Environmental Engineering Reference
In-Depth Information
characteristic of optical sensing made it the best choice for fast detection of
fl
ame
instability events.
The combustor was operated from stoichiometric condition up to complete
LBO, and the temporal behavior of the combustor was recorded at various
equivalence ratios (
). The equivalence ratio was lowered by keeping the air
fl
ow
Ф
rate constant and decreasing the fuel
flow rate. This ensured that the incoming
velocity was almost constant as air consists of bulk of reacting mixture compared to
the low fuel
fl
low rate. Chemiluminescence emission from the CH* radicals is
recorded on the photomultiplier tube (PMT) at various equivalence ratios obtained
till complete blowout of
fl
ame occurs.
Figure 7 shows the temporal recordings of the CH chemiluminescence signal
from the
fl
flame, acquired on photomultiplier tube (PMT) and the corresponding
power spectrum density (PSD) plots obtained after fast Fourier analysis. The air
fl
fl
flow rate used here is Q air = 80 LPM. The data length captured in each experiment
N = 32,768 (i.e., 2 15 points) data points with a sampling frequency of 2 kHz, and
the lowermost port (port 1, L fuel = 35 cm.) is used for injecting the fuel. The
blowout equivalence ratio observed in this test is just below 0.75 (i.e.,
Ф LBO = 0.75). The data are presented for different equivalence ratio conditions
which are normalized by the value at blowout (i.e.,
Ф LBO ). The normalization
facilitates the comparison of same state of combustor with different port conditions,
described subsequently below.
It is observed that away from
Ф
/
Φ LBO = 1.33),
CH* oscillations are coherent and strong in intensity. The corresponding PSD plot
revealed some dominating frequency around 20 Hz with adjacent bunch of fre-
quencies. Apart from this, the spectrum also shows some low-powered high fre-
quencies in a band of 50
Ф LBO at high equivalence ratios (
Φ
/
60 Hz. From the different CH time series plots, it is
observed that as the equivalence ratio is lowered, the mean value of CH chemi-
luminescence intensity decreases, which is expected as the fuel
-
flow is lowered to
obtain lower equivalence ratio which correspondingly gives lower heat release rate.
At
fl
Φ LBO = 1.08, the strength of oscillation at the dominating combustion fre-
quency
Φ
/
20 Hz diminishes and some high-powered low frequencies appear in
spectrum. As combustor approaches to LBO (i.e.,
*
ame
becomes weak in intensity with the increase in length due to lower reaction rate and
changes shape from conical to columnar and started showing some oscillations in
combustor. The corresponding CH intensity also decreases as
Φ
/
Φ LBO = 1.02, 1.01), the
fl
ame becomes weak
and exhibits some relatively high-amplitude bursts where the signal shows high
spike in both below and above the mean value which matches with the
fl
ame
oscillations. The corresponding PSD elucidates that high-powered low dominating
frequencies apparently match with
fl
fl
flame oscillations. At
Φ
/
Φ LBO = 1.0, just before
the LBO, the
flame lifts off from dump plane and starts oscillating in combustor.
The chemiluminescence intensity decreases and shows some relatively high-
amplitude bursts in CH time series plot, with the signal going both below and well
above the mean value, and occasionally drops to near-zero value. Often, these
bursts or events are characterized by an almost complete loss of chemiluminescence
signal quickly, followed by intense emission from an imaged region. These bursts
fl
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