Blockage Analysis and Mixing Enhancement Evaluation of Tabs , Vortex Generators , and Deflector Plates

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). This accounts for the apparent dramatic effect of the delta tab on jet mixing. In a parameter analysis of half delta-wing vortex generators, we found that the centerline velocity decay is a strong function of the generator’s height and angle of attack, yet not as sensitive to sweep angle variations The final technique considered in this report is the use of extremely large tabs, or deflector plates, to alter the trajectory and mixing characteristics of an axisymmetric jet. For deflector plates with a length of 1 jet diameter and blockage of approximately 25% of the jet exit area, the peak temperature is decreased by over 50% at a distance of 20 jet diameters. The decrease in temperature is found to be primarily a function of blockage, while the vectoring of the jet varies strongly with the shape and angle of the deflector plate. INTRODUCTION A jet flow is a continuous flow of fluid issuing from an orifice into a medium of lower speed fluid. As the jet fluid travels further away from its origin, it slows down due to the mixing with slower speed ambient fluid. This interaction between the jet and the ambient fluid forms the mixing layer, or shear layer. As the primary jet roll-up structures, or ring vortices, move downstream, they grow in size due to the entrainment of slower moving ambient fluid. The resulting jet decay is proportional to the velocity gradient across the shear layer and is a strong function of the distance downstream of the jet exit normalized by the orifice diameter. In a recent study on the near-field entrainment of round jets, Liepman and Gharib (1992) show that streamwise vorticity drastically alters the mass entrainment of a jet, and the efficiency of this vorticity in entraining fluid increases relative to the of azimuthal vorticity as the jet evolves downstream. There are numerous systems, especially in the aeronautical community, where the ability to enhance the mixing characteristics of a jet will greatly improve their performance. For example, by increasing the rate of mixing between air and fuel, the efficiency of a combustion cycle may be improved. Also, by increasing the rate of mixing with the ambient, the IR signature of the jet can be altered. In another application of reducing aircraft noise (Csanady, 1966 and Rogers & Parekh, 1994), alteration of the coherent structures of the jet can have a large influence on its far field noise. Research has also shown that an increase in jet mixing is essential to many ejector and thrust augmentation devices (Carletti et al., 1995). In general enhanced jet mixing techniques are divided into two categories: passive and active. Passive jet mixing techniques may be permanent or deployable, but have no moving parts during operation. They range from alterations in the exit shape of the jet nozzle (Ho & Gutmark, 1987, and Longmire et al., 1991) to the implementation of tooth-like tabs (Ahuja & Brown, 1989, Bradbury & Khadem, 1975, Samimy et al., 1991, Zaman, 1993, and Zaman et al., 1994) and vortex generators (Carletti & Rogers, 1994, Rogers & Parekh, 1994 and Surks et al. 1994) at the mouth of the jet. Active techniques, on the other hand, operate at a specified frequency and for the most part involve a more complex design and operation than passive techniques. Pulsed jets (Raman & Cornelius, 1995) and piezo-electric generators (Wiltse et al.,1994) are among the most effective active mixing techniques. As a simple retrofit solution to enhance mixing and reduce noise, many studies have focused on the placement of small tabs and vortex generators at the exit of axisymmetric and rectangular nozzles. Both tabs and vortex generators introduce streamwise vorticity to entrain low speed fluid while forcing out higher speed core fluid. The main difference between the two methods is in the vortex structures which are generated. In the case of tabs (which are placed at a 90° AOA to the flow), a pair of counter-rotating vortices is produced by each tab, where as a half delta-wing vortex generator (typically oriented at an AOA less than 30°) produces only a single vortex. Extensive research on tab flows has been completed. An early study Bradbury and Khadem (1975) found that two tabs placed 180° apart at the exit of an axisymmetric jet caused it to bifurcate, increasing the overall mass entrainment. Since this study several studies have reported results for variations in flow field conditions, as well as tab shape, size, number and angle. The following is a summary of published results (Ahuja & Brown, 1989, Bradbury & Khadem, 1975, Samimy et al., 1991, Zaman, 1993, and Zaman et al., 1994): Appreciably faster decay of the centerline velocity for 1,2,4, and 6 tab cases, relative to the reference case Mixing enhancement greater for supersonic cases than subsonic cases. Heating the jet provides no significant change in the effectiveness of the tabs. Smaller tabs have less effect in distorting the jet. Tab width is more critical than tab height. A tab height greater than the boundary layer is required to be effective. Tab shape (rectangular vs. triangular) has little effect on tab performance. Orientation or angle of the tab is more critical than shape, triangular tab leaning 45° downstream, referred to as a “delta” tab, has the greatest effect. The understanding of the performance of half delta-wing vortex generators is not as extensive, however, very promising due to their flexibility in vortex generation. The existing literature (Carletti & Rogers, 1994, Rogers & Parekh, 1994 and Surks et al. 1994) yields the following conclusions: Half delta-wing vortex generators allow for more flexibility in nozzle design in that they only produce a single vortex. Counter-rotating pairs may be produced by having two adjacent generators of opposing sign. Generator orientations producing co-rotating (remember tabs only produce counter-rotating vortices) vortices show increased mixing relative to counter-rotating configurations. Improved mixing is achieved for increased generator height and angle of attack to the oncoming flow. Generators smaller than the boundary layer thickness were found to have essentially no effect on the mass entrainment of an axisymmetric jet, at low speeds (Re=50,000). The following report focuses on the optimization of half delta-wing vortex generators and presents a comparison of their effectiveness to tab flows and much larger deflector plate flows. Although tabs and vortex generators are considered to be a proven technology, much work is needed in the area of optimizing generator parameters. It is necessary to take the blockage of the generator into account when evaluating its performance. A comparison of the velocity decay and blockage associated with tab (counter-rotating vortex pair) and half delta-wing vortex generator (single vortex) flows is presented for generators of varying size, shape, orientation and location relative to the jet’s exit plane. Finally, ignoring thrust loss considerations, the mixing and vectoring effects of large tabs, or deflector plates are examined for varying angles and spacings from the nozzle exit. EXPERIMENTAL APPARATUS The results presented in this report are a product of research conducted both at the Tufts University Fluid Mechanics Laboratory and the McDonnell Douglas Air Jet Facility. Nozzles implementing both tabs and vortex generators were compared at each of the two facilities. Tests done at Tufts and MDC used the same 2.54 cm diameter nozzles. The generators in these nozzles are permanently braised into place and a different nozzle exists for each parameter variation. Additional data was collected at MDC on a 6.35 cm diameter nozzle, which allowed for the variation of generator parameters (see figure 1). This nozzle not only allows for variation in the number and location of generators, but also their size, shape, and angle of attack. As illustrated in the figure, larger generators were rounded to mate the inner curvature of the nozzle for high angles of attack. All of the tabs and vortex generators were professionally machined to ensure generator uniformity. Tufts University Water Jet Facility Measurements taken at the water jet facility at Tufts University were acquired on the recirculating jet shown in figure 2, using a 2-component laser velocimetry (LV) probe. The mass flow rate of the jet is controlled by a 3 hp variable speed pump and can be held steady (±0.5%) from 2.5 to 7.5 m/s. The flow is conditioned as it passes through a flow straightener and a series of screens. Once uniformity is ensured the flow is sent through a 5th