Flow, Turbulence and Combustion最新文献

Flame acceleration plays an important role in determining the onset of deflagration-to-detonation transition (DDT) phenomenon that is relevant to novel pressure-gain propulsion and explosion safety research. Accordingly, this work explores the influence of the separation distance between obstacles (S) inside a 1050 mm closed duct on the acceleration of premixed flames fueled by a stoichiometric methane/air mixture at 40 kPa pressure. The studied duct geometry features a 96 mm x 96 mm square cross section and includes five obstacles along the wall with a 75% blockage ratio, each delineated by side dimensions of 96 mm x 96 mm and square holes of 48 mm x 48 mm. Experimental and direct numerical simulations (DNS) techniques are employed here to investigate the flame acceleration dynamics under different operating conditions. More specifically, high-speed video captures the dynamics of the flame front evolution from experiments, while DNS are carried out using the PeleC fully compressive Navier Stokes solver, including finite-rate chemistry and adaptive mesh refinement (AMR). A comparison between experimental and numerical results for S = 1.0 Dₕ shows reasonable agreement in flame tip velocity and reduced position, supporting the applicability of a two-dimensional DNS model like the one employed here. In contrast, for S = 1.5 Dₕ the numerical results fail to reproduce the experimentally observed flame structure and acceleration, likely due to missing three-dimensional effects. Numerical simulations for different S values ranging from 0.75 to 1.5 Dₕ reveal that obstacle spacing has a strong influence on flame acceleration mechanisms. As S increases indeed, the flame shifts from geometry-constrained jetting to instability-driven propagation involving vortex generation and pressure-wave interactions. The case with S = 1.25 Dₕ yields the highest flame tip velocity, even though the one with S = 1.5 Dₕ exhibits greater vorticity and pressure amplitudes. This is attributed to the reduced flame–vortex coupling coherence in the S = 1.5 Dₕ case, which results in more chaotic flame dynamics and lower flame acceleration efficiency. These results offer new insight into the mechanisms of flame acceleration under confinement and highlight obstacle spacing as a key design parameter for optimizing performance and safety in combustion systems.

On one hand, the combustion utilization of hydrogen and ammonia (carbon-free fuel) is a practical way to reduce CO₂ emissions. On the other hand, the ultra-lean premixed combustion near the flammability limit is a promising technology to achieve cleaner utilization of gaseous fuels. Therefore, this work investigates the ultra-lean premixed combustion characteristics of 40%H₂-60%NH₃-air by means of numerical simulation. It is observed that the 40%H₂-60%NH₃-air premixed flame can remain stable at the ultra-lean fuel condition, and its behavior and structure are revealed in detail. It is observed that the flame root is anchored by the recirculation zone. H₂ tends to be consumed more upstream compared with NH₃. Both H₂ and NH₃ arrive at the flame front primarily through diffusion rather than convection. The amount of H₂ arriving at the flame root and waist is larger than that of NH_3, primarily owing to its faster diffusion velocity. The preferential transport of H₂ contributes to the larger reaction rate of H₂ around these regions. In addition, the negative flame displacement speed appears in the vicinity of the flame root and tip. Then, the factors contributing to the residual flame stabilization are analyzed. For the flame root, the strong preferential transport effect moderates the decrease in the flame displacement speed with decreased equivalence ratio and generates a relative fuel-rich region contributing to the flame root stabilization. For the flame tip, the significant heat recirculation and preferential transport effects contribute to its stabilization. The present study helps us to further understand the ultra-lean H₂-NH₃-air premixed flame dynamics.

This study experimentally investigated the void fraction components in concurrent vertical upward air–water slug flow using a non-intrusive Infrared Optical Sensing technique. Experiments were conducted under ambient conditions in a transparent PVCu pipe with an internal diameter of 27.3 mm, using air and water as the working fluids. Time-averaged void fractions, Taylor bubble fractions, and entrained bubble fractions were measured across a range of liquid superficial velocities (v_SL = 0.15 m/s to 0.55 m/s) and gas superficial velocities (v_SG = 0.29 m/s to 1.18 m/s). The influence of superficial velocity ratios (v_SG/v_SL) and flow turbulence on slug flow void components was also explored. Results revealed that Taylor bubble formation begins when the superficial gas velocity is approximately half that of the liquid (v_SG ~ 0.5 v_SL). Taylor bubble fractions increased with gas velocity and reached a maximum of ~ 48%. While existing literature typically reports a monotonic increase in entrained bubble fraction up to a 25% cap, this study not only confirms the maximum value but also uncovers a previously unreported critical decline beyond that point. Specifically, once the superficial gas velocity approached twice the liquid superficial velocity (v_SG ~ 2v_SL), the entrained bubble fraction began to decrease due to enhanced bubble coalescence, a phenomenon not captured by current entrained bubble correlations. Additionally, bubble fractions characterizing the end of bubble flow, bubble-slug transition flow, and fully developed slug flow were observed and reported in this study. These findings address key gaps in current void fraction characterizations and provide improved benchmarks for the modeling, simulation, and operation of industrial multiphase flow systems.

The effect of non-parallel walls on the flame dynamics and the tulip flame formation in a narrow, closed volume channel is investigated experimentally using high-speed visualization techniques and pressure measurements. The angular channel is a (4^{circ}) planar diverging (D–C) or converging (C–C) channel ignited at one of the ends. The flame propagation is studied for a wide range of equivalence ratios ((phi=0.9-1.5)) of premixed LPG-air mixture in both the channel configurations, and the outcome is extended for a straight channel. The flame speed is higher in the D–C compared to the C–C and highest in the straight channel due to thermal expansion caused by the narrow volume and flame stretch in the varying cross-section. In addition, the pressure changes in the channels are recorded, and the results show that the rate of pressure varies with the flame growth and is not significantly affected by the flame surface area. Analysing the normalized position and time for the flame inversion for different (phi) and for all the channels, it is found that the flame inversion occurs prominently at halfway of the flame propagation and at about 1/3rd of the total time and it is true for both parallel and non-parallel walled channels at every equivalence ratio. Flame morphology with diverging and converging walls are presented. In particular, the flame deceleration process and tulip flame formation are visualized across different orthogonal cross-sections. Distorted tulip flame and the vortex flows are observed in the burned gas along with asymmetric tulip-shaped flame propagation at different equivalence ratios for the D–C and C–C.

Development of a generally applicable inlet condition generation method for Large Eddy Simulation (LES) is challenging and limits application to complex engineering flows. Inlet velocity time-series are required, at temporal/spatial resolutions consistent with LES numerics, covering the entire computational inlet plane, and for a time period allowing statistical stationarity. Ideally measurements would be used, but capture of large area, long duration time histories is problematic. Several generation techniques have been proposed, but compliance with measurements is normally guaranteed only for single point statistical data. The present work demonstrates how Stereoscopic Particle Image Velocimetry (SPIV) may be used to generate conditions simultaneously matching 1-point statistics, 2-point spatial correlations, and frequency spectra. A validation test case is selected containing complex flow structures typical of engineering applications. Linear Stochastic Estimation (LSE) and high-pass filtering are combined to match the LES inflow area with the smaller SPIV area required for accurate spatial resolution. A single synchronous velocity field is created from multiple non-concurrent SPIV fields. The method extends the inflow complexity that can be considered and provides improvement over existing methods.

In this work a new method of passive drag reduction on bluff bodies by tessellation is presented. Wind-tunnel measurements on tessellated spheres reveal that the variation of the drag coefficient is similar to other types of surface modifications with rotational symmetry such as dimples, manifested by a sudden decrease in the drag coefficient at a critical Reynolds number followed by a nearly constant drag value in the post-critical regime. However, tessellated spheres can achieve a further 10%-15% drag reduction compared to dimples without shifting the critical Reynolds number. To further investigate the underlying physics leading to this reduction we also conducted Direct Numerical Simulations of both tessellated and dimpled spheres at (Re=1.50times10^5). The predicted values of drag coefficient agree very well with the experiments and confirm the drag reduction. Analysis of the flow reveals that the tessellated panels introduce a smaller pressure “penalty” compared to dimples at the front part of the body. In addition, transition to turbulence occurs later and near the top of the body. As a result the boundary layer grows thinner and global separation is delayed by approximately (10^circ).

The underexpanded jet issuing from an elliptic convergent nozzle with an aspect ratio of 8 is quantitatively visualized using rainbow schlieren tomography. Flow visualizations are conducted at a nozzle pressure ratio of 4.0 to produce strong shocks within the jet plume. The Reynolds number, based on the equivalent diameter and flow properties at the nozzle exit, is 4.0(times)10(^5). Multi-view rainbow schlieren images with a horizontal filter setting are captured by rotating the nozzle around its central axis at an angular step of 5(^circ), ranging from 0(^circ) to 180(^circ). The jet density field is reconstructed at a spatial resolution of 13 (mu)m using the convolution back projection method. The near-field flow features of a shock-containing elliptic jet with a high aspect ratio are experimentally demonstrated for the first time. In addition, to examine the effects of angular steps on the reconstructed density field, the rainbow schlieren images taken at 10(^circ), 20(^circ), and 30(^circ) intervals are selected from a total of 37 rainbow schlieren images captured at 5(^circ) intervals. The density field is then reconstructed for these specific angular steps. The effects of angular steps are clarified on the two-dimensional density fields in the minor-axis and major-axis planes, as well as on the streamwise density profiles along the jet centerline and liplines in both the minor-axis and major-axis planes.

This study investigates the impact of (textrm{CO}_2) and (textrm{H}_{2}textrm{O}) dilution ratios on the characteristics of a premixed (textrm{C}_{3}textrm{H}_{8})/air turbulent flame in a swirled burner at atmospheric pressure. High-fidelity turbulence resolution is critical for capturing transient flame stabilization dynamics and pollutant formation in swirling flows. Therefore, Detached Eddy Simulation (DES) is employed to resolve large-scale unsteady turbulent structures, while the Eddy Dissipation Concept (EDC) models turbulence-chemistry interaction, incorporating a new reduced kinetic model made up of 36 species and 166 reactions. The study explores five volumetric fractions of (textrm{CO}_2) or (textrm{H}_{2}textrm{O}) dilution ((X_{textrm{CO}_2/textrm{H}_2textrm{O}} = 4%), 8%, 12%, 16%, and 20%), three swirl numbers ((Sn = 0), 0.6, and 1.05), and two equivalence ratios ((phi = 0.8) and 1). Validation against experimental data confirms the model’s accuracy in capturing flow fields and scalar distributions. The results show that (textrm{CO}_2) addition significantly lowers flame temperature and alters its shape, resulting in a major reduction in (textrm{NO}_{x}) concentrations at (X_{textrm{CO}_2} = 16%). Dilution by (textrm{H}_{2}textrm{O}) does not reduce (textrm{NO}_{x}) as noticeably, but still leads to somewhat more stable and thinner flames. Additionally, (textrm{CO}_2) is more effective than (textrm{H}_{2}textrm{O}) in suppressing flame flashback. This work provides interesting insights for optimizing swirl-stabilized flames.

Large uncertainties exist in the description of the turbulent premixed combustion regimes and their boundaries, especially the validity regime of flamelet assumptions, due to challenges involved in experimental studies. Our understanding of the physics of turbulent premixed combustion gets nebulous as the Karlovitz number increases (exceeds 500). A novel piloted burner design capable of generating high-intensity, homogeneous, axisymmetric turbulence is presented, and its flow characteristics are discussed. An impinging-jet injector is used to generate high-intensity turbulence. Axial flow measurements were made using hot-wire anemometry. Fluctuating rms velocities ranging from 1.4 m/s up to 30 m/s are reported for bulk velocities of 10 m/s to 60 m/s at the jet centerline. The measured velocity profiles were shown to be self-similar for all considered injector configurations. The limitations of hot-wire measurements in large turbulence and necessary corrections are discussed; the corrected length scales were compared to those obtained from scaling laws. Premixed methane-air flames exceeding 60 m/s of bulk velocities and equivalence ratios ranging from 0.65 to 1.0 can be stabilized using a large laminar pilot. The tested flame conditions span over two orders of magnitude in normalized turbulence velocities and three orders of magnitude in Karlovitz number. High-fidelity measurements of premixed flames over a broad range of turbulence can be realized using the burner.