Analysis of Toroidal Vortex Unsteadiness and Turbulence in a Confined Double Annular Jet

The flow in the downstream of the bluff body in a double annular confined jet is experimentally investigated over a range of Reynolds numbers. The near wake region of the inner annular jet is occupied by a laminar central toroidal vortex. A pair of contrarotating toroidal vortices occupies the region between the inner and outer jet. And a large reverse flow region occupies the space between the outer jet and the confining wall. The flow in the regions occupied by the toroidal vortices is investigated using flow visualization, particle image velocimetry and laser Doppler velocimetry at several Reynolds numbers ranging from the laminar to turbulent regimes. At the lowest Reynolds number the vortices are laminar, even at very low Reynolds number, unsteadiness sets in and break up these toroidal vortices into a number of large eddies. In the range of Reynolds number investigated, the time averaged velocity field preserves the same topology but the extent of the various regions gradually changes with Reynolds number. The re-circulation zones behind the bluff bodies' increase with increasing Reynolds number reaches a peak then gradually approaches an asymptotic value. INTRODUCTION Mixing of jets, often in enclosed space, plays a crucial role in a large number of industrial appliances. Unfortunately, it is still a poorly understood physical phenomenon and can not be predicted accurately. In the burners, the flow downstream of a bluff body is used as a flame holder. Frequently, the flame holders are in the form of the circular central body of an annular jet where a toroidal vortex forms. In these flow fields large temperature gradients exist which leads to large variation of local Reynolds number. To understand the behavior of such flows it is essential to study the unsteadiness and turbulence characteristics of the flow over a wide range of Reynolds numbers encountered in these flows. The fluid dynamic behavior of the flow inside a combustion chamber under operating condition and that of cold condition are shown to be similar when the nonisothermal Craya Courtet number (NCCN) in the two cases are same'. This criterion establishes the validity of cold flow experiments to explain the behavior of the flow field in combustion chambers. The instability and transition to turbulence of round jets are mainly through the formation of vortex rings and their interaction in the shear layers. This lead to the formation of large eddies significantly different from the vortex rings. The phenomena occur over a short distance from the exit of the nozzle. Yule studied the behavior of these large-scale structures and their distinctive characteristics in the transition and in the turbulent regions. Petersen reported the influence of forcing waves on the coalescence of these vortex rings and shear layer dispersion. Chan and Ko investigated the development of the outer mixing layers of annular jets and found it to be similar to that of a circular jet and independent of the bluff body shape enclosed by the jet. When a jet is enclosed by a concentric cylindrical surface the jet reattaches with the enclosing surface and forms an annular re-circulating region. Sheen et al investigated the behavior of an enclosed annular jet. They reported that the flow re-attachment with the enclosing surface exhibits a hysteresis loop with changing Reynolds number. The study revealed four distinct flow patterns for this enclosed jet over the range of Reynolds number investigated. The results of a large number of experimental investigations are available in the literature on the structures of the initial regions of annular jets and coaxial jets''''. From the published computational results''' it is clear that existing turbulence models are not suitable for prediction of all the aspects of the initial region. However, they are capable of revealing * Copyright © 2001 The American Institute of Aeronautics and Astronautics Inc. All rights reserved. 1 Scientific Researcher * Scientific Researcher, Member AIAA. § Professor, Member AIAA. 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. the topology of the flow under certain conditions. Reynolds stress algebraic equation model relating the Reynolds stress explicitly to the quadratic terms of the mean velocity gradients and ensure positiveness of each component of turbulent kinetic energy performed better than K-e model in predicting turbulent jets. It is stated that possibly there is no universally applicable K-e turbulence model. Large-eddy simulation is more promising in predicting the mixing of Coannular jets accurately. In all the computational investigations it has been reported that at low swirl number the reattachment length is only slightly effected by the swirl. In case of non-swirling annular jets Young et al has reported that the agreement between experiments and computations of the re-circulation lengths improve at higher Reynolds numbers. In this paper, the results of an investigation on the evolution of the flow structures in the near wake of the bluff bodies of a double annular confined jet are presented. The highest Reynolds number used for the investigation matches that in a prototype combustion chamber in operation. At the lowest value of the Reynolds number, the re-circulation zones behind the bluff bodies are clearly visible in flow visualization. EXPERIMENTAL ARRANGEMENT The flow field under study is the cold chamber simulation of a prototype natural gas burner. The primary air is admitted through the inner annular nozzle and the secondary air is admitted through the outer annular nozzle. Due to the constraints on space, the designed burner admits air perpendicular to the burner axis through two circular tubes. The two streams of air then enter two concentric annular ducts leading to the nozzles. A contraction ratio slightly greater than 2 is used between the duct and the nozzles. This arrangement, 90-degree flow turning and short annular duct, makes the exit flow field severely distorted from axisymmetry. Characterizing such a flow field is nearly impossible. Therefore, the two models used for this investigation are modified to admit air axially through smooth entry sections. The length of the burner is increased to obtain the length to width ratio of the flow passages close to 50 in order to ensure fully developed flow at the exit. The experiments are conducted in two test-beds. One uses water as the fluid medium the other uses air. The model used for the water tunnel is half the size (diameters) and the model used in the cold chamber test using air is twice the size of the prototype burner. The cold chamber diameter is selected to match the operating NCCN of 0.23. This violated the geometrical similarity of the combustion chamber in the process of preserving the dynamic similarity. The ratio between the jet diameter to the combustion chamber wall diameter is not maintained. This non-similarity affects only the outer reverse flow region, which is not under investigation. At present only the near wakes of the bluff bodies are studied, the non-similarity is assumed to have no effect on the results. Experiments in Water The model used in the water tunnel is shown in Fig. 1. The dimensions of the ducts, the nozzles and the cold chamber for this model are given in table 1. As mentioned above the diameters of the prototype is twice those given in this table. The dimensions of the model for air are 4 times the dimensions given in the table 1. The tolerance on dimension during manufacture is 1%. However, since the nozzles are machined from metal the tolerances on the annular openings of the nozzles are maintained within 0.01 mm. Table 1. Dimensions of the model for water tunnel. Component (Subscript used) Primary Nozzle (p) Primary Duct (^) Secondary nozzle (s) Secondary duct (sd) Cold chamber (c) Inner diameter, dmm