Abstract:
The mechanisms governing the formation and destruction of soot in
turbulent combustion are intimately coupled to thermal radiation due to the
strong dependence of sooting processes and radiative loss on temperature.
Detailed computational fluid dynamics (CFD) predictions of the radiative
and soot output from turbulent non-premixed flames are normally
performed by parabolic algorithms. However, the modelling of combustion
systems, such as furnaces and unwanted enclosure fires, often require a
fully elliptic description of the flow field and its related physical
phenomena. Thus, this thesis investigates the intimate coupling between
radiative energy exchange and the mechanisms governing soot formation
and destruction within a three-dimensional, general curvilinear CFD code.
Thermal radiation is modelled by the discrete transfer radiation model
(DTRM). Special emphasis is given to approximate solutions to the
radiative transfer equation encompassing various models for the radiative
properties of gases and soot. A new algorithm is presented, entitled the
differential total absorptivity (DTA) solution, which, unlike alternative
solutions, incorporates the source temperature dependence of absorption.
Additionally, a weighted sum of gray gases (WSGG) solution is described
which includes the treatment of gray boundaries. Whilst the DTA solution
is particularly recommended for systems comprising large temperature
differences, the WSGG solution is deemed most appropriate for numerical
simulation of lower temperature diffusion flames, due to its significant time
advantage.
The coupling between radiative loss and soot concentration is
investigated via a multiple laminar flamelet concept applied within the
CFD simulation of confined turbulent diffusion flames burning methane in
air at 1 and 3 atm. Flamelet families are employed relating individual
sooting mechanisms to the level of radiative loss, which is evaluated by the
DTRM formulated for emitting-absorbing mixtures of soot, C02 and H20.
Combustion heat release is described by an eddy break-up concept linked to
the k-c turbulence model, whilst temperature is evaluated from the solved
enthalpy field. Detailed comparisons between prediction and experiment
for the critical properties of mixture fraction, temperature and soot volume
fraction demonstrate the effectiveness of this novel, coupled strategy within
an elliptic flow field calculation.