Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 61

flame temperature and the oxygen concentration through exhaust gas recirculation.
Recently, in the furnace industry, a combustion method using high temperature air has
been used whereby air is preheated utilizing the sensible heat of high temperature
exhaust gas. 5 The flames are stable in high temperature air, and the recirculation of
the combustion gas lowers oxygen concentration, which is a principal agent in pro-
duction of NO x . Thus, it is possible to achieve both energy savings and NO x reduction
simultaneously. The high temperature air combustion brings about these advantages. 6-10
Sobiesiak et al. constructed a new type of burner, in which complex mixing is
done nonadiabatically, and realized low-pollution combustion with unusually low
temperature flames using highly preheated air. 6 Some numerical simulations 11,12 and
experimental research 13 have been reported on the flames in diesel engines where
fuel is injected into high temperature and high pressure air. The fundamentals of the
highly preheated air combustion have not been made clear. Therefore, we looked
into the flame extinction/stabilization mechanism and the NO x formation mechanism
under the influences of the strain rate of the flow field of nonpremixed flames, using
high temperature air.
2.3.1.1 Experimental Apparatus
Figure 2.35 shows the experimental setup. A rectangular combustion chamber with
a sectional area of 3600 mm 2 (30 × 120 mm) was used for the experiments. A porous
cylinder (diameter of 16 mm and a length of 30 mm) was installed at the center of
the combustion chamber and the fuel was injected through it. The air was heated
by 16 ceramic honeycomb elements (heat reservoirs), each 100 × 100 × 100 mm
large, and they were heated to a prescribed temperature by premixed burners inserted
upstream of the regenerator. The material of the heat reservoirs was aluminum
titanate. The air was heated to a prescribed temperature through this heat exchanger
with the heat reservoirs.
Downstream of the regenerator, comprising the heat reservoirs, the heated air
was accelerated by a converging nozzle and then is supplied to the combustion
chamber located vertically. The inner surfaces of the converging nozzle and a settling
chamber were coated with ceramic fiber to avoid heat loss. There were ceramic balls
and damping screens of fine mesh inside the settling chamber for eliminating large-
scale turbulence. Therefore, the airflow entering the combustion chamber had a
uniform velocity distribution and turbulence was negligible.
Figure 2.36 shows time histories of the temperature upstream of the temperature
at the nozzle exit from the beginning to the saturated condition. The temperature
downstream of the regenerator was set at 1490 K by regulating the fuel flow and
equivalence ratio of the premixed burners. Whereas the temperature downstream of
the regenerator quickly rose to the set value, the temperature at the nozzle exit rose
slowly and saturated at 1323 K. When the airflow velocity at room temperature was
4.06 m/s, the retention time of the experiment was about 15 min. The temperature
difference between the two curves is due to heat loss in the settling chamber and
the nozzle.