3.1. Effect of Inner Swirler Parameters on Flow Field
Figure 5 shows the effect of internal swirl parameters on the flow filed in Schemes S1–S4, the black line at the bottom half of the flow filed is an isoline with zero axial velocity. It could be seen that when the outer swirl number (
SNo) is fixed at 0.695, the Primary Recirculation Zone (PRZ) is gradually formed with the increase in the inner swirl number (
SNi). The vortex center of PRZ gradually approaches the downstream and radially outer sides of the combustion chamber, and the axial velocity at the downstream decreases gradually. As shown in
Figure 5a, when
SNi is 0.235, the airflow near the swirler outlet flows downstream directly. In this condition, the PRZ is not formed, but two Corner Recirculation Zones (CRZ) are formed near the wall of the combustor. The vortex center positions of CRZ are (57 mm, 36 mm) and (100 mm, 33 mm), respectively. However, as illustrated in
Figure 5b, as
SNi increases from 0.235 to 0.266, there is a distinct angle of the airflow at X = 90 mm near the centerline. Affected by the airflow radially expanding, the two corner vortices merge into one, and the vortex center is (78 mm, 35 mm). No PRZ is formed in this condition as well. The PRZ is formed as the
SNi increases from 0.235 to 0.402, as shown in
Figure 5c. The starting position of PRZ is X = 52 mm and vortex center is (115 mm, 30 mm). Meanwhile, the axial velocity downstream of swirler is greatly reduced. However, with
SNi increasing from 0.402 to 0.427, as illustrated in
Figure 5d, the axial velocity at the swirler outlet decreases slightly. The starting position of PRZ is X = 52 mm and the coordinate of vortex center is (127 mm, 35 mm). The axial length of the PRZ is 108 mm.
Figure 6 shows the comparison between the PRZ shape of Schemes S2 and S4. It could be seen that the starting position and expansion angle of the PRZ of these two Schemes are basically the same. As the air flows downstream, the radial width of the PRZ increases first and then decreases. Compared with Scheme S4, the PRZ size of Scheme S2 is longer and narrower. The flow topology of Scheme S4 is better than Scheme S2 because the shorter PRZ length of Scheme S4 helps to reduce the length of combustor.
Figure 7,
Figure 8 and
Figure 9 show the comparison of different velocity components between Schemes S1 to S4, including axial velocity, radial velocity and tangential velocity at different planes.
The velocity dimension at 2 mm downstream of swirler exit is large. As is shown in
Figure 7a, the peak values of axial velocity of Schemes S1–S4 are concentrated near the central axis of combustor. Among these schemes, the highest axial velocity could reach 70 m/s in Scheme S3; the reason is that the smaller blade angle of Scheme S3 leads to a stronger axial momentum of the airflow. In terms of radial velocity, compared with Schemes S1 and S3, the radial velocities near the wall of Schemes S2 and S4 are lower (see
Figure 8a). However, near the central axis, the radial velocities of Schemes S2 and S4 are higher. Meanwhile, the radial velocities at radially outside of swirler of Schemes S2 and S4 are mostly negative, which means that the air flows inward along the radial direction in these two schemes. For the tangential velocity, as is shown in
Figure 9a, with the increase in
SNi, the tangential velocity increases near the center axis at plane of
d = 2 mm.
At 10 mm downstream of the swirler exit, as illustrated in
Figure 7b, the negative axial velocity regions are observed on the central axis of Schemes S2 and S4, which proved that the PRZ are formed in the flow field. Compared with Schemes S1 and S3, the tangential velocities of Schemes S2 and S4 are significantly reduced and the radial velocities are significantly increased, see
Figure 8b and
Figure 9b. This indicates that part of tangential velocity converts into radial velocity, which forms a central low-pressure zone, thus promoting the formation of PRZ.
At 30 mm downstream of the exit, as shown in
Figure 7b,c, the radial widths of the PRZ increase under Schemes S2 and S4 compared with 10 mm downstream of swirler exit.
At 60 mm downstream, the radial width of the PRZ under Schemes S2 and S4 continues to increase (see
Figure 7d). In addition, a negative axial velocity region is formed in Scheme S4, resulting in a narrow PRZ. In terms of the tangential velocity, compared with Scheme S2, the tangential velocity of Scheme S4 decreases and the inverse pressure gradient decreases at this location (see
Figure 9d).
The developments of each velocity component near the swirler exit are summarized in
Figure 10. As is illustrated in
Figure 10c,d, the radial velocities of Schemes S2 and S4 (in which the PRZ have formed) raise along the central axis in proximity of the exit. Moreover, both the tangential velocities of outer-swirl flow and that of inner-swirl flow decrease. However, the tangential velocity of outer-swirl flow drops more rapidly than the inner-swirl tangential velocity. We infer that part of the outer-swirl tangential velocity counteracts the inner-swirl tangential velocity, and the other part of the outer-swirl tangential velocity converts to the radial velocity of main flow. This leads to an increasing trend of radial velocity of main flow, as is indicated in
Figure 10 c,d. The increase in radial velocity results in a trend that sees the airstream flow more easily away from the central axis, forming a local low-pressure zone. This local low-pressure zone sucks the air from the downstream, resulting in the formation of the PRZ. Thus, we conclude that it is the transition of tangential velocity to radial velocity that promotes the formation of the PRZ.
Figure 11 shows the radial profiles of turbulent kinetic energy at
d = 2 mm of Schemes S1–S4. It could be seen that the distribution trend of turbulent kinetic energy at
d = 2 mm is basically consistent at different inner swirl numbers. With the increase in
SNi, the turbulent kinetic energy decreases at the center axis of the swirler outlet and increases at the outer swirler exit. This indicates that the shear effect between internal and external flow increases with the increase in internal swirl intensity, and the momentum dissipation in the shear layer leads to the increase in turbulence intensity.
3.2. Effect of Outer Swirler Parameters on Flow Field
We have already discussed the effect of inner swirler structural parameters on the flow topology, by altering the installation angle and number of blades of inner swirler. In this section, we will focus on analyzing the flow filed with variations of outer swirler structural parameters.
Figure 12 shows the flow filed of the Schemes S5 and S6 (which represents the effect of blade installation angle of outer swirler). The black line at the bottom half of the figure is an isoline with zero axial velocity.
Figure 13 illustrates the shape of the PRZ of Schemes S5 and S6. Combined with
Figure 5a (Scheme S1), it can be observed that, keeping the inner swirl number
SNi at 0.235, the PRZ is gradually formed and the starting position of PRZ moves forward with the increase in the
SNo. As
SNo increases from 0.695 to 0.987, there is a distinct angle of airflow at the exit of swirler; this angle is more pronounced at the inner swirler exit. The PRZ is formed at 4 mm downstream of the swirler outlet, and the vortex center of PRZ is (123 mm, 26 mm). The axial length of PRZ extends to the combustor outlet. Furthermore, as
SNo increases from 0.987 to 1.434, the PRZ is formed at 2.5 mm downstream of the swirler outlet, which is earlier compared with Scheme S5. Moreover, the vortex center moves forward to (119 mm, 32 mm). The axial length of the PRZ shrinks to 120.5 mm with a larger angle of swirling jet flow.
Figure 14,
Figure 15 and
Figure 16 show the radial distribution of axial velocity, radial velocity and tangential velocity in planes at different distances from the swirler exit under Schemes S1, S5 and S6.
At 2 mm downstream of swirler exit, it is noticed in
Figure 14a and
Figure 15a that the axial velocity at the central axis gradually decreases with the increase in
SNo and the radial velocity gradually increases from negative to positive. This elucidates that the increase in
SNo alters the airflow direction at the swirler exit. Moreover, the larger
SNo is, the more obvious is the increase in radial velocity. In addition, the radial velocity of Scheme S6 increases more significantly and the PRZ is formed more upstream compared with Scheme S5 (see
Figure 15a). This phenomenon indicates that the earlier the tangential velocity is converted to radial velocity at the swirl exit, the easier the PRZ is formed.
It is noticed that at 10 mm downstream of swirler exit, the axial velocity and tangential velocity at central axis decrease further, while the radial velocity continues to increase, as indicated in
Figure 14b,
Figure 15b and
Figure 16b. The negative axial velocity region is observed near the central axis in Schemes S5 and S6, indicating that there is a PRZ in the flow field (see
Figure 14b). Moreover, the radial width of the PRZ increases with the increase in
SNo, which demonstrates that stronger
SNo results in a faster dissipation of the tangential momentum.
Figure 14c illustrates that at 30 mm downstream of swirler exit, the radial width of the PRZ continues to increase with the increase in
SNo. As indicated in
Figure 15c, compared with Schemes S1 and S6, the attenuation of radial momentum decreases under Scheme S5 at 30 mm downstream.
Figure 14d indicates that at 60 mm downstream of swirler outlet, the negative axial velocity at the central axis of Scheme S5 increases, compared with that at
d = 30 mm.
We infer that the reason why the radial velocity at 2 mm downstream of exit increases from negative (Scheme S1) to positive (Schemes S5 and S6) with the increase in
SNo is that a high positive tangential velocity dissipates the negative radial velocity in Scheme S5 and S6 (see
Figure 16a). After that, the rest of the tangential velocity transforms to positive radial velocity to support airstream flows toward the wall and then flows downstream, forming a relatively long and narrow PRZ. Thus, it can be inferred that the increase in
SNo makes the tangential velocity at the exit of the external swirler rapidly transform to the radial velocity, which changes the direction of the airflow at the swirler exit. This change makes the outer swirling air have a tendency to diffuse away from the center of combustor, forming a central low-pressure zone. The reverse flow formed in this way induces the airflow at the outer swirler exit to transform from tangential flow to radial flow, further promoting the formation of the PRZ. To some extent, this also indicates that the strong swirling flow is the main factor influencing the flow field structure of the swirler.
3.3. Effect of Sleeve Angles on Flow Field
In this section, we will discuss the effect of the sleeve angles of outer swirler on flow topology. Schemes S1 and S7 with sleeve angles of 30° and 15° were selected to carry out the numerical simulation of the flow filed. The obtained axial velocity contours and streamlines are shown in
Figure 17. And the tangential velocity contours are shown in
Figure 18. It is noticed from
Figure 17 that, as the sleeve angle changes from
β = 30° to
β = 15°, the flow field transforms from CRZ-dominated to PRZ-dominated. In the scheme of
β = 15°, the starting position of the PRZ is 15 mm from swirler exit, and the coordinate of vortex center is (96 mm, 26 mm). Moreover, in the scheme of
β = 30°, the larger sleeve angle makes the swirling air of the outer stage directly flow along the axis, without distinct diffusion angle. The reason for this phenomenon is that the outer stage airflow suppresses the inner-stage airflow flow along radially outer direction, which makes the inner-stage swirling air converges to the central axis, and the high-speed region is concentrated in the center of the combustor.
Figure 19,
Figure 20 and
Figure 21 show the radial distribution of axial velocity, radial velocity and tangential velocity in planes at different distances from the swirler exit under Schemes S1 and S7.
The radial velocity at 2 mm downstream of the outer swirler exit with
β = 15° is larger than that of the scheme of
β = 30°, as shown in
Figure 20a. This indicates that the dissipation effect between the internal and external counter-rotated swirling air is weakened, and part of the tangential velocity is converted to the radial velocity, which is conducive to the formation of the PRZ.
Then, at 10 mm downstream from the exit, with the increase in distance from swirler exit, the axial velocity and tangential velocity of Scheme S7 decrease significantly, while radial velocity of Scheme S7 increases obviously (see
Figure 19b,
Figure 20b and
Figure 21b).
At 30 mm downstream of the swirler exit, the negative axial velocity region is observed near the central axis, and the PRZ is formed, as shown in
Figure 19c. As the swirling airstream flows downstream, part of tangential momentum is dissipated, and part of it is converted into radial momentum, leading the airstream flow radially outward. This promotes the formation of PRZ.
From
Figure 17, we notice that the radial width of the PRZ increases significantly at 60 mm downstream in Scheme S7 with
β = 15°. The airflow recirculation is obvious. In conclusion, compared with altering the swirl number, the PRZ obtained by changing the sleeve angle is more downstream in the combustor, and the size of the PRZ is significantly reduced.