Page 129 - New Trends In Coal Conversion

92 New Trends in Coal Conversion

outperforms RANS simulation in terms of accuracy, at the expenses of computational
efficiency.
In a combustion furnace, turbulence plays the central role in mixing, and then in
heat and mass transfer, as well as chemical reactions. Accurate modeling of turbulence
is very important. From this point of view, one can expect an eventual switch from
RANS turbulence modeling to LES even for industrial cofiring CFD.

4.3.2 Fuel particle motion
Correct prediction of fuel particle trajectory in a furnace is important for air supply and
combustion performance. The particle tracking model most commonly used in CFD is
for small, heavy particles in dilute two-phase flows, for which it is sufficient to only
retain drag and gravity forces in the equation of motion of particles (Maxey and Riley,
1983),

1
dv
m p ¼ C D r A p ju vjðu vÞ þ r r V p g (4.1)
p
g
g
dt 2
|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl}
gravity
drag; F D
where m p , v, t, C D , r g , A p , u, r p , V p , and g represent particle mass, particle velocity
vector, time, drag coefficient, gas density, particle projection area normal to the drag
force, gas velocity vector, particle density, particle volume, and gravitational accel-
eration, respectively. This model is applicable to and is also commonly used in
suspension-firing of pulverized coal particles. However, in biomass cofiring, this
model framework may have to be extended to accommodate the relatively large and
highly nonspherical biomass particles, which will be elaborated in detail in Section
4.4.2.
In turbulent multiphase flows, the interaction between turbulent eddies and
immersed particles, known as turbulent particle dispersion, can be important. From
the modeling point of view, the turbulent particle dispersion can be addressed by using
either cloud tracking or stochastic tracking model. In the former, the mean fluid veloc-
ity u is used in Eq. (4.1) to obtain a mean trajectory, and the dispersive effect around
the mean trajectory is described by an assumed probability density function. In the
latter, a random fluid velocity fluctuation u following a certain profile is added to
0
the mean fluid velocity u to update the particle velocity and position at the current
0
time step. At the next time step, a new random velocity u will be used, until the par-
ticle tracking is finished.

4.3.3 Radiation heat transfer
Thermal radiation is often the dominant heat transfer mechanism in combustion fur-
naces. For a gray participating medium containing particles, the radiative transfer
equation (RTE) can be expressed as (Modest, 2003),