The Effects Of Air Motion On Combustion In The Mit Rapid Compression Machine

Author : Mickael Chomier
Publisher :
ISBN 13 :
Total Pages : 72 pages
Book Rating : 4.:/5 (884 download)

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Download or read book Effect of Vortex Roll-up and Crevice Mass Flow on Ignition in a Rapid Compression Machine written by Mickael Chomier and published by . This book was released on 2013 with total page 72 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of this thesis is to understand the influence of the non-ideal effects in Rapid Compression Machines (RCM), namely the vortex roll-up and mass flow into the crevice, on autoignition. The effect of the vortex roll-up is studied computationally using CFD simulations of autoignition in a RCM. Whereas, the effect of the crevice mass flow is investigated experimentally by studying isooctane autoignition. Over the last two decades, experimental data of the nature of species evolution profiles and ignition delays from RCMs has been used to develop and validate chemical kinetic mechanisms at low-to-intermediate temperatures and elevated pressures. A significant portion of this overall dataset is from RCMs that had not employed a creviced piston to contain the roll-up vortex. The detrimental influence of the roll-up vortex and the thermokinetic interactions due to the resulting temperature non-homogeneity during the negative temperature coefficient (ntc) regime have been documented in the literature. However, the adequacy of the homogeneous modeling of RCMs without creviced pistons during reactive conditions has not been investigated. In this work, computational fluid dynamics simulations of an RCM without a creviced piston are conducted for autoignition of n-heptane over the entire ntc regime over a range of compressed pressures from 5 to 18 bar. The results from the CFD simulations highlight the non-homogeneity of autoignition and reveal significant quantitative discrepancy in comparison to homogeneous modeling, particularly for the hot ignition delay in the ntc regime. Specifically, the roll-up vortex induced temperature non-homogeneity leads to diminution of the ntc behavior. The experimental data from RCMs without creviced piston needs to be taken with caution for quantitative validation and refinement of kinetic mechanism, particularly at conditions when ntc behavior is highly pronounced. Rapid Compression Machines (RCMs) often employ creviced pistons to suppress the formation of the roll-up vortex. However, the use of a creviced piston promotes mass flow into the crevice when heat release takes place in the main combustion chamber. This multi-dimensional effect is not accounted for in the prevalent volumetric expansion approach for modeling RCMs. The method of crevice containment, on the other hand, avoids post-compression mass flow into the crevice. In order to assess the effect of the crevice mass flow on ignition in a RCM, experiments were conducted for autoignition of isooctane in a RCM with creviced piston in the temperature range of 680-940 K and at compressed pressures of ~15.5 and 20.5 bar in two ways. In one situation, post-compression mass flow to the crevice is avoided by crevice containment and in other it is allowed. Experiments show that the crevice mass flow can lead to significantly longer ignition delays. Experimental data from both scenarios is modeled using adiabatic volumetric expansion approach and an available kinetic mechanism. The simulated results show less pronounced effect of crevice mass flow on ignition delay and highlight the deficiency of the volumetric expansion method owing to its inability to describe coupled physical-chemical processes in the presence of heat release. Results indicate that it is important to include crevice mass flow in the physical model for improved modeling of experimental data from RCMs for consistent interpretation of chemical kinetics. The use of crevice containment, however, avoids the issue of mass flow altogether and offers an alternative and sound approach.