OCR Text |
Show pulve~~ed coal flames. Type I flames, are usually observed at relatively high primary air yelOCltleS a~d as ~ result o! the large momentum associated with the primary air jet, the l~te~al reclfculatlon z<?ne IS penetrat~d by the primary air jet. Consequently, the primary alf Jet and the pulvenzed coal partIcles carried with it divide the IRZ into smaller segments, which mayor may not be equal in size depending on the symmetry of the flow field. Type II flames are observed at TYPE-1 TYPE-2 TYPE-2 TYPE-3 relatively low primary air (attached velocities and allow the formation of a larger IRZ, since in this case the primary Internal air jet no longer carries adequate momentum to penetrate the IRZ. Other researchers have identified a third type of flame referred to as Type ill, in which the degree of penetration of the IRZ by the primary jet differs '1l that of Type I. Hence, need for a third category of _Jdlnes (Truelove, Wall, Dixon, & Stewart, 1982; Truelove, 1984; Truelove, 1986; Truelove & Holcombe, Figure 1: Schematic Diagram of the Flow Types 1990; Truelove & Williams, (Lawn, 1987) 1988). In this investigation, the aforementioned flow types were employed to classify the flames issued by the burner (Figure 1). By staging the combustion air, a 10w-NOx burner allows for only a fraction of the combustion air to mix with the pulverized coal during (i <'Holatilization (Wendt, 1995). The remaining air is then mixed downstream in the flame omplete the process of combustion. As the overall mixing is reduced, the flame elope becomes longer, compared to non-staged flames. The role of the IRZ in low_ IIOX burners is extremely important (Lockwood & Mahmud, 1988). The objective in the design of a low N Ox burner is to generate an IRZ large and intense enough for all of the co 1 • rticles to devolatilize in the IRZ (Sharifi & Scaroni, 1995). The intensity of the IF .,haracterized by the rates of heat (J/s) and mass (kg/s) transferred from the tail end 01 .. .. Ilame toward the burner throat. The key parameter responsible for the formation of the IRZ is the swirl imparted to the combustion air, as characterized by the swirl number. In swirling free jets or flames, both axial flux of the angular momentum and the axial thrust are conserved (Beer & Chigier, 1972). These can be written as: Gcp = I: pu wr 2m dr = constant (1) Ox = foR pu u 21tr dr + foR p 21tr dr = cons! (2) \\'r "re u, wand p are the axial and tangential components of the velocity and static tre respectively, in any cross section of the jet. Since both of these momentum " can be considered to be characteristic of the aerodynamic behavior of the jet, the :imensional criterion of swirl number based on these quantities can be set up as: s=~ (3) GxR |