| OCR Text |
Show The most important aspect of Figure 3 is the limit on lean flame combustion. For the best case with the laboratory burner operating under laminar conditions, stable combustion is only possible down to an equivalence ratio of approximately 0.8 and only then for a very narrow range of input powers. For a laminar flame at 4>= 1, the maximum stable input power is about 10.8 kW, above which blowoff occurs. The minimum stable input power is 2.3 kW below which flashback occurs. Thus the turndown ratio of the rim stabilized burner is 10.8/2.3 or 4.9: 1 which is typical of many commercial burners. The turndown ratio is significantly reduced for lean flames. Figure 4 shows the stability limits for the ring stabilized burner in turbulent flow. The range of stable operation is significantly increased, well into the region of lean flame combustion. For ring G4, the lean blowofflimit is as low as 4>=0.55, which approaches the flammability limit for a mixture of natural gas and air. Also noteworthy is the dramatic increase in turndown ratio. The upper limit of the blowoff lines extends beyond the range of the current measurement equipment, but the turndown ratio at cI>=O.8 is conservatively . estimated at 20: 1 and could easily be 30: 1 or greater for ring G4. The traditional rim stabilized burner essentially has no turndown at 4>=0.8 and operation is fixed at an input power of 1.5 kW. This result is significant in tenns of commercial application. A burner with a large turndown can adapt to a wide range of heating loads and thus maintain efficient operation without wasting energy due to overcapacity. Although all four rings have significantly larger stable operating ranges than the rim stabilized burner, increasing the gap size, 0, clearly improves the blowoff limit. However, the effect of increased stability with increasing 0 is limited. The difference in cI> at blowoff for rings Gland G2 is greater than the difference for rings G3 and G4 even though there is a constant 0.8 mm increase in gap size between all four rings. Nevertheless, ring G4 offers optimum stability with an extremely high turndown ratio. The influence of gap size on emissions will be discussed in later pages. 8 |
