端渦旋與燃燒偏折噴流之交互作用

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(4) . will never lift the flame. A number of studies on the general characteristics, temperature, concentration, and velocity fields, as well as the stability properties of the lifted flames in a cross-flow have appeared in the literature in the past two decades [2-9]. The combusting flow field at high jet-to-wind momentum flux ratio was found to be characterized by a pair of streamwise-oriented kidney-shaped counter-rotating vortices in the downstream area [2]. Botros and Brzustowski [5] used a five-hole pitot tube to measure the average velocity distribution of the propane gas jet flame in a cross air flow in a wind tunnel at a jet-to-wind momentum flux ratio 230. Since the lifted flame stabilized in the downstream area, the streamwise-oriented counter-rotating vortices were the primary flow structure dominating the mixing and combustion processes. This paper presents the experimental results of flow visualization and LDV measurements of the flow field in the near wake regions of the various characteristic modes of wake-stabilized jet flames in a cross-flow. The flow pattern, velocity and vorticity fields, flame stability behavior, as well as the interaction of the cross-flow, gas jet, and burner tube are investigated and discussed.. !"#$%&'()*+,-./0$ 1/23456-.7809:;,0<=>6 ?@ABCDEFGH0<IJ>K:LMN0 <OEFPQRSTU:LVS6WX0<DY Z[\]^_`;abcd0ebfgb/ 0e6h/0Di[\]^_`jk,lmJ /0n%&,oVpVq)BDrs]^_`, ?@60<,tubTvdVb0wb5vb6 ixV6?@ABC,uyz{|}Z~6 #D (keywords: flame, tip vortex, jet) The flow fields of wake-stabilized jet flames in a transverse air stream have been studied experimentally employing reactive Mie scattering flow visualization and laser Doppler velocimetry. Five characteristic flow modes are identified in different ranges of jet-to-wind momentum flux ratio, R: down-wash (R<0.1), cross-flow dominated (0.1<R<1.6), transitional (1.6<R<3.0), jet dominated (3.0<R<10), and strong jet (R>10) modes. The streamlines, vorticity distributions, as well as flame appearance, which vary significantly in the different modes, are presented and discussed. The down-wash and reverse flow regions are recognized as the primary mechanisms related to the flame stability. The reduction of the down-wash effect with increase of the jet-to-wind momentum flux ratio and the change of vorticity from negative to positive in the near-wake region of the burner tip lead to blow-off of flames. The turbulence intensity in the wake of the jet is stronger than that in the wake of the tube. The values of Reynolds shear stress at the plane of symmetry are positive in the wake of the jet, but negative in the wake of the tube..

(5) The experiments were in an open loop wind tunnel with a test section 30 x 30 x 110cm in size, as shown in Fig. 1. The turbulence intensity was less than 0.2% at all wind speeds. The burner was a stainless tube with an inner diameter d = 5.0 mm, outer diameter D = 6.4 mm, and length L = 250 mm. The burner tube protruded perpendicularly 180 mm into the aluminum floor plate of the test section. Positions are described in terms of a rectangular coordinate system (x, y, z) as shown in the lower left corner of Fig. 1. The cross-flow velocity uw was set at 4.86 m/s. The cross-flow Reynolds number Rew based on D is thus 2074 which is in the subcritical range for a 2-D cylinder wake [18]. In this range the Strouhal number of fluctuated vortex shedding in the wake is about 0.21 and the boundary layer remains laminar. The average velocity of jet at exit, denoted by uj, was varied. The local velocities in the x, y and z directions were denoted by u, v and w, respectively. The origin of the coordinate system was centered at the exit plane of the burner tube. This tube was adapted to the tip of a nozzle assembly which served as a flow conditioning and measuring device. The turbulence intensity of the issuing jet at the exit plane, except in the shear-layer, was lower than 0.2%. The gas jet fuel was commercial-grade propane, composed of about 95.0% C3H8, 3.5% C2H6, and 1.5% C4H10. The reactive Mie scattering method using TiCl4 vapor contained in the propane gas was used to study the flow patterns. A laser power of 1.5 W was employed for photography. The velocity field was measured with a two-component LDV. Visualized Flow Patterns and Flame Appearances.

(6) A gaseous fuel jet discharging perpendicularly to a subsonic air stream can burn as a stable flame. There are many instances of such flames, ranging from the very large flames in a cross-wind, such as occur on refinery flare stacks during emergency blow-downs, to smaller flames in a cross-stream of air in a variety of industrial gas burners. The gaseous jet flames in a cross-flow can be categorized into lifted and wake-stabilized flames [1]. If the flame is ignited at a jet-to-wind momentum flux ratio larger than a critical value, increasing the fuel jet velocity may lift the flame base and stabilize it at a distance from the burner tip. The lifted flame surrounds the trajectory of the deflected gas jet and behaves essentially like a cross-flow-deflected lifted diffusion flame [2-4]. However, if the flame is ignited at a jet-to-wind momentum flux ratio lower than a critical value, the flame base stabilizes in the wake region of the burner tube instead of surrounding the deflected gas jet column as observed in the case of the lifted flames [1]. Increasing the fuel jet velocity to the stability limit 1.

(7) The appearances of jet and flame have complex variations in different ranges of jet-to-wind momentum flux ratio R, where ρj and ρw are mass densities of jet and cross-flow, respectively. Based on the features of the flow and flame patterns, five characteristic flow modes are identified: down-wash (R < 0.1), cross-flow dominated (0.1 < R < 1.6), transitional (1.6 < R < 3.0), jet dominated (3.0 < R < 10), and strong jet (R > 10) modes. Typical flow patterns of the symmetry plane (y = 0) of the jet and wake regions visualized by the titanium-tetrachloride method are shown in Figs. 2(a) to (f).. counter-clockwise-rotating large scale vortex motion is clearly observed by eye in the flow visualization experiments. Transitional mode At R = 2.47, Fig. 6, the down-wash effect on the jet fluids of the central plane is reduced. The boundary of the down-wash area described by the streamline ψ (1.8,1) reduces to (x, z) = (55, -40) mm. The size of the down-wash area decreases with increasing R for R > 1.6. Little of the jet fluid is entrained into the down-wash area so that the flame area in this zone is reduced. The recirculation bubble shrinks further and its limits become x = 20 mm and z = -22 mm. The vortex center of the bubble moves upward to the area of the wake of jet. According to Huang and Chang [1], in the transitional mode the “tail” flame splits into upper and lower parts. Owing to the tilt-up of the jet with increasing R, some of the fuel issuing from the upstream side of the burner exit is flushed to the downstream area to support the combustion of the upper flame. Some fuel from the downstream side of the burner exit is flushed to the lower levels to support the lower flame. The streamlines evolving from the downstream side of the burner exit move up faster with increasing R than those from the upstream side and eventually cause the merging and elongation of the flames.. Flow Structure in the Plane of Symmetry The velocity vectors and streamlines in the plane of symmetry corresponding to the flow visualization pictures of Figs. 2(a)-(f) are shown in Figs. 3-8, respectively. The streamlines are obtained by a shooting/interpolation method. Down-wash mode In Fig. 3, the down-wash mode at R = 0.04, the evolving from (x, z) streamlines ψ ( −3.2 ,1) and ψ = (-3.2, 1) and (-2.2, 1) mm tilt up a little, cross the bent jet, flush downwards, then turn to the downstream area. The streamline ψ ( −1.2,1) evolving from (x, z) = (-1.2, 1) mm does not extend to the downstream area. Instead, it curves to (x, z) = (38, -25) mm, then turns backwards and downwards to the burner, forming the boundary of the down-wash area. Almost all of the jet fluid is entrained into the down-wash area. For instance, the streamline ψ ( −0.2,1) evolving from (x, z) = (-0.2, 1) mm moves to the right, curves downwards at a large curvature into the down-wash area, and turns towards the burner. Between the streamlines ψ ( −0.2,1) and ψ ( −1.2,1) , the velocities progressively increase and then decrease to satisfy mass continuity. The streamline ψ ( 3.2 , 0.5 ) encloses the recirculation bubble indicated by the dashed line, which attains limits of x = 16 mm and z = -28 mm. The location of flame, which is represented by the shading, lies near the boundary of the recirculation bubble. ( −2. 2 ,1 ). Jet dominated mode In the jet dominated mode, all the streamlines evolving from the exit of burner deflected by the cross-flow flow directly downstream without being entrained into the down-wash area, as shown in Fig. 7 for R = 4.32. The jet momentum is large enough to overcome the down-wash effect. The boundary of the down-wash area reduced to (x, z) = (41, -30) mm is not a streamline evolving from the exit of jet. A recirculation bubble exists in the wake of the tube and jet, which shrinks drastically to limits of x = 15 mm and z = 5 mm. The flame stabilizes in the down-wash area. Strong jet mode In the strong jet mode the wake structures behind the jet and tube change significantly, as shown in Fig. 8 for R = 12.6. The recirculation bubble in the wakes of the jet and tube disappears. Instead, a singularity identified as an improper node or source point is observed at (x, z) = (34.6, 4.6) mm. The transverse stream passes around the jet column, forming the horse-shoe-like shear layer which merges at the singularity point. Upstream of the source point, the reverse flow above the level of the burner tip is attracted to the jet, then follows the bent jet stream under the effect of high shear. The reverse flow below the level z = 0 is flushed downwards into the down-wash area. The streamline evolving from the source point delineates the boundary of the down-wash area, which is smaller than that in the jet dominated mode. The area of the flame stabilized in the down-wash area shrinks further.. Cross-flow dominated mode At R = 0.16 in the jet dominated mode, the boundary of down-wash area evolving from (x, z) = (-0.2, 1) denoted by the streamline ψ ( −0.2 ,1) extends to (x, z) = (49, -30) mm before curving back, as shown in Fig. 4. The recirculation bubble also enlarges a little and has limits of x = 20 mm and z = -30 mm. The ”tail” flame stabilizes on the boundary of the down-wash area. When R is increased to 0.7, the boundary of the down-wash area becomes ψ ( 0.8,1) and the limits increase to (x, z) = (67, -38) mm, as shown in Fig. 5. The size of the down-wash area enlarges with increasing R for R < 1.6. The starting point of the down-wash boundary moves downstream with increasing jet momentum so that little jet fluid is entrained into the area. Most of the jet fluid is flushed to the downstream area with increasing R. The “tail” flame thus elongates with increasing R. The recirculation bubble enlarges to limits of x = 29 mm and z = -34 mm. The vortex center of the recirculation bubble is located at (x, z) = (0, 11) mm. There exists a four-way saddle denoted by the symbol “X” to satisfy the topological rule. Under the four-way saddle, the flow turns counter clockwise. Although not shown by pictures, the. Characteristic length scales The characteristic length scales of flame and flow fields vary significantly in the different ranges of jet-to-wind momentum flux ratios. Figure 9 shows the variations of the flame length, down-wash length, x-location of the visual merging point of shear layer, 2.

(8) and maximum extent of the recirculation bubble in the x-direction. All the distances were determined in the direction of the x axis from x = 0 and were obtained from the magnified film images at the two-second exposure. The flame length increases with increasing jet-to-wind momentum flux ratio in the cross-flow dominated modes. In the transitional mode, however, it shortens progressively with the increase of R in the range 1.6 < R < 2.24, and then elongates for R > 2.24. The x-limits of the down-wash area, shear-layer merging point, and recirculation bubble all increase with increasing R in the cross-flow dominated mode. They decrease drastically in the transitional mode with increasing R. For R > 3 in the jet dominated mode, the shortening is much slower with increasing R.. 29.44, as shown in Figs. 12(a) and (b), respectively. Just prior to the blow-off, however, the position of the source point changes drastically to (x, z) = (21.3, 0) at R = 35.20, as shown in Fig. 11(c) and Fig. 18. The strong entrainment and bluff-body effects of the high speed jet cause the pressure in the reverse flow region to decrease with increase of jet momentum, and thus shorten the region of the reverse flow in Fig. 11(c). Little fuel/air mixture is brought to the area near the burner tip via the small down-wash velocities to stabilize the flame, as shown in Figs. 12(a) and (b). The flammable area behind the burner tip reduces with increasing jet-to-wind momentum flux ratio so that the flame stabilization region shrinks and moves up progressively, as shown in Figs. 11(a) and (b). Prior to blow-off, the position of the source point lowers as shown in Fig. 17(c), which causes the velocity vectors behind the jet column around the burner tip to tilt up. The down-wash effect near the burner tip also becomes unimportant so that very little mixture is brought to that region, as shown in Fig. 11(c). The flame thus stabilizes only on the burner tip, as shown in Fig. 11(c). Further increasing the jet-to-wind momentum flux ratio makes the flame blow out at R = 35.80. The vorticity contours in Figs. 13(a)-(c) delineate the tilt-up and evolution of the constant vorticity contours in the jet streams. The region of counter-clockwise rotating vorticity surrounding the contour of value -40 on the lee side of the burner tip enlarges with increase of jet-to-wind momentum flux ratio, Figs. 13(a) and (b). Prior to blow-off, due to the change of the flow pattern shown in Fig. 11(c), the region of the counter-clockwise rotating vorticity in the lee side of the burner tip enlarges, becomes stronger, and eventually moves up to the wake of the jet, Fig. 13(c). A region with positive values of vorticity contours appears behind the burner tip. The diffusion of the mixture in the reverse flow region of the wake of the jet would decrease due to the change of the vorticity distribution.. Vorticity Field in the Symmetry Plane The vorticity distribution, Ω , in the plane of symmetry is evaluated and an interpolation technique from the measured velocity data. Contours of constant vorticity are shown in Figs. 10(a)-(f). In the down-wash mode in Fig. 10(a), the family of constant vorticity contours concentrated on the contour of value +130 delineates the down-wash affected area. In the cross-flow dominated mode, shown in Fig. 10(b), the down-wash affected area in the wake of the tube, surrounding the contour of value +30, enlarges. In addition, a vorticity concentrated region centered on the contour of value +130 formed by the bent jet is observed. The down-wash affected area in the wake of the tube and the vorticity concentrated region formed by the bent jet enlarge with further increases of jet-to-wind momentum flux ratio, as shown in Fig. 10(c). The flames in the wake of the tube are enclosed basically in the down-wash area. However, “tail” flames do stem from the vorticity concentrated region formed by the bent jet. In the transitional mode, with the tilt-up of the jet the vorticity concentrated region surrounding the contour of value +100 formed by the bent jet moves up, as shown in Fig. 10(d). The down-wash affected area surrounding the contour of value +20 in the wake of the tube shrinks. Between the main streams of the bent jet and the down-wash area, a vorticity concentrated region enclosing the contour of value -20 appears. The upper flame of the dual-flame mode observed by Huang and Chang [1] stems apparently from the main streams of the bent jet, while the lower flame originates from the vorticity concentrated region in the wake of the jet. With the increase of the jet-to-wind momentum flux ratio to the jet dominated mode, the vorticity concentrated region in the wake of the jet disappears, as shown in Fig. 10(e). The down-wash affected area reduces. In the strong jet mode, shown in Fig. 10 (f), in addition to the shrinkage of the down-wash area, a contour with a counter-clockwise rotating vorticity -60 appears in a small region near the lee side of the burner tip. The flame appearances are apparently associated closely with variations of the vorticity field.. [1] Huang, R. F. and Chang, J. M., Combust. Flame 98:267-278 (1994). [2] Gollahalli, S. R., Brzustowski, T. A., and Sullivan, H. F., Trans. CSME 3:205-214 (1975). [3] Brzustowski, T. A., Gollahalli, S. R., and Sullivan, H. F., Combust. Sci. and Technol. 11:29-33 (1975). [4] Brzustowski, T. A., AIAA Paper 77-222 (1977). [5] Botros, P. E. and Brzustowski, T. A., Seventeenth Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, 1978, p. 389-397. [6] Kalghatgi, G. T., Combust. Sci. and Technol. 26:241-244 (1981). [7] Birch, A. D., Brown, D. R., Fairweather, M., and Hargrave, G. K., Combust. Sci. and Technol. 66:217-232 (1989). [8] Askari, A., Bullman, S. J., Fairweather, M., and Swaffield, F., Combust. Sci. and Technol. 73:463-478 (1990). [9] Ellzey, J. L., Berbee, J. G., Tay, Z. F., and Foster, D. E., Combust. Sci. and Technol. 71:41-52 (1990).. Flame Stability The features of the flow patterns do not change significantly with increasing R except just prior to blow-off, as shown in Figs. 11(a)-(c). The source point moves up and to the right a little from (x, z) = (34.6, 4.6) mm at R = 12.60 to (x, z) = (38.8, 10.0) mm at R = 23.21 and (x, z) = (39.9, 12.3) mm at R = 3.

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(10) FIGURE CAPTIONS Fig. 1 Apparatus. Fig. 2 Side view of flow patterns of symmetry plane (y = 0) using titanium-tetrachloride flow visualization method. Fig. 3 Down-wash mode. Fig. 4 Cross-flow dominated mode. Fig. 5 Cross-flow dominated mode. Fig. 6 Transitional mode. Fig. 7 Jet dominated mode. Fig. 8 Strong jet mode. Fig. 9 Variations of flame length, down-wash area, visual merging point of shear layer, and recirculation bubble in x-direction. Fig. 10 Contours of constant vorticity lines. Fig. 11 Velocity vector field and streamlines of symmetry plane (y = 0) prior to blow-off. Fig. 12 Position variation of source point prior to blow-off. Fig. 13 Contours of constant vorticity lines of symmetry plane (y = 0) prior to blow-off.. 5.

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