High Pressure Ratio Centrifugal Compressor Research Articles

Investigation of the Map Width Improvement in a Radially Reduced Diffuser Design Concept for a Centrifugal Compressor

Abstract Often, an improvement in performance at one operating point in a compressor/turbine operating map will lead to a reduction in performance at another area. For example, improving the efficiency of a compressor at design point (DP) may result in a reduction of efficiency at other operating points or it may result in a reduced stable operating range. In order to reduce the compressor cost and weight, a 15% radial reduction was applied to the diffuser passage length of a high pressure ratio centrifugal compressor. The diffuser endwall was diverged to achieve an appropriate area ratio from diffuser inlet to outlet. Two different vane shapes were considered with the radially reduced diffuser passage. The first diffuser vane was a scaled version of the baseline airfoil vane shape and this was used to develop an understanding of the impact of the passage length reduction and diverging endwall. The second vane geometry was parameterized using an unconventional method derived from published literature and optimized to improve performance at DP. Increased losses were generated through the diffuser passages of the radially reduced designs, but this was compensated for by lower losses through the volute due to more desirable flow at volute inlet. As a result, the radially reduced designs not only matched the performance of the baseline configuration, but also succeeded in achieving a marginal efficiency improvement. Furthermore, despite not being one of the goals of the optimization procedure, an extension of the stable operating range of 36% and 20% at the highest speed condition was achieved for the scaled and optimized designs, respectively. The current study focuses on understanding the physical reasons that generated this significant enhancement to the compressor operating range. In order to acquire the desired understanding, experimental data is presented to validate the Computational Fluid Dynamics (CFD) model and a detailed numerical investigation is presented to explain the changes in the flow field within the radially reduced designs. The study focuses on low flow rate operating points, necessitating the use of an Unsteady Reynolds Averaged Navier-Stokes (URANS) approach to analyse the flow. Detailed scrutiny of the flow field revealed a secondary flow feature in the diffuser passage which resulted in higher diffuser passage losses at DP but stabilised the flow as it developed for lower mass flow rate operating points.

Performance Improvement of a High Loading Centrifugal Compressor with Vaned Diffuser by Hub Contour Optimization

High-pressure ratio centrifugal compressors’ diffusers face challenges from high-velocity, non-uniform flow at the impeller outlet, decreasing efficiency and stall margin. To address this, this paper presents a novel vaned diffuser passage design method that successfully improved the compressor’s performance. An optimization method using axisymmetric hub contours and NURBS curves was applied to modify the diffuser’s design. After optimization, centrifugal compressor peak efficiency increased by 0.78%, and stall margin expanded from 12.8% to 20.4%. Analysis at the peak efficiency point showed loss reduction mainly from decreased recirculation and mixing losses in the diffuser’s vaneless and semi-vaneless spaces. Furthermore, correlation analysis and Mach number distribution revealed that flow behavior at the diffuser’s leading edge significantly influences efficiency. Consequently, design principles emphasize satisfying specific Mach number distribution rules at the diffuser’s leading edge under certain inflow conditions for optimal performance.

The investigation of mechanisms on pipe diffuser leading edge vortex generation and development in centrifugal compressor

Pipe diffuser can adapt the high Mach number distorted flow in high pressure ratio centrifugal compressors because of its special scallop leading edge. However, the special leading edge could generate pair vortices and it is not clear if the pair vortices have positive or negative effect on pipe diffuser performance. Besides, the generation and development of pair vortices are also need to be investigated. This work is aimed at uncovering the influence of vortices on aerodynamic loss in pipe diffuser. Pipe diffusers with several different geometries are designed for a high pressure ratio centrifugal compressor used in a small gas turbine. The generation and evolution of pair vortices in pipe diffuser passages are studied by validated state-of-the-art CFD method. Detailed understandings of the influence of different geometries on pair vortices and pipe diffuser performance are provided. Through these works, it is found that the pair vortices could enhance the flow mixing of high and low energy flow in pipe diffuser passage near throat, thus the boundary layer becomes thin, and the flow separation is suppressed near pipe diffuser throat under large negative incidence. However, pair vortices also move low momentum flow to the pressure side and induced separation in pipe diffuser cone after throat, eventually. The diffuser inlet-to-impeller exit radius ratio and diffuser cross section shape could affect the compressor performance by changing the intensity of pair vortices. The diffuser with inlet-to-impeller exit radius ratio 1.02 and the pipe diffuser with the cross section with big fillets such as 3.37 mm are recommended for their good performance. The diffuser throat area and passage count can also have impact on pair vortices, but the compressor performance is not only influenced by pair vortices when these two parameters are changed.

Numerical and experimental investigation of a radially reduced diffuser design concept for a centrifugal compressor performance at design point

Large scale, high pressure ratio (PR) centrifugal compressors are commonly made up of a radial impeller with a vaned diffuser. In research to date, the majority of research and design has focused on extending the operating range or improving the compressor efficiency. However, a cost and weight saving can be achieved by reducing the compressor dimensions. This is a study of a vaned diffuser design concept aiming to reduce the overall radial dimensions of the compressor without any sacrifice of compressor performance at design point (DP).The study parameterized a diverging endwall diffuser concept and optimized it to achieve a similar performance within a reduced radial outlet dimension. A metamodel assisted Multi Objective Genetic Algorithm (MOGA) method has been used. A numerical approach has been used to investigate how the flow physics within the diffuser passage changed with the new geometry and detailed experimental measurements have been used to validate the numerical approach. The result of this study is a summary of the impact of vane geometry defining parameters within a diverging endwall concept which offers design guidance for application of the concept. The measured performance showed that the same efficiency performance was achieved within a 15% radially reduced diffuser passage using the diverging endwall concept.

Stability Improvement of a High-Pressure Ratio Centrifugal Compressor by Flow Injection

AbstractHigh-pressure ratio centrifugal compressors are required in modern turboshaft engines with the requirements of a high power-to-weight ratio and low fuel consumption. The reduction of the st...

Novel Simplified Blade Hub Line Optimization of High-Pressure Ratio Centrifugal Compressor Design Using Numerical Simulations

The focus of this research is to numerically investigate the effect of blade hub line variation on the performance (Total pressure-ratio and Isentropic-Efficiency) of centrifugal compressor from stall to choke to study the operating range and stall margin. An optimization technique is carried out in which the meridional profile hub line is modified and compared with the high Mach number SRV2 compressor designed and fabricated by DLR (German Aerospace Center). Numerical simulations showed significant increase in the stall margin and operating range by bargaining on pressure ratio and isentropic efficiency. Reynolds Averaged Navier Stokes (RANS) based k-ε model is used to predict turbulence using numerical simulations. The value of Y plus for the structured mesh near the boundaries is kept 35. Blade hub line being the important parameter, has substantial performance improvement of centrifugal compressor. The novel design improved the stall margin by 44 percent while operating range from stall to choke has been upgraded by 7.5 percent.

Numerical Analysis of Flow through High Pressure Ratio Centrifugal Compressor Impeller and Effect of Varying Diffuser Exit Width on Performance

Numerical Analysis of Flow through High Pressure Ratio Centrifugal Compressor Impeller and Effect of Varying Diffuser Exit Width on Performance

Effects of a slotted diffuser on the aerodynamic performance of a highly loaded centrifugal compressor

High-pressure ratio centrifugal compressors usually adopt vaned diffusers to reach high efficiency. Nevertheless, the compressor operating range might be narrow on account of the diffuser stall resulting from the large flow separations in diffuser passages at low flow rates. Flow control techniques, aimed at expanding the compressor operating range, are required to suppress these flow separations. In this paper, the flow control strategy, in terms of the slotted diffuser was used to widen the operating range for a highly loaded centrifugal compressor. The main focus of the research is to address the effects of the slotted diffuser vane on the aerodynamic performance, including the underlay flow physics of the compressor. In the case of the baseline compressor, a large flow separation exists near the pressure surface between the hub and approximately 40% of the span inside diffuser passages over the entire flow range. In addition, flow separation appears at the junction of hub and suction surface at low flow rates, which promotes compressor stall. Therefore, a hub-side slotted diffuser is utilized to mitigate the flow separations inside diffuser passages. Through application of the slotted diffuser, the stall margin is improved a lot without performance decline as a result of the improvement of the flow field within diffuser passages. The flow separation on the suction surface is suppressed by a vortex induced by the leakage flow. Additionally, the flow angle at the diffuser inlet becomes more circumferential owing to the leakage flow passing through the slot; thus, the incidence angle is increased, which results in mitigation of the flow separation at the junction of hub and pressure surface. As a result, a 13.5% stall margin increment is gained in the presence of the slotted diffuser vane.

Roles of vanes in diffuser on stability of centrifugal compressor

The stability considerations of centrifugal compressors become increasingly severe with the high pressure ratios, especially in aero-engines. Diffuser is the major subcomponent of centrifugal compressor, and its performance greatly influences the stability of compressor. This paper experimentally investigates the roles of vanes in diffuser on component instability and compression system instability. High pressure ratio centrifugal compressors with and without vanes in diffuser are tested and analyzed. Rig tests are carried out to obtain the compressor performance map. Dynamic pressure measurements and relevant Fourier analysis are performed to identify complex instability phenomena in the time domain and frequency domain, including rotating instability, stall, and surge. For component instability, vanes in diffuser are capable of suppressing the emergence of rotating stall in the diffuser at full speeds, but barely affect the characteristics of rotating instability in the impeller at low and middle speeds. For compression system instability, it is shown that the use of vanes in diffuser can effectively postpone the occurrence of compression system surge at full speeds. According to the experimental results and the one-dimensional flow theory, vanes in diffuser turn the diffuser pressure rise slope more negative and thus improve the stability of compressor stage, which means lower surge mass flow rate.

Preliminary design of centrifugal compressor using multidisciplinary optimization method

Centrifugal compressor is widely used in turbochargers in which the aerodynamic performance and strength are invariable among the important design objectives. As high pressure ratio centrifugal compressor develops, the interaction between multiple disciplines should be involved in the preliminary design process. A strength prediction method was presented and the prediction error was less than 3% compared with the 3D finite element calculation. The preliminary design method was established with consideration of multidisciplinary couplings. Then, a centrifugal compressor with the lowest pressure ratio of 4.4 was designed based on the method. The optimal results showed that the aerodynamic efficiency increases by 2.245% compared with the initial design results. Finally, the 3D validation was carried out including aerodynamic analysis and strength calculation, which showed good agreement with the optimal results of the preliminary design.

Design of a High Pressure Ratio Centrifugal Compressor

High pressure ratio centrifugal compressor has gained widely attention recently. In this paper, a pressure ratio 11 centrifugal compressor with an inlet mach number of about 1.6 is preliminary designed by computer. The complex shock boundary layer interaction, low energy region in the vicinity of the casing and the outlet flow conditions are analyzed. The vaneless and vaned diffusers are also designed. CFD results show that the vaned diffuser can effectively improve the compressor efficiency.

Roles and mechanisms of casing treatment on different scales of flow instability in high pressure ratio centrifugal compressors

The development of single-stage high pressure ratio centrifugal compressors for small gas turbine engines is hindered by compressor flow instabilities with various temporal and spatial scales. In this paper, the effect of a self-recirculation casing treatment device for suppressing various scales of flow instability has been discussed. Rig tests of a high pressure ratio centrifugal compressor with and without a casing treatment device are performed, and transient pressure signals are measured at endwalls by fast response pressure transducers. By analyzing measured pressure signals in both time and frequency domain, the casing treatment device is found effective in eliminating rotating instability and stall at the impeller inlet at low speeds, as well as suppressing surge of the compression system at middle and high speeds. The flow recirculation process generated inside the casing treatment device removes the vortical structures at the impeller inlet tip section, therefore eliminating the regional rotating instability and stall. Besides, the casing treatment device turns the pressure rise characteristics of the impeller more negative by enhancing the impeller work input, but turns that of the vaned diffuser more positive by increasing the incidence at the vaned diffuser inlet. Combining both effects, mild surge and the deep surge are suppressed.

Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser

Compressor stability is an essential issue for developing single-stage high pressure ratio centrifugal compressors. In this paper, a comprehensive stability map of a centrifugal compressor stage with the peak pressure ratio of 6.2 has been illustrated. Fast response pressure transducers are mounted inside the casing or the back-plate wall to measure internal transient behaviors. From Fourier analysis of pressure signals, various instabilities across time and length scales are identified. At low speeds, the impeller inlet rotating instability develops in scale and evolves into the inducer stall, which eventually induces the mild surge and deep surge of the compression system. At middle speeds, the compressor successively experiences stable state, mild surge, rotating instability, and deep surge. The mild surge region coincides with the dip region of the compressor S-shape pressure rise curve, and deep surge occurs when the compressor pressure rise raises to the second peak. At high speeds, mild surge and deep surge abruptly occur without preceding stall or rotating instability. To explain the complex surge behavior, the mechanical analogy between the compression system and the mass-spring-damper system is applied. Both mild surge and deep surge transients are found to belong to the dynamic instability of the mass-spring-damper system, and the exact form of the surge state will be determined by eigenvalues of the system governing equation.

The influence of wedge diffuser blade number and divergence angle on the performance of a high pressure ratio centrifugal compressor

Wedge diffuser is widely used in centrifugal compressors due to its high performance and compact size. This paper is aimed to research the influence of wedge diffuser blade number and divergence angle on centrifugal compressor performance. The impact of wedge diffuser blade number on compressor stage performance is investigated, and then the wedge diffusers with different divergence angle are studied by varying diffuser wedge angle and blade number simultaneously. It is found that wedge diffuser with 27 blades could have about 0.8% higher adiabatic efficiency and 0.14 higher total pressure ratio than the wedge diffuser with 19 blades and the best compressor performance is achieved when diffuser divergence angle is 8.3°.These results could give some advices on centrifugal compressor design.

High-pressure ratio centrifugal compressor with two different fishtail pipe diffuser configurations

To improve fishtail pipe diffuser outflow uniformity, deswirlers and splitters are added inside and after fishtail pipe diffuser passages for a compact centrifugal compressor. The influence of clocking effects between the deswirler and fishtail pipe diffuser passage is first studied to find the optimum clocking position. The performances of these two configurations are then compared, and their loss mechanisms are determined. Finally, the compressor exit flow conditions are estimated. Either adding the deswirler after the fishtail passages or adding splitters inside the fishtail passages tends to degrade the compressor performance. The intensity of the vortices in the fishtail passages is increased; therefore, losses are induced when a deswirler is added after the fishtail passage. As splitters are added inside the fishtail passages, flow separation is observed at the splitter suction side near the hub as a result of the large incidence angle near the splitter hub and results in large losses. Adding a deswirler after fishtail passages has a negative effect on the compressor outlet flow uniformity but can eliminate the residual swirl angle. Adding splitters in fishtail diffuser passages is recommended to decrease the compressor outlet non-uniformity and residual swirl angle. Through this work, physical insight into complex flows in these two configurations is obtained to provide guidelines on a diffusing system design with a fishtail pipe diffuser.

Investigation of a High Pressure Ratio Centrifugal Compressor with Wedge Diffuser and Pipe Diffuser

Abstract This present work is aimed at providing detailed understanding of the flow mechanisms in a highly loaded centrifugal compressor with different diffusers. Performance comparison between compressor stages with pipe diffuser and wedge diffuser was conducted by a validated flow solver. Stage with pipe diffuser achieved a better performance above 80 % rotating speed but a worse performance at lower rotating speeds near surge. Four operating points including the design point were analyzed in detail. The inherent diffuser leading edge of pipe diffuser could create a better operating condition for the downstream diffusion, which reduced the possibility of flow separation in discrete passages at design rotating speed. At 60 % rotating speed operating point, there was a large negative incidence angle. The sharp leading edge of pipe diffuser could largely accommodate this negative incidence as comparison of the round leading edge of wedge diffuser. As a result, a better performance was achieved in the pipe diffuser. At 60 % rotating speed near surge, performance of the pipe diffuser dropped below wedge diffuser. Total pressure loss of pipe diffuser exceeded that of the wedge diffuser due to the larger friction loss near wall at throat and ineffective static pressure recovery.

Effects of Radial Gap Ratio between Impeller and Vaned Diffuser on Performance of Centrifugal Compressors

A high-performance centrifugal compressor is needed for numerous industry applications nowadays. The radial gap ratio between the impeller and the diffuser vanes plays an important role in the improvement of the compressor performance. In this paper, the effects of the radial gap ratio on a high-pressure ratio centrifugal compressor are investigated using numerical simulations. The performance and the flow field are compared for six different radial gap ratios and five rotational speeds. The minimal radial gap ratio was 1.04 and the maximal was 1.14. Results showed that reducing the radial gap ratio decreases the choke mass flow rate. For the tip-speed Mach number (impeller inlet) with Mu < 1, the pressure recovery and the loss coefficients are not sensitive to the radial gap ratio. However, for Mu ≥ 1, the best radial gap ratio is 1.08 for the pressure recovery and the loss coefficients. Furthermore, the impeller pressure ratio and efficiency are reduced by increasing the radial gap ratio. Finally, the compressor efficiency was compared for different radial gap ratios. For Mu < 1, the radial gap ratio does not have noticeable effects. In comparison, the radial gap ratio of 1.08 has the best performance for Mu ≥ 1.

Comparison of Cooling Different Parts in a High Pressure Ratio Centrifugal Compressor

Cooling in a centrifugal compressor can improve the performance and reduce the impeller temperature. In a centrifugal compressor, external walls can be cool down, which is known as the shell cooling. This method avoids undesirable effects induced by other cooling methods. Cooling can be applied on different external walls, such as the shroud, diffuser or the back plate. This paper focuses on seeking the most effective cooling place to increase the performance and reduce the impeller temperature. It is found that shroud cooling improves the compressor performance the most. Shroud cooling with 2400 W of cooling power increases the pressure ratio by 4.6% and efficiency by 1.49%. Each 500 W increase in the shroud cooling power, increases the efficiency by 0.3%. Diffuser cooling and back plate cooling have an identical effect on the polytropic efficiency. However, back plate cooling increases the pressure ratio more than diffuser cooling. Furthermore, only back plate cooling reduces the impeller temperature, and with 2400 W of cooling power, the impeller temperature reduces by 45 K.

Area Schedule Based Design of High-Pressure Recovery Radial Diffusion Systems

High-pressure ratio centrifugal compressors require advanced diffusion systems to achieve enhanced efficiencies set by future turbocharger applications. To address the shortcomings of the commonly used channel diffuser and airfoil cascade design perspectives, a streamtube based area schedule is adopted paying special attention to the diffuser entry region. It is shown that the diffusion in the semivaneless space, controlled chiefly by inlet flow angle and the vane suction side geometry, plays a key role in improving diffuser performance. Removing excess thickness from the suction side eliminates flow overspeed, increases effective diffusion length, and leads to higher pressure recovery at reduced stagnation pressure loss. The pressure side thickness distribution controls the channel area schedule. Thin leading edges (LEs) ensure smooth flow area transition into the channel and reduce the vane upstream influence, mitigating high-cycle fatigue related mechanical issues.

Effect of internal heat leakage on the performance of a high pressure ratio centrifugal compressor

Centrifugal compressors are widely used in compact gas turbines in industrial and military applications where a high pressure ratio in small size is needed. The trend in centrifugal compressor is high pressure ratios and high efficiencies. However, higher pressure ratio increases the temperature difference between upstream flow and downstream flow which leads to a heat leakage from downstream to upstream through the solid parts. This heat leakage negatively affects the performance as well as the reliability of the compressor. In this study the effect of heat leakage through solid impeller and casing on the compressor performance has been studied for different pressure ratios. The compressor performance has been calculated using a three dimensional numerical model. Conjugate Heat Transfer (CHT) method has been used to calculate the temperature in the solid parts. The results show that the internal heat leakage through both the impeller and casing reduces the efficiency by 2.5% and the total pressure ratio by about 0.83 at pressure ratio up to 11. The effect of the internal heat transfer on the compressor performance is more noticeable at pressure ratios higher than 5. Meanwhile, heat transfer inside the solid impeller alone from the hot region to the cool region reduces the maximum impeller temperature while this heat leakage has a small effect on the compressor performance. However, heat leakage inside the casing alone slightly increases the impeller temperature while it strongly affects the compressor performance.