NEAR-FIELD CHARACTERISTICS OF A JET WITH A COIL-INSERT INJECTOR Hamid R. Rahai 1 Center for Energy and Environmental Research and Services (CEERS) Mechanical and Aerospace Engineering (MAE) Department California State University, Long Beach Long Beach, CA. 90840. Ayaz Alware 2 , Daniel Carpio 2 , and Eyass Khansa 2 CEERS Mechanical and Aerospace Engineering (MAE) Department California State University, Long Beach Long Beach, CA. 90840. 1 Professor and Director 2 Graduate Assistant ABSTRACT Simultaneous time resolved measurements of two components of turbulent velocity and their cross moments are made at the exit and downstream of an axisymmetric jet with a coil-insert injector. The coil-insert injector is a coil shaped tube with ratios of coil diameter, pitch spacing and length to the jet inside diameter of 0.1, 1.0, and 1.5 respectively. The coil had three round holes of 0.2 mm diameter at the middle of each pitch for radial injection. The volume flow ratios of the radial blowing to the axial flow were 0.075, 0.10, 0.125, and 0.15. Results indicate that the radial blowing enhances asymmetry and increased generation of turbulence intensities at the jet outlet. However, increased entrainment and mixing between the injected flow and the axial flow reduces the asymmetry downstream, resulting in relatively constant intensities in the region with high axial momentum. INTRODUCTION Vortex generators or tabs have been used to enhance mixing process in a turbulent jet (e.x. Bradbury and Khadem (1975), Samimy etal (1993), Zaman et al (1994)). The increase in the mixing process is believed to be due to the stirring action of the trailing vortex motion shed from the side of the tabs. Another study by Bell and Mehta ((1993) on the effect of different spanwise perturbations on the mixing layer growth has shown that, these mechanisms result in regular array of counter rotating vortices, which enhance the mixing layer growth rate in the near field. Gutmark and Grinstein (1999) presented a review of previous investigations of flow control with non-circular jets and showed that the initial conditions play significant roles on evolution of the jets and are responsible for the structure of the asymmetric vortices that roll up in the near field. The non- circular jets have higher entrainment and increased mixing as compared to the circular jet which is the result of interactions between streamwise and azimuthal vortices. Mi et al (2000) compared mean and turbulence characteristics of nine different shaped jets and found that non- circular jets decay more rapidly than the circular jet and the highest mixing rates among the shapes investigated was for an isosceles triangle. However, no significant change in the far filed mixing rates was found between the circular, square, star, and cross-shaped jets. Seidel et al (2005) performed experimental study of the effects of multiple radial blowing around a circular jet on its centerline velocity decay and turbulence and found that non- equally distributed blowing performs better than the equally distributed one in enhancing the mixing process. In the present investigations, a coil insert injector is used for enhancing the mixing process in the near field of an axisymmetric jet. Previous experimental investigations by Hoang et al (2001) , Rahai and Wong (2002), and Hoang and Rahai (2002a), have shown than the when coil pitch spacing is near one diameter, the axial location where mixing enhancement is obtained vary with coil wire diameter. When the coil wire diameter is less than 0.1, the coil insert induces enhanced mixing immediately downstream of the jet outlet. However, when the coil wire diameter is near or larger than 0.1, the mixing process is delayed to an axial distance between X/D=1 to 3. Hoang and Rahai (2002b) performed further numerical investigations of a coil-inserted tube with ratios of coil pitch and coil wire diameter to the jet inside diameter of 1.2 and and o.11 respectively, using a standard ε − k turbulence model. Their results indicate that the coil insert generate asymmetric streamwise vortices that enhances mixing in the near field of the jet From these studies it was conjectured that for small wire diameter, the free streamwise vortices generated off the coil are the mechanism behind the mixing process. However, as the wire diameter is increased, a weak swirl generated by the coil insert prevents the mixing enhancement process by the free 1 Copyright © 2006 by ASME Proceedings of IMECE2006 2006 ASME International Mechanical Engineering Congress and Exposition November 5-10, 2006, Chicago, Illinois, USA IMECE2006-13728 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 10/13/2014 Terms of Use: http://asme.org/terms