Flow regimes of large-velocity-ratio coaxial jets

An investigation of the near-eld flow structure of coaxial jets with large outer to inner velocity ratio ru has been conducted. Since in all cases ru > 1, the outer jet dominates the near-eld flow structure. Two flow regimes are identied depending on whether ru is larger or smaller than a critical value ruc. When ru r uc, the inner potential cone is truncated and is followed by an unsteady recirculation bubble with low-frequency oscillation. The transition from one regime to another is explained by a simple model whose ingredients are the turbulent entrainment rate, governed by the outer-jet mixing layers and mass conservation. This model satisfactorily predicts the dependence of the inner potential cone length on ru and the critical velocity ratio ruc. The recirculation bubble has a wake-type instability. It oscillates at a low frequency and a large amplitude compared to the Kelvin{Helmholtz mode. Angular cross-correlations in the plane parallel to the jet outlet show moreover that this oscillation displays an azimuthal precession such that the rotation time of the phase of the oscillation equals the oscillation period. These salient features are discussed in the framework of the nonlinear delayed saturation (NLDS) model. Coaxial jets are a simple way by which two fluid streams can be mixed and this conguration is used for instance in combustion chambers of rocket engines. Often, one of the jets (the inner one) is in a liquid state and has to be atomized by a high-speed annular gas jet. This process, known as airblast atomization, has received considerable attention (Lefebvre 1989) during the past few decades. Most of the time the experiments have been aimed at characterizing the spray and have not allowed an analysis of the near-eld flow structure and the instabilities in any detail. Leaving aside surface tension eects, the important parameters in this problem are the momentum flux ratio between the two streams M = 2U 2=1U 2 and the ratio of the outer to the inner nozzle diameters = D2=D1. When the fluid densities are the same, the momentum flux ratio reduces to the velocity ratio of the outer to inner jet ru = U2=U1. The near-eld flow structure of coaxial jets in homogeneous fluids is, therefore, expected to be relevant to the understanding of liquid jet atomization. In the coaxial water jets studied here, quantitative flow visualizations can be used which are particularly helpful in the understanding of the interaction of dierent mixing layers present in the near eld. This is well demonstrated by the laser-induced-fluorescence visualizations of coaxial water jets with 0:59 6 ru 6 4:16, performed by Dahm,