| OCR Text |
Show heights of the precessing flames can be compared with the data of R¢kke et al. (1994) for conventional turbulent jet flames. This comparison shows that the effect of jet precession is to reduce liftoff height by factors of one-fifth to one-tenth for the same uJde. This suggests, in agreement with the earlier comments regarding the scale of mixing in the flame, that the actual strain rate in the region of the base of the precessing jet flame is significantly less than the exit strain rate. It also suggests that the effective strain rate at the base of the flame and, hence, the Damk5hler number is a function of Stp. Peters (1984) developed a theory of partially premixed laminar flamelets in which the mechanism determining liftoff depends on a critical scalar dissipation rate, Xc, which scales with ue/de. For X < Xc, the position of the flame front is controlled by the amount of air which has been entrained to enable a premixed flame to propagate against the flowing jet. For X > Xc, the liftoff height is controlled by the extinction of partially premixed flamelets, which, in turn, depends on the rate of scalar dissipation. R¢kke et. al (1992) propose that liftoff scales with a modified strain rate of uef dgtl, where the effective diameter is defmed as decc = de (PC I Pair)O.5. However, flame stability is also known to be increased by the presence of adverse pressure gradients and recirculated flow, as in a swirl burner (Syred & Beer, 1974). Recent cold flow measurements in the high-Stp precessing jet flow (Schneider et al., 1995) have revealed the presence of a recirculation flow zone whose position is instantaneously asymmetric and precesses with the jet Such a flow, if also present in a flame, can be expected to generate large scale turbulent motions, increased entrainment of air, and some local upstream transport of combustion products. It can be argued, then, that the observed reduction in liftoff height produced by jet precession is caused by some combination of the following: increased 'premixing 'of fuel 'and air within the region at the base of the flame - possibly augmented by the presence of a recirculated flow zone; reduced effective strain rate at the base of the flame by generation of an increase in the dominant scale of mixing; the presence of an adverse pressure gradient (which also precesses) and its associated flow field (e.g., flow recirculation), somewhat analogous to the effect of swirl (Claypole and Syred, 1982). CONCLUSIONS An investigation was conducted to characterize the shape and size, the global radiation, and the CO and NOx emissions of flames whose turbulent flow fields do not have the self-similar characteristics of conventional, free, turbulent jet diffusion flames. Deviations from conventional 13 |
