OCR Text |
Show The observation of changes of to from 0.001 to ~(Figure 4(c» indicates as to increases, with dramatic changes in heat convection curve. to ~ 0.01 the heat convection curve displays a linear relationship. to > 0.01 the heat convection shows a dented, and gradually increase tendency. Besides the greater the to value, the greater the curvature. Figure 4( d) indicates limited changes of heat convection as a result of changes in the optical properties, 000, of medium particles in the preheat zone. The flux generally displays a dented and gradually increasing curve. The above analyses show that parameter N" has the most significant effects on heat convection followed by to and N 1 , with ID 0 trailing the last in influence. In a very slow flow velocity, the heat convection may be almost neglected in the preheat zone,. But with the increase in flow velocity, the closer the flow to the flame, the more energy taken away. Then the observation of changes in the optical thickness, to, reveal that the large quantity energy taken away by the fuel flow only occurs in the extremely thin zone close to the flame front. Given enough distance away from this zone, the heat convection is abruptly reduced to zero. 3.4 The Effects on the Radiation Heat Flux Figure 5(a) indicates that as Nl increases (i.e. the role of conduction becomes more important), the decrease of heat radiation slows down, but then accelerates again when approaching the flame sheet. Nt ~ 1.0 changes in the radiation heat flux are not very distinctive. Figure 5(b) shows that as N" increases, the decrease of heat radiation curve becomes even mor pronounced. But the opposite behavior of upward increase occur when approaching the flamelet. N" ~ 0.1 ,N" ~ 15 the heat radiation changes very little. The observation of changes of to from 0.001 to 4.0(Figure 5(c» indicates that as to increases, the distribution curve of heat radiation changes gradually from a horizontal line to a mostly decreasing curve with increases close to the flamelet. the decreases of to has dramatic effects on heat radiation. to ~ 0.01 the heat radiation curve is almost a horizontal line without any changes. to > 0.01 the radiation heat flux is a curve with gradual decrease first, followed by a gradual increase. This effect also intensifies as the to value increases. Figure 5( d) indicates that the radiation heat flux curve display two different behaviors as a result of an increase in medium particle 000 . o ~ 000 ~ 0.80 the radiation heat flux is a curve with a gradual decrease followed by a gradual increase, which intensifies as 000 decreases. 0.80 < 000 ~ 1.00 the radiation heat flux is a linear and increasing line. The above analyses demonstrate that, regardless of changes of any parameter, the minimum value of heat radiation curve occurs roughly at t = 0.9to(the neglect of the negative sign means the maximum heat radiation at the spot). Among the parameters discussed, to and IDo have the most significant effects on heat radiation, followed by N" and Nt . We also discover an important phenomenon, in which the maximum value of heat radiation in the preheat zone does not occur on the flame sheet, but close to the flame front. We also note that when the preheat thickness is too thin, the heat radiation in this zone becomes consistent without any changes. Besides, as the emissio characteristics of the medium in this zone intensify, the difference in radiation heat between the fuel surface and the flame sheet is gradually reduced. 8 |