| OCR Text |
Show moderate when compared with the cases for SFG whereby the emissivity of combustion gas seems to control heat transfer to the furnace wall. A rough estimation reveals that the gas emissivity is proport ional to Pl· 2 - 0 .3 [5] for CO2 and H20 which coincides with the order of n for SFG flames. However, note that for flames with Type D burner, n=O.l independent of the fuel type. NOx Emission : As a rough trend, NOx emission increases with increasing pressure, as shown in Figure 5. The exponent of pressure is reported as being near 0.5 for gas turbine combusters. [6] The present result is in accordance with this in terms of its global trend. However, NOx emission is also dependent on flame structure, as it relates to maximum flame temperature and residence time in flame. The scattering of plots can be attributed to this fact. 3. Theoretical Consideration 3.1 Dimensionless Parameter Analysis Non-dimensionalization of governing equations for a steady flow combustion system derives some important parameters which determine flame characteristics. They are Reynolds number, Froude number, Eular number and Damkohler number. The boyancy effect is assumed to be marginal under the present consideration, so that the Froude number is omitted here. Eular number describes the relative importance of pressure force against inertial force in the equation of motion. As it does not depend on level of absolute pressure as long as fluid can be regarded as ideal gas, the effect of pressure on flames cannot be determined by this parameter. As the variation of fluid viscosity is marginal for pressure levels less than 10 MPa, Reynolds number can also be neglected, as the density change cancels out velocity change. Furthermore, Reynolds number is generally large for industrial furnace conditions, indicating that the viscous force is insigificant when compared with the inertial force. Thus, no consideration is necessary with regard to Reynolds number. Damkohler number is probably the most important in describing the effect of pressure. It can be defined as a ratio between characteristic time for chemical reaction and residense time of fluid in a reactor by convection. According to the molecular kinetics, the effect of pressure on the rate of reaction is apparent and can be expressed in by following equation. 9 |
