Environmental Engineering Reference
In-Depth Information
controlled to produce the reactant mixture near the lean blow-off (LBO) limit which
favors lean premixed (LPM) combustion producing very low emissions of harmful
species.
3 Liquid Fuel Atomization
A spray serves as the heart of almost every type of liquid-fueled combustion
system. In what Lefebvre (
1989
,
1999
) has de
atomization, a
liquid jet or sheet, subjected to destabilizing forces, breaks up after it leaves the
injector. In the classical atomization, a liquid jet or sheet initially disintegrates into
ligaments which in turn break up into droplets, whose size depends on the char-
acteristic length scale of the liquid jet or sheet. If the Weber number, the ratio of the
jet
ned as
“
classical
”
is kinetic energy to its surface tension, is small, surface instabilities control the
breakup. The
'
fl
uid
'
s viscosity and the interaction at the air
-
liquid interface become
in
”
atomization as spray production immediately at the injector exit or even within the
injector, depending on the internal structure of the injector. Prompt atomization also
occurs with the interaction of the liquid with a source of highly energetic air.
Excellent reviews concerning the preparation of fuel sprays optimized for lean
combustion include those written by McDonell and Samuelsen (
1991
), Razdan
(
1998
), Mansour (
2005
), and Nakamura et al. (
2006
). Some of the characteristics
Lefebvre (
1989
,
1999
) listed for an ideal fuel injector include good atomization
over a wide range of fuel
fl
uential with larger Weber numbers. Lefebvre (
1989
,
1999
) denotes
“
prompt
fl
flow rates either steady or transient, unaffected by
fl
ow
instabilities,
low power
requirements, scalability,
resistant
to blockages, and
delivering a uniform spray.
A high pressure gradient across the injector will ensure a large velocity differ-
ential at the jet
air interface, which, in turn, leads to rapid classical or even
prompt breakup in pressure atomizers. This simple arrangement can be problematic
if the fuel
'
s liquid
-
fl
flow rate is to extend over a large range. The ori
ce size and pressure
gradient must accommodate the largest anticipated fuel
fl
ow rate, and the pressure
gradient cannot be maintained for low fuel
low rates. A second method transfers
the kinetic energy from the surrounding air to the fuel jet through the use of air
assist or air blast. The air-assist method employs a low volume
fl
low rate of high-
velocity atomizing air to break up the fuel jet. The air-blast atomizer delivers a large
volume
fl
flow rate of low-velocity atomizing air to both break up the fuel jet or sheet
and to deliver the resulting spray to the combustion zone. For the air-assist and air-
blast (AB) atomization procedures, the production of either small volumes of high-
pressure air or large volumes of low-pressure air can be expensive in terms of
power requirements. Pre
fl
lming the fuel represents a third type of process in which
the liquid fuel
flows over a surface in such a manner that it leaves the surface as a
sheet. The thin liquid sheet can subsequently be subjected to air blast on both sides
to affect
fl
lming reduces the volume of air needed to atomize
the liquid fuel. Combinations of these three processes in a wide variety of
fine droplet spray. Pre
Search WWH ::
Custom Search