4.3. Temperature characteristics
Figs. 6–8 show the temperature characteristics of the fuel nozzle
(Tn), dump plane (Td) and exit gas (Te) of the original and the
modified nozzles for the fuel of methane and DME.
As shown in Fig. 6, Tn of DME was slightly higher than that of
methane due to the higher turbulent burning velocity
(ST;CH4 ¼ 167 cm=s, ST,DME = 176 cm/s [11]) and the lower injection
velocity of DME fuel. The injection velocities of DME and methane
are easily calculated from the heating values based on volume, and
the velocity of methane is 1.65 times higher than that of DME. Fuel
nozzle temperatures of the DME and methane in the case of original
nozzle were higher than that of modified nozzle by more than
40 C for all heat inputs. This anti-flash-back characteristic results
from the improved mixing and heat transfer. Enhanced fuel/air
mixing caused by the doubled number of holes lowers the equivalence
ratio in the combustion area, and thus decreases the local
flame speed near the fuel nozzle. The doubled number of holes also
enhances the heat transfer from heated nozzle to fuel gas because
of an increased contact area between fuel gas flow and fuel nozzle.
Fig. 9 also supports the fact that flame seldom attaches to the modified
nozzle in comparison to the original nozzle since the weak
luminescence near the modified is observed. From these results,
it can be concluded that the design of the modified nozzle prevents
flash-back to the fuel nozzle.