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I. Size effects in combustion of isolated methanol droplets II. Autoignition in unstrained, laminar mixing layers
The droplet simulations focus on the effects of droplet size variation on its combustion characteristics. A gradual transition from a kinetically controlled regime of small droplets to a diffusion controlled regime of large droplets is shown. With reduction in the droplet size, the flame-sheet approximation breaks down, and the flame becomes a larger reactive zone relative to the droplet, along with a concomitant reduction in the flame temperature. Fuel vapour accumulation and depletion affects the entire burning history significantly, and combustion characteristics, such as burning rate, and flame stand-off ratio are time dependent, with increasing unsteadiness with reduction in size. The mixing layer simulations focus on the time evolution of the ignition kernel of different fuels in the oxidizer stream of standard air at 1200 K. Detailed investigations are presented for methanol/air and hydrogen/air mixing layers. Limited investigations for acetylene, ethylene, ethanol, n-heptane, and n-decane are also presented. The volumetric heat release rate during ignition is used to delineate the evolution of the ignition kernel. The results demonstrate a continuous evolution of partially-premixed flame structures into a diffusion flame. For low enough fuel stream temperatures (400 K) ignition is caused by the deflagration travelling from the hot oxidizer stream to the cold fuel stream, as predicted in the literature. With equal fuel and oxidizer stream temperatures (1200 K), for methanol, ethanol, acetylene and ethylene, an additional deflagration is predicted for the first time. Effects on the ignition kernel due to (i) varying fuel stream temperature (400 K to 1200 K), (ii) varying operating pressure (1 bar to 40 bars) and, (iii) varying fuel stream dilution with N2 are also presented.
Chemical engineering|Mechanical engineering
Awasthi, Inkant, "I. Size effects in combustion of isolated methanol droplets II. Autoignition in unstrained, laminar mixing layers" (2013). ETD collection for University of Nebraska - Lincoln. AAI3588423.