Swirling flow in a research combustor

Measurements of turbulent, confined, swirling flows have been obtained by means of a two-color laser Doppler velocimeter in a research combustor and compared with other experimental data and numerical results obtained by means of two two-equation models of turbulence. The combustor consists of two confined, con- centric, swirling jets whose mass flow rates and swirl numbers can be controlled independently, and which can be used to study cold flow, premixed and non-premixed reactive flows, and two-phase flows. Results are reported for cold flow conditions under co- and counterswirl. It is shown that under both conditions a closed recirculation zone is created at the combustor centerline. This zone is characterized by the presence of a one-celled toroidal vortex, low tangential velocities, high turbulent intensities, and large dissipation rates of turbulence kinetic energy. The experimental data agree satisfactorily with the numerical results, but do not agree with other ex- perimental data under coswirl flow conditions. The reasons for the discrepancies are discussed. N important application of swirling flows lies in its use to provide flame stabilization and improved mixing in many combustion chambers.1 In order to achieve enhanced flame stabilization and better control of the mixing process, multiple coaxial swirling streams can be introduced in swirl combustors. Yetter and Gouldin2 carried out a series of experiments on a model combustor which uses two coaxial swirling jets. They found that the fluid mechanical aspects play an important role in determining the operation of the combustor; combustion under coswirl conditions (jets rotating in the same direction) was found to be significantly different from combustion under counterswirl conditions. Vu and Gouldin3 performed experiments in a model combustor composed of two confined coaxial swirling jets under nonreacting conditions. They found that a recirculation zone occurs only with counterswirl near the exit of the inner jet. The recirculation zone was in the form of a one-celled toroidal vortex having very low swirl velocities. Habib and Whitelaw4 performed similar experiments in confined coaxial jets although under weak swirl conditions, i.e., no recir- culation zone at the combustor centerline appeared in their experimental work. Gouldin et al. 5 performed experiments similar to those reported here and in Ref. 3 by means of a laser Doppler velocimeter (LDV) and found that a recir- culation zone is created at the combustor centerline only under counterswirl flow conditions. The results of Gouldin et al.5 and Vu and Gouldin3 indicate that, in the geometrical arrangement employed by these investigators, coswirl does not result in a recirculation zone under incompressible flow conditions. The results presented in this paper show that a recirculation does exist under both co- and counterswirl flow conditions. Experimental work on turbulent, confined, swirling flows has also been performed by Rhode,6 Rhode et al.,7 Gupta et al.,8 and Yoon and Lilley.9 Rhode6 and Rhode et al.7 studied swirling flows in a combustor provided with a sudden ex- pansion by means of a visualization technique in which neutrally buoyant helium filled soap bubbles were employed. The results of the visualization technique were compared with a two-equation model of turbulence and showed that the model predicts the gross features of the flowfield. Gupta et