Environmental Engineering Reference
In-Depth Information
4 Creating Spherically Symmetric Droplet Burning
Conditions
The computational simplicity of the one-dimensional droplet
fl
flame in Fig.
4
belies
the experimental dif
culty of creating this con
guration; it is arguably the most
dif
guration to create in all of combustion science. To create
spherically symmetric droplet burning conditions, an environment is required where
the dynamic parameters that control convection are
cult combustion con
“
small
”
: The Rayleigh number
and Reynolds number Re
are most relevant, where g,
g
bD
TD
3
¼
U
m
Ra
¼
β
,
D
T,
am
D,
a
U
∞
, and v are gravity, thermal expansion coef
cient, characteristic temperature
difference (e.g.,
flame and droplet temperatures), droplet diameter, gas thermal
diffusivity, relative velocity between the droplet and surrounding gas, and kinematic
viscosity, respectively (Df
f
may also be used as the characteristic dimension though
it is related to the droplet diameter). It is seen that there are a number of ways that
convective in
fl
uences on the burning process can be reduced. The gravitational
level is typically reduced to promote a small Ra, while various experimental con-
fl
figurations are designed to form, deploy, and ignite the droplet with minimal dis-
turbances and residual droplet motion to make Re suitable small.
Reduced gravity is created by performing the experiments in a free fall facility,
either on Earth over a distance that determines the experimental time [i.e., a drop
tower (Avedisian et al.
1988
)] or in an orbiting craft [e.g., the International Space
Station (ISS) (Dietrich et al.
2014
)] which is essentially in free fall as it orbits the
Earth.
The experimental design must form, deploy, and ignite the test droplet without
any signi
cant accompanying motion to these operations. The droplet can either be
physically restricted by some sort of a support
fiber attached to the droplet
(Avedisian and Jackson
2000
) or free of any support structures that do not impart
any residual motion to the droplet during the deployment process (Avedisian et al.
1988
; Okajima and Kumagai
1975
; Hara and Kumagai
1990
; Liu et al.
2014b
;
Avedisian and Callahan
2000
; Jackson et al.
1991
).
The experimental designs are broadly tailored to
“
small
”
droplets corresponding
to initial diameters (D
o
)ofD
o
< 1 mm and
droplets corresponding to
D
o
> 1.5 mm. Small droplet experiments are most suitable for experimental times
on the order of several seconds (i.e., as in drop towers). Large droplet experiments
will typically require several tens of seconds to observe the complete droplet
burning history and are best examined in the environment of the ISS where the
experimental time is unlimited.
For small droplets, radiative effects are not important. For large droplets, radi-
ation can be important to promote extinction with the possibility of
“
large
”
”
(Nayagam et al.
2012
; Farouk and Dryer
2013
; Dietrich et al.
2014
) that are
characterized by a low-temperature combustion regime of burning. By varying D
o
from about 0.5 mm up to about 6 mm, the complete range of physical phenomenon
a liquid fuel could experience in a combustion environment can be accessed.
“
cool
fl
ames
Search WWH ::
Custom Search