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Figure 9. Analysis of the passage flow structures showing comparisons of the vortex breakdown and of the low-velocity region. (a) Streamlines and static pressure contours on 95% span surface (AP = 10kPa), BL (OP-D),; (b) relative Mach number contours on 95% span surface, BL (OP-D); (c) streamlines and static pressure contours on 95% span surface (AP = 10kPa), SG (OP-D); (d) relative Mach number contours on 95% span surface, SG (OP-D); (e) streamlines and static pressure contours on 95% span surface (AP = 10kPa), RC (OP-D); (f) relative Mach number contours on 95% span surface, RC (OP-D). neighboring passage, as highlighted by red lines of 0 = 110 degrees in Figure 10c. Figure 10d shows that the recirculation channel retains its effectiveness at mitigating the flow blockage at the NS conditions.

Figure 9 Analysis of the passage flow structures showing comparisons of the vortex breakdown and of the low-velocity region. (a) Streamlines and static pressure contours on 95% span surface (AP = 10kPa), BL (OP-D),; (b) relative Mach number contours on 95% span surface, BL (OP-D); (c) streamlines and static pressure contours on 95% span surface (AP = 10kPa), SG (OP-D); (d) relative Mach number contours on 95% span surface, SG (OP-D); (e) streamlines and static pressure contours on 95% span surface (AP = 10kPa), RC (OP-D); (f) relative Mach number contours on 95% span surface, RC (OP-D). neighboring passage, as highlighted by red lines of 0 = 110 degrees in Figure 10c. Figure 10d shows that the recirculation channel retains its effectiveness at mitigating the flow blockage at the NS conditions.

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Figure 1. The concept of the self-adjusting passive control technique. (a) Conceptual sketch of the recirculation channel (RC) for the axial compressor rotor; (b) meridional cross-section of the recirculation channel. All dimensions are normalized by the rotor blade tip chord cz and all axial locations are referenced to the rotor blade tip Leading Edge (LE). Figure 1. The concept of the self-adjusting passive control technique. (a) Conceptual sketch of Rotor blades and axial flow compressors can feature a rotor tip type stall, by which reverse flow at the rotor tip progressively extend s over the blad e span. Rotor tip leakage promotes the onset of this reverse flow, as explained in Section 4. By controlling the rotor tip leakage, a delay in the tip stall onset is sought. Figure 1 describes the concept of a self-adjusting passive control technique for the rotor tip leakage. An axisymmetric The size of the Recirculation Channe channel was d (RC) was smal of the RC was only 8.5% of the rotor broadly draws from a detail of a one-way valve by Nikola Tesla [24]. The angles of suction and induced in this RC by the pressure blade. This pressure difference was injection to the casing flow were set at 45°. A recirculating flow was difference between the suction side and the pressure side of a rotor enhanced by the axial pressure gradient through the rotor passage. The induced flow was intend blade tip axial c to obtain a high momentum with a small recirculation mass flow rate. The axial component of t high momentum flow was in the positive axial direction. This was opposite to the axial velocity of esigned on the casing wall over the rotor blade compared to the rotor blade, and the axial length hord c, as shown in Figure 1. This channel design of ed to seal the tip leakage flow near the casing wall. The channel outlet was of a subsonic nozzle shape his the eakage flow. The high momentum flow therefore opposed the motion of the leakage flow. This leakage flow mitigation approach did not require reducing the rotor tip clearance. Therefore, this design was ess likely to suffer from blade tip rubbing issues than more established flow resistive type approaches such as labyrinth seals, which rely on small tip clearances. This recirculation channel design does not exclude in principle the use of abradable coatings on the casing surface.
Figure 1. The concept of the self-adjusting passive control technique. (a) Conceptual sketch of the recirculation channel (RC) for the axial compressor rotor; (b) meridional cross-section of the recirculation channel. All dimensions are normalized by the rotor blade tip chord cz and all axial locations are referenced to the rotor blade tip Leading Edge (LE). Figure 1. The concept of the self-adjusting passive control technique. (a) Conceptual sketch of Rotor blades and axial flow compressors can feature a rotor tip type stall, by which reverse flow at the rotor tip progressively extend s over the blad e span. Rotor tip leakage promotes the onset of this reverse flow, as explained in Section 4. By controlling the rotor tip leakage, a delay in the tip stall onset is sought. Figure 1 describes the concept of a self-adjusting passive control technique for the rotor tip leakage. An axisymmetric The size of the Recirculation Channe channel was d (RC) was smal of the RC was only 8.5% of the rotor broadly draws from a detail of a one-way valve by Nikola Tesla [24]. The angles of suction and induced in this RC by the pressure blade. This pressure difference was injection to the casing flow were set at 45°. A recirculating flow was difference between the suction side and the pressure side of a rotor enhanced by the axial pressure gradient through the rotor passage. The induced flow was intend blade tip axial c to obtain a high momentum with a small recirculation mass flow rate. The axial component of t high momentum flow was in the positive axial direction. This was opposite to the axial velocity of esigned on the casing wall over the rotor blade compared to the rotor blade, and the axial length hord c, as shown in Figure 1. This channel design of ed to seal the tip leakage flow near the casing wall. The channel outlet was of a subsonic nozzle shape his the eakage flow. The high momentum flow therefore opposed the motion of the leakage flow. This leakage flow mitigation approach did not require reducing the rotor tip clearance. Therefore, this design was ess likely to suffer from blade tip rubbing issues than more established flow resistive type approaches such as labyrinth seals, which rely on small tip clearances. This recirculation channel design does not exclude in principle the use of abradable coatings on the casing surface.
Figure 2. Meridional sketch of the NASA Rotor 37 used as the benchmark axial rotor test case [26] as Es a aaa as ae that several flow features are still not entirely explained. As the stage design total pressure ratio of 2.1 is relatively high compared to current axial compressors, this compressor geometry is representative of highly loaded compressors, such as the ones used in high-specific thrust engines of high-speed aircraft. Figure 3 shows one flow passage of the NASA Rotor 37, rendered in three dimensions. The casing includes two treatments that are absent in the original geometry. The treatment in Figure 3a is a simple square groove used in previous work and the treatment in Figure 3b is that of the recirculation channel from the previous section. In this work, a circumferential square groove was selected as the reference casing treatment case because it is able to avoid the addition of unwanted adiabatic efficiency penalties by its application [9]. Both casing treatments were applied at approximately 20% axial chords downstream from the blade leading edge in the axial direction, since in Reference [18] this gave the best stall margin among the investigated axial locations. The height of the rotor blade tip clearance was set to 0.356 mm in all cases.
Figure 2. Meridional sketch of the NASA Rotor 37 used as the benchmark axial rotor test case [26] as Es a aaa as ae that several flow features are still not entirely explained. As the stage design total pressure ratio of 2.1 is relatively high compared to current axial compressors, this compressor geometry is representative of highly loaded compressors, such as the ones used in high-specific thrust engines of high-speed aircraft. Figure 3 shows one flow passage of the NASA Rotor 37, rendered in three dimensions. The casing includes two treatments that are absent in the original geometry. The treatment in Figure 3a is a simple square groove used in previous work and the treatment in Figure 3b is that of the recirculation channel from the previous section. In this work, a circumferential square groove was selected as the reference casing treatment case because it is able to avoid the addition of unwanted adiabatic efficiency penalties by its application [9]. Both casing treatments were applied at approximately 20% axial chords downstream from the blade leading edge in the axial direction, since in Reference [18] this gave the best stall margin among the investigated axial locations. The height of the rotor blade tip clearance was set to 0.356 mm in all cases.
Figure 3. Overview of the casing treatments applied over the NASA stage 37 rotor. (a) Square groove [18]; (b) recirculation channel.
Figure 3. Overview of the casing treatments applied over the NASA stage 37 rotor. (a) Square groove [18]; (b) recirculation channel.
Figure 4. NASA Rotor 37 passage computational domain and mesh with circumferential recirculation channel.
Figure 4. NASA Rotor 37 passage computational domain and mesh with circumferential recirculation channel.
Figure 5. Error reduction with increasing spatial resolution based on Richardson’s extrapolation. (a) Adiabatic efficiency; (b) local static pressure near the rotor pressure side tip.
Figure 5. Error reduction with increasing spatial resolution based on Richardson’s extrapolation. (a) Adiabatic efficiency; (b) local static pressure near the rotor pressure side tip.
Table 1. Flow conditions used for the numerical model.
Table 1. Flow conditions used for the numerical model.
Table 2. Comparison of choked mass flow rate and of mass flow rate at the near-stall condition. Figure 7 compares the radial profiles of mass-averaged pressure, velocity, and flow angle at Station 4 of Figure 4 between the CFD and the experiment. These profiles are in good agreement with the experimental results, shown by the symbols. The results shown in Figure 6, Figure 7, and
Table 2. Comparison of choked mass flow rate and of mass flow rate at the near-stall condition. Figure 7 compares the radial profiles of mass-averaged pressure, velocity, and flow angle at Station 4 of Figure 4 between the CFD and the experiment. These profiles are in good agreement with the experimental results, shown by the symbols. The results shown in Figure 6, Figure 7, and
Figure 6. Overall performance characteristics of the NASA Rotor 37. Comparison between the current predictions and the exnerimental data. (a) Total pressure ratio: (h) adiabatic efficiency.
Figure 6. Overall performance characteristics of the NASA Rotor 37. Comparison between the current predictions and the exnerimental data. (a) Total pressure ratio: (h) adiabatic efficiency.
Figure 7. Circumferentially mass-averaged radial profiles at Station 4 of Figure 4. Comparison between CFD and experimental data reported in [26,27]. (a) Normalized total pressure at Station 1; (b) normalized total pressure at Station 4; (c) normalized tangential velocity at Station 4; (d) flow angle at Station 4. Figure 7. Circumferentially mass-averaged radial profiles at Station 4 of Figure 4. Comparison between Table 2 indicate that the current steady RANS simulation with the realizable k-e turbulence model was capable of evaluating the performance of the NASA Rotor 37 for the purpose of assessing different casing treatments.
Figure 7. Circumferentially mass-averaged radial profiles at Station 4 of Figure 4. Comparison between CFD and experimental data reported in [26,27]. (a) Normalized total pressure at Station 1; (b) normalized total pressure at Station 4; (c) normalized tangential velocity at Station 4; (d) flow angle at Station 4. Figure 7. Circumferentially mass-averaged radial profiles at Station 4 of Figure 4. Comparison between Table 2 indicate that the current steady RANS simulation with the realizable k-e turbulence model was capable of evaluating the performance of the NASA Rotor 37 for the purpose of assessing different casing treatments.
Figure 8. Overall compressor performance characteristics showing the impact of the investigated marian rerbnibowrenkenn LaX"Tixtkal waanncesu: wnbiicn: newune bla wnkewnsce Tle acditalandtin «EEDA woneer: Past work on the development of effective casing treatments for extending the stall margin of axial mpressors reported component efficiency penalties associated with this approach [16]. Figure 8b very encouraging as it shows that the casing treatments considered herein are predicted to have a iall positive effect on the rotor adiabatic efficiency. Hathaway [21] applied a recirculating casing ~atment to Rotor 67 and showed a larger stall margin improvement than this study. Since the casing ~atment applied in [21] was longer in the axial direction than in the current study, it is thought that a ‘ger pressure difference between the channe inlet and the channel outlet was obtained in [21], which ay explain the larger improvement in the compressor stall margin reported in [21].
Figure 8. Overall compressor performance characteristics showing the impact of the investigated marian rerbnibowrenkenn LaX"Tixtkal waanncesu: wnbiicn: newune bla wnkewnsce Tle acditalandtin «EEDA woneer: Past work on the development of effective casing treatments for extending the stall margin of axial mpressors reported component efficiency penalties associated with this approach [16]. Figure 8b very encouraging as it shows that the casing treatments considered herein are predicted to have a iall positive effect on the rotor adiabatic efficiency. Hathaway [21] applied a recirculating casing ~atment to Rotor 67 and showed a larger stall margin improvement than this study. Since the casing ~atment applied in [21] was longer in the axial direction than in the current study, it is thought that a ‘ger pressure difference between the channe inlet and the channel outlet was obtained in [21], which ay explain the larger improvement in the compressor stall margin reported in [21].
BL: Baseline casing; SG: Casing with Square Groove [12]; RC: Casing with Recirculation Channel. Table 3. Summary of the overall performance obtained by steady RANS simulations with and without the casing treatments. 5.2. Analysis of the Passage Flow Structures
BL: Baseline casing; SG: Casing with Square Groove [12]; RC: Casing with Recirculation Channel. Table 3. Summary of the overall performance obtained by steady RANS simulations with and without the casing treatments. 5.2. Analysis of the Passage Flow Structures
Figure 10. Contours of flow angle showing the flow structure near the rotor blade LE at 0.99 blade span. (a) BL (OP-D); (b) SG (OP-D); (c) RC (OP-D); (d) RC (NS).
Figure 10. Contours of flow angle showing the flow structure near the rotor blade LE at 0.99 blade span. (a) BL (OP-D); (b) SG (OP-D); (c) RC (OP-D); (d) RC (NS).
Figure 11. Radial profiles of Reynolds-Averaged Navier-Stokes axial velocity predictions near the casing wall showing the change with the rotor loading. These radial profiles were extracted at the constant pitch-wise angle of Position (1) shown in the sketch above the graphs. It is of interest to study the effect of the casing treatment on the rotor outflow, with the aim of investigating whether this outflow would have any negative effect on the operation of any stator blade row that may be downstream of it. For this purpose, Figure 12a shows the velocity distributions at the axial plane 1 mm downstream from he rotor blade row exit axial plane. The distribution of axial velocity at the peak efficiency condition OP-A indicates that the influence of the RC on the exit flow was negligible. This is confirmed by Figure 12b, which shows the circumferentially averaged profiles of tangential velocity and of axial veloci y at the same test plane. Along the blade span, the changes in normalized u tan and U,y; Were small between the BL case and the RC case. This was because the recirculation channel applied in this study only affected the near-casing flow and left the remainder of the velocity profile substantially unchanged. Furthermore, the injection was not intense away from flow stall, at the operating condition (PE), at which the typical engine operating line is set [38]. Specifically, the normalized momentum condition, as reported in Section 5.3. injected in the axial direction M was 0.102 at the OP-A (PE)
Figure 11. Radial profiles of Reynolds-Averaged Navier-Stokes axial velocity predictions near the casing wall showing the change with the rotor loading. These radial profiles were extracted at the constant pitch-wise angle of Position (1) shown in the sketch above the graphs. It is of interest to study the effect of the casing treatment on the rotor outflow, with the aim of investigating whether this outflow would have any negative effect on the operation of any stator blade row that may be downstream of it. For this purpose, Figure 12a shows the velocity distributions at the axial plane 1 mm downstream from he rotor blade row exit axial plane. The distribution of axial velocity at the peak efficiency condition OP-A indicates that the influence of the RC on the exit flow was negligible. This is confirmed by Figure 12b, which shows the circumferentially averaged profiles of tangential velocity and of axial veloci y at the same test plane. Along the blade span, the changes in normalized u tan and U,y; Were small between the BL case and the RC case. This was because the recirculation channel applied in this study only affected the near-casing flow and left the remainder of the velocity profile substantially unchanged. Furthermore, the injection was not intense away from flow stall, at the operating condition (PE), at which the typical engine operating line is set [38]. Specifically, the normalized momentum condition, as reported in Section 5.3. injected in the axial direction M was 0.102 at the OP-A (PE)
Figure 12. Axial and tangential velocity distributions across the axial plane 1 mm downstream of the rotor blade row exit axial plane. (a) Contours of axial velocity viewed from downstream of the rotor; (b) circumferentially mass-averaged profiles of tangential velocity and of axial velocity. Figure 12. Axial and tangential velocity distributions across the axial plane 1 mm downstream of the location of the passage shockwave shown in Figure 9c. This was actually a positive feature of this axisymmetric casing treatment, as it provided the controlling flow just over a passing rotor blade to control locally the tip leakage flow, and allowed the main passage flow to flow through unobstructed.
Figure 12. Axial and tangential velocity distributions across the axial plane 1 mm downstream of the rotor blade row exit axial plane. (a) Contours of axial velocity viewed from downstream of the rotor; (b) circumferentially mass-averaged profiles of tangential velocity and of axial velocity. Figure 12. Axial and tangential velocity distributions across the axial plane 1 mm downstream of the location of the passage shockwave shown in Figure 9c. This was actually a positive feature of this axisymmetric casing treatment, as it provided the controlling flow just over a passing rotor blade to control locally the tip leakage flow, and allowed the main passage flow to flow through unobstructed.
Figure 14 shows the distribution of the norma inlet and outlet. The normalized momentum was calculated as M = puyaquaxi/ Pre ¢(Umia)?, where pref was the inlet ambient condition and U,,;q was the inlet and outlet where u,,q was negative and the casing recess are shown by a uniform grey tone. tip leakage flow by the mechanism described in by levels of M. Areas where u,,q was positive and provide mitigation by the same mechanism, theref ized axial momentum at the recirculation channe blade mid-span velocity. The portion of the channe flow injected into the main passage was contoured the flow moved from the main passage into the ow injection into the main passage mitigated the Section 5.5. Areas where u;,q was positive cannot ore they have no obvious role to play in controlling the tip leakage flow. For comparison purposes, the equivalent results from the square groove casing treatment are presented in the same figure, showing the surface where the groove opens into the rotor
Figure 14 shows the distribution of the norma inlet and outlet. The normalized momentum was calculated as M = puyaquaxi/ Pre ¢(Umia)?, where pref was the inlet ambient condition and U,,;q was the inlet and outlet where u,,q was negative and the casing recess are shown by a uniform grey tone. tip leakage flow by the mechanism described in by levels of M. Areas where u,,q was positive and provide mitigation by the same mechanism, theref ized axial momentum at the recirculation channe blade mid-span velocity. The portion of the channe flow injected into the main passage was contoured the flow moved from the main passage into the ow injection into the main passage mitigated the Section 5.5. Areas where u;,q was positive cannot ore they have no obvious role to play in controlling the tip leakage flow. For comparison purposes, the equivalent results from the square groove casing treatment are presented in the same figure, showing the surface where the groove opens into the rotor
main passage. Compared to the square groove case, far more axial momentum was added at the channel outlet near the blade SS by using a recirculation channel, as highlighted by the magnified detail of the recirculation channel inlet and outlet in Figure 14. This result reinforces the evidence of the recirculation channel being a self-activating passive flow control system for rotor tip leakage.
main passage. Compared to the square groove case, far more axial momentum was added at the channel outlet near the blade SS by using a recirculation channel, as highlighted by the magnified detail of the recirculation channel inlet and outlet in Figure 14. This result reinforces the evidence of the recirculation channel being a self-activating passive flow control system for rotor tip leakage.
Static pressure at the domain outlet normalized by ambient pressure at a greater axial distance from the rotor blade leading edge axial plane. The second interface was formed where the flow injection from the casing treatment collided with the tip leakage flow. This second interface was generated by the physical requirement of separating flows of opposing directions, namely, the tip leakage flow and the injected flow, by a zero-velocity boundary. The flow blockage region, which the entropy iso-levels indicate as developing downstream of the interface axial location in Figure 16a,b, is shown to have been reduced in Figure 16b. Flow injection from the RC mitigates the reverse flow near the casing and allows the blade’s leading ledge shockwave to move axially downstream. The interaction between this shockwave and the tip leakage vortex was, therefore, also delayed axially, mitigating the vortex breakdown, which was a significant entropy production process near the pressure side of the blade.
Static pressure at the domain outlet normalized by ambient pressure at a greater axial distance from the rotor blade leading edge axial plane. The second interface was formed where the flow injection from the casing treatment collided with the tip leakage flow. This second interface was generated by the physical requirement of separating flows of opposing directions, namely, the tip leakage flow and the injected flow, by a zero-velocity boundary. The flow blockage region, which the entropy iso-levels indicate as developing downstream of the interface axial location in Figure 16a,b, is shown to have been reduced in Figure 16b. Flow injection from the RC mitigates the reverse flow near the casing and allows the blade’s leading ledge shockwave to move axially downstream. The interaction between this shockwave and the tip leakage vortex was, therefore, also delayed axially, mitigating the vortex breakdown, which was a significant entropy production process near the pressure side of the blade.
Figure 16. Surface streamlines and the entropy contours near the casing wall showing the creation of flow interfaces near the rotor LE. (a) BL (OP-D); (b) RC (OP-D). Figure 15. Variation in the injected flow momentum M, showing that M increases with compressor loading.
Figure 16. Surface streamlines and the entropy contours near the casing wall showing the creation of flow interfaces near the rotor LE. (a) BL (OP-D); (b) RC (OP-D). Figure 15. Variation in the injected flow momentum M, showing that M increases with compressor loading.
the casing wall. In turn, this provides a benefit to the stall margin of Rotor 37. with the positive axial static pressure gradient to drive fluid from the channel inlet to the channel outlet, where the nozzle shape of the channel generates an outflow of high-axial momentum in the positive axial direction. This induced flow is illustrated in Figure 17b, which is a magnification of the dashed-line inset of Figure 17a. As the working fluid runs through the recirculation channel in the casing, it spreads pitch-wise outwards from the blade camber line, affecting the incoming flow across a larger portion of the blade pitch. Close to the untreated casing, an interface sets between the oncoming and tip leakage flow along the dashed line labelled “Interface” in Figure 17. By adding the recirculation channel treatment, the dashed-line interface is replaced by two interfaces, shown by continuous lines in Figure 17, one of which is further downstream. This new downstream flow structure constrains the region of reverse flow, relocating it further downstream in the main passage. This reduces the volume of reverse flow over the casing wall, which reduces the flow blockage near the casing wall. In turn, this provides a benefit to the stall margin of Rotor 37. These changes are beneficial to the rotor performance. Adding axial momentum through the channel outlet reduced the extent of the reverse flow near the casing wall. This addition of axial momentum also reduced the size of the vortex breakdown, which occurred due to the interaction between the tip leakage vortex and the passage shockwave. This reduced the flow incidence angle at
the casing wall. In turn, this provides a benefit to the stall margin of Rotor 37. with the positive axial static pressure gradient to drive fluid from the channel inlet to the channel outlet, where the nozzle shape of the channel generates an outflow of high-axial momentum in the positive axial direction. This induced flow is illustrated in Figure 17b, which is a magnification of the dashed-line inset of Figure 17a. As the working fluid runs through the recirculation channel in the casing, it spreads pitch-wise outwards from the blade camber line, affecting the incoming flow across a larger portion of the blade pitch. Close to the untreated casing, an interface sets between the oncoming and tip leakage flow along the dashed line labelled “Interface” in Figure 17. By adding the recirculation channel treatment, the dashed-line interface is replaced by two interfaces, shown by continuous lines in Figure 17, one of which is further downstream. This new downstream flow structure constrains the region of reverse flow, relocating it further downstream in the main passage. This reduces the volume of reverse flow over the casing wall, which reduces the flow blockage near the casing wall. In turn, this provides a benefit to the stall margin of Rotor 37. These changes are beneficial to the rotor performance. Adding axial momentum through the channel outlet reduced the extent of the reverse flow near the casing wall. This addition of axial momentum also reduced the size of the vortex breakdown, which occurred due to the interaction between the tip leakage vortex and the passage shockwave. This reduced the flow incidence angle at
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Materials ScienceMechanicsAxial CompressorsAxial Compressorcasing treatment
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