OCR Text |
Show - 15 - concentration in a turbulent flow, Cj. In the computational model, this transport equation has the form: (3) where Ui is the velocity in the Xi direction, p is the gas density, Sc is the Schmidt number and fJe is the effective viscosity defined as: (4) with c~ equal to 0.09 [12]. The left hand side of Equation 3 represents the convective mixing of the species in the flow. A characteristic time scale of convective mixing, L/U, may be derived from a dimensional analysis of Equation 3. Here, L represents a characteristic distance between reburn fuel and primary combustion products and U is the characteristic velocity of the flow. The first term on the right hand side of Equation 3 represents the turbulent diffusion mixing of the species in the flow. A characteristic time scale for the turbulent diffusion mixing is Vefk2 where k, the turbulent kinetic energy, and e, the turbulent dissipation, are calculated from the computational modelling. The second term on the right hand side of Equation 3 represents a source or sink tenn for the species due to reaction in the flow. Assuming that the rate of reaction is controlled by mixing of reactants inside a fluid eddy, a characteristic time scale for Sj, kje, may be deduced from the eddy break-up model [13]. The magnitude of these three time scales are compared for the two experiments using length and velocity scales estimated from the experiments. The characteristic distance between the reburn fuel and primary combustion products, L, was estimated as the radial distance between the reburn gun and the bulk of the forward flow, Figure 13. The characteristic velocity, U, was estimated as the average velocity of the primary combustion products at the point of reburn fuel injection. The turbulent energy, k, and dissipation, e, were approximated from the calculated values for the forward flow near the point of reburn fuel injection. The characteristic times for mixing by convection, turbulent diffusion and reaction are shown in Table 2 for the reb urn zone experiments and the low and high mixing tests in furnace experiments. The mixing rate of reburn fuel with primary combustion products was significantly faster in the reburn zone experiments, using the IPFR, compared to the furnace experiments, using Furnace No. 1. In the IPFR, the convective mixing time was short (0.01 s) due to rapid impingement of primary combustion products on the reburn coal jet. The impingement generated a high level of turbulence and the characteristic reaction time, kje, was estimated at 0.01 s. These mixing times were of a similar order of magnitude to the characteristic times for nitrogen chemistry reactions [14]. For the high mixing case in Furnace No.1, the convective mixing time was short due to the proximity of the reburn fuel in the forward flow. However, the reaction time was estimated to be 0.08 s which was an order of magnitude longer than characteristic times for nitrogen chemistry reactions [14]. |