OCR Text |
Show o 0 10 30 40 50 ratio of auxiliary gas (%) Figure 14 NOx concentration standardized 11 % 02 condition in the divided supply systems NUMERICAL SllvfULA TION OF A LARGE REHEATING FURNACE The experimental results mentioned above lead to strong possibility of the application of oxygen/COG combustion to a reheating furnace. What is now important is to consider the application of the new system in an industrial scale reheating furnace, where the thermal-fluid flow condition is different from that in the cylindrical furnace used in the experiment. Numerical simulations for an industrial scale reheating furnace applying the oxygen/COO combustion system have been carried out by the mathematical model developed in Nippon Steel. This mathematical model, named PROHEAT, is three-dimensional unsteady incompressible fluid flow and heat transfer model as follows; Turbulence Model: LES(Large Eddy Simulation), Deardorff(1970), which deals with a fluid flow in the filtered field (large scale) and in the sub-grid scale field separately, is employed for turbulence of the flue gas. The model has advantages in analyzing strong shear stress flows, like jets, or unsteady flows, compared with Reynolds averaged models, for example k-e model (Kobayashi 1986: ). The non-dimensionalized equations for the conservation of momentum and mass after filtering can be written as -ai +iI -a- ( -u·-u. ) =-a-- p +1- -a- [a- u-J' +a-i -iI 1- -a, (u-.u ),. +-G-r 0. -T a t a x . I J a x . Re a x. a x . a x . a x. \ 1 J Re 2 i3 J I J I J J a ii. -=J 0 ax . • (1) (2) where the index i = 1,2,3 refers to the x,y and z direction, respectively, and repeated indices im I summations. The Smagorinsky model is adopted for the sub-grid scale model, p y -uiuj = -;kOij +2v~ij' vT = (cst/ J2&'jSij (3) |