Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 33

those obtained with 4% oxygen in air at 1200˚C are sho wn in Figure 2.3b . In both

cases, the air flows vertically upward as shown by the three arrows in the figure and
the gaseous fuel flows from right to left in the

X -direction at a position of Y = 0 mm,

as shown by a single horizontal arrow in Figure 2.3 .
Temperature profiles were measured for the two different flames obtained with
air at 35˚C and 21% O (as in the case of ordinary comb ustion) and preheated air

2

at 1200˚C with 4% O The latter conditions represent highly preheated and diluted

air combustion conditions. It can be seen from Figure 2.3a t hat the peak temperature
is located near the vicinity of the fuel nozzle when air with 21% oxygen is used
without preheating. However, in contrast, when low oxygen concentration air is
preheated to very high temperatures, a far more uniform distribution of the thermal
field can be observed; see temperature profiles shown in Figure 2.3b .
Temperature fluctuations were also measured at the spatial position where the
mean temperature was found to be maximum. These temperature fluctuations were
also measured using an R-type thermocouple (Pt-Pt/Rh 13%) with a wire diameter
of 0.05 mm. It is recognized that this size of thermocouple would not capture the
very high frequency turbulent fluctuations in the flow. The results showed that
temperature fluctuations decrease drastically when the combustion air conditions
were changed from 21% oxygen in air at 35˚C to 4% oxygen in air at 1200˚C. The
results shown in Figure 2.4 clearly indicate the high levels of temperature fluctuations
(root mean square value of about 197˚C ) associated with the ordinary combustion
conditions; see Figure 2.4a . Temperature fluctuation levels of around 50 to 100˚C
are characteristic of many turbulent diffusion flames.
The results obtained here show a significant reduction in the rms value of
temperature fluctuations (less than 5˚C) with the flame having 4% oxygen in air at
1200˚C; see Figure 2.4b . The characteristic features of the results obtained at two
additional data points, measured upstream and downstream of the maximum flame
temperature position, remained very similar to the results shown in Figure 2.4b . This
suggests that turbulent mixing does not appear to be governed by the high temper-
ature and low O

.

2

ustion process. This requires further clarification
since the local velocity and fluctuation levels are not known.

content air comb

2

2000 2000

1500 1500

1000 1000

T a =20 ° C, O 2 =21%, N 2 ,(x,y)=(100,47)
T mean =1318 ° C, Tr ms =197 ° C

T a =1200 ° C, O 2 =4%, N 2 ,(x,y)=(80,107)
T mean =1288 ° C, Tr ms =3.7 ° C

500

500

0

0

0 50 100 150 200

0 50 100 150 200

Time ms Time ms

(a) Low temperature/normal air (b) High temperature/low oxygen conc. air

FIGURE 2.4 T emperature fluctuation.