dependence on the thermal properties of the bluff body. We analyze the influence of flow strain and heat losses on the flame, and show that the flame stretch increases sharply at the flame leading edge, and this high stretch rate, together with heat losses, dictate the flame anchoring location. By analyzing the impact of the flame on the flow field we reveal that the strong dependence of vorticity dilatation on the flame location leads to high impact of the flame anchoring location on the flow and flame stretch downstream. This study sheds light on the impact of heat losses to the flame holder on the flame–flow feedback mechanism in lean premixed flames.
