15th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC-15 February 24 - 28, 2014, Honolulu, HI, USA EXPERIMENTAL INVESTIGATION OF TRANSITION AND SEPARATION PHENOMENA ON AN INLET GUIDE VANE WITH SYMMETRIC PROFILE AT DIFFERENT STAGGER ANGLES AND REYNOLDS NUMBERS David Händel1*, Uwe Rockstroh2, Reinhard Niehuis1 1 Universität der Bundeswehr München, Institute of Jet Propulsion, Werner-Heisenberg-Weg 39, D-85577 Neubiberg, Germany david.haendel@unibw.de 2 MAN Diesel & Turbo SE D-13507 Berlin, Germany Introduction Inlet Guide Vanes (IGVs) are used to deliver a defined preswirl for the following rotor and can be used to adjust the stage’s aerodynamic performance. Using Variable Inlet Guide Vanes (VIGVs) the preswirl can be varied in a wide range in order to control the compressor’s operating point. Figure 1 shows a VIGV as applied in multishaft compressors. These machines are used for compression of gases in several industrial fields like fertilizer industry, oil and gas or power generation ([12], [13]). They provide a pressure ratio up to 150 and in most applications they run with a constant shaft speed. Hence, these compressors are adjusted basically by VIGVs (see [3]). Vanes with symmetric profiles are often found as their stagger angle can be changed in both positive and negative direction without preference of one of the turning directions. Disadvantages of symmetric VIGVs are high losses at relatively small turning angles and eventually a high sensitivity for stall occurrence ([5], [9]). In order to reduce the losses and to optimize the blade performance, it is essential to know details of the flow on the vane (e.g. the presence of a laminar or turbulent boundary layer, flow separation, transition etc.). Experimental investigations with a VIGV linear cascade have been conducted at the Institute of Jet Propulsion at the Universität der Bundeswehr München. The effects of varying stagger angle and Reynolds number on the profile losses are investigated and the detailed analysis is supported by measurements inside the boundary layers on the vane surface. For this reason a Preston probe was used to determine the near wall flow and to perform traverses along the boundary layer. Nomenclature l H12 Ma p q Re t x [m] [-] [-] [Pa] [Pa] [-] [m] [m] [°] chord length shape factor Mach number pressure dynamic pressure Reynolds number pitch position in axial direction flow angle Fig. 1 Variable Inlet Guide Vane *corresponding author 1 1 2 [-] [m] [m] [m] [%] [m] Subscript ax is S t 1 2 Acronyms (V)IGV heat capacity ratio boundary layer thickness displacement thickness momentum thickness loss coefficient distance to the surface axial isentropic stagger total inlet outlet Test Cascade and Measurement Techniques A linear cascade of the VIGV profile has been manufactured for experimental investigations. The cascade consists of five pivotable vanes. Two of them are instrumented with pressure taps. Thereby the static pressure distribution can be measured on a number of positions on the surface of the vane. In figure 3 it is depicted a sketch of the investigated VIGV cascade. The stagger angle is named with βS und and flow angles are defined with β1,2. (Variable) Inlet Guide Vane Experimental Setup The High-Speed Cascade Wind Tunnel All measurements have been conducted at the High-Speed Cascade Wind Tunnel of the Institute of Jet Propulsion which is depicted in figure 2. It is a continuous operating open-loop wind tunnel inside a pressure tank as described by Sturm and Fottner [11]. A 1.3 MW electric motor drives an axial compressor. Inside the pressure tank, the static pressure can be varied between 3 500 Pa and 120 000 Pa by externally placed vacuum pumps. In combination with the controlled temperature by the main flow cooler, this enables an independent variation of Reynolds and Mach number. Therefore, experiments can be performed under a wide range of realistic turbomachinery conditions and varied geometric scales. Fig. 3 Sketch of VIGV cascade [3] The static pressure measurements shown in this paper are presented in terms of isentropic Mach number distributions as defined below: Mais xax 1 pt1 2 lax 1 . (1) 1 p xax lax With a five-hole probe, wake traverses behind the cascade have been conducted to investigate the outlet values of the cascade, like turning, Mach Fig. 2 The High-Speed Cascade Wind Tunnel 2 number, and losses. The losses are quantified by the following equation: pt1 pt 2 . pt1 p1 qprobe/q∞,x ≈1 < 1, > 0 (2) For more details to the High-Speed Cascade Wind Tunnel, the VIGV cascade and measurement techniques see Händel et al. [3]. Preston Probe For a detailed investigation of the boundary layer on the vane surface a Preston probe was used. This is a flattened total head or Pitot probe. In the sketch on the left hand side of figure 4 the dimensions of the probe head are illustrated. On the right hand side a picture of the probe is displayed. ≈0 explanation pprobe ≈ pt1; probe head is at the border or outside of the boundary layer pt1 > pprobe > p; probe head is inside the boundary layer pprobe ≈ p; probe head is at the point of separation or inside a separation Tab. 1 Explanation of pressure ratio as defined in equation 3 The sketch on the top in figure 5 explains the Preston probe investigations in principle. The pressure at the probe head is measured at different positions along the vane surface. For the results presented in this paper the pressure was only measured at the positions of the pressure taps because the static pressure at the surface is known there. By passing the transitional region downstream a fairly sudden increase of qprobe/q∞,x (named with q/q1 in figure 5) can be observed as illustrated in the chart in figure 5. Fig. 4 Sketch of probe head and picture of Preston probe In order to investigate the position of the region of transition the Preston probe was moved downstream along the surface of the vane. If the static pressure on the surface is known a pressure ratio can be calculated by equation 3 to analyze the boundary layer state. q probe q , x p probe ( x, ) p( x) pt1 p( x) (3) Thereby the total pressure at the inlet of the cascade is named with pt1 and the static pressure which depends on the position on the surface is named with p(x). The pressure determined by the Preston probe is pprobe(x,η). This value is dependent on the one hand to the position x of the probe head at the surface and on the other hand to the distance from the surface η. The pressure ratio in equation 3 relates the dynamic pressure at the position of the Preston probe inside the boundary layer to the dynamic pressure of the undisturbed flow at this position. Table 1 summarizes the three significant values of the pressure ratio. Fig. 5 Explanation of Preston probe measurements [7] 3 Apart from Preston probe measurements along the surface traverses through the boundary layer on selected locations on the surface have been conducted. Thereby, starting at the surface, the probe head was moved perpendicular to the vane surface in steps of 0.1 mm to investigate the boundary layer state and thickness at this position. In figure 6 (top) the schematic shape of the boundary layer near a point of separation (S) is shown. A point of inflexion (PI) inside the velocity distribution indicates a separation point, if the velocity gradient at the wall is zero, or a separation, if the value is below zero. In the chart in figure 6 the change in the shape factor for a flat plate in the transition region (measured by Schubauer and Klebanoff [8]) is depicted. The shape factor (H12) represents the ratio between the displacement thickness (δ1) and the momentum thickness (δ2) of a boundary layer. For a flat plate the ratio decreases from H12 ≈ 2.6 for a laminar region to H12 ≈ 1.4 for a turbulent region. Fig. 6 Boundary layer (top) at the point of separation [7] and shape factor (bottom) [6] For further information to investigations of the boundary layer state on the surface it is referenced in general to Hoeger [4] and to Stotz et al. [10] who compared Preston probe measurements with hot film anemometry. Results Measurements at a stagger angle of βS=90° In figure 7 it is depicted the isentropic Mach number distribution along the surface of the vane for different Reynolds numbers and a completely open VIGV (βS=90°, refer to figure 3). For the lowest investigated Reynolds number of 100 000 a plateau can be detected xax/lax = 0.65 to 0.73. This is an indication of a flow separation on the surface as explained by Cumpsty [2]. For Re1 = 200 000 there is no plateau but a change in the gradient of the Mach number distribution. The two highest investigated Reynolds number show a continuously decrease of the isentropic Mach number. But the Mach number distribution delivers no information about the exact beginning or end of the separation bubble or the following boundary layer, especially for higher Reynolds numbers. For this reason Preston probe measurements have been conducted. In figure 7 also the previous described pressure ratio distribution is plotted. For a Reynolds number of 100 000 the pressure ratio decreases until xax/lax = 0.65. The next two measurement points are close to zero. This indicates a flow separation in this region on the surface. From xax/lax = 0.82 until the trailing edge of the vane the pressure ratio is rising indicating a transitional boundary layer. Hence, for the smallest Reynolds number there is a laminar boundary layer for the first 65% of the vane surface flowed by a separation and a transitional region. For a Reynolds number of 200 000 a flow separation is not present since the pressure ratio is considerably higher than zero. From xax/lax = 0.65 to 0.82 the ratio rises followed by a decreasing of the ratio. Therefore, the investigations at this particular Reynolds number reveal no flow separation but a turbulent boundary layer at the trailing edge of the vane. For a Reynolds number of 300 000 a transitional region cannot be detected anymore. Reason for this is an already small transitional area which cannot be resolved due to a large distance between the pressure taps on the surface. But an incipient transition can be supposed at xax/lax = 0.57. For the highest Reynolds number of 500 000 considered here even an incipient transition cannot be resolved anymore. The 4 pressure ratio is close to one until xax/lax = 0.5. After this the ratio decreases linearly up to the trailing edge of the vane. These results can be confirmed by Barthmes et al. [1], who conducted numerical investigations of the previously described VIGV. The flow separation observed in the isentropic Mach number distribution for the lowest Reynolds number can be found in the pressure ratio distribution, too. Because of the symmetric profile and a stagger angle of βS = 90° the boundary layer state on the pressure and suction side of the vane can be considered as the same. most evident for the lowest Reynolds number from xax/lax = 0.25 to 0.57. Looking at the isentropic Mach number distribution it is difficult to estimate, whether the flow is separated or not. For all Reynolds numbers the pressure ratio for this stagger angle is depicted in figure 8, too. Compared to a completely open VIGV (figure 7) the separation at Re1 = 100 000 becomes smaller and a turbulent boundary layer occurs downstream of the transition zone. Reason for this is a shifting of all flow phenomena towards the leading edge. This effect could be observed for all higher Reynolds numbers, too. Compared to a completely open VIGV the transition regime can be obviously determined for Re1 = 300 000. For the suction side at this stagger angle all Reynolds number result in turbulent boundary layers close to the trailing edge of the vane. Fig. 7 Pressure ratio and isentropic Mach number distribution at βS=90° Measurements at a stagger angle of βS=80° The pressure ratio and isentropic Mach number distribution on the suction side of the vane for a ten degree lower stagger angle is depicted figure 8. Multi shaft compressors operate with a constant shaft speed for most applications. Hence a lower stagger angle correlates with a lower mass flow rate due to a decreased power conversion. This leads to a lower velocity at the inlet of the VIGV. For a stagger angle of βS = 80° the Mach number decreases to Ma1 = 0.29. A shifting to the leading edge as well as an increasing of the peak Mach number can be observed for all Reynolds numbers. The highest Reynolds number shows a continuously decreasing Mach number. For Re1 = 300 000 and 200 000 a discontinuity in the gradient of the isentropic Mach number distribution can be observed, indicating a transitional regime. This discontinuity becomes Fig. 8 Pressure ratio and isentropic Mach number distribution at βS=80°, suction side In figure 9 the distributions for the pressure side of the vane for βS = 80° are depicted. Due to difficulties with the accessibility of the Preston probe to the pressure side only the rear part of the vane could be investigated. The isentropic Mach number shows a plateau beginning at xax/lax = 0.73 for Re1 = 100 000. Hence a flow separation occurs at this position. All Reynolds numbers reach the same value at the last pressure tap except of the lowest Reynolds number. This is an indication for a separation of the flow without reattaching before reaching the trailing edge for Re1 = 100 000. In comparison to the suction side a lower stagger angle leads to a shifting of the flow phenomena 5 towards the trailing edge on the pressure side as shown in the distribution of the pressure ratio. The flow separates from the vane without reattaching before reaching the trailing edge for the lowest Reynolds number. For Re1 = 200 000 the pressure ratio is increasing again at the last pressure tap. Hence the flow is not fully separated and the boundary layer becomes transitional. The same behavior of the pressure ratio distribution occurs for Re1 = 300 000. A complete transition to a turbulent boundary layer state can only be observed for the highest Reynolds number. traverses at xax/lax = 0.74 and 0.98 deliver a shape factors as they occur for a turbulent boundary layer. The first traverse on the pressure side (figure 10, chart on the right hand side) downstream indicates a laminar boundary layer. The point of inflexion in the second traverse (xax/lax = 0.73) is an indication of the beginning or already existing separation. The boundary layer measurements match very well with the previously observed pressure ratio measurements. Fig. 10 Boundary layer measurements at βS=80° Measurements at a stagger angle of βS=70° Fig. 9 Pressure ratio and isentropic Mach number distribution at βS=80°, pressure side In order to determine the local state of the boundary layer, traverses perpendicular to the surface have been conducted for Re1 = 100 000 with the Preston probe. The grey bars on the abscissa of figure 8 and 9 indicate the positions of the boundary layer traverses. These traverses normalized to the corresponding boundary layer thickness are presented in figure 10 for βS = 80°. Due to the finite thickness of the Preston probe the first measurement point of a normalized traverse is higher in a boundary layer with a small thickness compared to a boundary layer with a higher thickness. This is the reason for the different initial values of η/δ in figure 10. The chart on the left hand side represents the suction side. At the first position xax/lax = 0.24 the shape factor of H12 = 2.75 indicates a laminar boundary layer (refer to figure 6). In the next traverse downstream on the surface a point of inflexion occurs. This is an indication for a separation. The following two The third investigated stagger angle has been at βS = 70°. As mentioned before a lower stagger angle leads to a decreasing velocity at the inlet of the cascade. The Mach number at the inlet is Ma1 = 0.27 for this case. The results for this angle on the suction side are depicted in figure 11. Fig. 11 Pressure ratio and isentropic Mach number distribution at βS=70°, suction side 6 The isentropic Mach number distribution shows on the one hand a significant increase of the peak Mach number and on the other hand a shifting of all flow phenomena closer towards the leading edge of the vane. For the lowest Reynolds number obviously a separation bubble can be detected from xax/lax = 0.08 to 0.2. From xax/lax = 0.3 until the trailing edge the Mach number is continuously decreasing for all Reynolds numbers. The pressure ratio measurements show now a strongly upstream shifted transitional regime. This leads to a turbulent boundary layer along 90% of the axial chord length for Re1 = 500 000. A transition region can only be detected for the lowest two Reynolds numbers. At Re1 = 100 000 a separation bubble is present close to the leading edge of the vane. As already observed in figure 9 the pressure ratios for all four Reynolds numbers provide the same values if the boundary layer is turbulent. to zero at xax/lax = 0.81 and does not increase before reaching the trailing edge. Hence the ratio decreases for the entire surface length for the two highest Reynolds numbers which means that the boundary layer remains laminar. At βS = 70° traverses through the boundary layer also been conducted. They are presented in figure 13. The first traverse on the suction side (chart on the left hand side) at xax/lax = 0.17 exhibits a point of inflexion and a high shape factor. Both indicate a flow separation at this point. The more downstream located two traverses deliver a low shape factor. Hence the boundary layer is turbulent for at least 66% of the axial chord length. The first traverse on the pressure side (chart on the right hand side) of the vane indicates a laminar boundary layer. In the second traverse at xax/lax = 0.81 a point of inflexion can be observed. In combination with figure 12 this location seems to be very close to the point of separation. The last traverse is located close to the trailing edge of the vane and delivers a pressure ratio of zero for almost 50% of the normalized height. The shape factor exhibits a value that is significant higher than expected for a laminar boundary layer. Hence, in can be concluded that there is an open separation at the trailing edge. The boundary layer measurements match again very well with the previously observed pressure ratio measurements. Fig. 12 Pressure ratio and isentropic Mach number distribution at βS=70°; pressure side The isentropic Mach number and pressure ratio distribution for the pressure side at βS = 70° is depicted in figure 12. The flow accelerates up to about 50% of the chord length beginning from the leading edge. Due to this acceleration the boundary layer on the surface of the vane can be considered as laminar. For Re1 = 500 000 and 300 000 the isentropic Mach number exhibits a similarly value at the last pressure tap. The two lowest Reynolds numbers, however, reveal higher values. This indicates a separation of the flow without reattaching. These results can also be confirmed by the pressure ratio measurements. The ratio is close Fig. 13 Boundary layer measurements at βS=70° Discussion In figure 14 the main findings of the previously discussed results are summarized. The development of the boundary layer state along the surface of the vane is shown in dependence of the Reynolds number and of the stagger angle. The first case (case a) represents the highest investigated Reynolds number and a completely 7 open VIGV. The boundary layer is laminar on both sides of the airfoil followed by a small transitional region and a turbulent boundary layer until the end of the vane. A decreasing Reynolds number (case a→b) leads to a longer laminar boundary layer followed by a flow separation which reattaches before reaching the trailing edge. Until the end of the vane a transitional regime occurs. A turbulent boundary state was not detected. This is linked with a significant increase of the total pressure losses (ζ). A 10°-change of the stagger angle (case a→c) leads to a shift of the transition point on the suction side towards the leading edge and on the pressure side towards the trailing edge. A reduced Reynolds number for βS = 80° (case d) again is combined with a longer laminar boundary state on the suction side. A small separation bubble occurs, followed by a transitional region and a turbulent boundary layer. On the pressure side a flow separation comes up without reattaching to the surface. This is combined with an overturning of the flow as described in more detail in [3]. A further decrease of the stagger angle to βS = 70° (case e) leads to a shifting of the isentropic peak Mach number even closer towards the leading edge on the suction side followed by transition into a turbulent boundary layer. On the pressure side, the flow accelerates almost along the entire length of the vane resulting in a laminar boundary layer. At βS = 70° and the lowest Reynolds number (case f) the highest total pressure losses were measured. This is caused by a flow separation downstream at the end of the vane close to the trailing edge of the pressure side and a separation bubble combined with a turbulent boundary layer state covering a large portion of the suction side. Conclusions This paper deals with the experimental investigation of a VIGV at different stagger angles and Reynolds numbers as they appear in technical applications. It is focused on surface measurements using a Preston probe to investigate the boundary layer state. The main results can be summarized as follows: Preston probe measurements are very useful to detect the location and length of laminar and turbulent boundary layer state as well as separation and transition regime for many relevant cases. For the airfoil under consideration here a decreasing Reynolds number leads to a separation bubble and therefore to a massive loss increase for βS = 90°. At stagger angles βS < 90° the boundary layer state (laminar/turbulent, transition, separation) moves towards the leading edge on the suction side and towards the trailing edge on the pressure side. At βS < 90° and low Reynolds numbers a flow separation occurs on the pressure side trailing edge without reattaching. For a better understanding of the aerodynamically behavior, these experimental results have been used to generate a validation basis for further numerical studies. All the findings will be utilized to design an enhanced VIGV profile in order to reduce the total pressure losses and increase the working range. Acknowledgments The experimental investigations presented here are part of the AG TURBO 2020 project no. 1.3.2 “Optimierung großer Mehrwellenkompressoren für CCS-Anwendungen” funded by the German Ministry of Economy and Technology (BMWi). The project was performed in collaboration with MAN Diesel & Turbo SE. References [1] Fig. 14 Boundary layer (b. l.) state on blade (schematically) Barthmes, S.; Händel, D.; Niehuis, R.; Wacker, C.; Klausmann, J.; 2013, “2D Investigation of the Flow Through a Symmetric Variable Inlet Guide Vane, Part 2: Numerical Analysis”, American Institute 8 of Aeronautics and Astronautics, AIAA 2013-3683 [2] Cumpsty, N. 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