Environmental Engineering Reference
In-Depth Information
Table 1 Experimental details
Mixing length (L
fuel
)
Port number
L
fuel
(cm)
1
35
2
30
3
25
4
20
5
15
Air flow
(low flow rates)
Q
air
(LPM)
Re
70
6,645
80
7,480
85
7,948
(high
fl
ow rates)
150
13,982
175
16,312
200
18,643
Sampling rate from PMT
2 kHz.
A/D digitization resolution
16 bit, range
±
5V
allows characterizing the
flame dynamics in the regimes of different degrees of
premixing. The underline strategy was the higher premixing length (L
fuel
) that
allows greater time for fuel/air mixing and thus improves air
fl
fuel premixing.
-
The experiment involved
first using stoichiometric air
fuel mixture (
Φ
= 1).
-
Equivalence ratio was then gradually reduced in higher steps (
0.05) initially and
*
then in smaller steps (
0.02) closer to
Φ
LBO
. The reduced equivalence ratio was
*
obtained by reducing the fuel
ow rate constant.
This ensured that the incoming velocity is almost constant since the air consists of
the bulk of the reacting mixture compared to the low fuel
fl
flow rate while keeping the air
fl
ow rate.
Figures
4
and
5
plot a sequence of images captured at 30 frames per second for a
stable equivalence ratio far from LBO, i.e.,
fl
Φ
= 1.0 and
Φ
= 0.81, and one close to
LBO, i.e.,
= 1.0 case, the combustion is seen to be
relatively steady and occupies the whole combustor and the shape of the
Φ
= 0.75, respectively. In
Φ
fl
flame is
observed to be conical. As the equivalence ratio is reduced, for
Φ
= 0.81 case, there
is a signi
cant change in
fl
flame color and
fl
flame becomes bluish with reddish
tip. Though the
fl
flame zone changes from image to image, there is always a well-
de
ned combustion region and the
fl
flame looks more stable (Fig.
4
b). For conditions
Φ
LBO
= 0.75), the
close to blowout (
flame shape changes from conical to columnar
and the length increases due to reduced reaction rate and burning velocity of
fl
fl
ame
near LBO (Chaudhari
2011
).
Near LBO limit, there are random instances where the
fl
flame exhibits oscillation
in the combustor and
fl
flame lifts off from the dump plane. These oscillations produce
near
flame loss event in combustor. The combustor initially has a spatially com-
bustion zone. Then, the
fl
flame detaches from the center body, showing weak reac-
tion, and moves further downstream from the combustor inlet and stabilizes there.
The
fl
fl
flame begins to disappear from the
field of view, and there is almost complete
loss of
fl
flame, suggesting extinction event. From downstream, the
fl
ame packets are
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