PASSIVE CONTROL ON JET MIXING FLOWS BY USING VORTEX GENERATORS

Transcription

1 Proceedings of the Sixth Triennial International Symposium on Fluid Control, Measurement and Visualization, Sherbrooke, Canada, August -7,. PASSIVE CONTROL ON JET MIXING FLOWS BY USING VORTEX GENERATORS Hui HU, Tetsuo SAGA, Toshio KOBAYASHI and Nobuyuki TANIGUCHI Institute of Industrial Science,University of Tokyo Roppongi 7--, Minato-Ku, Tokyo , Japan ABSTRACT In an effort to increase mixing in jet flows, a passive control method, using vortex generators in the form of mechanical tabs or small protrusions at the exit of a nozzle had been investigated experimentally in the present paper. The techniques of Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) were used to accomplish flow visualization and velocity field measurements of the jet mixing flows simultaneously. The vortical and turbulent structure changes in the near field of a jet mixing flow caused by mechanical tabs were studied based on the PLIF and PIV simultaneous measurement results. The experimental results revealed the great changes of vortical and turbulent structures in the near field of jet flow due to the mechanical tabs intrusion. Compared with a nature jet flow (a circular jet flow without mechanical tabs intrusion), the tabbed jet flow was found to have shorter laminar length, smaller size of the spanwise Kelvin-Helmholtz vortices, earlier appearance of small scale turbulent structures. The existence of the streamwise vortices generated by mechanical tabs was verified clearly from both PLIF flow visualization and simultaneous PIV measurement results. Due to the engulf of the streamwise vortices, an inward indentation of ambient flow into core jet flow and the outward ejection of core jet flow into the ambient flow was found in the cross planes of the tabbed jet. As the streamwise distance increasing, the cross section form of the tabbed jet flow was found to change from round to oval gradually. Keywords: tabbed jet flow, passive control of jet mixing, mixing enhancement, PIV and LIF technique INTRODUCTION In an effort to increase mixing in jet flows, a passive control method, using vortex generators in the form of mechanical tabs or small protrusions at the exit of a nozzle had been under investigation in the past several years. Bradbury and Khadem [] were the first to study the effect of mechanical tabs on jet flows in detail. They reported that, for the low speed jet, mechanical tabs or small protrusions in the jet flows at the exit of a nozzle can increase jet spread rate significantly, reduce the potential core length and even bifurcate the jet flows. Ahuja et al.[,] and Zaman et al.[-8] began to investigate the mixing enhancement performance of mechanical tabs systematically. They found that mechanical tabs not only can increase the jet mixing for the low speed jets, but also have good mixing enhancement performance at high speed and high temperature jet flows as well. The mechanical tabs had been used to suppress the jet noise of the air breathe engines by Ahuja et al.[,] and Zaman et al.[-8]. More recently, tabbed nozzle was also found to be used as a fuel injector nozzle (Glawe et al.[9]) in supersonic combustion chamber to enhance the mixing of fuel with supersonic air. About the fundamental research of how and why mechanical tabs can enhance jet mixing process, Zaman et al.[7] reported that large scale streamwise vortices can be generated by mechanical tabs in jet flows and two sources of the streamwise vortices were postulated in their paper. In the research of the molecular mixing in a jet mixing flow with mechanical tabs, Zhang et al.[] found that mechanical tabs can reduce jet transitional Reynolds number and increase the molecular mixing about 5% at the downstream location of six diameter of the nozzle. The work of the Reeder and Samimy [] revealed more detail about the vortices and turbulent structures in tabbed jet flows by using flow visualization and Laser Doppler Velocimeter (LDV) measurements. They confirmed the existence of the large-scale streamwise vortices caused by mechanical tabs and reported the higher Reynolds stress levels in the tabbed jet flow. Although many important results have been obtained through those previous works, much work still need in order to understand the fluid dynamic mechanism of the enhancement of mixing caused by mechanical tabs more clearly. Regarding to the vortical and turbulent structure changes in the near field of a jet flow caused by the intrusion of mechanical tabs, the quantitative data are far more limited. In the meanwhile, most of the previous experiments were conducted by using Pitot probe, LDV or Hot Wire Anemometer, with which it is very hard to reveal the vortical and turbulent structures in tabbed jet mixing flows instantaneously and globally due to the limitations of these experimental techniques. In the present study, the optical whole-field diagnostic techniques of Particle

2 Imaging Velocity (PIV) and Planar Laser Induced Fluorescence (PLIF) were used to conduct flow visualization and instantaneous velocity field measurement of the tabbed jet mixing flow simultaneously. By using the directly perceived PLIF flow visualization images and the simultaneous velocity and vorticity distributions of the PIV measurement results, the characteristics of the evolution of the various turbulent and vortical structures in the near field of tabbed jet mixing flows were discussed in detail. EXPERIMENTAL SYSTEM AND TECHNIQUES Figure shows the experimental set-up used in the present research schematically. The test circular nozzle was fixed in the middle of a water tank (6mm*6mm*mm). Fluorescent dye (Rhodamine B) for PLIF or PIV tracers (hollow glass particles, d=8- µm) was premixed with water in a 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, was measured by a flow meter. A cylindrical plenum chamber with honeycomb structures was installed at the upstream of test nozzles to insure the turbulent levels of the core jet flows at the exit of test nozzles were less than %. An overflow system was used to keep the water level in the test tank to be constant during the experiment. The pulsed illumination laser sheet was generated by a double-pulsed Nd:YAG Laser system. After passing through a second harmonic generator cell, the wavelength of the light beams emitted from the double-pulsed Nd:YAG Laser system is 5nm. By using a set of optics (cylindrical lens and mirrors), the laser beam was bundled in a planar laser sheet with thickness being about.5 mm. The frequency of the double-pulsed illumination is Hz. The pulsed illumination duration is ns, and power is mj/pulse. The time interval between the two pulses is adjustable, which was set as ms in the present study. In order to achieve the PLIF flow visualization and PIV simultaneous measurement, a simultaneous image recording system was designed by using at set of optics and two high-resolution CCD cameras (TSI PIVCAM -, K by K resolution). The diagram of the simultaneous image recording system was shown on the right upper corner of the Figure. Rhodamine B was used as fluorescent dye in the present study. It was well known that the emission peak of Rhodamine B is about 59nm, and the wavelength of the illuminating laser light scattered by the PIV tracer particles is 5nm. Two kinds of optical filters were used in the present study to separate the LIF light from the scattered laser light, and then recorded them separately to obtain PLIF and PIV image simultaneously. As shown in the left upper corner of the Figure, the combined light including both LIF light (peak at 59nm) and scattered laser light (5nm) were divided into two light beams by using a beam splitter. Once light beam is go straight to CCD camera # for PIV image recording. A bend pass optical filter (5nm±5) was installed at the head of the camera #. Therefore, only the scattered laser light is transmissible to generate PIV images in the camera #, the LIF light is blocked out. Another light beam from the beam splitter is reflected by a mirror, then, goes into the camera #. A high pass filter (>58nm pass) was installed in the head of the camera # to filter out the scattered laser light (wavelength 5nm). The LIF light (peak at 59nm) passed through the high-pass optical filter to generate LIF image in camera #. More detail about the simultaneous PLIF and PIV image recording system can be got from Hu et al.[]. Laser sheet mixing region Double-pulsed Nd:YAG Laser Overflow system Mirror for cross plane measurement water tank A cylinderica plenum chamber mirror CCD camera # for PLIF High pass optical filter Test nozzle Beam splitter low pass optical filter CCD camera # for PIV synchronizer jet supply tank pump Flowmeter Figure. Experimental set up

3 top view 8mm mm Z 9 side view 5 Y a. test nozzle with mechanica tabs b. mechanical tab c. the two veiws Figure. Test nozzle and delta tabs The double-pulsed Nd:YAG Laser and the simultaneous image recording cameras were connected to a host computer via a synchronizer, which controls the timing of laser illumination and image acquisition. The host computer is composed of two high-speed CPU (8MHz, Pentium III, CPU), colossal image memory and Hard disk (GB RAM, Hard Disk GB). It can acquire the continuous PIV and PLIF image pairs up to 5 frames every time at the framing frequency of Hz. Figure shows the test nozzle and mechanical tabs used in the present research. The diameter of the circular nozzle at exit was D=mm, and the mechanical tabs are the delta tabs suggested by Zaman et al.[7]. Each mechanical tab had about.5% blockage area after mounted on the nozzle exit. During the experiment, the core jet velocity was about U =.m/s and.7m/s. The Reynolds Number of the jet flows, based on the circular nozzle exit diameter and core jet velocity, were about, and 8,. For the PIV image processing, rather than tracking individual particle, cross correlation method [] was used in the present study to obtain the average displacement of the ensemble particles. The images were divided into by pixel interrogation windows, and 5% overlap grids were employed. The resolution the PIV images for the present research is about µm/pixel. The postprocessing procedures which including sub-pixel interpolation[] and spurious velocity deletion [5] were used to improve the accuracy of the PIV result. RESULTS AND DISCUSSIONS ). In the axial sections PLIF flow visualization and simultaneous PIV measurement results for a natural jet flow (circular jet flow without mechanical tab intrusion) were shown in the Figure. It can be seen that, for a natural jet flow, three different regions, which are laminar, transition and turbulent regions, can be identified clearly in the figures. A laminar region exists at the downstream of the circular nozzle trailing edge. At the end of laminar region, spanwise Kelvin-Helmholtz vortices were found to roll up. The pairing and combining of the spanwise Kelvin- Helmholtz vortices conducted in the transition region. In much downstream, the big Kelvin-Helmholtz roller was found to break down, many small-scale turbulent and vortical structures were found in the flow field, and jet flow transited to turbulence. All these process can be seen clearly from both the PLIF visualization image(fig.(a)) and simultaneous PIV measurement result (Fig. (b)). frames of instantaneous PIV results were used to calculated the ensemble-averaged values of the flow parameters, which include mean velocity, turbulent intensity distribution and Reynolds stress field. The mean velocity field of the nature jet flow was given on the Figure (c). It can be seen that the nature jet flow expanded linearly along the downstream. Figure gives the PLIF flow visualization image and simultaneous PIV measurement results in the side view slice (X-Z plane, Fig (c)) of the tabbed jet mixing flow. Compared with the natural jet flow, the laminar region of the tabbed jet flow in this slice was not straight. It became a convergent region instead. This can be explained by that there was a local contraction at the exit of the nozzle due to the existence of mechanical tabs. The length of the laminar region was much less than that in the natural jet flow. It also can be found that the scale of spanwise vortices rolled up by the Kelvin-Helmholtz instability was much smaller, and the transition region also became much shorter. The small-scale vortices appeared earlier in the tabbed jet flow and the flow field is much more zigzag instead of symmetrical in the natural jet flow. From the ensemble-averaged PIV results, the expend angle of the tabbed jet in the side view section is found to be smaller than that of the nature jet. Figure 5 shows the PLIF flow visualization image and simultaneous PIV measurement results for the tabbed jet flow in the top view section (X-Y plane, Fig.(c)). It can be seen that, similar to the natural jet flow, a straight laminar region can be found in the tabbed jet flow at this section, but it was a bit shorter than that in the nature jet. Small scale turbulence and vortical structures appeared earlier in the jet flow. From the ensemble averaged results,

4 the trailing edge of the mechanical tabs. The existence of the streamwise vortex pairs generated by mechanical tabs can be seen clearly from the simultaneous PIV measurement result and ensemble-averaged velocity distributions. Due to the engulf effect of the streamwise vortices generated by the mechanical tabs, the ambient flow was pumped into the core jet in the mechanical tab intrusion plane and core jet flow was extracted out in the plane normal to the mechanical tab intrusion. So, an inward indentation of ambient flow into core jet flow and the outward ejection of core jet flow into the ambient flow can be observed in the ensemble averaged PIV result. it can also be seen that the spread angle of the tabbed jet in this axial section was much bigger that in the side view slice and that in the nature jet flow. ). In cross planes Figure 6 shows the PLIF flow visualization and the simultaneous PIV measurement results in the X/D=.5 cross plane of the tabbed jet mixing flow. An inward indentation at the downstream of the mechanical tab intrusion locations can be seen clearly from the PLIF flow visualization image. Small-scale turbulent structures were found to appear in the inward indentations, which is due to the earlier shedding of the Kelvin-Helmholtz vortices from. m/s m/s b. simultaneous PIV velocity vectors d. ensemble averaged velocity Figure. PLIF and PIV simultaneous measurement results in the axial section of the nature jet (Re=8,). m/s m/s b. simultaneous PIV velocity vectors d. ensemble averaged velocity Figure. PLIF and PIV simultaneous measurement results in the side view section of the tabbed jet (Re=8,). m/s m/s b. simultaneous PIV velocity vectors d. ensemble averaged velocity Figure 5. PLIF and PIV simultaneous measurement results in the top view section of the tabbed jet (Re=8,)

5 .5 m/s.5.5.5m/s Figure 6. PLIF and PIV simultaneous measurement results in the X/D=.5 cross section of the tabbed jet (Re=,).5 m/s.5.5.5m/s Figure 7. PLIF and PIV simultaneous measurement results in the X/D=. cross section of the tabbed jet (Re=,).5 m/s.5.5.5m/s Figure 8. PLIF and PIV simultaneous measurement results in the X/D=. cross section of the tabbed (Re=,).5 m/s.5.5.5m/s Figure 9. PLIF and PIV simultaneous measurement results in the X/D=. cross section of the tabbed jet (Re=,)

6 At X/D=. cross plane(figure 7), the inward indentations in to the core jet flow due to the intrusion of the mechanical tabs were found to become bigger. Much more small-scale turbulent and vortical structures at the inward indentations can be found from the PLIF flow visualization image. The mushroom structures can also be seen in the flow field due to the growth up of the azimuthal instability[6]. The simultaneous PIV measurement result shows that the jet flow will be more turbulent than that in the X/D=.5 cross plane. Due to the "engulf effect" of the streamwise vortices, the jet flow was found to expend rapidly in the plane normal to the mechanical intrusion section. This also verified the measurement results of that the tabbed jet flow was found to spread more rapidly in the top view plane than that in the side view plane, which were given in Fig. and Fig.5. As the streamwise distance increased to X/D=. (Figure 8) and X/D=.(Figure 9), small-scale turbulent and vortical structures was found to almost fill the whole flow field from the PLIF visualization results. The instantaneous velocity vector fields of the simultaneous PIV measurement also show that the flow fields become much more turbulent as the downstream distance increasing, and many streamwise vortices can be identified in the instantaneous velocity vector fields, which may be due to the growth of the azimuthal instability. Form the ensemble-averaged results of the PIV measurement, it can be seen that the cross section form of the tabbed jet flow was found to change from round to oval gradually due to the "engulf effect" of the streamwise vortices generated by the mechanical tabs. CONCLUSIONS The PLIF and simultaneous PIV measurement results of the present investigation revealed the great changes of the turbulent and vortical structures in the near field of a jet flow by the intrusion of the mechanical tabs. Compared with a natural jet flow without the mechanical tab intrusion, the tabbed jet flow was found to have shorter laminar region length, smaller scale of the spanwise Kelvin- Helmholtz vortices and earlier appearance of small-scale turbulent and vortical structures in the axial planes. Jet spread angle in the mechanical tab intrusion plane was found to be smaller than that in the plane normal to the mechanical tab intrusion. The existence of the streamwise vortices generated by the mechanical tabs can be seen clearly from both PLIF flow visualization and simultaneous PIV measurement results in the cross planes of the tabbed jet flow. Due to the engulf of the streamwise vortices, an inward indentation of ambient flow into core jet flow and the outward ejection of core jet flow into the ambient flow occurred in the tabbed jet. As the streamwise distance increasing, the cross section form of the tabbed jet flow was found to change from round to oval gradually. 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