An Experimental Study of Turbulent Mixing Enhancement with a 10 deg Deflection of the Annular Stream

Abstract

†Turbulent mixing enhancement of two co-axial jets having a low annular to core area ratio is studied using a fiber-optic LDV system. Mixing of jets with low area ratio finds application in the mixing chamber of low bypass turbofan engines used in military aircraft. A chute mixer geometry was employed to enhance mixing of the jets. Experiments, at a velocity ratio of 1.8, which is close to a typical value of a real low bypass engine, were conducted in probably the biggest facility of this kind to study the effect of injecting part of the annular stream at 10 o towards the core region on the turbulent mixing process. Contours of mean velocity and streamwise and transverse turbulence intensities were obtained by making measurements close to the point of injection with a very fine grid to gain a deeper insight into the mixing enhancement process. Measurements were made at several thousand locations and at each location, a large number of samples was obtained. Mean velocity and turbulence intensity distributions were obtained at different axial locations within a relatively short length of 5 mixing duct radii. Total and static pressure measurements were made to study the total pressure loss and the static pressure variation due to mixing. The high velocity annular stream was found to quickly diffuse after entering through the chutes and get mixed with the core stream due to higher levels of turbulence generated in that region. A strong transverse turbulence component is observed close to the injection points, which enhances the mixing of the penetrating stream. Profiles at the downstream locations show a faster approach towards uniform mean velocity and homogeneous turbulence intensity profiles. With the aid of the chute model, nearly complete mixing is achieved over a length of 5 duct radii. However, a higher total pressure loss of about 1.38% is the penalty to be paid for enhanced mixing of jets.