OCR Text |
Show distance from the center of the CO2 laser beam. From these distributions, it is found that the ignition can not occur near the surface because near the center where the temperature is sufficiently high for spontaneous ignition, the fuel concentration is too high to ignite and around the irradiated boundary where the fuel concentration is within the flammability limits, the temperature is too low to ignite. In this case, ignition occurs at the center far from the PMMA surface, where both the temperature and fuel concentration are in the ranges sufficient for spontaneous ignition. Figure 9 ---- 1.0 0) 1000 - -~ ....... - . -7- -:. - '-- - -'-- '-- - 0.8 800 ] --- j 0.6 .rradiated 600 15 boundary ~ • E - '+- ~ U - - 0.4 .. ... _- .- - ~ 0.2 0 .5 1.0 r(Cm) (a) (b) Typical interferogram(a) and temperature and fuel concentration distributions at the distance 0.25 cm from the PMMA surface(b) deduced from it [9] • Left and right sides of the interferogram indicate that for the wavelength of 4416 and 6328 A• , respectively. Radiant flux I = 65 J/(cm2s), Irradiated period t = 0.382 s. Velocity Profiles Figure 10 shows a typical photograph of particle tracks which indicate the flow field near the edge of a diffusion flame established in a laminar air flow along a flat porous plate, from the surface of which methane is injected[18]. At the station near the leading flame edge; stream lines are largely distorted, due to the flame reaction, and deceleration and accelera tion of the gas stream are observed. Understanding the mechanism responsible for the velocity change depends considerably on understanding the aerodynamic structure of this region. Because of fairly well arrangement and opera tion of the recording system in this case, reliable velocity profiles could be obtained, and based on the veloci ty profiles, the pressure distribution near the leading flame edge could be deduced as shown in Fig. 11, which made clear the distortion of the aerodynamic structure caused by the flame [18] • 9 |