Parametric study of jet mixing enhancement by vortex generators, tabs, and deflector plates (original) (raw)
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Blockage Analysis and Mixing Enhancement Evaluation of Tabs, Vortex Generators, and Deflector Plates
ABSTRACT This report provides a comparison of three passive mixing techniques employed on an axisymmetric jet. In the first part of this report, blockage, velocity, and average vorticity data are presented for delta tabs and half delta-wing vortex generators. It is determined that the blockage associated with a half delta-wing vortex generator (h/D= 0.2, angle of attack= 30, and sweep angle= 60) is approximately 1/3 of the blockage of a delta tab of equivalent projected frontal area blockage (2% of jet exit area).
INCAS BULLETIN
Jet mixing becomes necessary for its wide range of applications from household appliances to modern high technology rockets. Various researchers have studied the enhancement of jet mixing and concluded that the most effective jet mixing is due to the engagement of a vortex generator at the exit plane of the nozzle, thereby creating vortices of different sizes to enhance the mixing. To intensify the jet mixing, two similar innovative vortex generators with a total blockage ratio of 3.5% are placed diametrically opposite locations of the convergent nozzle. The Aspect Ratio of the convergent nozzle is 1. A numerical investigation is carried out to assess the effectiveness of the vortex generator for Mach numbers of 0.4, 0.5, 0.6, 0.7 and 0.8. The centerline Pitot pressure decay was calculated and found to exhibit the core length reduction due to the introduction of the Vortex generator. To measure the effectiveness of the jet mixing using vortex generators, the results are compared wit...
Experimental study on enhancement of supersonic twin-jet mixing by vortex generators
Aerospace Science and Technology, 2019
Experimental results on the mean flow evolution and the control of single and twin compressible jets at Mach 1.6 are presented. The jets issue from conical CD nozzles closely placed side-by-side resembling the twin nozzle configuration of supersonic aircrafts. The results are relevant to scenarios where turbulent jet mixing, supersonic core length, thermal radiation and acoustic loading are of concern. Experiments show that closely spaced twin jets grow, merge and interact near the inter-nozzle region that influences the characteristic decay and jet spread. Moreover, the deviation in centerline characteristic decay is more significant at off-design conditions. Vortex generators in the form of small metallic rectangular tabs mounted at the nozzle exit plane in different azimuthal orientations are used to control the mixing characteristics and the spread of these jets. Abrupt reduction in the core length and suppression of shock cell structure is achieved in over to under-expanded conditions. Furthermore, the orientation of vortex generators is found to significantly influence the development of jet flow field. The jet bifurcation and formation of daughter streams with distorted quasi-periodic shock cells structure are visualized using the schlieren technique. The underlying mechanisms for the observed effects and the behavior of daughter streams are discussed.
Transverse jet mixing characteristics
This experimental study explores and quantifies mixing characteristics associated with a gaseous round jet injected perpendicularly into cross-flow for a range of flow and injection conditions. The study utilizes acetone planar laser-induced fluorescence imaging to determine mixing metrics in both centreplane and cross-sectional planes of the jet, for a range of jet-to-cross-flow momentum flux ratios (2 J 41), density ratios (0.35 S 1.0) and injector configurations (flush nozzle, flush pipe and elevated nozzle), all at a fixed jet Reynolds number of 1900. For the majority of conditions explored, there is a direct correspondence between the nature of the jet's upstream shear layer instabilities and structure, as documented in detail in Getsinger et al. (J. Fluid Mech., vol. 760, 2014, pp. 342–367), and the jet's mixing characteristics, consistent with diffusion-dominated processes, but with a few notable exceptions. When quantified as a function of distance along the jet trajectory, mixing metrics for jets in cross-flow with an absolutely unstable upstream shear layer and relatively symmetric counter-rotating vortex pair cross-sectional structure tend to show better local molecular mixing than for jets with convectively unstable upstream shear layers and generally asymmetric cross-sectional structures. Yet the spatial evolution of mixing with downstream distance can be greater for a few specific convectively unstable conditions, apparently associated with the initiation and nature of shear layer rollup as a trigger for improved mixing. A notable exception to these trends concerns conditions where the equidensity jet in cross-flow has an upstream shear layer that is already absolutely unstable, and the jet density is then reduced in comparison with that of the cross-flow. Here, density ratios below unity tend to mix less well than for equidensity conditions, demonstrated to result from differences in the nature of higher-density cross-flow entrainment into lower-density shear layer vortices.
Influence of Issued Jet Conditions on Mixing of Confined Flows
The development of the velocity and scalar fields in a coaxial jet mixer has been experimentally investigated applying simultaneously Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) methods. Mixing of a turbulent jet (water solution of Rhodamin 6G) issued from a long round nozzle (l/d = 60) with co-flow (water) was studied. Because of the nozzle length the boundary layers of the inner flow already merged upstream of the jet exit. The issued jet conditions were changed installing vortex generators (tabs) at the nozzle exit. The tabs of rectangular and triangular forms with heights of 13 15 % the inner nozzle diameter accelerated mixing significantly but the scalar field developed to the homogeneous state faster than the velocity one. Turbulent characteristics measured downstream of the jet exit gave evidence the mixing specific behind the rectangular and triangular tabs.
Journal of Fluid Science and Technology, 2011
In order to develop a new mixing procedure, we conduct DNS (direct numerical simulation) of vector controlled free jets. The inflow velocity of jet is periodically oscillated perpendicular to the jet axis. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which Fourier spectral and 6th order compact scheme are adopted. From view of instantaneous vortex structures, it is found that the flow pattern considerably changes according to the oscillating frequency, i.e., according to the increasing the frequency, wave like mode, bifurcation mode and flapping mode appear in turn. In contours of ensemble averaged streamwise velocity and turbulence kinetic energy, the jet diffuses largely in the oscillating direction. On the other hand, the jet width in the perpendicular to the oscillating direction is similar to that of uncontrolled jet. Further in order to quantify the mixing efficiency under the vector control, as the mixing measure, the statistical entropy is investigated. Compared to the uncontrolled jet, the mixing efficiency is improved in order of the wave-, the flapping-and the bifurcating mode. Thus the vector control can be expected for the improvement of mixing efficiency.
The Time Evolution of the Flow Fields and Mixing Characteristics of Non-Reactive Jets
2010
Numerical simulations are used to study an under-expanded sonic jet injected into a supersonic crossflow and an over-expanded supersonic jet injected into a subsonic crossflow. A finite volume compressible Navier–Stokes solver developed by Park & Mahesh (2007) for unstructured grids is used. The flow conditions are based on Santiago et al.’s (1997) and Beresh et al.’s (2005) experiments for sonic and supersonic injection, respectively. The simulations successfully reproduce experimentally observed flow vortical structures and shock systems such as the barrel shock, Mach disk, horseshoe vortices that wrap up in front of the jet and the counter rotating vortex pair (CVP) downstream of the jet. The time averaged flow fields are compared to the experimental results, and reasonable agreement is observed.
Predictions of mixing enhancement for jets in Cross Flows
The efficiency of a set of mixing enhancement devices on an array of Jets In Cross-Flow (JICF) is studied using Large Eddy Simulation (LES). The baseline flow is a rectangular channel flow on which five JICF's are installed on each wall (upper and lower). Mixing devices are fixed tabs installed upstream of the jets. Instantaneous analyses of the LES fields reveal two large vortical structures developing downstream of the mixing device. These structures strongly enhance mixing. Comparisons of the statistically averaged LES predictions against experimental results validate the predictions. The mixing devices provide a better spatial and temporal homogeneity of the gas mixture at the exit of the main duct. Even though full temporal and spatial homogeneity of the gas mixture prior to combustion is not guaranteed with this design, the probability of finding strong inhomogeneous zones is reduced. More generally, this study confirms the power of LES to help design actuating devices for flow and mixing control.