Abstract: The image series of the diffusion flames of the moving fire in linear motion at the speeds of 3. 6,5. 4 and 6. 5m/s2 were acquired by using a high-speed video camera,and the structure charts of the flame images of different projected grayscales were acquired through the image processing.Then,the fractal structure characteristic of the projected grayscale was investigated.The results show that ( 1) the fractal dimension of the diffusion flame of the moving fire can be changed by the combined effects of both the buoyancy generated by combustion and the tangential force generated by the flame speed and its acceleration,and the structure of moving flames is complicated than that of static flames; ( 2) as the flame speed increases,the fractal dimension of the diffusion flames of different projected grayscales will increase in the combustion control zone,but decrease in the transition zone and fluctuate within a small range in the cross flow control zone; ( 3) the speeds of moving fire,at which the fractal dimension of the flame images of different projected grayscales reaches the maximum and the minimum,are greatly different,and the time to reach the maximum fractal dimension is shorter; and ( 4) the fractal dimension of the flame images projected grayscale decrease with the increase of grayscale,but the decrease is weakened when the acceleration and speed of the moving fire are higher.

Key words: moving fire, diffusion flame, projected grayscale, fractal characteristic