FIGS. 2A, 2B and 2C are a perspective view illustrating a first embodiment of the present invention; is a top view of the embodiment; and a fireball generated in the embodiment of FIG. 2A, respectively;FIG. 3 is a sectional view illustrating vertical arrangement of inner water walls and cooling air holes;FIG. 4 are sectional views illustrating a horizontal arrangement of inner water walls;FIG. 5 a