Design Guidelines for High Performance Industrial Furnaces - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 261

as an elliptic slit. The diluted condition near the burners was enhanced by the
increased inducement of the furnace gas. The flow volume and the velocity were
unchanged, but instead of the original round port of 162 mm diameter, a slit-shaped
port of 55 mm in width was used, and the length of the potential core was about
one third of the core of the round port. The direction of the fuel jet was also changed
to 10 ˚ inward to the center axis of the burner. The flame thus formed was highly
transparent by visual observation but the steel temperature near the skids and the
ion current remained similar to those measured in the case of the original burner
shape. This can be interpreted to show that such high temperature air combustion,
which allows detection of the ion current of 20 mV, is feasible even at locations far
from the burners by controlling the feeding method of air and fuel, even when their
mixing near the burners is enhanced. Thus, this method is considered effective as a
practical method to perform uniform heating, while preventing the escape from the
furnace of the gas during the combustion process.
The forced recirculation of exhaust gas near and under the skids improved
heating performance near the skids. Because there was a slight increase of ion density
compared with the other cases of high temperature air combustion, it is conceivable
that, aside from the effect of the increased convection heat transfer due to the greater
flow speed of the furnace gas, the increase of ions carried toward the skids by the
increased flux near the location where the probe was inserted also contributed to the
improvement. However, considering that the temperature difference under normal
combustion also decreased, the effect of convection heat transfer was seen to be
greater in this test. Quantitative comparison of the effects of each factor was not
carried out, but this method showed the possibility of heating control in the domain
where it was difficult to control the flow by the flame from burners or the momentum
of combustion gas alone ( Figures 5.24 and 5.25 ) .

1060

A Burner Side

HiTAC with Agitator

1040

Conventional with Agitator

1020

T=50˚C

1000

980 T=69˚C

960

a b c d e

940

-400 -200 0 200 400

Position (mm)

FIGURE 5.24 Comparison of temperatures on the steel surface near skids when the forced
circulation of furnace gas is added.