Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 118

the fuel can be determined according to the air dilution rate, it can be considered
that these curves are dependent only on the fuel. Even in this case, the type of
measurement device should be designated.

2.4.1.4.2 Changes in Flame Form and Flame Color
The flame was like a lifted flame at some distance from the nozzle, and a red flame
was generated on top of a violet flame. At first, an orange flame was generated, and
then a violet flame was generated from the bottom part. As the temperature fell, the
violet flame expanded gradually to the upper side, and the flame changed to all
violet. This violet flame was clearly stable compared with the other.
The blue-green color originates from a C 2 ·CH radical emission. The color of the
flame with the strong C 2 radical emission is a rich green, and the color of the flame
with the strongCH radical emission is a rich blue. Further, it has been recognized
that, as the air ratio decreases, the C 2 radical emission increases and the CH radical
emission decreases. The blue-green flame obtained in the high temperature preheated
diluted air has a wide distribution of the C 2 radical emission. This had been already
confirmed by experiments carried out before.
From the results obtained in observing changes of the flame state, shown in
Figure 2.79 , it has been confirmed that around the central portion of the flame the
temperature is higher, and there is little difference in temperature between these
flames. Further, it has been made clear that the external appearance of the flame
changes greatly under the conditions at a temperature higher than 1000˚C and at an
oxygen content lower than 5%.

2.4.1.4.3 Spray Combustion in the High Temperature Preheated
Diluted Air
It has been confirmed that the combustion stabilizes when the fuel oil droplets
become finer. Therefore, to make the fuel oil droplets fine, the spraying pressure
should be increased. However, the fuel flow rate is already very high for this small
scale experimental apparatus due to limitations in the size of the atomizer. Thus
spraying pressure could not be increased, and proper atomization of the droplets
was not possible in this case. To decrease droplet size, the fuel flow rate in the in-
furnace flowing field should be increased more than usual, or the rated capacity of
the nozzle should be lowered if a sufficiently small atomizer can be manufactured.
Further a spraying installation that is resistant against spraying pressure should be
required. Figure 2.81 shows the forms of flame by each oxygen content at 0.3 MPa
of spray pressure in high temperature preheated and diluted air.

2.4.1.5 Summary

Observation of the flames reveals that the spray combustion in high temperature
preheated, diluted air has its own shape and its own combustion mechanism. These
are both quite different from the normal spray combustion. The characteristics of
the spray combustion are as follows:

1. Changes of NO x value. NO x emissions reduce in the same manner as gaseous

fuel when the highly preheated air is lower than 15% in oxygen content.