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Show Takeno [24] evaluated the effects of chemical reaction on laminar/transitional jet flow structure. Takeno [24] found that chemical reaction (e.g., in diffusion flames) delayed the transition of a gas jet from the laminar to the turbulent regime when compared to cold gas jets at similar Reynolds numbers. Stabilization of the jet flow resulted from an increase in viscosity and an increase in the diffusion coefficient (producing a thick viscous layer) due to the higher temperatures resulting from the combustion event. In addition, Takeno [24] suggested that the reduction in density associated with the higher temperatures diminished the jet's ability to expand laterally. Takagi et ale [25] evaluated the effect that chemical reaction plays on the flow structure of turbulent jets. Through schlieren photographs of reacting and nonreacting jets at the same Reynolds number, Takagi et al. [25] demonstrated the laminarization of the jet's upstream outer edges under combusting conditions, which would tend to retard entrainment. However, the laminarization region for the combusting cases decreased with increasing Reynolds number. This laminarization layer was not evident under cold-flow conditions. In addition, for the reacting case, Takagi et al. [25] showed that turbulence was suppressed in the high-temperature upstream region of the jet as compared to the cold-flow jet due to the increase in viscosity associated with the temperature increase. Here, the observation of a lower jet mass entrainment rate with increased surrounding-gas oxygen content is consistent with the current view of turbulent diffusion flames. Flame Lift-Off Height A series of flame lift -off height measurements were made for a natural gas jet in a high-temperature oxidant using the bench-scale apparatus shown in Fig. 2. The entrainment chamber with windows was employed for this test series, hence the hot surrounding-gas flow 70 60 :::..,.0 50 .r. 40 30 20 10 o ~--~~~~~~~~~ 0 .0 0 .2 0 .4 0 .6 0 .8 1.0 ~o Fig. 7. N ondimensional flame lift-off height, hldo' of a natural gas jet in a high-temperature oxidant as a function of jet Mach number, Mo' oxidant oxygen content and oxidant temperature, Ts. The orifice diameter was 1.59 nun. was coaxial with the natural gas jet flow. The flame lift -off height, h, defmed as the distance from the injector exit to the base of the luminous flame, was measured as a function of jet velocity, surrounding-gas oxygen content and surrounding-gas temperature, as shown in Fig. 7. The velocity of the surrounding gas (i.e., the oxidant) emanating from the porous disk was low (-0.35 mls), and more than enough surrounding gas was supplied to the natural gas jet to satisfy its entrainment appetite. As a matter of reference, the height of the entrainment chamber (the distance from the injector exit to the Inconel insert opening) was 202 mm, or x/do = 127. The oxygen concentrations given in Fig. 7 are on a volume and wet basis, with the balance species of the oxidant being primarily CO2 and H20. The flame lift-off height increased with increasing jet velocity, decreasing oxidant oxygen concentration and decreasing oxidant temperature. It is interesting to 10 |
