OCR Text |
Show studies [2, 5], pool frres [6-10] and shear layers [11 , 12]. In addition, recent turbulent natural gas combustion modeling efforts have employed mass entrainment correlations [13]. . In a classic study, the mass entrainment of both nonreacting and reacting turb~lent. Jets were measured by Ricou and Spalding [2] . The jet under study was enclosed in a cyhndncal chamber having an opening to atmosphere large enough for the jet flow to exit the ~~~r uninhibited. Surrounding gas was fed around the jet through a porous cylindrical rmg lDSlde the chamber. The differential pressure across the exit of the chamber was used as an indication of the entrainment capacity of the jet. When the differential pressure across the chamber opening was zero, Ricou and Spalding [2] noted that the supplied surrounding gas flow matched the mass flow that can be entrained by the jet under study. From their experimental results, Ricou and Spalding [2] developed an expression for the mass entrained by an isothermal gas jet, given by, mt = ms +mo = 0.32 [.K.] J p, mo mo do Po Eq. 1 where ~ is the total mass flow rate, Ills is the surrounding-gas mass flow rate, IIlo is the jet mass flow rate at the injector exit, x is the axial distance from the injector exit, do is the injector diameter, Ps is the surrounding-gas density and Po is the jet-gas density. Ricou and Spalding [2] also noted that buoyancy-dominated jets have a larger entrainment rate than momentum-dominated jets, and that combustion tends to lower the entrainment capacity of a jet. Becker and Yamazaki [5] experimentally investigated the jet spread, entrainment and momentum flux associated with turbulent diffusion flames. The reactants were commercial propane and air, and values of the mean temperature and mean axial velocity within the flame region were obtained by using a thermocouple and a slender pitot tube, respectively. Correlations were developed for a number of quantities, including the mass entrainment ratio, which had a dependency on the Richardson number (:=buoyancy/input momentum flux). The form of the developed mass entrainment correlation revealed an increase in the mass entrainment capacity of the jet as buoyancy became important. More specifically, the entrainment ratio in a buoyancy -dominated regime increased by a factor of 5.8 over that in a momentum-dominated regime [5]. To a large extent, the mass entrainment correlation in the momentum-dominated regime developed by Becker and Yamazaki [5] agreed with that formulated by Ricou and Spalding [2]. Zhou and Gore [6] studied the flow field associated with a 71 mm toluene pool frre using a laser Doppler velocimeter (LDV). The authors [6] noted that only a small amount of the air being entrained by the pool flame actually crossed the visible flame boundary. The detailed LDV measurements also confrrmed instances of radial outflow from the flame as well as negative axial flow (towards the pool of toluene). The work brings to focus the potential importance of transient phenomena with regards to understanding entrainment behavior. Flame Lift-Off Heights A number of studies treating diffusion flame lift -off heights [14-17] and blowout stability limits [18-20] have been pursued. An excellent review of previous flame lift-off and 3 |