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Flameless “Cool” Combustion in Multi-phase Configuration☆

Published on Jan 1, 2015in Procedia Engineering
· DOI :10.1016/j.proeng.2015.05.085
Tanvir Farouk18
Estimated H-index: 18
(USC: University of South Carolina)
Abstract
Abstract Cool flames are commonly associated with engine-knock phenomenon in spark ignition engines and autoignition in diesel engines, and results from low temperature partial oxidation of the fuel air mixture that eventually leads to hot flame ignition. However, the possibility of a cool flame supporting quasi-steady combustion of a fuel droplet has never been speculated, let alone been experimentally observed. Recent microgravity droplet combustion onboard the International Space Station (ISS) under the Flame Extinguishment (FLEX) Experiment Program showed anomalous combustion of n -heptane droplets; high temperature combustion followed by radiative visible extinction and a transition to a second stage burn characterized by loss of visible flame emission. In the second stage the droplet regression continues eventually resulting in extinction diameters characteristic of diffusive extinction. Experimental examples of the two stage burning and extinction characteristics of isolated n -heptane droplets under microgravity conditions are presented and analyzed numerically. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “ Cool Flame ” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continue to burn and regress in size. Factors leading to initiation of the second stage burning phenomena are also investigated. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.
  • References (8)
  • Citations (2)
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References8
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#1Tanvir Farouk (Princeton University)H-Index: 18
#2Yu Cheng Liu (Cornell University)H-Index: 11
Last. Frederick L. Dryer (Princeton University)H-Index: 68
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Abstract Combustion characteristics of isolated sub-millimeter sized methyl butanoate (MB) droplets are studied at low gravity (10 −4 m/s 2 ) in a 1.2 s drop tower. In the experiments, droplets were grown and deployed onto the intersection of two 14 μm silicon carbide fibers in a cross-string arrangement and exposed to symmetrically placed spark ignition sources. The initial droplet diameter was fixed at 0.54 ± 0.01 mm, and experiments were carried out in room temperature air at atmospheric pres...
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#1Tanvir Farouk (Princeton University)H-Index: 18
#2Frederick L. Dryer (Princeton University)H-Index: 68
Abstract Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water dropl...
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#2Frederick L. Dryer (Princeton University)H-Index: 68
Abstract Tethered methanol droplet combustion in carbon dioxide enriched environment is simulated using a transient one-dimensional spherosymmetric droplet combustion model that includes the effects of tethering. A priori numerical predictions are compared against recent experimental data. The numerical predictions compare favorably with the experimental results and show significant effects of tethering on the experimental observations. The presence of a relatively large quartz fiber tether incr...
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#2Frederick L. Dryer (Princeton University)H-Index: 68
Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with publishe...
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A chemical kinetic mechanism has been developed to describe the high-temperature oxidation and pyrolysis of n-heptane, iso-octane, and their mixtures. An approach previously developed by this laboratory was used here to partially reduce the mechanism while maintaining a desired level of detailed reaction information. The relevant mechanism involves 107 species undergoing 723 reactions and has been validated against an extensive set of experimental data gathered from the literature that includes ...
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