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Show However, w e surmise that in a furnace large enough to avoid this impingement, the zone length would approximate the value given by linearly extrapolating the pre-impingement data in Fig. 12 to XCH = 0. A s shown in the figure, w e also linearly fit (using intuitive judgment) the sparse data for the two higher temperatures. W e thus estimate combustion-zone or "flame" lengths L/, measured from the burner wall (x = 0), of 2.6, 2.05 and 1.65 m for the three operating conditions. The transverse scale of the combustion zone may be measured by the breadth of the C H4 profiles from fringe to fringe, in the horizontal midplane of the furnace (z = 0.5 m ) or in the vertical plane containing the burner axis (y = 0.75 m ) . These horizontal and vertical measures roughly agree, indicating approximate cylindrical symmetry. The profile breadths divided by two are thus effective combustion zone or flame radii, R/. Figure 13 shows these radii as a function of distance x from the burner wall and gives indications of the flame shape. In its early development, the boundary is plausibly virtually the same in all three cases, determined by jet aerodynamics and near constancy of the jet spreading rate. The effects of gas temperature level on the combustion zone boundary are manifested toward the end, with burnout occurring sooner at the higher temperatures, as might be expected.The combustion zone at the lowest temperature level, as already noted, impinged upon the furnace boundaries. In the figure, w e have drawn from imagination an approximation to the likely flame shape in a furnace large enough to avoid impingement. The maximum flame radii, as estimated from the figure, are related to the flame length by Rf= 0.24 Lf. (6) The flame volumes approximate Vf= 0.095 4. (7) We can accordingly estimate volumetric combustion intensities, mf(-Ahc)/Vf, i.e., the combustion heat release per unit flame volume. The numerical results on this, Rf, I/and V/SLTQ summarized in Table 6, together with the operating conditions of the trials. It should be remembered that the combustion air temperature, Ta, was not constant in these trials, Table 6. Because of the the use of recuperators for air preheating, the exhaust gas temperature Te rose and fell with Ta. In the Discussion (§ 8), w e develop a flame length formula that takes into acount both Ta and Te. 7.4. Flame appearance and combustion stability 7.4.1. Effects of furnace load The CGRI burner is basically suited to operating in reasonably hot furnaces, but not in cold furnaces. This does not mean that there is generally a difficulty in getting the furnace running from a cold start. The most critical factor is the amount of sink, or "furnace load", available. W h e n there is so much load that steady state operation would be unstable, because adequate temperature levels would not be reached, then start-up is also problematic and it is typically impossible to get above low fire because of spatial and temporal discontinuities in combustion (intermittent failures of ignition and flame propagation), causing, at worst, soft localized explosions and heavy pressure surges, prompting shutdown for safety reasons. The 12 |