| OCR Text |
Show 25 o 20 E ~ EO 15 + { 10 °2, :t (\101. w.,) • 6.S1 • 9 .00 ... 11 .8 • 1 • •• a 17 .1 o 19 .• 6 22 .• 'Q 24 .S o 26.S 20 - Rlcou and Spalding 60 Fig. 6. The mass entrainment ratio, 100 entrainment measurements agree well WI· th the predictions obtained from Eq. 1, thus giving confidence in the acetylene torch measurement technique. A series of entrainment measurements were made for several combusting-flow cases. In particular, the entrainment characteristi~s of a natural gas jet in a high-temperature OXIdant (7-27% O2 vol. wet) were studied. Throughout the test series, the natural gas jet Reynolds number (Reo = 2.9.104) and the surrounding gas temperature (Ts = 1368 K) were held constant. Variation in the nondimensional height, (x/do)(p/ Po)o.s, was achieved through changes in the injector diameter, the entrainment chamber height and the surrounding -gas molecular weight. Recall that the temperature of the natural gas jet was maintained at 298 K as a result of the water-cooled injector employed. (Ills + Illo)/Illo, plotted as a function of a nondimensional height and the oxygen content of the surrounding gas. The solid line represents the mass entrainment correlation developed by Ricou and Spalding [2]. The temperature of the surrounding gas was 1368 K. The mass entrainment ratio, (Ills +11\,)/1110, plotted as a function of oxygen content in the surrounding gas and (x/do)(p/po)o.s is shown in Fig. 6 along with the predictions made using Eq. 1. The oxygen concentration in Fig. 6 is given on a volume and wet basis and the balance species in the surrounding gas were primarily CO2 and H20. It is interesting to note that the mass entrainment ratio decreases with increasing oxygen content in the surrounding gas. The mass entrainment predicted by Eq. 1 more closely matches the low-oxygen content results. As the oxygen content of the surrounding gas increased, the flame moved closer to the water-cooled injector. In other words, the flame lift-off distance decreased with increasing oxygen content in the surrounding gas. Thus, it is not unexpected that the correlation developed for nonreacting jets matches the low-oxygen content data since chemical reactions are probably taking place well downstream of the injector. The effect is similar to having a natural gas jet entering into a hot surrounding atmosphere without chemical reactions occurring. As the oxygen level increases, the flame lift-off distance decreases, yielding a combination of a reacting and nonreacting jet inside the entrainment chamber. It is evident from the data in Fig. 6 that an increased degree of reaction leads to lower jet entrainment rates. In general, the entrainment rate decreased by about 30% with an increase in oxygen content from 6.5 to 26.5%. The observation of a decreasing entrainment rate is not unprecedented as Ricou and Spalding [2] noted a similar trend. Clemens and Paul [23] in their study of heat release effects on hydrogen diffusion flames provided evidence of a lengthening potential core for reacting conditions as opposed to nonreacting. A longer potential core suggests a decrease in the overall jet entrainment rate. 9 |
