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Show Detailed structural investigations of turbulent flames and studies of the flow fields effects have recently become feasible by means of modern laser diagnostics. In spite of the availability of these techniques, very little detailed experimental information exists on turbulent premixed flame in a combustor with lateral injection. The recent experimental and numerical results of Montazel [15] and Veynante et al. [23] will be systematically compared to the predictions of the model described above. The experimental work of Montazel consisted in flame front visualisation by tomography imaging using C C D camera, heat release rate measurements derived from C H radical emission field and temperature measurements derived from gas composition analysis. Figure 2 is a picture of the experimental combustion chamber showing the type of flame which is obtained with this arrangement: two reaction zones can be identified, one which is located up-stream with respect to the injection of the fresh gases (called the dome) and the other one downstream from the injection ports. In the experimental work of Veynante et al. [23], flame front visualisation is obtained by tomographic imaging (Figure 3.a) the flow being seeded with small droplets of groundnut oil burning at the flame front. W h e n the flow is illuminated by a laser sheet, only fresh gases, including the droplets, diffuse the laser light, whereas burnt gases appear as dark area. The C H radical emission field is shown in Figure 3.b. This radical emission, generally assumed to be proportional to the reaction heat release, is obtained from a C D D camera including a narrow band filter centered at 413 nm. The C H radical field and the flame surface field are quite different. T w o combustion zones are apparent in the C H emission field. The first one is just upstream from the interaction of the two jets. Following Veynante et al. [22], the downstream reaction zone is quite longer and larger for radical emission than for flame surface. These descrepancies may be explained by various reasons. First, tomographic measurements do not provide the reaction zones but only isothermal lines where oil droplets are vaporized. In this experiment, where walls reach high temperature and where recirculation zones are important, droplets m a y be vaporized far from the flame front. The upstream reaction zone, apparent in C H radical emisssion, takes place in a recirculation zone that may not be very well seeded by droplets. It is important to note that the heat release source term is far more interesting than temperature for modelling purposes. Following Candel et al. [6], many observations indicate that light emitted by the combustion zone is related to the reaction intensity and hence to the heat-release process. Certain radicals like C 2 and C H appear almost exclusively in the reactive zone and their concentration is always small. Hence, the self absorption of the light emitted by these radicals is not important and the radiated light is directly related to the reaction rate or equivalently to the heat release rate. While a linear relation between the heat release source term and the light emission from free radicals has been proved in some special situations, it m a y be safely stated that a monotonic and more or less linear-relation exists between these two quantities. The measurements of the heat release source term relies on this assumption. The calibration of the radical light emission m a y be obtained from additional information (for instance gas analysis in the exit plane of the combustor) providing the global heat release rate in the combustion chamber. Figure 4 presents the calculated velocity field. O n e can notice on this figure the presence of recirculation zones of the hot combustion gases which contribute to stabilize this type of flame. The maximum velocity at the exit of the combustion chamber is 22 m/s. Figure 5 is a comparison between heat release rate (in W / m 3 ) field calculations from this work (Figure 5.b) and the experimental (Figure 5.a) and numerical (Figure 5.c) results of Montazel [15]. It can be seen that the present work produces better qualitative and quantitative predictions. The numerical results generate the main features observed in real situation : the two separate reaction zones in the d o m e and downstream from the injection ports, the minimum heat release rate obtained in the zone where the injection jets meet and the combustion taking place in the mixing zones on both sides of the fresh gas jets near the injectors. The location of the m a x i m u m heat release rate is also well reproduced. About his numerical results, Montazel admits the following : 1- the combustion intensity in the dome and downstream is considerably overestimated (a calculated maximum of 2.12 108 W / m 3 compared with an experimental result of 1.02 108 W / m 3 ), 2 - combustion downstream from the injection ports is not well reproduced : the reaction zone predicted numerically is less extended and the intensity of combustion is weaker than what is obtained experimentally, 3 - combustion on both sides of the merging jets is noticeably overestimated and the calculated results do not reproduce the minimum heat releases measured in the fresh gas jets. 7 |