RESEARCH ON THE MIXING ENHANCEMENT PERFORMANCE OF LOBED NOZZLES BY USING PIV AND LIF

Size: px

Start display at page:

Download "RESEARCH ON THE MIXING ENHANCEMENT PERFORMANCE OF LOBED NOZZLES BY USING PIV AND LIF"

Brett Webb
6 years ago
Views:

1 Proceedings of FEDSM ASME Fluids Engineering Division Summer Meeting June 21-25,1998, Washington, DC FEDSM RESEARCH ON THE MIXING ENHANCEMENT PERFORMANCE OF LOBED NOZZLES BY USING PIV AND LIF Hu Hui, Toshio Kobayashi, Tatsuo Saga Nobuyuki Taniguchi, Sigeki Segawa and Akira Ono Institute of Industrial Science, University of Tokyo 7-22-l Roppongi, Tokyo 106, Japan ABSTRACT An experimental investigation of the vertical and turbulent structures in the jet mixing flows of lobed nozzles had been conducted. The techniques of Laser Induced Fluoresce (LIF) and Particle Image Velocimetry (PIV) were used to accomplish the flow visualization, instantaneous quantitative concentration measurements and velocity field measurements of the lobed jet mixing flows. The experimental results showed that, besides the existence of the well known streamwise vortices, compared with a circular jet flow, lobed jet flows were found to have shorter laminar region, smaller scale of the spanwise Kelvin-Helmholtz vortices, earlier appearance of small scale turbulent structures and bigger turbulent intensity. Based on the LIF and PIV results, two aspects of the mechanism of mixing enhancement of a lobed nozzle are suggested. One is that a lobed nozzle can accelerate the cut and connect process of large scale spanwise Kelvin- Helmholtz vertical rings. Another is that the stretch effect of streamwise vortices generated by the lobed nozzle on the spanwise Kelvin-Helmhotz vertical rings also enhanced the energy cascade process of turbulence. Both of them can result in the creation of much small scale intense turbulence and enhances the mixing of jet flow with ambient flow. INTRODUCTION A lobed nozzle which consists of a splitter plate with convoluted trailing edge is extraordinary fluid mechanic device for efficient mixing of two co-flow streams with different velocity, temperature and/or spices. Such device had been known since the earliest days of jet engines and received considerable attention for reducing jet noise during the 1960 s. More recently, it has emerged as an attractive approach for mixing core and bypass streams of turbofan engine to improve propulsion efficiency, reduce the specific fuel consumption (SFC) and suppress the infrared radiation emission (Power et al.1994, Presz et al.1994 and Hu Hui et a1.1996). Lobed nozzle/mixer has also been received attention for using in supersonic ejectors for jet noise reduction at aircraft take off and landing as well as in combustion chamber for enhancing mixing between fuel and air (Tillman et al.1993 and Smith et a1.1997). About the mechanism of the mixing enhancement of a lobed nozzle, many work had been conducted in the past, such as Paterson (1982), Werle et a1.(1987), Elliott et a1.(1992) and McCormick et a1.(1993). In the work of the Belovich et al.( 1997), the results of these researches were summarized by that the mixing process in a lobed nozzle/mixer is controlled by three primary elements. The first is the streamwise vortices generated due to the lobed shape. The second is the increase in interfacial area between the two flows due to the special geometry of the lobed structure, and the third is the Brown- Roshiko type structures which occurring in any shear layer due to the Kelvin-Helmholtz instabilities. Although many important results had been got through these investigations, much work still need to understand the fluid dynamic mechanism of mixing enhancement by a lobed nozzle more clearly, especially regarding to the vertical and turbulent structures changes in the jet flow caused by a lobed nozzle and the mechanism of how the streamwise vortices caused by a lobed nozzle enhance jet flow mixing process. In the meanwhile, most of the previous researches were conducted by using Pitot probe, Laser Doppler Velocimetry(LDV) or Hot Film Anemeter (HFA), which are very hard to reveal the vertical and turbulent structures in jet mixing flows instantaneously and globally due to the limitation of these experimental techniques. In the present study, both Laser Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) techniques were used to studied lobed jet mixing flows instantaneously and globally. 1 Copyright by ASME

2 EXPERIMENTAL SYSTEM AND TECHNIQUES 8/ Lase Figure 1. Experimental Setup Figure 1 shows the schematically experimental facility used in the present research. The tested nozzles were stalled in the middle of the water tank (550mm*550mm*600mm). Fluorescent dye for LIF or PIV tracers (polystyrene particles of d=looum) was premixed with the water in the jet supply tank, and jet flow was supplied by a pump. The flowrate of the jet flow, which was used to calculate the representative velocity and Reynolds numbers of the jet flow, was measured by a flow meter. A honeycomb structure was installed in the entrance of the tested nozzle to insure the uniform flow entrance. A beam of argon ion laser passed an optical system to form a plane sheet. The investigated area was focused on a CCD camera and then, recorded by Laser Videodisc Recorder. The images were digitized by a s-bit gray-scale image processing system at a spatial resolution of 640 by 480 pixels, which also can be stored on a PC (host computer) and displayed on a PC monitor. a. circular nozzle A outer pcnetratton ang c b. lobed nozzle B c. lobed nozzle C d. three tested axial slices - Figure 2. Studied tested nozzles Figure 2 shows the three tested nozzles: a baseline circular nozzle A and two lobed nozzles with different lobe numbers. The equivalent diameters of these nozzles at the exit were the same, i.e. D=40mm. In the present study, the jet velocities were O.O7m/s and 0.2Om/s, and the Reynolds Number of the jet flows, based on the nozzle exit diameter, were about 2,000 and 6,000. In the present research, LIF technique was used to conducted flow visualization and instantaneous concentration measurement. Rhodamine B was used as fluorescent dye and the fluorescent light was separated from the scattered laser light with an optical filter. Rhodamine B solution in the jet supply box has a low concentration (0.5mg/l) to insure the strength of fluorescent light being linear with the concentration of the fluorescent dye and the effect of laser light attenuation being negligible as the laser light sheet propagated through the flow. The cross correlation method was used in the present study to conduct PIV image processing. The post-processing procedures including subgrid interpolation (Hu Hui et al. 1998), velocity outliner deletion (Zhou et al. 1996), and field smoothing (Willert et a1.1991) were used to improve the accuracy of the PIV measurements. RESULTS AND DISCUSSIONS Form the result of the flow visualization and instantaneous concentration field measured for the circular jet flow (Fig, 3(a)), it can be seen that three different regions, which are laminar, transition and turbulent regions, can be identified clearly in the figure. At the end of laminar region (X/D=l.O), spanwise Kelvin-Helmholtz vortices were found to roll up. The pairing and combining of these spanwise vortices were conducted in the transition region. In the downstream of X/D=3.0, the small scale mixing (low concentration region) began to occur and much small scale turbulence and vertical structures began to appear in the flow field. Figure 3(b) shows the result of the flow visualization and instantaneous concentration field measured in the axial slice of the lobe trough (Fig.2(d)) for the lobed nozzle B. Compared with the circular jet flow, the jet flow for lobed nozzle B at this axial slice had a shorter laminar region (X/D=O.2). The scale of the spanwise Kelvin-Helmholtz vortices of the jet flow was much smaller. The transition region in which the pairing and combining process of the spanwise vortices were accomplished also became shorter. In the downstream of the locationx/d=o.& much small scale mixing and small scale turbulent structures (low concentration region) were found to appear. Figure 3(c) shows the result of the flow visualization and instantaneous concentration field measurements in the axial slice of the lobe peak (Fig.2(d)) for the lobed nozzle B. From the figure, it can be seen that: the laminar region was not a straight cylinder like that in the circular jet, and looked like a expansive cut-off cone along the downstream of the lobed structure instead. Compared with the flow structures shown in Fig.3(b), the laminar region in the lobe peak axial slice was a bit longer (X/D=0.4), but it was still shorter than that in the circular jet flow. This is caused by the different thickness of the boundary layer at the exit of the lobed nozzle (the work of the Brink et al.(1993), had verified that the thickness of the Copyright by ASME

3 boundary layer at the lobed trough is smaller than that in the lobed peak), and the thicker boundary layer at the lobe peak need a longer streamwise distance to roll-up the Kelvin- Helmholtz vortices (Hussain et al. 1989). In this axial slice, it can also be seen that small scale mixing and small scale turbulent structures (low concentration region) were found to appear about in the down stream of location X/D=0.8. Figure 3.(d) gives the result of flow visualization and instantaneous concentration field measurement in the axial slice of the lobe side (Fig.2(d)) for the jet flow of lobed nozzle B. In this axial slice, streak flow structures can be seen in the near field of the jet flow. These structures were the Kelvin- Helmholtz vertical tubes shed periodically from the lobe training edge, which was observed and called normal vortex by McCormick et a1.(1993). At the downstream of the location X/D = 0.8, small scale mixing and small scale turbulent structures (low concentration region) were found to appear in the flow field. Figure 4 is the flow visualization of the jet flow for the lobed nozzle C in four cross planes. From the figures it can be seen that, at X=lOmm (X/D=0.25, Fig.4(a)), the existence of the streamwise vortices in the form of 12 petal mushrooms can be seen clearly in the jet flow. As the streamwise distance increased to X=20mm(X/D=OS), the mushrooms grew up (Fig.4(b)), which indicated the intersection and enhancement of the streamwise vortices generated by lobed nozzle. As the streamwise vortices intensified, some small scale structures began to appear in the flow and the interaction between the streamwise vortices and Kelvin-Helmholtz vortices made adjacent mushrooms merging with each other (X=30mm, Fig.4(c), which indicated the process that the streamwise vortices deform the Kelvin-Helmholtz vertical tube into pinchoff structure suggested by McCormick et a1.(1993). At X= 40mm (X/D=I.O, Fig.4(d)), the mushroom shape structures almost disappeared and the flow was almost fully filled with small turbulent structures. 2). PIV results The PIV measurement results of the lobed jet flow and circular jet flow were showed in the Fig. 5 to Fig. 8. The instantaneous flow fields showed on these figures were obtained at the frequency of 30 Hz and the mean flow fields were got by the average of 100 frames of the instantaneous flow fields. The instantaneous spanwise vorticity (Wzi,j,, ), mean velocity (U i,j and V i,j ) and mean turbulent intensity T, shown on the figures were defined as: r&zyg7 ~j(.~,,,, -I/,.,)? +(v,,,,,-y,,) T = /=I I 100 = 100 In which ui,j,r and Vi,j,r are the instantaneous velocities in the X and Y directions respectively, while u i,j,t and v i,j,t are the instantaneous turbulent velocities. From these figures, it can be seen that, compared with the mean flow fields, the instantaneous velocity and vorticity fields revealed the existence of many small scale turbulent in the flow field, which cannot be observed in the mean flow fields. This is also the advantage of the instantaneous full field experimental technique like PIV over conventional experimental techniques. From the mean turbulent intensity fields, it can also be seen that, the bigger turbulent intensity regions in the three axial slices of the lobed jet were located almost in the near down stream of the lobed nozzle, which means that the most of the mixing process between the jet and ambient flows were completed in near down stream region (X/D<3.0). While, for the flow field of the circular jet, it is still in its core potential region (Fig. 8). From the comparison the maximum values of the turbulent intensity fields in the lobed jet (which are n/s, 0.1 lom/s and O.lOOm/s in the axial slice of lobe trough slice, lobe peak slice and side of the lobe slice respectively) and circular jet flow (which is O.O6Om/s), it also can be seen that lobed jet flows were much more turbulent than the circular jet, which also indicted the mixing enhancement performance of the lobed nozzle. Figure 9 shows the 100 frames PIV instantaneous velocity fields in the axial slice passing the lobe trough of the lobe nozzle B. In the figure, Z direction indicates the time step. The iso-surfaces of the spanwise vorticity Wz=l.O(red) and Wz=l.O(blue) was also showed in the figure. From the figure it can be seen that, at the entrance of the measurement region, for the iso-surface of the spanwise vorticity Wz= I.O(red) and Wz=-1.0 (blue), some periodical structures were found along the time direction (Z direction), which were corresponded to the periodically rolled up process of the spanwise vortices due to the Kelvin-Helmholtz instability in the shear layer. The size of the iso-surface of these spanwise vortices decreased along the flow direction (Y direction) due to the mixing process in the jet flow. In the exit of the measurement region, the scale of these iso-surface structures were much smaller than that in the entrance of the measurement region. 3). Mechanism of the mixing enhancement of lobed nozzles From above LIF and PIV results, it can be seen that, compared with the circular jet flow, lobed nozzles can reduce the scale of Kelvin-Helmholtz vortices, accelerate the process of vortices pairing, produce streamwise vortices in the jet flow. Small scale turbulence appeared earlier and bigger turbulent intensity was found in the lobed jet. All these indicated the mixing enhancement performance of lobed nozzles. However, how the lobed nozzles enhance fluid mixing in the jet flow? McCormick et a1.(1993) suggested that the interaction of Kelvin-Helmholtz vortices with streamwise vortices generated by lobed nozzles produces high levels of mixing which is the mainly responsible for the enhanced mixing, but by what means were these processes conducted? They did not explain it. Copyright by ASME

4 completed in the near field region (X/D<3.0) of the lobe jet flow. stretched Doint Kelvin- Helmhotlz spanwise streamwise vortices Figure 10. Idealization of the vertical evolution in a circular jet flow conjectured by Hussain (1986) It was well known that, for a circular jet, just like described in the article of Hussain (1986, Fig.10) spanwise vortex rings will be rolled up firstly due to the Kelvin-Helmholtz instability (first instability) existed at any shear layer. As these spanwise vortex rings move downsteam, they can not be two dimensional vertical rings due to the self-interaction effect and cross-interaction effect between them. They will be the combinations of helical vertical tubes, i.e. toroidal vertical rings through the effect of an additional instability (secondary instability or azimuthal helical instability model). So, the two dimensional spanwise vortex rings caused by the Kelvin- Helmholtz instability will be wrapped and developed into three dimensional structures through secondary instability. With undergoing interactions, the large scale toroidal vertical rings will be broken down into many substructures through the cut and connect process, which may be responsible for the avalanche of three dimensional and smaller scale motions and for the generation of high turbulence and Reynolds stress. However, in a circular jet flow, the evolution of such process will need a very long streamwise distance to complete. For a lobed nozzle, because of its special geometry, it can cause big perturbation along the azimuth of the jet flow, such as the non-uniform momentum thickness of the boundary layer at the exit of the lobed nozzle. The streamwise vortices produced by the lobed nozzle enlarge the azimuthal perturbation by the means of deforming the Kelvin-Helmholtz vertical tubes into pinch-off structures (suggested by McCormick et al.( 1993) and visualized in the Fig.4). All these enhance the creation of the complex three dimensional vortices and the helical instability of the jet flow. i. e., the toroidal effect of the spanwise vertical structures is enlarged, and then the cut and connect process of the vertical rings is accelerated (which is the merging process of the adjacent mushroom observed on the flow visualization in the cross plane, Fig. 4), This means that the process of a large-scale vertical structure breaking into smaller scale vertical structure is conducted more rapidly, therefore, the mixing of the jet flow with ambient flow is enhanced. All these processes can be stretchid point Figure1 1. Stretch effect of streamwise vortices on the spanwise Kelvin-Helmhotlz vertical tube Besides this, the interaction between the large scale streamwise vortices produced by the lobed nozzles and the spanwise vortices caused by the Kelvin-Helmholtz instability also results in that the spanwise vertical rings are stretched(fig. 11). According to the Helmholtz vorticity conservation law, the scale of the vortices will be reduced when the vortices are stretched, These also enhance the energy cascade process of turbulence, and result in rapid reduction of the scale of the spanwise instability (this is the reason why the scale of spanwise Kelvin-Helmholtz vortices is smaller in the lobed jet flows than that in the circular jet flow showed in the above LIF results). These also result in the creation of much small scale intense turbulence and the mixing enhancement of the jet flow with ambient flow. CONCLUSION The LIF and PIV results of the investigation revealed the great differences of the turbulent structure and vortex scale between the lobed jet and circular jet mixing flow. Compared with the circular jet flow, the lobed jet flow had shorter laminar instability region, smaller scale of the spanwise Kelvin-Helmholtz vorices, earlier appearance of the small scale turbulent structures and bigger turbulent intensity. All these indicated the mixing enhancement performances of a lobed nozzle over a circular nozzle. Based on LIF and PIV measurement results, two aspects of the mechanism of the mixing enhancement of a lobed nozzle are suggested: One is that a lobed nozzle can cause azimuthal perturbations in the jet flow,and the streamwise vortices produced by the lobed nozzle enhanced these azimuthal perturbations. These accelerate the cut and connect process of the large scale spanwise Kelvin-Helmholtz vortex rings to transfer the energy and vorticity from large scale vortices to small scale vortices. Another is that the interaction between the streamwise vortices and spanwise Kelvin-Helmholtz vortices also enhanced the energy cascade process of the turbulence, which also resulted in the creation of much smaller scale intense turbulence and mixing enhancement of the jet flow with ambient flow. 4 Copyright by ASME

Proceedings of 3rd International Workshop on PIV, Santa Barbara, USA, Sep.16-18, 1999 PIV and LIF Measurements on the Lobed Jet Mixing Flows

Proceedings of 3rd International Workshop on PIV, Santa Barbara, USA, Sep.16-18, 1999 PIV and LIF Measurements on the Lobed Jet Mixing Flows Proceedings of rd International Workshop on PIV, Santa Barbara, USA, Sep.68, 999 PIV and LIF Measurements on the Lobed Jet Mixing Flows Hui HU, Toshio KOBAASHI, Tetsuo SAGA, Shigeki SEGAWA and Nobuyuki