Unlike Fig. 2(b), Fig. 2(c)
shows the flame with light-blue color. The flame temperature
of Fig. 2(b) and (c) at the point of L0 in Fig. 1 is ∼1590
and 1250 K, and the visual flame length of Fig. 2(b) and (c)
is 18 and 11 cm, respectively. From another experiment result
of CH4 microwave plasma burner not shown in this paper,
it is identified that CH4 injection rate up to 30 L/min (near
0.36 g/s) is reasonable at 1.2-kW microwave plasma torch
and stoichiometric fuel/air mixture.