Diffusion flame

About: Diffusion flame is a research topic. Over the lifetime, 9266 publications have been published within this topic receiving 233522 citations.

Controlling Mechanisms of Flame Spread

Abstract: Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that in most practical situations, the spread of fire in opposed flows occurs at near extinction or non-propagation conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemical kinetics is unimportant in the flame spread process, it is important in the establishment and extension of the diffusion flame that generates the spread process. The current experimental observations, although still in need of further verification, provide insight toward the development of accurate descriptions of the flame spread process.

Two-Dimensional Visualization of Premixed-Charge Flame Structure in an Ic Engine

Abstract: Spark kernels and flame fronts in a premixed-charge, spark-ignition, ported engine were examined for the first time with two-dimensional spatial resolution. The most important conclusions are: (1) the flame fronts in this engine are in the reaction sheet regime; (2) the flame fronts become more convoluted and disrupted as the engine speed is increased and eventually become multiply connected in the plane of the laser sheet; (3) near the lean limit, the flame fronts become more distorted and the results strongly suggest that local extinction is taking place; (4) the turbulent flow field affects all the ignition kernels examined, which included kernels as small as 2 mm and times after ignition as short as 100 microseconds; (5) the distortion and cycle-to-cycle variation in the ignition kernel increase with engine speed; (6) the estimated increase in surface area of the flame sheet scales almost linearly with engine speed between 1200 and 2400 rpm but only a two-fold increase in the surface area of the flame sheet was observed between 300 and 1200 rpm. This agrees well with corresponding variations in combustion duration; (7) the results lend strong support to the theory that in the reaction sheet regime the increase of themore » turbulent flame speed over the laminar flame speed is caused primarily by an increase in the surface area of the flame sheet; (8) the thickness of the turbulent flame zone was found to increase somewhat with both mass fraction burnt and engine speed.« less

The interaction of flame and flow field in a lean premixed swirl-stabilized combustor operated on H2/CH4/air

Abstract: The interaction of flame and flow field was studied in an enclosed lean premixed swirl-stabilized combustor operated on methane (CH 4 )- and hydrogen (H 2 )- enriched CH 4 . This was accomplished by simultaneous measurements of the 2D velocity field using particle image velocimetry and planar laser induced fluorescence imaging of the hydroxyl (OH) radical. Experiments were conducted at an inlet Reynolds number of 13,500 and a theoretical swirl number of 1.5. The fuel was pure CH 4 or a mixture of 60% CH 4 and 40% H 2 by volume. The adiabatic flame temperature was 1350 ± 20 °C for both fuels. Results show that the addition of H 2 changes the shape of the reaction zone, producing a shorter, and a more robust flame. Fuel composition also affected the location of the reaction zone, which in turn, influenced both instantaneous and time-averaged flow fields. Time-resolved measurements show vortical structures containing regions of high OH concentration around their edges. The combustion stabilized in regions of high compressive strain for the CH 4 flame but not for the H 2 -enriched CH 4 flame. In the CH 4 flame, reactions extinguished in the high velocity inlet jet region, where the H 2 -enriched CH 4 flame was able to sustain combustion.

A simple model for the filtered density function for passive scalar combustion LES

Abstract: LES models for turbulent non-premixed combustion usually require knowledge of the filtered density function of the conserved scalar, and we propose to use a simple top-hat function. Such top-hat distributions were developed as probability density functions for RANS applications in the 1970s but were soon surpassed by the β function. We find that in the context of LES, the top-hat distribution provides an excellent alternative to the now much more common β function. The top-hat function is assessed through a phenomenological analysis of Direct Numerical Simulation (DNS) data from a planar jet and of experimental data from a turbulent opposed jet. The approach is then tested a posteriori for a piloted diffusion flame (Sandia Flame D). Advantages of the top-hat function are the ease of implementation and the reduced dimensionality of look-up tables. The present paper also discusses inconsistencies of sub-grid β-FDFs, the FDFs sensitivity on implicit filtering, and the regime in which a β assumption can be a ...