Diffusion flame

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

Multistage oscillatory “Cool Flame” behavior for isolated alkane droplet combustion in elevated pressure microgravity condition

Abstract: Recently, large diameter, isolated n-heptane droplet experiments under microgravity conditions (aboard the International Space Station) exhibited “Cool Flame” burning behavior, resulting from a heat loss mechanism that extinguishes hot combustion and a transition into a sustained, low temperature second stage combustion. In atmospheric pressure air, a single combustion mode transition to “Cool Flame” burning is followed by diffusive extinction. But with increasing pressure, multiple cycles of hot initiation followed by transition to “Cool Flame” burning are observed. This paper reports experimental observations that characterize the transition time histories of this multi-cycle, multi-stage behavior. Transient sphero-symmetric droplet combustion modeling that considers multi-stage detailed kinetics, multi-component diffusion, and spectral radiation is applied to analyze the experimental observations. The simulations indicate that as parameters change the chemical time scales dictating low temperature degenerate chain branching, multiple hot/cool flame burning transitions are induced by increasing the cool flame burning heat generation rate compared to the diffusive loss rate. The balance of these terms in the negative temperature coefficient kinetic regime defines whether reactions accelerate into re-ignition of a hot flame event, burn quasi-steadily in the cool flame mode, or diffusively extinguish. The rate of reactions controlling ketohydroperoxide formation and destruction are shown to be key re-ignition of hot combustion from the cool flame mode. Predictions are found to be in good agreement with the experimental measurements. Modeling is further applied to determine how these observations are dependent on initial experimental conditions, including pressure, and diluent species.

The effect of global mixing on soot volume fraction: measurements in simple jet, precessing jet, and bluff body flames

Abstract: Measurements of soot volume fraction were performed in three different turbulent diffusion flames using laser-induced incandescence (LII) to provide insight into the effect of global mixing rate on the instantaneous local and the average soot volume fraction. The three flames, produced by simple jet, precessing jet, and bluff-body jet burners, are chosen because they span a wide range of different global mixing rates for the same fuel, nozzle diameter, and flow rate, and because global measurements of their radiation, NOx emissions, and residence times are available. The measurements reveal an inverse relationship between global mixing rate and both the total amount of soot in the flame and local instantaneous soot volume fractions, broadly consistent with trends in laminar flames. However, probability density functions reveal that the relationship between local instantaneous and time-averaged soot volume fraction is different in the lower and upper regions of each flame. In the lower regions, the instantaneous and average volume fractions increase together with distance from the burner. However, the upper regions exhibit a bi-modal distribution in local, instantaneous volume fraction. This suggests that burnout in the tip region occurs predominantly by reducing the proportion of the flame being occupied by the sheets of soot, rather than by reducing the instantaneous volume fraction within the sheets. In addition, the shape of the pdf is different for each flame, showing a dependence on the turbulent characteristics, and that care must be taken in applying measurements obtained from one class of burner to another.