1,721,035 research outputs found
Effect of an inlet temperature disturbance on the propagation of methane-air premixed flames in small tubes
No full textA flame stabilized in a tube is affected by the temperature disturbance and velocity profile at the inlet boundary. Thus, a multi-dimensional analysis is necessary near the flame. The deviation between one-dimensional and two-dimensional analyses near the flame was investigated quantitatively. The temperature profile in the radial direction was varied to investigate its effects on the propagation of methane-air premixed flames in small tubes. A numerical experiment with Navier-Stokes equations, an energy equation and species equations was conducted coupled with a single-step global-reaction model. Three different temperature profiles were examined for slip and no-slip wall boundary conditions. The effect of temperature profiles on the flame propagation velocity and flame shapes was not negligible depending on the magnitude of the temperature deviation and the tube diameter. This study evaluated a critical length scale of a computational domain or a thermal entrance length of a premixed flame over which the inlet temperature disturbance does not affect the flame characteristics. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved
A numerical study on propagation of premixed flames in small tubes
No full textA premixed flame in a tube suffers strong variation in its shape and structure depending on boundary conditions. The effects of thermal boundary conditions and flow fields on flame propagation are numerically investigated. This study employs eight combinations of thermal and velocity boundary conditions. Navier-Stokes equations and species equations are solved with a one-step irreversible global reaction model of methane-air mixtures. Finite volume method using an adaptive grid method is applied to investigate the flame structure. In the case of an adiabatic wall, friction force on the wall significantly affected the flame structure while in the case of an isothermal wall, local quenching near the wall dominated flame shape and propagation. In both cases, variations of flow fields. occurred not only in the near field of the flame but also within the flame itself, which affected propagation velocities. Near the quenching conditions, strong similarity in the flame structure was found regardless of the boundary velocity profiles due to self-induced velocity deformation. This study provides an overview of the characteristics of flames in small tubes at a steady state. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Experimental Study on Breakup and Ignition of Single Droplet of Water Emulsified Dodecane Using Two Pulse-Lasers
No full textNumerical study of opposed non-premixed jet flames of methane in a coaxial narrow air tube
No full textIn a previous study, an opposed non-premixed jet flame (ONPJF) in a coaxial narrow air tube was described, and various flame structures and their flame extinction limits were evaluated experimentally. In this study, flame structures and flow variation near the ONPJF of methane in a narrow air tube were numerically investigated using a one-step reaction model. Boundary conditions were examined for various tube sizes: cold or adiabatic walls and no-slip or slip walls. The flame extinction limits were numerically evaluated and classified into three modes: a higher air limit (HA-limit), a lower fuel limit (LF-limit), and a lower air limit (LA-limit). The HA-limits were determined by flame stretch, while the LF-limits and IA-limits were determined by the thermal quenching effect. The extinction mechanism at the LA-limit was investigated in detail. The structural transition was observed, and an enclosed edge flame structure was observed at the LA-limits. These results will be helpful in understanding the overall behavior of opposed non-premixed flames in narrow spaces, and in designing small-scale combustors with better stabilization performance. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Improved stabilization mechanisms of CH4+H2 lifted flames using laminar non-premixed jets with fuel dilution
No full textThe stabilization mechanisms of lifted methane flames were investigated by employing fuel-dilution with hydrogen and various inert gases (He, N2, and CO2). The lift-off heights were measured, and the flame shapes were discussed. Representative flame structures were visualized using images obtained simultaneously from shadowgraph and OH-PLIF. When the Schmidt number was larger than unity, ordinary laminar lifted flames with similarity solutions could be formed far downstream using small tubes. Additional laminar lifted flames could be formed when the lift-off heights were smaller than the theoretical laminar mixing core. These were classified into a lower jet velocity (LJV) regime and a higher jet velocity (HJV) regime. In the LJV regime, fuel-dilution helped flame stabilization. In the HJV regime, jet break-up occurred, and the flow transferred to turbulence. Nevertheless, the OH-PLIF image at the flame front showed sharp edge structures in most cases, and the flame stabilization could be explained based on laminar flame theories. Lifted flames could be stabilized more easily in the He-dilution cases. In the LJV regime, lifted stable flames could be explained based on the effective (or local) Schmidt number, considering local concentration, which was larger than unity near the fuel tube. In the HJV regime, flame quenching and flow re-direction in the mixing layer induced lift-off in the N2-and CO2-dilution cases. Finally, an improved stabilization map with six modes was proposed for lifted flames in the laminar fuel jets.
Stabilization mechanism for Non-premixed Lifted Flames in a Methane-Hydrogen Blended Jet through the Laminar to Turbulent Transition
No full textSurface tension, light absorbance, and effective viscosity of single droplets of water-emulsified n-decane, n-dodecane, and n-hexadecane
No full textWater-emulsified liquid fuels have been used occasionally in combustion systems to control flame temperature and NOx emission. Atomization characteristics are important for efficient combustion of emulsified fuels, and they are affected by the mechanical properties of the emulsion. The dynamic behavior of an emulsion droplet is affected by the micro-droplets dispersed within the emulsion, and the actual properties of the emulsion are of interest. Recently, a new dynamic method using a pulse laser was developed based on the Taylor's analogy breakup model. In this study, surface tension, light absorbance, and viscosity were evaluated for two emulsified fuels (n-decane and n-hexadecane), and their results compared with those of n-dodecane from the previous study. Three independent ordinary methods and the dynamic method were employed to measure the emulsion properties. Actual light energy absorbance, more reliable surface tension, and effective viscosity of emulsion could be predicted through the dynamic method. Finally, a simple empirical equation was suggested for the prediction of the effective viscosity of emulsion. The applicability of the proposed dynamic method was verified conclusively, and the most effective properties of single emulsion droplets were determined.
Experimental Study on Breakup and Ignition of Water Emulsified n-Dodecane Impinging on a Hot Surface
No full textCellular Structures of Propane/air Premixed Flames Propagting in a Disk Burner of Variable Mesoscale Gaps
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