Static Pressure Recovery Coefficient Research Articles

Parametric optimization applied to design a high-performance vaneless-diffuser for CO2 centrifugal compressor

ABSTRACT Carbon-dioxide (CO2) centrifugal compressors are machines with high potential of usage in power generation plants and oil industry as it achieves high thermal efficiency in the Brayton cycles and contributes to oil production through the capture and storage of CO2 (CCS) in EOR (Enhanced Oil Recovery) systems. High levels of static pressure at the outlet are desired for EOR applications, which are usually obtained through the insertion of vanes in the diffuser. This work intends to increase vaneless-diffuser static pressure recovery by modifying only its meridional profile, ensuring a broader range of off-design operation when compared to vaned diffusers and attending to the fluctuations of mass flow and rotation expected in EOR practical applications. Therefore, a parametric optimization through surrogate model coupled to CFD was performed with three different objective functions that were submitted to single-optimization through the NSGA-II method: Maximize total-to-total polytropic efficiency, minimize total pressure loss coefficient or maximize static pressure recovery coefficient. Additionally, a sensitivity analysis was conducted using Morris Elementary Effects and SS-ANOVA. The results indicated that the optimized geometries increased the total-to-total polytropic efficiency by 2.9%, reduced the total pressure loss coefficient by 24.0% and increased the static pressure recovery coefficient at the design point by 11.4%, which is discussed in detail after a careful phenomenology assessment. The strategy adopted in the present work through a combination of Sensitivity Analysis, surrogate models and CFD increased the vaneless-diffuser static pressure recovery without the need of inserting vanes in the diffuser, which avoid instabilities in the equipment and would restrict its range of off-design operation.

Investigations on the Splitter Structure to Improve the Aerodynamic Performance of Gas Turbine Exhaust Diffuser at Different Swirl Angles

Abstract To weaken the flow separation in the exhaust diffuser under real operating conditions and increase the static pressure recovery coefficient of the exhaust diffuser, an exhaust diffuser structure with splitters was proposed. Based on verifying the reliability of the numerical method, the paper numerically explores the influence of the height and width of the splitter on the aerodynamic performance of the exhaust diffuser under three different inlet swirl angles by solving three-dimensional Reynolds-Averaged Navier–Stokes (RANS). The results show that the introduction of the splitter structure into the exhaust diffuser can effectively suppress the separation vortex near the strut, improve the flow state at the exhaust diffuser outlet, and further increase the static pressure recovery coefficient of the exhaust diffuser. The optimal splitter structure can be obtained by comparing the effects of the splitter height and width on the static pressure recovery performance of the exhaust diffuser under different inlet swirl angles. When the inlet swirl angles are 16 degs, 28 degs, and 38 degs, compared with the original exhaust diffuser without splitters, the optimal splitter structure can increase the static pressure recovery coefficient of the exhaust diffuser by 0.036, 0.096, and 0.146, respectively, with a relative increase of 7.3%, 34.2%, and 76.2%. The introduction of the splitter structure helps to improve the aerodynamic performance of the exhaust diffuser.

Investigations on the Aerodynamic Performance of a Gas Turbine Exhaust Diffuser With the Modified Shell Profile

Abstract In this paper, a shell profile design method based on diffuser flow area curve is proposed to improve the static pressure recovery coefficient of the exhaust diffuser over a range of inlet swirl angles. Based on verifying the reliability of the numerical method, the paper first explored the aerodynamic performance of the exhaust diffuser with the original structure and the modified structure by solving three-dimensional Reynolds-Averaged Navier–Stokes. Numerical results well capture the pressure distribution and flow characteristics in the exhaust diffuser. The numerical results show that the shell profile modification method effectively improves the aerodynamic performance of the exhaust diffuser. Compared with the original exhaust diffuser, the secondary vortex near the outer shell of the modified exhaust diffuser is weakened and the axial reverse pressure gradient near the strut is reduced. The static pressure recovery coefficient of the modified exhaust diffuser is significantly increased under different inlet swirl angles. Especially, under the normal working condition with a medium swirl angle (22 deg), the static pressure recovery coefficient increases by 0.034 with a relative increase of 7.6%. Under the extreme working conditions of a large swirl angle (35 deg), the static pressure recovery coefficient increases by 0.074 with a relative increase of 39%. A test rig for measuring the aerodynamic performance of the exhaust diffuser was built, and the validity of the shell profile design method is further verified through experimental measurements.

Performance Enhancement of a Centrifugal Compressor by Designing a Tandem Cascade Diffuser

The performance of a vane diffuser plays an important role in transforming the high kinetic energy into pressure energy at the impeller outlet of a centrifugal compressor. In this study, the performance enhancement of a centrifugal compressor is achieved by designing a tandem cascade diffuser. An optimum value of the total bending angle of a tandem cascade diffuser is obtained through numerical simulation. The total bending angle is defined as the angle between the tangent line of the front blade arc line at the leading edge point and the tangent line of the rear blade arc line at the trailing edge point. The range of the total bending angle increases from zero to 21°. The simulation results show that the variation of the total bending angle has a great impact on the performances of the compressor stage. The best performance is achieved by the 10° model, by which the minimum total pressure loss coefficient and the maximum static pressure recovery coefficient are obtained. The mechanism of performance improvement by choosing a reasonable total bending angle is that the separated flow zone in the diffuser is constrained and the distribution of radial velocity at the inlet and outlet of the diffuser is more uniformed. With the 10° model, the separated zone almost completely disappears in the diffuser and a distribution of more uniform radial velocity along the meridional section is obtained. Compared with the stage of the prototype with a single row of vane diffuser, the stage with the newly designed tandem vane diffuser (10° model) achieved performance improvement, the efficiency increased about 6%, and the total pressure ratio increased about 3.5% at the flow coefficient of 0.15.

Effects of the number of blades on impeller-volute interaction and flow instability of a centrifugal pump

Effects of the number of blades on the impeller-volute interaction and flow instability were studied for a centrifugal pump. The hydraulic performance and time-dependent flow field were analyzed by using the unsteady three-dimensional Reynolds-averaged Navier-Stokes equations with the k-ω based shear stress transport turbulence model. The grid dependence and temporal resolution were tested to evaluate the numerical uncertainties, and the numerical results were validated using experimental data. As the performance parameters, the stage total-to-static head coefficient, the impeller total-to-static head coefficient, and the volute static pressure recovery coefficient were selected to classify the cause of the impeller-volute interaction and flow instability observed inside components. Using the fast Fourier transform, fluctuations of the performance parameters were divided into three categories: the blade passing frequency and its harmonic frequencies, the fluctuations in the broadband frequency region, and the fluctuations in the low-frequency region. The results showed that the fluctuations of the performance parameters and local pressure were affected in various ways by the number of blades. The time-dependent flow structure was investigated to study the root causes of the differences in the impeller-volute interaction and flow instability by the number of blades.

Experimental and Numerical Investigation of Clearance Effects in a Transonic Centrifugal Compressor

Extensive research studying tip clearance effects on compressor performance has been conducted during the past few decades with significant progress achieved. However, most of the previous investigations focus on stage-level analysis at the design condition, with very limited studies at the component level or part speed conditions. To fill this gap, this paper investigates the effects of impeller tip clearance on the performance of a high-speed centrifugal compressor, at both stage and component levels, covering a variety of operating speeds resulting in subsonic to transonic inlet conditions. Experiments were conducted on the Single Stage Centrifugal Compressor facility at two different running tip clearances. With increased clearance, the stage and impeller pressure ratio, isentropic efficiency, and work factor drop, but the static pressure recovery coefficient in the diffuser increases. As the compressor transitions from subsonic to transonic operation from 60 to 85% corrected speed, the sensitivities of stage and impeller pressure ratio and efficiency reach their peak values. Lastly, numerical simulations at design speed showed that the computational fluid dynamics (CFD) model sufficiently predicts the sensitivities due to tip clearance for most of the parameters at the stage and component levels, indicating the potential of using CFD to investigate the flow physics.

Optimization of Sweep and Blade Lean for Diffuser to Suppress Hub Corner Vortex in Multistage Pump

The bowl diffuser is the main flow component in multistage submersible pumps; however, secondary flow fields can easily induce a separation vortex in the hub corner region of the bowl diffuser during normal operation. To explore the flow mechanism of the hub corner separation vortex and develop a method for suppressing hub corner separation vortices, the lean and sweep of the diffuser blade were optimized using computational fluid dynamics (CFD) simulations and central composite design. Diffuser efficiency, static pressure recovery coefficient, and non-uniformity were selected as the optimization objectives. Details of the internal flow were revealed and the collaborative response relationships between blade lean/sweep parameter equations and optimization objectives were established. The optimization results show that a greater pressure difference between the pressure surface and suction surface (PS–SS) at the inlet can offset transverse secondary flow, whereas a lower PS–SS pressure difference will cause a drop in low-energy fluid in the diffuser mid-section. The blade’s lean scheme suppresses the hub corner separation vortex, leading to an increase in pressure recovery and diffuser efficiency. Moreover, optimizing the sweep scheme can reduce the shroud–hub pressure difference at the inlet to offset spanwise secondary flow and enhance the hub–shroud pressure difference at the outlet, thus driving low-energy fluid further downstream. The sweep scheme suppresses the hub corner vortex, with a resulting drop in non-uniformity of 13.1%. Therefore, optimization of the diffuser blade’s lean and sweep can result in less low-energy fluid or drive it further away from hub, thereby suppressing the hub corner vortex and improving hydraulic performance. The outcomes of this work are relevant to the advanced design of bowl diffusers for multistage submersible pumps.

An Investigation of the Tapered Strut on Aerodynamic Performance of the Exhaust Diffuser Under Different Swirls

Abstract The strut structure directly affects the flow-field characteristics and aerodynamic performance of the gas turbine exhaust diffuser. Based on verifying the reliability of the numerical method, the exhaust diffuser with different struts was numerically calculated by solving three-dimensional Reynolds-averaged Navier–Stokes. The flow process and flow loss mechanism in the diffuser were analyzed, the influence of two different structures of tapered struts on the aerodynamic performance of the exhaust diffuser under different inlet preswirls was explored, and the aerodynamic performance of the exhaust diffuser with tapered struts was compared with a conventional exhaust diffuser with linear struts. The results show that, compared with the conventional linear strut, under different inlet preswirls, two different tapered struts can both weaken the flow separation in the exhaust diffuser, thereby reducing the total pressure loss. When the inlet preswirl is greater than 0.35, the total pressure loss coefficient of the exhaust diffuser with structure-C tapered struts decreases by up to 0.07. The two types of tapered struts also change the flow structure at the exhaust diffuser outlet, which affects the uniformity of the outlet airflow, and then affect the static pressure recovery coefficient. Under different inlet preswirls, two types of tapered struts can be effective to increase the static pressure recovery coefficient of the exhaust diffuser, for the exhaust diffuser with structure-C tapered struts, the static pressure recovery coefficient can be increased by up to 0.065, relative increase of 20%. The research in this paper shows that the tapered structure can significantly improve the aerodynamic performance of the exhaust diffuser under different inlet preswirls.

Effect of swirl flow on the performance of parallel hub axial annular diffuser

In the present work, the parallel hub axial flow annular diffuser's performance characteristics with divergent casing varying between equivalent cone angle (10°, 15°, and 20°) with area ratio 3 have been evaluated computationally as well as experimentally. The performance of three diffusers were tested at different inlet swirl angles (from 0° to 25°) for swirling and non-swirling flow. Simulations have been carried out on a fully developed flow at Reynolds number 2.5×105. The results were analyzed based on the velocity profiles, static pressure recovery coefficient, and the total pressure loss coefficient. The result analysis shows that the inlet swirl flow improves the recovery of pressure and also delays the flow separation on the casing. Moreover,the findings also show that the best performance was achieved in equivalent cone angle 10° at the inlet swirl angle of 7.5° compared to other diffusers.

Effects of struts profiles and skewed angles on the aerodynamic performance of gas turbine exhaust diffuser

The strut structure directly affects the flow field characteristics and aerodynamic performance of the gas turbine exhaust diffuser. The effects of the strut profiles and strut skewed angles on the aerodynamic performance of the exhaust diffuser at different inlet pre-swirls were numerically investigated using three-dimensional Reynolds-Averaged Navier-Stokes(RANS) and Realizable k-ε turbulence model. The numerical static pressure recovery coefficient of the exhaust diffuser is in agreement with the experimental data well. The reliability of the numerical method for the exhaust diffuser performance analysis was demonstrated. Exhaust diffusers with four kinds of vertical strut profiles obtain the highest static pressure recovery coefficient at the inlet pre-swirl of 0.35. The similar static pressure recovery coefficient of exhaust diffusers with four kinds of vertical strut airfoils are observed when the inlet pre-swirl is less than 0.48. The static pressure recovery coefficient of exhaust diffusers with vertical b1 and b2 struts are higher than that with the a1 and a2 struts when the inlet pre-swirl is greater than 0.48. At the inlet pre-swirl of 0.35, The static pressure recovery coefficient of the exhaust diffuser with the a1 strut decreases with the increasing of the strut skewed angles. The static pressure recovery coefficient of the exhaust diffuser with the b1 strut increases with the increasing of the strut skewed angles, and the static pressure recovery coefficient increases by 3.6% compared with the vertical design when the skewed angle of b1 strut is 40[Formula: see text]. At the inlet pre-swirl of 0.64. The static pressure recovery coefficient of the exhaust diffuser with the a1 strut increases by 8.7% compared with the vertical design when the skewed angle of a1 strut is greater than 20°. In addition, the static pressure recovery coefficient of the exhaust diffuser with the b1 strut decreases by 3.8% compared with the vertical design when the skewed angle of b1 strut is 40°. The method to improve the aerodynamic performance of the exhaust diffuser by appropriate increase the strut maximum thickness and design the strut skewed angle is proposed in this work.

Investigation of entropy generation, efficiency, static and ideal pressure recovery coefficient in curved annular diffusers

The annular diffuser is used mostly in turbomachines for recovery of the discharge kinetic energy and increasing the total pressure. The performance of diffuser is strongly dependent on the geometry and inlet conditions. Therefore, the optimum design of the diffuser shape is highly important. On the other hand, it seems necessary to work on entropy generation in different geometries of diffusers to reach the maximum efficiency. In the present study, the turbulent flow in curved annular diffuser is numerically investigated for different area ratios (AR = 1.74, 2 and 2.3) and different turning angles (θ = 60°, 80° and 90°). Reynolds number based on mass averaged flow at the inlet is 136,822. In addition, the entropy generation rate due to turbulent and viscous dissipation, total entropy, volumetric entropy generation, static pressure recovery coefficient, ideal pressure recovery coefficient and diffuser effectiveness are studied. The results show that in order to design the curved annular diffusers to reach the maximum efficiency and minimum entropy generation in air-conditioning systems, the lowest value of area ratio and highest value of turning angle should be considered, respectively. Moreover, it was obtained that at constant area ratio, the entropy due to viscous dissipation and total entropy decrease with increasing the turning angle, while the diffuser effectiveness and static pressure recovery coefficient increase with increasing the turning angle. Also, by increasing the area ratio at constant turning angle, the volumetric entropy generation, total entropy and entropy due to turbulent dissipation increase.

Effects of casing angle on the performance of parallel hub axial annular diffuser

AbstractIn this paper, the effects of non-swirling and swirling flow on the performance of parallel hub axial annular diffuser has been investigated. The study was conducted on a fully developed swirling flow and non-swirling flow to predict the separation of the flow from the wall. Three different annular diffusers were used with casing wall angles of 3°, 6°, and 9°. Furthermore, various swirl angles (0–25°) at the inlet of diffusers have been investigated to analyze the performance across the length. It was found that parallel hub axial annular diffuser performance increases up to a certain length as the inlet swirl angle increases. However, the performance also improves as the diffuser area ratio (AR) increases. The performance is evaluated based on the static pressure recovery coefficient (Cp) and the total pressure loss coefficient (CTL). The highest possible pressure recovery is achieved by the 12° swirl angle with a casing angle of 6°.

Effects of casing angle on the performance of parallel hub axial annular diffuser

Abstract In this paper, the effects of non-swirling and swirling flow on the performance of parallel hub axial annular diffuser has been investigated. The study was conducted on a fully developed swirling flow and non-swirling flow to predict the separation of the flow from the wall. Three different annular diffusers were used with casing wall angles of 3°, 6°, and 9°. Furthermore, various swirl angles (0–25°) at the inlet of diffusers have been investigated to analyze the performance across the length. It was found that parallel hub axial annular diffuser performance increases up to a certain length as the inlet swirl angle increases. However, the performance also improves as the diffuser area ratio (AR) increases. The performance is evaluated based on the static pressure recovery coefficient (Cp) and the total pressure loss coefficient (CTL). The highest possible pressure recovery is achieved by the 12° swirl angle with a casing angle of 6°.

Relationship Between Flow Instability and Performance of a Centrifugal Pump With a Volute

Abstract Flow instability and its correlations with performance characteristics were investigated for a centrifugal pump with a volute. Unsteady three-dimensional Reynolds-averaged Navier–Stokes analysis was performed to analyze the flow and performance characteristics using the shear stress transport (SST) turbulence model. The grid dependence and temporal resolution were tested to evaluate the numerical uncertainties, and the numerical solutions were validated using experimental data. The total-to-static head coefficient, the impeller's total-to-static head coefficient, and the volute static pressure recovery coefficient were selected as performance parameters. To identify the flow instability, pressure fluctuations were monitored upstream of the impeller, at the volute inlet, and on the shroud wall of the impeller. Three different types of flow instability were detected in partial-load conditions: inside the volute, upstream of the impeller, and at the interface between the impeller and volute. The time-dependent flow structures were investigated to obtain insight into the onset of the flow instability. The correlation of the onset of the flow instability with the performance curves was discussed.

Aerodynamic performance change of exhaust passage in steam turbine under low-load conditions

The exhaust passage is a passage which connect the exhaust port of steam turbine to the inlet of condenser. With the increase of new energy power generation, the coal-fired power units of greater capacity are demanded to participate in peak load regulation, so the steam turbine usually operates under low-load conditions. However, there are lack of literatures that comprehensive analysis aerodynamic performance of exhaust passage under low-load conditions. So in this paper, an exhaust passage coupling with the whole last-stage was numerically simulated by CFX software. The k-ω SST turbulence model and phase transformation model considering the spontaneous condensation flow of wet steam were used. The results show that the inlet swirl flow intensity of exhaust passage increases with the decrease of load. At the 10% turbine heat acceptance(THA) condition, the swirl angle which is more than 90° hold the range of 0% - 64% in blade height. Under low-load conditions, there are six different types of reverse vortices increasing the energy loss of exhaust passage. The total pressure drop of exhaust passage reaches to the maximum of 1671.24 Pa at 10% THA condition. Below the 50%THA condition, the static pressure recovery coefficient cannot be used as the indicator for evaluating the aerodynamic performance of exhaust passage.

Unsteady flow control of a plane diffuser based on a Karman-vortex generator

Implicit large-eddy simulation is employed to simulate the flow in an asymmetric plane diffuser at Re=9000. Flow separation exists near the throat and evolves to large-scale, unsteady separation in the expansion section and the downstream region. An unconventional flow control method, namely, a cylindrical Karman-vortex generator (KVG) with different sizes and locations that induces periodic spanwise vortex shedding, is set upstream of the throat to suppress the flow separation. An appropriately designed KVG can enhance the mixing of the outer flow and the low energy fluid near the wall region by the periodic shedding Karman-vortices, and effectively reduce the separation bubble size. For the present optimal case, the length and height of the separation bubble are decreased 50.4% and 90.9%, respectively. The static pressure recovery coefficient is also increased by about 50%. Moreover, the velocity and total pressure distributions at the end of the expansion section are more uniform with lower fluctuation in the case with KVG installed. An optimal KVG diameter DK is suggested to be 3–4% of the expansion section length LE. The gap ratio to the lower wall G/DK and the length ratio to the throat Lt/DK are suggested to be 2.0–3.0 and 5.0–10.0, respectively.

Numerical Analysis of Aerodynamic Characteristics of Exhaust Passage with Consideration of Wet Steam Effect in a Supercritical Steam Turbine

To investigate the aerodynamic performance of exhaust passage under multi-phase flow, an actual case is conducted in the low-pressure double exhaust passages of 600 MW steam turbine. Then, the flow field is compared and analyzed with and without the built-in extraction pipelines based on the Eulerian–Eulerian homogenous medium multiphase method. Results show that the upstream swirling flow and downstream mixed swirling flow are the main causes to induce the entropy-increase in the exhaust passage. Moreover, the flow loss and static-pressure recovery ability in the exhaust hood are greater than those in the condenser neck. Compared with the flow field without the steam extraction pipelines, the entropy-increase increases, the static pressure recovery coefficient decreases, and the spontaneous condensation rates of wet steam decrease in the downstream area of the pipelines. With the increase of steam turbine loads, an increment in entropy-increase in the exhaust passage is 0.98 J/(kg·K) lower than that without steam extraction pipelines. Moreover, the incrementing range of uniformity coefficient is increased from 14.5% to 40.9% at the condenser neck outlet. It can be concluded that the built-in exhaustion pipeline can improve the aerodynamic performance of exhaust passage and better reflect the real state of the flow field. These research results can serve as a reference for turbine passage design.

A Comparison of Circular and Slotted Synthetic Jets for Flow Control in a Twin Air Intake

The performance of an aircraft engine depends on air flow quality at the engine face / the exit of the air-intake also known as aerodynamic inlet plane (AIP). A single-engine aircraft has complex Y-shaped twin air-intake which causes severe flow separation, distortion and flow non-uniformity at the AIP. The present study compares the efficacy of slotted synthetic jet and a row of four circular synthetic jets attached to inner faces of a twin air-intake to improve aerodynamic performance at the AIP. The results are obtained using computational fluid dynamics. The velocity and vorticity plots show that lateral spread of the circular jets is limited as compared to the slotted jet. The circular jets are found to be weak as compared to slotted jet to prevent separation of main flow occurring in the twin air-intake. The various aerodynamic performance parameters, such as static pressure recovery coefficient, total pressure loss coefficient, distortion coefficient and secondary flow uniformity are compared for both the cases, exhibiting marked improvement in all these parameters. The study demonstrates that the slotted synthetic jets is a better option for controlling flow in twin air-intake as compared to a row of circular synthetic jets.

Outlet Surface Area Influence in Spiral Casing Design on Centrifugal Fan Performance

In this paper, the effect of the outlet surface area of the spiral casing on the performance of a centrifugal fan was investigated using open source CFD software OpenFOAM [1]. An automized loop with RANS and data post-processing is set up using Matlab, for allowing a large number of parameter variations. The effect was analyzed as a function of total pressure loss and static pressure recovery coefficient and on total efficiency as well.

Outlet Surface Area Influence in Spiral Casing Design on Centrifugal Fan Performance

In this paper, the effect of the outlet surface area of the spiral casing on the performance of a centrifugal fan was investigated using open source CFD software OpenFOAM [1]. An automized loop with RANS and data post-processing is set up using Matlab, for allowing a large number of parameter variations. The effect was analyzed as a function of total pressure loss and static pressure recovery coefficient and on total efficiency as well.