Absolute Flow Angle Research Articles

Numerical study of performance curve incline and instability flow phenomena in low flow rates according to absolute flow angle change at axial-flow pump inlet

This study predicted the performance of an inlet guide vane to be installed later by applying an absolute flow angle (ß) at the axial pump inlet. The pump’s performance was predicted based on changes in inlet ß, and the influence of variations in inlet ß on the low-flow area was analyzed. For turbulence flow analyses, the Reynolds-averaged Navier–Stokes equations were discretized based on the finite volume method. The internal flow was analyzed in detail through transient analysis. Furthermore, the impact of changes in the absolute flow angle at the inlet on the rotating stall occurring inside the pump in the low-flow-rate region was analyzed. The analysis of the internal flow field confirmed that the absolute flow angle at the inlet affects the stability of the internal flow. Results showed that energy reduction and efficient operation scenarios can be established compared with the existing valve control method by changing inlet ß. Through fast Fourier-transform analysis, it was confirmed that when the inlet angle (θ) is changed in the low-flow-rate region, a more stable operation is possible compared with the existing one.

Numerical investigation of energy dissipation and vortex characteristics on the S-shaped region of a reversible pump-turbine

In order to meet the regulation needs of power system, the pump-turbine is prone to operate in the S-shaped region during frequent start-up, leading to unit vibration and grid connection failure. In this paper, the S characteristic experiment was carried out, and the entropy production, unstable flow and vortex structure under turbine, runaway, turbine breaking, reverse pump conditions with optimum guide vane opening are detailed analyzed. The relationship between the energy dissipation and flow field characteristics is clarified. The results show that the runner entropy production is the largest in all components, and the wall entropy production is greater than the fluctuating entropy production under turbine condition. With the decrease in discharge, the fluctuating entropy production and wall entropy production first increase and then decrease, reaching the maximum values in the runaway condition. The circumferential movement trend of water enhances, and forms circumferential flow in the vaneless region. The separation vortex formed on the blade suction surface extends from the blade inlet to the middle of the flow channel, inducing a high entropy production. Under the turbine braking condition, the tangential velocity increases rapidly in the vaneless region. The circumferential flow develops into water ring, blocking the water flows into the runner. Under the reverse pump condition, the water flows in the opposite direction, with negative tangential and streamwise velocities. The absolute flow angle becomes obtuse, and a large-scale reverse vortex is formed. The runner inlet is blocked by the water ring, and the flow channel is almost completely filled with the low-speed region. The proportion of the guide vanes entropy production reaches maximum. Unstable flow leads to the increase in vorticity. Separation vortex, bypassed vortex, and reverse flow vortex leads to an increase in VST, forming a high VST region at corresponding positions. Due to the rotational motion accompanying the development of separated vortex, it leads to the increase in CFT. In addition, the circumferential flow and water ring have higher tangential velocities, which can lead to high CFT regions.

Non-uniform flow characteristics and rotating instability of a transonic high-pressure compressor rotor with cavity bleed

The presence of bleed in an aero engine’s compressor can significantly impact its flow characteristics and contribute to rotating instability. This study focuses on the impact of a typical cavity bleed structure on the internal flow characteristics of a compressor, specifically its circumferential non-uniformity and aerodynamic stability. A numerical simulation involving multiple flow passages was conducted on the transonic high-pressure rotor of the E3 compressor, considering its typical bleed structure. The study delves deep into the non-uniform flow characteristics and the mechanisms behind their generation in the compressor flow field. Furthermore, the influence of bleed on the rotating instability of the compressor is explored by comparing changes in compressor instability and adiabatic efficiency under uniform and non-uniform flow field conditions. The findings indicate that the axial position of the cavity bleed structure plays a crucial role in influencing key parameters such as rotor stall margin, peak efficiency, and total pressure ratio under near-stall conditions. The circumferential non-uniformity, resulting from the presence of the cavity bleed, intensifies with higher bleed air flow rates. For the upstream bleed configuration applied to the rotor, with a total bleed rate of 5%, the maximum variation in absolute flow angle at the inlet of different rotor channels can reach up to 1°. Additionally, the maximum difference in inlet flow coefficient can reach 0.0392. These findings demonstrate that the non-uniformity caused by the typical bleed structure leads to a loss in stall margin for the rotor when compared to a uniform flow field scheme.

Numerical study on the hydrodynamic performance and internal flow of an axial-flow pump with various inlet flow angles

In this study, numerical simulation was employed to predict the performance and internal flow characteristics of the inlet of an axial-flow pump by assigning an absolute flow angle to the inlet guide vane (IGV) trailing-edge flow. Further, the finite volume method based on the three-dimensional Reynolds-averaged Navier–Stokes equations was employed to discretize the governing equations. The shear stress transport model was used as the turbulence model, and an appropriate number of nodes were selected for the hexahedral grid system through a grid-dependency test. The performance curve and changes in the internal flow field were investigated based on the variation in the flow angle at the inlet of the axial-flow pump. These results can be used to establish an efficient operational plan by adjusting the IGV angle of IGV when installing a variable IGV for an axial-flow pump.

A New Influence Mechanism of Clocking Effect in Subsonic Compressor

This paper investigates the clocking effect in subsonic compressor element stages and the influence of design parameters on the flow mechanism. We focus on the relationship between the wake-induced separation loss and wake mixing loss and the unsteady mechanism in the wake flow process without considering the transition through several steady and unsteady numerical simulations aimed at a series of subsonic compressor element stages. The simulation results indicate that the performance difference at various indexing positions depends on the relationship between wake mixing loss and wake-induced separation loss for different compressor designs and operating conditions. Furthermore, the pressure transport caused by the negative jet of the Stator 0 wake in Rotor 1 creates a local acceleration region called SFAF, and a decrease in its absolute flow angle reduces the Stator 1 separation. Sufficient rim work of the rotor at highly loaded operating conditions is the basis for generating an effective SFAF. Furthermore, the fore-loading blade of Rotor 1 significantly reduces suction surface pressure drop, and a small angle between the stagger angles of Stator 0 and Rotor 1 increases the unsteady rotor load caused by the upstream wake to the total rotor load, both of which enhance SFAF.

A comparative study of mean-line models based on enthalpy loss and analysis of a cavity-structured radial turbine for solar hybrid microturbine applications

Lightweight components are essential for the future of the energy and aerospace fields. This paper presents an innovative radial turbine design that utilizes a cavity structure on the rotor for solar hybrid microturbines. The performance of the cavity-structured radial turbine is studied using mean-line models, computational fluid dynamics (CFD), and finite element analysis (FEA) simulations. The radial turbine is compared using air and supercritical carbon dioxide (sCO2) as the working fluid. The mean-line models based on enthalpy losses from recent literature are validated using commercial code Rital and CFD results. The impact of the isentropic velocity ratio (νs) and absolute flow angle at the rotor inlet (α1) on the optimum design for the rotor radial turbine is also demonstrated. A comparative loss enthalpy breakdown on the rotor is examined, such as incidence loss, passage loss, trailing edge loss, clearance loss, exit loss, and windage loss. The results show that the recent mean-line models based on enthalpy losses can accurately estimate the stage efficiency for both air and sCO2 turbines. By comparing the simulation results obtained from 3D-CFD and mean-line models, the efficiency prediction deviation is within an acceptable range of tolerance for both turbines when comparing the results obtained from 3D-CFD and mean-line models. The FE analysis reveals that the maximum stress decreased significantly for the air and sCO2 turbine after the rotor blade was added using the cavity structure inside. The maximum stress value on the rotor with the cavity structure design is still below the yield strength of the material limits. Furthermore, after the variations of the cavity structure design were added to the rotor blade of both turbines, the results were promising when the dangerous resonance at below 4EO (Engine Order) was minimized. The results of this study provide a promising direction for the development of high-efficiency and lightweight turbines in the energy and aerospace fields.

축류펌프의 입구 안내깃 모사에 따른 성능 및 내부 유동장에 관한 수치해석적 연구

In this study, an absolute flow angle was numerically assigned to the inlet of an axial pump in order to predict its performance and internal flow field prior to the design process of an inlet guide vane. The finite volume method, which is an approximate analysis method based on the three-dimensional Reynolds-averaged Navier-Stokes (RANS) equation, was applied for discretization of governing equations. The shear stress transport (SST) model was used as the turbulence model and the hexahedral grid system was selected as the appropriate number of nodes through the grid dependency test. The change in the performance curve and internal flow field was found with respect to the difference in the absolute flow angle at the impeller inlet. When installing a variable inlet guide vane of an axial flow pump, an efficient operation plan by adjusting the angle of an inlet guide vane might be established with the results of this study.

Influences of main design parameters on the aerodynamic performance of a micro-radial inflow turbine

The micro-radial inflow turbine (MRIT) is not only an important component of a micro-gas turbine but also the core part of a vehicle exhaust gas turbocharger and a small-scale organic Rankine circulatory system. Therefore, designing an MRIT with good performance is of great importance. The values of some main turbine design parameters can significantly influence its aerodynamic performance and structural size. In accordance with the structural characteristics of an MRIT, the relationships between the aerodynamic performance of a turbine and the absolute flow angle at the rotor inlet, the relative flow angle at the rotor outlet, the nozzle and rotor blade velocity coefficients, the ratio of the wheel diameter, velocity ratios, and the degree of reaction were examined. This was done through theoretical analysis combined with experience in the design of a conventional radial inflow turbine. On this basis, the influencing laws and degrees of the seven main design parameters on the aerodynamic performance of an MRIT during aerodynamic design were analyzed. The selection principles of these main design parameters were also summarized. The reasonableness of the above-mentioned selection principles was verified through an aerodynamic design case study of an MRIT combined with a 3D numerical simulation. It was found that the 3D numerical simulation results agreed well with the 1D aerodynamic design. The summarized selection principles of the main design parameters can provide a reference for the successful aerodynamic design of an MRIT.

On Choosing the Optimal Impeller Exit Velocity Triangles in Preliminary Design

Abstract Impeller discharge flow plays an important role in centrifugal compressor performance and operability for two reasons. First, it determines the work factor and relative diffusion for the impeller. Second, it sets the flow into the downstream stationary diffusion system. The choice made in the preliminary design phase for the impeller exit velocity triangle is crucial for a successful design. The state-of-the-art design approach for determining the impeller exit velocity triangle in the preliminary design phase relies on several empirical guidelines, i.e., maximum work factor and diffusion ratio for an impeller, the optimal range of absolute flow angle, etc. However, as modern compressors continue pushing toward higher efficiency and higher work factor, this design approach falls short in providing exact guidance for choosing an optimal impeller exit velocity triangles due to its empirical nature as well as the competing mechanism of the two trends. In light of this challenge, this paper introduces a reduced-dimension, deterministic approach for the design of the impeller exit velocity triangle. The method gauges the design of the impeller exit velocity triangle from a different design philosophy using a relative diffusion effectiveness parameter and is validated using six impeller designs, representative of applications in both turbochargers and aero engines. Furthermore, with the deterministic method in place, optimal impeller exit velocity triangles are explored over a broad design space, and a one-to-one mapping from a selection of impeller total-to-total pressure ratios and backsweep angles to a unique optimal impeller exit velocity triangle is provided. This new approach is demonstrated, and discussions regarding the influences of impeller total-to-total pressure ratio, isentropic efficiency, and backsweep angle on the optimal impeller exit velocity triangle are presented.

Correlation between the Internal Flow Pattern and the Blade Load Distribution of the Centrifugal Impeller

The blade load distributions reflect the working characteristics of centrifugal impellers, and the vortexes in the impeller channel affect the blade load distribution, but the mechanism of this phenomenon is still unclear. In this study, particle image velocimetry (PIV) was adopted to clarify the correlation between the internal flow pattern and the blade load distribution. The internal flow pattern and the blade load distribution were presented under different working conditions to study the influence of the internal flow pattern on the blade load. Results showed that the vortexes in the flow channel redistributed the blade load. The clockwise vortex made the position of the maximum blade load closer to the outlet, while the counterclockwise vortex had the opposite effect. Meanwhile, the vortexes caused the blade load distribution to be steeper, which reduced energy conversion efficiency. Moreover, the mean absolute flow angle was introduced to explain the mechanism of the effects of vortexes on blade load. The results can be used as a theoretical basis for the design of high-performance impellers.

Influence of geometric factors at runner outlet on the hump characteristics of a pump-turbine

In order to investigate the influence of geometric factors at runner outlet on the hump characteristic of pump-turbines, three different distribution plans of geometric factors at runner outlet are proposed and numerical simulated using the SST (shear stress transport) k-ω turbulence model. Numerical results show good agreement with the experimental data. Different distribution plans show large difference at the hump region and of which the plan with large radius and small outlet angle near the shroud can improve the hump safety margin from 4% to 5.82%. The causes are analyzed through the input energy (the Euler head) and the hydraulic loss respectively. The analysis results show that the best distribution plan can provide small absolute flow angle near the hub and the middle spanwise at the runner outlet, leading to the increase of the Euler head. Moreover, for the best distribution plan, the hydraulic loss distribute more evenly at part load operating points, resulting in the movement of the hump region to smaller discharge operating points and the increase of the hump safety margin. Detailed local entropy production rate (LEPR) analyses reveal that when the pump-turbine operates in hump region, the hydraulic loss is concentrate in vaneless region and show different variation rules in discharge varying direction.

Flow behavior in a contra-rotating transonic axial compressor

This study investigated a three-dimensional flow analysis on a two-stage contra-rotating axial compressor using the Navier–Stokes, continuity, and energy equations with Ansys CFX commercial software. In order to validate the obtained results, the absolute and relative flow angles curves for each rotor in radial direction were extracted and compared with the other investigation results, indicating good agreement. The compressor efficiency curve also was extracted by varying the compressor pressure ratio and compressor efficiency against mass flow rate. The flow results revealed that further distortion of the flow structure in the second rotor imposed a greater increase in the amount of entropy, especially at near-stall conditions. The increase of entropy in the second rotor is due to the interference of the tip leakage flow with the main flow which consequently caused more drops in the second rotor, suggesting that more efficacy of flow control methods occurred in the second rotor than in the first rotor.

Effects of Inlet Guide Vanes on the Performance and Stability of an Aeronautical Centrifugal Compressor

Abstract A research centrifugal compressor stage designed and built by Safran Helicopter Engines is tested at three inlet guide vanes (IGV) stagger angles. The compressor stage includes four blade rows: axial inlet guide vanes, a backswept splittered impeller, a splittered vaned radial diffuser (RD), and axial outlet guide vanes (OGVs). The methodology for calculating the performance is detailed, including the consideration of humidity in order to minimize errors related in particular to operating atmospheric conditions. The shift of the surge line toward lower mass flow rate as the IGV stagger angle increases highly depends on the rotation speed. The surge line shift is very small at low rotation speeds, whereas it significantly increases at high rotation speeds. A first-order stability analysis of the impeller and diffuser sub-components shows that the diffuser (resp. impeller) is the first unstable component at low (resp. high) rotation speeds. This situation is unaltered by increasing the IGV stagger angle. At low rotation speeds below a given mass flow rate, rotating instabilities (RI) at the impeller inlet are detected at zero IGV stagger angle. Their occurrence is conditioned by the relative flow angle at the tip of the leading edge of the impeller. As the IGV stagger angle increases, the mass flow decreases to maintain a given inlet flow angle. Therefore, the onset of the rotating instabilities is delayed toward lower mass flow rates. At high rotation speeds, the absolute flow angle at the diffuser inlet near surge decreases as the IGV stagger angle increases. As a result, the flow is highly alternate over two adjacent channels of the radial diffuser beyond the surge line at IGV stagger angle of 0 deg.

Numerical analysis of the formation mechanism and suppression method of the reverse flow in a semi-open centrifugal pump

Reverse flow has a detrimental effect on the stable and safe operation of centrifugal pumps. To study the formation mechanism and suppression of the reverse flow, a semi-open centrifugal pump with circumferential groove in the shroud was simulated. Then, the flow field and pressure fluctuation were analysed. The absolute flow angle at the blade inlet nearing the shroud was close to 180° because of the joint action of the leakage flow and blade inlet impact under low flow rate. This phenomenon resulted in the formation of a low-speed region and the reverse flow and low-frequency pressure fluctuation. The circumferential groove provided a channel for the leakage flow, which could quickly pass through the groove, and reduced the absolute flow angle at the blade inlet nearing the shroud and weakened the trend of the tip leakage flow to upstream. The low-frequency pressure pulsation was eliminated, and the amplitude of the blade passing frequency was reduced under 0.7 Qd (Qd is the design flow rate). The reverse flow thickness coefficient became zero with the circumferential groove. The proportion of the reverse flow volume to the volume of inlet pipe decreased from 14.7 % to 2.2 % under 0.4 Qd. This research indicated that the circumferential groove arranged in the shroud could effectively suppress or eliminate reverse flow.

Optimal design of dual-pressure turbine in OTEC system based on constructal theory

Based on constructal theory, the optimal design of a dual-pressure turbine in OTEC system is performed under the condition of fixed total volume of the turbines. The total power output of the turbines is chosen as the optimization objective, and the volume fraction, ratio of wheel diameter and relative flow angle at rotor outlet are employed as the optimization variables. The optimal performance and optimal constructs of the dual-pressure turbine with the same and different structural parameters are obtained, respectively. Besides, the influences of five parameters on the constructal optimization results of the dual-pressure turbines are analyzed. The results show that compared with the initial design point, the total power output of the turbines after primary, twice and triple constructal optimizations are increased by 0.69%, 1.82% and 2.02%, respectively. The optimal volume fraction, ratio of wheel diameter and relative flow angle at rotor outlet of the turbines after triple constructal optimization are 0.243, 0.49 and 28°, respectively. The total power output of the turbines will increase with the increases of the inlet pressure of the low-pressure turbine, mass flow rate ratio of the working fluids, total volume of the turbines and absolute flow angle at rotor inlet, and will decrease with the increase of the reaction degree. The constructal optimization results of the high-pressure and low-pressure turbines with the same and different structural parameters are similar. The maximum total power outputs of the two cases are both 50.38 kW, and the largest difference is that the optimal volume fraction is reduced by 4.33%. The obtained results can provide theoretical guidelines for the optimal designs of the dual-pressure turbines in OTEC systems.

Assessment of turbine performance under swirling inflow conditions

Under the pressure of increasingly stringent emission regulations, highly efficient turbocharger turbines are desired to achieve high performance of internal combustion engines. Turbines suffer from significant performance deterioration under realistic inlet conditions distorted with swirls, yet relevant researches are quite limited. Little understanding has been gained about how the inlet swirls influence the turbine performance.This paper investigated turbine performance under swirling inflow conditions through a numerical method. Results show that swirling inflow has a considerable effect on the turbine swallowing capacity and efficiency, and swirling intensity correlates with the efficiency reduction. A simple theoretical analysis reveals that swirling inflow causes high loss of the circumferential velocity in the turbine housing. Consequently, it leads to a reduction of the absolute flow angle at rotor inlet and a large deviation of the relative flow angle at rotor inlet from its optimum value, which are the main causes of the swirling flow effect on turbine swallowing capacity and efficiency respectively. Swirling direction has a remarkable effect on turbine loss mechanisms. Opposite swirl directions result in different vortex movements and deformation around the volute periphery, which leads to different circumferential flow distributions at rotor inlet and inserts distinguishing influence on rotor performance.

Influence of the volute cross-sectional shape on mixed inflow turbine performances

The volute is an essential element in the centrifugal machines. Improving its performance is an effective way to improve the total performance of the turbine. The purpose of this study is to replace the accelerating and guiding nozzle vanes by exploring different design possibilities on the cross-sectional area convergence of the volute, since a decreasing area is then associated with expansion in the subsonic regime. The work is extended to a mixed inflow turbine using the new volute cross sections under pulsating regimes for turbocharging. The numerical simulation results show larger accelerations [Formula: see text] and lesser losses in the case of sections with flatter area in the radial direction and without vaneless space between the volute and the rotor; but this combination has an effect on the exit absolute flow angle which is less uniform.

Experimental study of self-recirculating casing treatment in a subsonic axial flow compressor

Parametric studies of recirculating casing treatment were experimentally performed in a subsonic axial flow compressor. The recirculating casing treatment was parameterized with injector throat height, injection position, and circumferential coverage percentage. Eighteen recirculating casing treatments were tested to study the effects on compressor stability and on the compressor overall performance at three blade speeds. The profiles of recirculating casing treatment were optimized to minimize the losses generated by air recirculation. In the experiment, the stalling mass flow rate, total pressure ratio, and adiabatic efficiency of the compressor were measured to study the steady-state effects on the compressor performance of recirculating casing treatments, and static pressure disturbances on the casing wall were monitored to study the influence on stall dynamics. Results indicate that both the compressor stability and overall performance can be improved through recirculating casing treatment with appropriate geometrical parameters for all the test speeds. The influence on stall margin of one geometric parameter often depends on the choice of others, i.e. the interaction effects exist. In general, the recirculating casing treatment with a moderate injector throat and a large circumferential coverage is the optimal choice to enhance compressor stability. The injector of recirculating casing treatment should be placed upstream of the blade tip leading edge and the injector throat height should be lower than four times the rotor tip gap for the benefits of compressor efficiency. At 71% speed, the blade tip loading is decreased through recirculating casing treatment at the operating condition of near peak efficiency and increased near stall. Moreover, the outlet absolute flow angle is reduced in the tip region and enhanced at lower blade spans for both operating conditions. The stall inceptions are not changed with the implementation of recirculating casing treatment for all the test speeds, but the stall patterns are altered at 33% and 53% speeds, i.e. the stall with two cells is detected in the recirculating casing treatment compared with the solid casing with only one stall cell.

Non-axisymmetric flow characteristics in centrifugal compressor

The flow field distribution in centrifugal compressor is significantly affected by the non-axisymmetric geometry structure of the volute. The experimental and numerical simulation methods were adopted in this work to study the compressor flow field distribution with different flow conditions. The results show that the pressure distribution in volute is characterized by the circumferential non-uniform phenomenon and the pressure fluctuation on the high static pressure zone propagates reversely to upstream, which results in the non-axisymmetric flow inside the compressor. The non-uniform level of pressure distribution in large flow condition is higher than that in small flow condition, its effect on the upstream flow field is also stronger. Additionally, the non-uniform circumferential pressure distribution in volute brings the non-axisymmetric flow at impeller outlet. In different flow conditions, the circumferential variation of the absolute flow angle at impeller outlet is also different. Meanwhile, the non-axisymmetric flow characteristics in internal impeller can be also reflected by the distribution of the mass flow. The high static pressure region of the volute corresponds to the decrease of mass flow in upstream blade channel, while the low static pressure zone of the volute corresponds to the increase of the mass flow. In small flow condition, the mass flow difference in the blade channel is bigger than that in the large flow condition.

LDV measurements of the velocity field on the inlet section of a pumped storage equipped with a symmetrical suction elbow for variable discharge values

The storage pumps are equipped with various types of inlet casings. The flow nonuniformity is generated by the suction elbows being ingested by the impeller leading to unsteady phenomena and worse cavitational behaviour. A symmetrical suction elbow model corresponding to the double flux storage pump was manufactured and installed on the test rig in order to assess the flow field at the pump inlet. The experimental investigations are performed for 9 discharge values from 0.5 to 1.3 of nominal discharge. LDV measurements are performed on the annular section of the pump inlet in order to quantify the flow non-uniformity generated by the symmetrical suction elbow. Both axial and circumferential velocity components are simultaneously measured on the half plane (180°) of the annular inlet section along to 19 survey axis with 62 points on each. The flow field on the next half plane is determined tacking into account the symmetry. As a result, the flow map on the pump inlet annular section is reconstructed revealing a significant variation of the circumferential velocity component. The absolute flow angle is computed showing a significant variation of ±38°.