OCR Text |
Show calculated with above combustion model for the same zone. An empirical value was used for the constant of proportionality. Thermal ~oundary Conditions Prescription of thermal boundary conditions for the combustion space must distinguish between the refractory surfaces above the glass melt, and the port openings as well as the glass bath itself. All three sections require either calculation or prescription of effective temperatures and effective emissivities of the radiating boundaries. Heat Losses through Refractory Walls Local surface temperatures of the refractories exposed to the combustion space were calculated with the zone model and with a 1-D model for heat conduction through the refractory walls using effective values for the ratio k/s of conductance to refractory thickness and estimations for outside refractory temperatures. Calculation of Port Heat Losses Net radiation through the port openings constitutes a major heat loss of the combustion space. In the case of 02-combustion, the ports were also considered to be open, since the experimental set-up for 02-conversion of the furnace considered in inis study was planned for all ports to be left open. The analysis of port radiat jo~ was separately carried out by applying the zone model for a rt;:fi"'~sentative section of a port and the upper ·,-c,~ -e-~~ erator. The port analysis yielded ultimately port view factors, which differed between the cycles for air combustion and also for 02-combustion. The view factors Ff-->rg are defined by Eq. (5)., which relate the net heat flux Qp through the combustion space port opening with area Ap to the incident radiation qp,in from the combustion space and to an effective black regenerator temperature Trg,eff: 4 Qp = Ff->rg Ap (qp,in - aT rg,eff> (5) Boundary Conditions at Glass Surface In prescribing thermal boundary conditions at the melter surface, it was assumed that the glass of the end-section was batch-free. Two different types of thermal boundary condjtions were then studied. 7 |