Autoignition of high pressure methane jets at engine relveant conditions within a shock tube
is investigated using Conditional Moment Closure (CMC). The impact of two commonly
used approximations applied in previous work is examined, the assumption of homogeneous
turbulence in the closure of the micro-mixing term and the assumption of negligible radial
variation of terms within the CMC equations. In the present work two formulations of
an inhomogeneous mixing model are implemented, both utilizing the β -PDF, but differing
in the respective conditional velocity closure that is applied. The common linear model
for conditional velocity is considered, in addition to the gradient diffusion model. The
validity of cross-stream averaging the CMC equations is examined by comparing results
from two-dimensional (axial and radial) solution of the CMC equations with cross-stream
averaged results.
The CMC equations are presented and all terms requiring closure are discussed. So-
lution of the CMC equations is decoupled from the flow field solution using the frozen
mixing assumption. Detailed chemical kinetics are implemented. The CMC equations are
discretized using finite differences and solved using a fractional step method. To maintain
consistency between the mixing model and the mixture fraction variance equation, the
scalar dissipation rate from both implementations of the inhomogeneous model are scaled.
The autoignition results for five air temperatures are compared with results obtained using
homogeneous mixing models and experimental data.
The gradient diffusion conditional velocity model is shown to produce diverging be-
haviour in low probability regions. The corresponding profiles of conditional scalar dis-
sipation rate are negatively impacted with the use of the gradient model, as unphysical
behaviour at lean mixtures within the core of the fuel jet is observed. The predictions of
ignition delay and location from the Inhomogeneous-Linear model are very close to the
homogeneous mixing model results. The Inhomogeneous-Gradient model yields longer ig-
nition delays and ignition locations further downstream. This is influenced by the higher
scalar dissipation rates at lean mixtures resulting from the divergent behaviour of the
gradient conditional velocity model. The ignition delays obtained by solving the CMC
equations in two dimensions are in excellent agreement with the cross-stream averaged
values, but the ignition locations are predicted closer to the injector.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/5102 |
| Date | 26 April 2010 |
| Creators | Milford, Adrian |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
Page generated in 0.0024 seconds