OCR Text |
Show follow the turbulent flow. The lumps are tracked within the zone arrangement. The lifetime of individual lumps is statistically calculated with help of weighted random numbers in such a way that .the unburnt matter of a large assembly gas fuel lumps of same origin is an exponential function of residence time t : = (4) QUllb 0 is the sum of chemical heat of all gaseous fuel lumps or1ginating from the same burner inlet zone, and Qunp is the corresponding amount after time t. Accumulation of chem1cal heat from all fuel lumps reacted in a volume zone yields the chemical source term Qch needed for the total energy balance (Eq. 1). By using the value 0.1448 for the constant cb' tb can be identified as the time needed to achieve (at least on average) burn-out down to 99.9%. In the current study, the heat release distribution in the air flames was calculated using tb = 0.75 sand m = 2. The value for tb was chosen so, that 99.9% burn-out was achieved just at the flue gas port. Thus the observed flame length was matched. The time constant tb for the 02 flames was chosen so that the length of the 02 flames observed in pilot scale experiments was matched. Fig. 2 shows measured axial values of unburnt fuel and of 02-concentration dependent on distance from the 02 burner normalized with an equivalent nozzle diameter deq • The distance Z/deq = 121.2 represents the width of the furnace investigated in the current study. Also plotted into Fig. 2 are burn-out distributions based on Eq. 4 and used in the model. The curves denoted by A5 represent an 02 flame with a load 3.71 higher than the fldm~ represented by curves A6 (see also below). Both sets of curves were obtained for m = 3. For flame A5, tb = 0.045 s was utilized and for the lower load flame, tb = 0.167 s, i.e. the ratio for tb of both flames was assumed to be proportional to their load ratio. The heat release model described above is coupled with species transport equations which allow calculation of zonal concentrations of major combustion products and 02 needed to compute specific heats. The zonal concentrations of CO2 and H20 are also used to calculate zonal absorption coefficients Kn as required by Eq. (2). The current radiation model requires additionally estimation of flame soot concentrations. Whereas the O~ flames are soot free, luminous radiation plays a certain role 1n air flames. The natural gas/air flames considered in this study were very sooty. In order to calculate soot absorption coefficients in these flames, it was assumed that flame soot is present in a zone in proportion to unburnt fuel 6 |