Proceedings of ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition GT2016 June 13-17, 2016, Seoul, South Korea

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1 Proceedings of ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition GT2016 June 13-17, 2016, Seoul, South Korea GT INVESTIGATION OF VANED DIFFUSER SPLITTERS ON THE PERFORMANCE OF A AND FLOW CONTROL HIGH PRESSURE RATIO CENTRIFUGAL COMPRESSOR Xiao HE, Xinqian ZHENG*, Jie WEI, Hanxuan ZENG Turbomachinery Laboratory, State Key Laboratory of Automotive Safety and Energy Tsinghua University, Beijing, , China. zhengxq@tsinghua.edu.cn ABSTRACT High pressure ratio centrifugal compressors with vaned diffusers are required in modern gas turbine engines. A potential method to further improve the performance of such compressors is to apply splitters to the vaned diffuser. This paper numerically investigates design choices of splitters in a vaned diffuser of a high pressure ratio centrifugal compressor. The splitters are derived from the main vane but with different chord length. The total vane number has been adjusted to maintain the same diffuser flow capability relative to the impeller. When the splitter is designed in the vaned region of the datum diffuser, the flow separation has been successfully suppressed with increasing relative splitter chord length from 0.10 to At high mass flow rate conditions, the stage efficiency and the diffuser pressure recovery coefficient increases by 1.0% and 0.04 respectively due to the suppression of the flow separation. However, at design mass flow rate, the stage efficiency and the diffuser pressure recovery coefficient drops slightly by 0.3% and 0.01 respectively because the enhanced friction loss of vane surfaces is now dominant. When the splitter vane reaches the semi-vaneless region by further increasing its chord length from 0.8 to 0.95, the shock structure changes from two parallel shocks to two sequential shocks at first, but turns to the datum shock structure at last. The stage efficiency and the diffuser pressure recovery coefficient of the sequential shock structural case significantly drops by 0.7% and 0.03 respectively due to the increased shock strength and thus the higher shock loss. It is indicated that the application of splitter vanes in the vaned region of the diffuser is beneficial when the induced friction loss from splitter surfaces is less significant or the reduced loss from the suppressed flow separation is more dominant, which is the case that the flow separation happens relatively downstream or the case operating at large incidence conditions. NOMENCLATURE A passage area C p pressure recovery coefficient C s relative chord length of splitter vanes H enthalpy m mass flow rate r distance in the radial direction U blade speed y + normalized wall distance Z number of blades β blade angle η total-to-total isentropic efficiency π total-to-total pressure ratio ρ density ω loss coefficient Subscripts 1 impeller inlet 2 impeller exit 3 diffuser inlet 4 diffuser exit d design condition Definition of non-dimensional parameters φ flow coefficient m 4ρ 1 U 2 r 2 2 ψ pressure coefficient h 4s h 1 U 2 2 N s specific speed 2φ1/2 ψ 3/4 1 Copyright 2016 by ASME

2 1 INTRODUCTION Centrifugal compressors are widely applied to turbochargers and gas turbine engines. For reduction of manufacturing cost and engine weight, high pressure ratio centrifugal compressors are especially preferred in the aviation industry [1]. However, due to the rapid drop of stable flow range [2] and efficiency [3] with increasing pressure ratio, the application of such compressors is still limited. For the past twenty years, institutes including DLR [4] and MHI [5] have conducted plenty of researches on designing high performance and high pressure ratio centrifugal compressors, as well as understanding their flow phenomena (i.e. shock-boundary layer interaction and shock-leakage interaction). Recently, with the raising demand for commercial helicopters, researches on this topic are growing in popularity in China. Several attempts have been made on designing centrifugal compressors with pressure ratios higher than 7 [6], [7]. Vaned diffusers are common choices for maximizing stage efficiency of centrifugal compressors. To further improve the vaned diffuser performance, a potential method that has been widely used in impeller designs is adopting splitters. Actually such an idea is not new, as it has been claimed to have benefits from preventing the flow separation [8] and controlling the shock structure [9] in early patents. However, limited supportive data of performance and flow field are available in open literatures at that time. In recent researches, there are several design cases applying splitter vanes to the diffuser. An integrated curved diffuser with splitters has been developed by Chen [10] to replace the wedge diffuser and the deswirler in the centrifugal compressor of a micro turbine engine, which significantly increased the pressure recovery coefficient by Medic [11] has adopted the vaned diffuser with splitters to replace the wedge diffuser in a high efficiency and high pressure ratio centrifugal compressor, and the pressure recovery coefficient was found to increase by Although the above cases showed improvement in pressure recovery coefficient, modifications in stagger angle, endwall contours and diffuser types were also involved. Therefore, it is difficult to distinguish the pure benefit of introducing splitter vanes. Effects of splitter vanes have been well studied in low pressure ratio centrifugal fans and compressors. For centrifugal fans, Konishi [12] tested diffusers with different arrangements of the guide vanes, and an increase of 5% in diffuser efficiency was obtained at low flow coefficient conditions. Sharma [13] numerically investigated diffusers with different arrangements of splitter vanes. The splitter located in the rear part of the diffuser with about one quarter of the main vane chord length was found to increase the pressure recovery coefficient by 0.05 and decrease the loss coefficient by For centrifugal compressors, Drtina [14] investigated splitter vanes with different circumferential leading edge positions through both experiments and CFD. It was found that the pressure recovery coefficient increased significantly by 0.1 at low flow coefficient conditions when the splitter leading edge was positioned closer to the suction side of the main vane, which was resulted from the suppression of the flow separation. Similar results were obtained from experiments by Abdel-Hafez [15], where the pressure coefficient slightly increases at low flow coefficient conditions when the splitter vane located near the pressure side of the main vane. For high pressure ratio centrifugal compressors, however, little discussion about diffuser splitter vanes has been found from open literatures. This paper numerically investigates design choices of splitters in a vaned diffuser of a high pressure ratio centrifugal compressor. The basic configuration of the splitters is derived from the main vane. To ensure appropriate stage matching, the choke mass flow rate of the diffuser is maintained nearly unchanged. Diffusers with different splitter chord length as well as different diffuser total vane numbers have been investigated. Effects of different designs of splitter vanes on the performance and flow fields have been discussed. 2 CASE DESCRIPTION The datum high pressure ratio centrifugal compressor named TTL-1 is a prototype design, whose specifications are presented in Table 1. The impeller is constructed through the linear connection between points of hub and shroud sections, and no state-of-the-art features (i.e. splitter blades, 3D stacking, etc.) have been involved yet. The vaned diffuser downstream of the impeller is a two-dimensional design, which has the same blade profile along the span. The motivation of developing the TTL compressor series is to evaluate the potential and mechanisms of several design features (i.e. splitter blades, tandem blades, 3D features, casing treatment, etc.) in improving the performance of high pressure ratio centrifugal compressors. In this paper, the potential and mechanisms of vaned diffuser splitters have been investigated. Table 1 Specification of datum compressor TTL-1 Impeller Number of blades Z I = 24 Normalized leading edge hub radius r 1h / r 2 = 0.26 Normalized leading edge tip radius r 1t / r 2 = 0.65 Diffuser Number of vanes Z D = 19 Area ratio A 4 / A throat = 2.0 Blade turning β 4 β 3 = 15 Normalized leading edge radius r 3 / r 2 = 1.10 Normalized trailing edge radius r 4 / r 2 = 1.48 Stage performance Peak pressure ratio max π 1-4 = 8.05 Peak isentropic efficiency max η 1-4 = 79.9% Flow coefficient φ = 0.08 Pressure coefficient ψ = 0.63 Specific speed N s = Copyright 2016 by ASME

3 The definition of splitter vanes is illustrated in Fig. 1. It has the same blade angle distribution along the chord direction and the same thickness distribution along the relative chord length. Its trailing edge locates at the same radius of that of the main vane, and its circumferential position is kept at the middle of the passage surrounded by two adjacent main vanes. When the leading edge of the splitter vane locates at the vaned region downstream the diffuser throat, the only parameter of investigated cases is the relative splitter chord length. When the leading edge of splitter appears in the semi-vaneless region upstream the diffuser throat, the total vane number should be changed according with the different splitter chord length so as to keep the same flow capability of the diffuser. The splitter designs in the following discussion are organized regarding to the above two situations. as shown in Fig. 2. The same mesh topology and size have been adopted for all cases in the study. Total pressure, total temperature and velocity components are imposed at the inlet, and an average static pressure is imposed at the exit. The blades and casing surface are defined as rotational non-slip solid boundary and static non-slip solid boundary, respectively. The non-reflecting 2D model is applied to the rotor/stator interface. The former converged result is used as the initial solution for the next case with a higher backpressure. The step rise in backpressure at the near-surge condition is kept as 1 kpa. If no signs of numerical instabilities occur after 10,000 iterations, the solution is considered to be converged. Fig. 1 Schematic of splitter vanes design 3 NUMERICIAL METHODS AND EVALUATION 3.1 Numerical Methods The simulation by computational fluid dynamics (CFD) is done with the solver EURANUS [16] based on a threedimensional steady, compressible, finite volume scheme to solve the Reynolds-averaged Navier-Stokes equations in the conservative formulation. The spatial and temporal discretization are achieved with the modified Jameson central difference scheme and the fourth-order Runge-Kutta scheme, respectively. The Spalart-Allamaras (SA) one-equation model [17] is adopted as the turbulence model, and the real gas model is adopted since the change of the gas property due to the high diffuser temperature is significant. The compressor is meshed by a multi-block structured grid with O4H topology. The mesh of the impeller consists of 118, 73, 43 nodes in the streamwise, spanwise and pitchwise directions, and the diffuser mesh contains 74, 73, 47 nodes, respectively. The tip clearance is set constant to 5.6% of exit blade height. To predict a boundary layer resolution sufficiently fine for the Spalart-Allamaras (SA) model, the scalar averaged y + is set around 1.6, and its maximum value is kept below 10. The final grid of the datum compressor has 1.0 million nodes for one impeller blade passage and one diffuser vane passage, Fig. 2 Domains of the numerical model 3.2 Grid Independence Study The grid independence is studied before large amounts of simulations being carried out. Four different types of mesh of the compressor are investigated, whose number of nodes vary from 0.6 to 1.4 million. The calculated performance of all meshes at design mass flow rate is summarized in Table 2 and plotted in Fig. 3. It is illustrated that the CFD simulation produces consistent results when the mesh nodes surpass 1.0 million. Hence, the numerical method that adopts a mesh with 1.0 million nodes is independent of the mesh resolution. Impeller nodes Table 2 Mesh overview Diffuser Min. nodes Skewness π 1-4 η , , , , , , , , Copyright 2016 by ASME

4 Table 3 Turbulence model overview Models Equation numbers Time (10,000 iterations per core) π 1-4 η 1-4 SA hour SARC hour k-ε hour SST hour Fig. 3 Performance at design condition predicted by different mesh sizes 3.3 Evaluation of Turbulence Models Besides the adopted SA model, three other turbulence models namely SARC [18], k-ε with extended wall function [19] and SST [20] are evaluated in the high pressure ratio centrifugal compressor stage. No complex models with more equations have been tested, since more detailed resolution of the boundary layer seems not to bring obvious differences in predicting the flow field at design condition in high pressure ratio centrifugal compressors [21]. Results at design speed are illustrated in Fig. 4, and results at design condition are summarized in Table 3. It can be seen that results predicted by the SA model locates in the middle among that of all cases while costing the least time. Thus the application of SA in this case is considered acceptable and efficient. 4 ANALYSIS OF THE DATUM DIFFUSER The datum diffuser performance at design condition is presented in Fig. 5. It is illustrated that about 70% of the pressure recovery coefficient rise and about 50% of the loss coefficient rise are obtained in the vaneless and the semivaneless region. Thus, the design of the front part of the diffuser is crucial. At the exit of the diffuser, the pressure recovery coefficient and the loss coefficient reaches 0.70 and 0.21, respectively. Fig. 4 Performance at design speed predicted by different turbulence models Fig. 5 Radial distribution of pressure recovery coefficient and loss coefficient of the datum diffuser at design condition To understand the flow pattern at the front part of the diffuser, the pitch-averaged incidence and the pitch-averaged absolute Mach number are plotted against spanwise direction in Fig. 6. A broad region of high Mach number is observed from the hub section to the mid-span section, which implies a shock at the leading edge of the diffuser vane. It is also shown that the spanwise distribution of the incidence is highly distorted. Because the datum diffuser has the same blade profile along the spanwise direction, such distortion comes directly from the non-uniform flow out of the impeller. The positive incidence at high blade span may trigger the diffuser instability, and the negative incidence at low blade span will result in the flow separation at the pressure side of diffuser vanes. 4 Copyright 2016 by ASME

5 suppressing the flow separation. Cases that differed in relative splitter chord length (from 0.10 to 0.70) with doubled total vane number have been investigated. The stage and component performance versus scaled mass flow rate are presented in Fig. 8. At high mass flow rate conditions, the stage efficiency and the diffuser pressure recovery coefficient are found to increase by 1.0% and 0.04 respectively as the splitter chord length increases from 0.10 to At design mass flow rate, however, the stage efficiency drops by 0.3% and the pressure recovery coefficient drops by Since the impeller performance keeps unchanged, the variation of diffuser performance is responsible for the change of the stage performance. Fig. 6 Pitch-averaged spanwise distribution of incidence and absolute Mach number at r/r2 = 1.08 The flow separation and the shock are illustrated in detail in Fig. 7. The shock forms ahead of the vane leading edge and impinging on the suction surface of the neighboring vane. The absolute Mach number at the shock front is about 1.3. A low momentum flow region is observed at the rear part of the pressure side of the vane. The major source of the low momentum flow comes from the flow separation due to the large negative incidence from the hub section to the mid-span section. The rest part comes from the flow downstream the shock near the shroud section. Such a region of low momentum flow not only generates extra loss, but results in the heavy blockage that limits the pressure recovery coefficient. Fig. 7 Absolute Mach number contour at 50% span superimposed with streamlines 5 DESIGN OF SPLITTER VANES IN THE VANED REGION OF THE DIFFUSER The application of splitter vanes in the vaned region of the diffuser is expected to improve the performance through Fig. 8 Stage and component performance at design speed of vaned region cases with different relative chord length The effect of splitter vanes on suppressing the flow separation is illustrated in Fig. 9, where the absolute Mach number contours superimposed with isolines at different cross 5 Copyright 2016 by ASME

6 flow planes of the diffuser are presented. It can be seen that the region of the low momentum flow near the pressure side of main vanes has been weakened with the increasing splitter chord length from 0.30 to This is supposed to increase the pressure recovery coefficient and decrease the loss coefficient. In the contrary, the performance at the design condition is in fact deteriorated in comparison with the datum as illustrated in Fig. 8. It implies that suppressing the low momentum flow near the pressure side of main vanes does not necessarily improve the performance, and other factors may be functioning as well. the vaned region (with r/r 2 from 1.2 to 1.48) of the diffuser, sudden changes in the parameters happen corresponding to the leading edge location of splitter vanes. The loss coefficient then gradually increases with the radius, which is caused by the friction on the blade surfaces. The pressure recovery coefficient drops at first and then recovers after the leading edge of splitter vanes, which is due to the change of both the passage area ratio and the blockage. In the vaneless region downstream diffuser vanes (with r/r 2 from 1.48 to 1.8), the loss coefficient and the pressure recovery coefficient slightly decreases and increases respectively due to the more uniform flow pattern downstream the vanes. Fig. 9 Absolute Mach number contour superimposed with isolines at design condition of vaned region cases To evaluate the developing processes of the pressure recovery and the loss generation, differences of the relevant coefficients between the datum and the cases with splitters are shown in Fig. 10. In the vaneless and the semi-vaneless region (with r/r 2 from 1 to 1.2) of the diffuser, little change is observed because the geometry upstream the throat area is unchanged. In Fig. 10 Radial distribution of the (a) difference of pressure recovery coefficient and (b) difference of loss coefficient at design condition of vaned region cases 6 Copyright 2016 by ASME

7 6 DESIGN OF SPLITTER VANES IN THE SEMI- VANELESS REGION OF THE DIFFUSER The application of splitter vanes in the semi-vaneless region of the diffuser is expected to influence the performance through changing the shock structure. Cases that differed both in relative splitter chord length (from 0.80 to 0.95) and total vane numbers have been investigated. The choke mass flow rate of the diffuser is kept nearly unchanged to maintain the same matching between the impeller and the diffuser. 0.80, the stage efficiency recovers by 0.2% at design condition, and the loss coefficient and the pressure recovery coefficient decreases and increases by 0.01 respectively. It is also noticed that trends of the stage efficiency curve and the diffuser performance curves of the C s =0.90 case is quite unique among these cases. The effect of splitter vanes on the shock structure is shown in Fig. 12, in which the absolute Mach number contours superimposed with isolines at the mid-span of the diffuser are presented. The shock structure has been changed from two parallel shocks at C s =0.95 to two sequential shocks at C s =0.90, where the shock strength of the main vane and the splitter vane gradually increases and decreases, respectively. When the splitter chord length further decreases to C s =0.80, the shock structure turns back to the datum shock pattern, where the shock of the diffuser vane is eliminated. The unique sequential shock structure is responsible for the different performance trends of the C s =0.90 case in Fig. 11. Fig. 11 Stage and component performance at design speed of semi-vaneless region cases with different relative chord length and total vane number The stage and component performance versus scaled mass flow rate at the rotor-stator interface of all cases are presented in Fig. 11. As the relative splitter chord length varies from 0.95 to 0.90, the stage efficiency at design condition drops by 0.7% due to the raise of diffuser loss coefficient by The pressure recovery coefficient drops by 0.03 as well. When further decreasing the relative splitter chord length from 0.90 to Fig. 12 Absolute Mach number contour at diffuser 50% span superimposed with isolines, design condition of semivaneless region cases To quantitatively compare the changes of shock structures, the blade loading distributions of main vanes and splitter vanes are presented in Fig. 13 respectively. From the blade loading distribution of the main vane in Fig. 13(a), the downstream movement of the shock front and the enhanced shock strength of the C s =0.90 case is observed, which can be denoted by the position of the sudden pressure rise and the value of pressure increment respectively. The enhanced shock strength will result in higher shock loss, which causes the sudden drop of diffuser performance of the C s =0.90 case in Fig. 11. From the blade loading of splitters in Fig. 13(b), the decreased shock strength from 0.95 to 0.80 of the relative splitter chord length is well 7 Copyright 2016 by ASME

8 captured. A negative loading is observed in the C s =0.90 case, which is due to the two sequential shocks that rises the pressure at the suction side of splitter vanes to a very high value. (with r/r 2 from 1.2 to 1.48) of the diffuser due to the friction of blade surfaces, and the pressure recovery coefficient deficit in the semi-vaneless region is gradually weakened due to the recovery of the shock structure. Fig. 13 Blade loading of (a) main vanes and (b) splitter vanes at diffuser 50% span, design condition of semivaneless region cases To evaluate the pressure recovery coefficient and the loss coefficient in detail, the difference of the above parameters between the datum case and cases with splitters has been plotted against the radial direction in Fig. 14. For the C s =0.90 case with two sequential shocks, it is illustrated that most of the performance deficit exists in the semi-vaneless region (with r/r 2 from 1.1 to 1.2) of the diffuser. For the C s =0.85 and the C s =0.80 cases, the loss coefficient increases mainly in the vaned region Fig. 14 Radial distribution of the (a) difference of pressure recovery coefficient and (b) difference of loss coefficient at design condition of semi-vaneless region cases 7 DISCUSSION The design choices of splitter vanes in the vaned region and the semi-vaneless region of the diffuser have been discussed respectively. Although the flow field has been changed, little performance improvement at design condition is obtained, which is in line with the conclusion of Abdel-Hafez [15] but against that of Drtina [14]. Therefore, the effectiveness of applying splitter vanes should be discussed case by case. 8 Copyright 2016 by ASME

9 For the design of splitters at the vaned region of the diffuser, the improvement in stage efficiency depends on both the reduced loss due to the suppression of flow separation and the induced friction loss from extra blade surfaces. If the induced friction loss is less significant or the reduced loss related to the suppression of flow separation is more dominant, an improvement in the stage efficiency is expected. For cases that the flow separation occurs at the rear part of the diffuser, the induced friction loss is less significant because splitter vanes have relatively shorter chord length to control the flow separation. On the contrary, if the flow separation happens at the front part, the induced friction loss will limit the performance improvement, which is the case of the investigated datum diffuser. For cases that operating at conditions with significant incidences, the loss related to the flow separation is therefore very dominant. In the case investigated by Drtina [14], the diffuser suffers from the flow separation at suction side of diffuser vanes. After adjusting the leading edge location of splitter vanes, the performance increases especially at low flow coefficient or high positive incidence conditions. It is also found in the current case that the performance at high flow coefficient or high negative incidence conditions is improved, where the datum compressor suffers from the flow separation at the pressure side of diffuser vanes. For the design choice of splitters at the semi-vaneless region of the diffuser, the passage area in front of the shock front is generally enlarged, which leads to an enhanced shock strength and a sequential shock structure, and thus deteriorated the performance. It is therefore not recommend in designs. 8 CONCLUDING REMARKS This paper numerically investigates design choices of splitters in a vaned diffuser of an in-house high pressure ratio centrifugal compressor stage. The splitter is defined inherited from the main vanes. To ensure appropriate stage matching, the choke mass flow rate of the diffuser is maintained unchanged. Splitters staying in the vaned region and intruding to the semivaneless region of the diffuser have been investigated. Conclusions are drawn as follows: (1) The flow field of the vaned diffuser in the high pressure ratio centrifugal compressor is governed by the shock wave and the flow separation. It is crucial to suppress the flow separation and to optimize the shock structure to further improve the performance of the datum diffuser. (2) For the splitter designs in the vaned region of the diffuser, the flow separation has been successfully suppressed with increasing splitter chord length from 0.10 to At high mass flow rate conditions, the stage efficiency and the diffuser pressure recovery coefficient increases by 1.0% and 0.04 respectively due to the suppressed flow separation. However, at design mass flow rate, the stage efficiency and the pressure recovery coefficient slightly drops by 0.3% and 0.01 respectively because of the enhanced friction loss of vane surfaces. (3) For the splitter designs in the semi-vaneless region of the diffuser, when declining relative splitter chord length from 0.95 to 0.90, the shock structure changes from two parallel shocks to two sequential shocks. The stage efficiency and the diffuser pressure recovery coefficient significantly drops by 0.7% and 0.03 due to the enhanced shock strength. When further decreasing the splitter chord length from 0.90 to 0.80, the shock structure turns back to the datum shock structure, and the stage efficiency and the diffuser pressure recovery coefficient recovers by 0.2% and (4) It is proposed that the application of splitter vanes in the vaned region of the diffuser is beneficial when the induced friction loss is less significant or the reduced loss from the suppressed flow separation is more dominant, which is the case that the flow separation happens relatively downstream or the case operating at large incidence conditions. ACKNOWLEDGEMENTS This research was supported by the National Natural Science Foundation of China (Grant No ). REFERENCES [1] Krain, H., 2005, Review of Centrifugal Compressor s Application and Development, ASME J. 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