Inlet Flow Field Research Articles

Turbulence Measurements at the Inlet of an Automotive Turbocharger Compressor Using Stereoscopic Particle Image Velocimetry

Abstract The present work uses stereoscopic particle image velocimetry (SPIV) to analyze the compressor inlet flow field, with specific emphasis on its turbulence characteristics during flow reversal in order to gain further insight into the inlet flow structures. SPIV experiments were carried out at the inlet of a centrifugal compressor without any recirculation channel at four different rotational speeds (from 80 to 140 krpm) and over the entire mass flow range (from choke to surge) at each speed. Detailed analyses have been carried out for the mean velocity field, the mean vorticity field, and the turbulent statistics including turbulent kinetic energy, Reynolds stress, and the one-dimensional energy spectra. The turbulent kinetic energy at the compressor inlet was observed to increase rapidly along a speed line with decreasing mass flowrate once flow reversal started, and the turbulence became more anisotropic. As the flowrate was reduced (along a speed line), the zone with maximum turbulent kinetic energy moved from the periphery toward the center of the inlet duct and also occurred further upstream from the impeller. The Reynolds stress distributions suggest that the Boussinesq assumption of an isotropic eddy viscosity may not be appropriate after the detection of flow reversal. The Reynolds shear stresses were observed to change signs with their corresponding velocity gradients at the tested mass flowrates at different rotational speeds. At the investigated flowrates, the radial gradients in the axial and tangential velocities were found to be most dominant.

Three-dimensional CFD investigation of a scramjet inlet under different freestream conditions

Scramjet (supersonic combustion ramjet) engines are often seen as a promising alternative to place payloads in the Earth's orbit. Such air-breathing engines have a simple structure and few moving parts. On the other hand, high heat fluxes and pressure loads on the walls, shock-wave-boundary-layer interactions, and the risk of choked flow inside the isolator channel are a few examples of obstacles that need to be addressed during scramjet design. Therefore, this study aims to evaluate of different air freestream conditions on the flow field in a scramjet inlet (compression ramps and isolator) through detailed three-dimensional computational fluid dynamics (CFD) simulations. The Mach number, flight altitude, and angle of attack were the evaluated conditions. Moreover, this study also focuses on the behavior of the boundary layer separation located at the compression ramp corners, isolator entrance, and scramjet sidewalls. Regarding the Mach number variation, the results showed that high Mach numbers yielded high-pressure levels throughout the engine. Furthermore, a higher altitude promoted a lower total pressure field. Considering the angle of attack changes, it is evident that a higher angle of attack results in a decrease in airflow pressure, while an increase in the total pressure along the walls is observed. By investigating these freestream parameters, this work can contribute to the early phase of engine design, avoiding critical failures in the scramjet structure due to aerodynamic load and thermal stress.

Valve Plate Structural Optimal Design and Flow Field Analysis for the Aviation Bidirectional Three-Port Piston Pump

This paper designed and optimized a bidirectional three-port valve plate structure for solving the matching problem of flow rate and pressure in the aerospace pump-controlled differential hydraulic cylinder. This design aims to make the valve plate work well under the bidirectional high-speed condition. The model was set up using dynamic mesh and sliding mesh, and the simulation is conducted by FLUENT. In addition, the flow field of inlet and outlet flow rate pulsations, pressure pulsation in cylinder, and non-dead-point transition zone of four cases are analyzed to optimize the valve plate in this work. The numerical results show that different angles of non-dead-point transition zones of the valve plate have a big impact on the performance of the piston pump. For example, the flow rate pulsation reaches the minimum when the angle of non-dead point transition zone is greater than or equal to the angle of a cylinder port. However, at this time, the closed compression would occur and the pressure inside the cylinder would rise rapidly as the piston moves to the non-dead point zone, thus resulting in serious pressure overshoot. In addition, if the angle of non-dead point transition zone is reduced within a certain range, the pressure overshoot will be reduced drastically, and the flow pulsation rate will rise a bit. The study suggests that it is necessary to adjust the angle of non-dead point transition zone to balance the pressure overshoot and flow pulsation of the pump to obtain the optimal kidney structure of the valve plate.

Influence of turbulent inlet conditions on the flow inside a bulb turbine draft tube using Large-Eddy Simulations

The head losses inside the draft tube of a bulb turbine can represent an important portion of the total energy losses due to the low heads at which these machines operate. However, the complexity of the flow makes it a numerical challenge to simulate. Previous studies have shown that Large Eddy Simulations (LES) based on mean inlet conditions improve flow prediction inside the draft tube’s cone but diverge further downstream [4]. Moreover, uncertainties and the lack of detailed experimental data at the inlet have been identified as the main reason for these discrepancies and precise measurements close to the walls are necessary to correctly validate the numerical simulations [8, 4]. Thanks to detailed experimental measurements, the objective of this paper is to enhance the accuracy of previous results by performing LES computations to numerically simulate the flow inside the draft tube of a bulb turbine, and to investigate the influence of inlet conditions using an innovative approach. A two-criteria based mesh adaptation [14] along with an element-wise masking strategy is used to assure a good spatial discretization level while reducing the computational cost. Partial experimental data imposed at the inlet are often not sufficient to achieve a proper downstream flow prediction with a LES. The real challenge thus consists in the economical generation of proper mean and fluctuating inlet flow fields. We first show that a simple homogeneous and isotropic synthetic turbulence field added to the mean experimental profiles may improve the prediction of the downstream flow, but this is only achieved through empirical adjustments. Therefore, we also investigate the use of machine learning procedure to automatically generate proper inlet mean and fluctuating fields.

Optimization of the Hollow Rectification Sill in the Forebay of the Pump Station Based on the PSO-GP Collaborative Algorithm

The layout of the pump station is easily affected by topography, site, and other factors, resulting in poor inlet flow patterns in the forebay, which seriously affect the normal operation of the pump station. To optimize its inlet flow pattern, the size of the hollow rectification sill has been continuously improved through physical model tests to meet the requirements of the required pump station inlet flow field. In this paper, particle swarm optimization (PSO) was combined with the Gaussian process (GP) to establish a particle swarm-Gaussian process (PSO-GP) model to predict the velocity uniformity of the inlet sump of pump stations with different hole-to-height ratios, hole-to-width ratios, upper-to-lower sill length ratios, and sill height-to-water table ratios. Finally, the hollow rectification sill with the optimal size was obtained and tested in the physical model to compare the rectification effect with other sizes of hollow sills. The results show that the algorithm model can help the traditional physical experiment quickly predict the velocity uniformity of the inlet sump of the pump station. Through the optimization by the PSO-GP algorithm, we can get the optimal size of the hollow rectification sill. Its hole-to-height ratio is 0.62, its hole-to-width ratio is 0.37, upper-to-lower sill length ratio is 0.63, and sill height-to-water table ratio is 0.23. It shows that this method is practical in the optimization design of the hollow rectification sill and provides a new method for the optimization of the flow field in the forebay of the pump station.

Buzz characteristics and separation bubble dynamics in supersonic intake

The flow field in an axisymmetric supersonic mixed-compression inlet is numerically solved to understand the separation bubble dynamics at different freestream Mach numbers of M∞ = 2.0, 3.0, 5.0, and throttling ratios of ψ = 0.515, 0.40, and 0.35. The two-dimensional compressible Reynolds Averaged Navier Stokes equations were solved in a finite volume framework. The computational results were validated with the wind tunnel experimental data for steady as well as unsteady operating conditions. Throttling conditions are simulated by varying the exit area of the model in the form of plug inserts. The analysis of the flow features in the complete computation domain indicates the formation of multiple separation bubbles near the exit plug as well as near the isolator entrance of the inlet. The frequency of unsteadiness (f) associated with the ramp surface pressure decreases with a decrease in throttling ratio ψ. The possibility of a common origin that is regardless of any buzz criteria is analyzed by considering separation and reattachment zones in the flow field in terms of wall shear stress variation. Inside the supersonic intake, the multiple shock waves interact with the wall boundary layer that can lead to a large separation bubble. This bubble is always unstable inside the intake flow passage and can generate the source of instability. The propagation of a pressure disturbance is analyzed by the temporal variation of the pressure on the ramp wall. The cross-correlation coefficient Cij(t) of the pressure signal on the ramp surface suggests the upstream propagation of the disturbance generated from the separation zone near the exit plug. The numerical Schlieren provides further information on the formation and propagation of the separation bubble near the exit plug.

Modelling and dynamic mode analysis of compressor impeller spike-type stall with global stability approach

Flow instability associated with spike-stall restricts the development of advanced aero-engines with a high-speed centrifugal compressor. However, given the irregular structure of centrifugal compressor, few approaches can be applied to stability analysis. Considering the typical features of spike-stall, an implement of stability analysis method on the flow field of impeller inlet was proposed innovatively. Thus, in-depth comprehension of stall perturbation structure and accurate prediction of stall perturbation propagation can be obtained. In this work, an unsteady full annular simulation of a centrifugal compressor with a volute coupled with experimental validations was carried out by using the Reynolds averaged Navier–Stokes technique. Then, an advanced dynamic mode decomposition method named compressed DMD was employed to extract and analyze the dominant stall perturbation structure. Finally, considering the effects of eddy viscosity and molecular viscosity, a two-dimensional global stability analysis method based on the θ-z cylindrical surface at the impeller inlet is established to reveal the propagation behaviors of stall perturbation. The results indicate that the compressed DMD approach can capture low-frequency perturbation caused by stall accurately but not dependent on samplings. The flow structure near stall can be decomposed into the influenced mean flow, flow perturbation associated with rotor frequency and blade passing frequency with the frequencies of 1333 Hz and 9333 Hz, respectively, and stall perturbation with the frequency of 510 Hz. In the view of perturbation propagation features, it is mainly two propagations along the θ-z cylindrical surface. One is the reverse propagation to the upstream axially; the other is the secondary propagation to the blade leading edge with the effect of the main flow during the first propagation process. Both propagation paths deflect to the rotation direction, and the deflection degree of the second propagation paths is larger than that of the first one, thus, a triangular instability region is formed. The researches not only provide a new thought for the application of stability analysis in the centrifugal compressor but also clarify the propagation characteristics of stall disturbance, which lays a foundation for accurate flow control.

Research on the ground vortex and inlet flow field under the ground crosswind condition

The ground vortex field and nacelle inlet flow field under ground crosswind condition are investigated experimentally and numerically. A test rig with ground plane and crosswind blowing devices have been built, the experiments have been carried out by DSA3217 Pressure Scanner and Hot Wire Anemometer to get inlet total pressure distribution and the unsteady characteristics of ground vortex, respectively. The characteristics of ground vortex are quantitatively analyzed respect to vortex intensity, frequency, and dynamics, then the study crosswind interact on ground vortex is conducted, by considering the direct influence of crosswind on inlet distortion and the indirect effect of crosswind on ground vortex. Finally, it is found that due to the ground vortex, the process that inlet distortion increases with crosswind is unsteady and unstable. According to the variation of vortex intensity and the influence level that ground vortex acted on the inlet flow field, the process of blowing away the ground vortex from the beginning to the end can be divided into four phases, the encouraging phase, the initial blowing phase, the rapid blowing phase and the end blowing phase.

RBF-POD reduced-order modeling of flow field in the curved shock compression inlet

The ramjet/scramjet engines require the control-oriented model to predict the inlet flow field in less than a few seconds. However, it is challenging for these kinds of inlets which utilize curved shock waves to compress the air flow. In this paper, a reduced-order model based on the computational fluid dynamics, the proper orthogonal decomposition theory, and the radial basis function interpolation method is developed. After that, the curved shock waves dominated flow fields of a ramjet inlet under different angles of attack and free stream Mach numbers are predicted with this reduced-order model and compared to the full order computational fluid dynamics solutions. The results show that this reduced-order model can successfully predict the curved shock waves, the curved shock wave/boundary layer interactions, and the shock trains caused by a back pressure with high accuracies. The consumed time is only 0.11 s. The performance parameters are also predicted with the relative errors no more than 2%.

Synthetic Database of Aortic Morphometry and Hemodynamics: Overcoming Medical Imaging Data Availability.

Modeling of hemodynamics and artificial intelligence have great potential to support clinical diagnosis and decision making. While hemodynamics modeling is extremely time- and resource-consuming, machine learning (ML) typically requires large training data that are often unavailable. The aim of this study was to develop and evaluate a novel methodology generating a large database of synthetic cases with characteristics similar to clinical cohorts of patients with coarctation of the aorta (CoA), a congenital heart disease associated with abnormal hemodynamics. Synthetic data allows use of ML approaches to investigate aortic morphometric pathology and its influence on hemodynamics. Magnetic resonance imaging data (154 patients as well as of healthy subjects) of aortic shape and flow were used to statistically characterize the clinical cohort. The methodology generating the synthetic cohort combined statistical shape modeling of aortic morphometry and aorta inlet flow fields and numerical flow simulations. Hierarchical clustering and non-linear regression analysis were successfully used to investigate the relationship between morphometry and hemodynamics and to demonstrate credibility of the synthetic cohort by comparison with a clinical cohort. A database of 2652 synthetic cases with realistic shape and hemodynamic properties was generated. Three shape clusters and respective differences in hemodynamics were identified. The novel model predicts the CoA pressure gradient with a root mean square error of 4.6 mmHg. In conclusion, synthetic data for anatomy and hemodynamics is a suitable means to address the lack of large datasets and provide a powerful basis for ML to gain new insights into cardiovascular diseases.

Influence of inlet flow field uniformity on the performance of the nuclear main pump

A test loop system with adjustable spoiler function and visual structure was built in this paper. A PIV test system was adopted to investigate the influence of the uniformity of the inlet flow field on the performance of a nuclear main pump scale model, and the NUMECA software was used to numerically simulate the test cases. The research results show that when the rotation direction of the spoiler blade is opposite to that of the impeller, the greater the deviation of the spoiler blade from the axial angle, the more the pump efficiency and head drop; proper positive pre-rotation will not reduce the pump efficiency, but will increase the pump head; for the same spoiler blade deflection angle, when its structure produces irregular flow field disturbances under non-centrosymmetric conditions, the flow field downstream of it will not produce clear swirling flow, and it will not have more impact on the performance of the pump; the axial velocity distribution uniformity δ and the velocity weighted average angle θ, θ’ have obvious direct effects on efficiency, and in the δ interval of each working condition in this paper (the difference between the maximum value and the minimum value of δ is 0.093), the speed weighted average angle has a greater impact on efficiency.Nomenclature.Q Rate of flowδ Axial velocity distribution uniformityθ Weighted average angle parameter of velocityθi Absolute axial velocity angleθ′ Weighted average angle parameter of velocityη EfficiencyH Head

On the unsteady throttling dynamics and scaling analysis in a typical hypersonic inlet–isolator flow

The flow field in a two-dimensional three-ramp hypersonic mixed-compression inlet in a freestream Mach number of M∞ = 5 is numerically solved to understand the unsteady throttling dynamics. Throttling conditions are simulated by varying the exit area of the isolator in the form of plug insets. Different throttling ratios between 0 ≤ ζ ≤ 0.7 in steps of 0.1 are considered. No unsteadiness is observed for ζ ≤ 0.2, and severe unsteadiness is found for 0.3 ≤ ζ ≤ 0.7. The frequency of unsteadiness (f) increases rapidly with ζ. As ζ increases, the amount of reversed mass inside the isolator scales with the frequency and the exit mass flow rate. A general framework is attempted to scale the unsteady events based on the gathered knowledge from the numerical study. The inlet–isolator flow is modeled as an oscillating flow through a duct with known upstream design conditions such as the freestream Mach number (M∞) and the isolator inlet Mach number (Mi). Factors such as the mass occupied by the duct volume, the characteristic unsteady frequency, the throttling ratio, and the exit mass flow rate through the duct are used to form a non-dimensional parameter β, which scales with the upstream design parameter ξ = Mi/M∞. The scaling parameters are further exploited to formulate a semi-empirical relation using the existing experimental results at different throttling ratios from the open literature. The unsteady frequencies from the present two-dimensional numerical exercise are also shown to agree with the proposed scaling and the resulting semi-empirical relation.

Dependence of wind turbine loads on inlet flow field

In wind farm simulations, the inflow wind field plays a crucial role in the accuracy of both power production, structural load predictions and the turbulent wake development behind wind turbines. Three different inflow wind field generation techniques, namely the Mann model, a reduced order based model described herein and LES data, are used in this study to characterise the relation between the inflow and the structural response of the wind turbine. In addition, the wake development under different inflow conditions are studied. The turbulence statistics of the reduced-order model and the LES data are similar to each other while the Mann turbulence has different turbulence profiles and spectral characteristics. An in-house developed aeroelastic code, 3Dfloat, is used for structural response analysis. The differences between the inflow fields are mainly attributed to the turbulence intensity profiles, and differences in their spectral characteristics.

Numerical investigation of internal flow characteristics in a mixed-flow pump with eccentric impeller

In order to study the internal flow fields of mixed-flow pump with eccentric impeller, the numerical simulation and experiment have been conducted based on the dynamic sliding multi-region method. During simulation, the impeller domain has been separated by three sub-domains to apply the whirl effect. The pressure, turbulent kinetic dissipation and streamline distribution in the tip clearance were analyzed. The results show that the energy performance of calculation is basically consistent with the experimental results, which indicates the accuracy of energy performance prediction. The efficiency and head at the design flow condition drop about 20% and 11%, respectively, when e = 0.5 mm. The circumferential pressure distribution of impeller inlet and outlet is influenced greatly by the eccentric impeller, but the middle part of impeller suffered little effects as a result of the rotation effect of impeller. When the impeller is eccentric, the tip leakage flow and tip leakage vortex (TLV) are restrained at small tip clearance but enhance at large tip clearance. The angle between the TLV core and blade rim also increases at large tip clearance but decreases at small tip clearance. More energy has dissipated in the first half part of impeller channel within tip region, and the impacting depth increases with the increase in eccentricity, which affect the inlet flow fields of impeller at large eccentricity. The turbulent kinetic dissipation and hydraulic losses increase with the increase in eccentricity, which is the main factor for the decrease in pump efficiency.

Effects of an Inlet Vortex on the Performance of an Axial-Flow Pump

The formation of an inlet vortex seriously restricts axial-flow pump device performance and poses a great threat to the safe and stable operation of the entire system. In this study, the change trends of an inlet vortex and its influence on an axial-flow pump are investigated numerically and experimentally in a vertical axial-flow pump device. Four groups of fixed vortex generators (VGs) are installed in front of the impeller to create stable vortices at the impeller inlet. The vortex influence on the performance of pump device is qualitatively and quantitatively analyzed. The vortex patterns at different positions and moments in the pump device are explored to reveal the vortex shape change trend in the impeller and the pressure fluctuation induced by the vortex. The reliability and accuracy of steady and unsteady numerical results are verified by external characteristics and pressure fluctuation experimental results. Results show that it is feasible to install VGs before the impeller inlet to generate stable vortices. The vortex disturbs the inlet flow fields of the impeller, resulting in significant reductions of the axial velocity weighted average angle and the axial velocity uniformity. The vortex increases the inlet passage hydraulic loss and reduces the impeller efficiency, while it only slightly affects the guide vane and outlet passage performance. The vortex causes a low-frequency pressure pulsation and interacts with the impeller. The closer the vortex is to the impeller inlet, the more significant the impeller influence on the vortex. The blade cuts off the vortex in the impeller; afterwards, the vortex follows the blade rotation, and its strength weakens.

Numerical Simulations Study on the Effect of Propeller Slipstream on Inlet Flow Field for a Turboprop Aircraft

Numerical Simulations Study on the Effect of Propeller Slipstream on Inlet Flow Field for a Turboprop Aircraft

Axial-Swept Influence on Inner Flow Performance of HP Steam Turbine Based on CFD

Swept blade technology was used to redesign the supercritical steam turbine sets that could improve the inner-efficiency of turbine sets and decrease the consumption of coal the noxious gas such as NOx. The eighth high-pressure stages, including static and rotor cascades, were selected as tested prototypes that were blew in the low-velocity wind tunnel. We used the five-hole ball head needle to measure the aerodynamic parameters distribution along the width and span direction of the high-pressure stage cascades. With the inkblot display technology, the limit flow spectrums were displaced in the blade surface and the endwalls. These tested data could be used to check the simulation results of CFD software. To improve the efficiency of the steam turbine high-pressure (HP) stage, we selected the supercritical steam turbine HP stage cascade blade as the prototype to research into its inner flow performance of the axial-swept blade by the CFD software. Two different redesigned blades, with ±20°, swept angle, and 30% swept height, named axial fore-swept and axial aft-swept, were built up. The stage passages flow field of the prototype blade, and the two redesigned swept blades were simulated using CFD software with stage interface planes between the stages. The CFD simulation results indicated that the leading edge of swept blades influenced the inlet flow field; the pressure in aft-swept blade stage in both endwalls was higher than in the middle and was beneficial to improve the passage flow properties of HP stage. But for the fore-swept HP stage, its pressure distribution was lower in both endwalls than in the middle and not beneficial to passage flow.

Parameterization and optimization design of a hypersonic inward turning inlet

To improve the performance of a hypersonic inward turning inlet, an optimization design method was proposed. A parameterization methodology was developed for an axisymmetric basic flowfield, and the geometry of a basic flowfield can be easily generated in several parameters. Preliminarily, an optimization targeting on the basic flowfield performance was conducted. As a result, the total pressure recovery of the basic flowfield was improved 7.65% compared with the referred one under the same constraints. The corresponding stream-traced inlet performance was also improved 5.65%. For further optimization, an optimization directly targeting on the 3-D inlet performance was carried out. The method of calculating along streamlines (MCS), which is for the fast calculation of a stream-traced inlet flowfield, was proposed and confirmed before this optimization. The result shows that the 3-D inlet total pressure can be further enhanced 6.74% than the previous one, and it is 12.78% higher than the referred one. That is to say, it is more reasonable to target on the inlet performance directly for an optimization, rather than targeting on the basic flowfield performance. By statistically analyzing the optimization data, it was found that the opinion “A better basic flowfield means a better stream-traced inlet” is statistically correct, but not exactly correct. The steam-traced inlet performance is not exactly determined by a basic flowfield, and it is determined by a combination of all the flow tubes the inlet captures.

Study on the Validity of NOX Uniformity Thermal State Optimization Adjustment Strategy for SCR Denitration Unit of Coal-fired Power Plant

In order to make the NOx stable discharge standard after the ultra-low emission reform of coal-fired power plants, SCR denitration unit safe and economical operation. Taking a 600MW unit as the research object, the effectiveness of the NOx uniformity thermal state optimization adjustment strategy at the outlet of the denitration unit was studied, and the denitration unit inlet and outlet flow fields and NOx concentration field were tested to analyze the existing problems and formulate an optimization plan. The thermal state optimization adjustment, the NOx uniformity of the denitration unit outlet is significantly improved, the standard deviation of the NOx concentration at each outlet of the load section is reduced to below 15% and the ammonia escape concentration is reduced to below 3ppm. The research shows that the thermal optimization adjustment strategy is reliable and effective for improving the NOx uniformity of the SCR denitration unit, and the unit can operate safely and economically.

Particle image velocimetry experiment of the inlet flow field in a mixed-flow pump during the startup period

The transient characteristics of the inlet flow field of mixed-flow pump during start-up process are hard to be measured experimentally. The elbow structure with viewport is designed before the mixed-flow pump for capturing the transient inlet flow fields of the mixed-flow pump. Meanwhile, the synchronous data acceptance system is also designed to collect the data from different sensors synchronously. Besides, uncertainty analysis is carried out after the experiment to ensure the accuracy and reliability of this test system. The transient external characteristics are obtained and the velocity distribution of a mixed-flow pump during the startup period is measured using particle image velocimetry technique. The results show that the overall uncertainty for the external characteristics was 0.2345%, which indicates the high accuracy of this test bench for capturing the transient flow condition in the mixed-flow pump during the startup period. At the beginning of the start-up process, the head of the mixed-flow pump increases and reaches the maximum once the rotation speed meets the maximum value and an instantaneous impulsive head appears at the end of the accelerating process. However, with the increase of the rotating speed, wall boundary layer gradually develops from laminar to turbulent flow, and the flow moves continuously from the hub center to the inlet. At the end stage of the accelerating process, more intensive entrainment effect of the impeller is found. When the rotation speed is steady, the influence of the elbow on the internal flow fields gradually decreases, compared with the startup process. The results can serve as a fundamental reference for the investigation of transient flow characteristics of the mixed-flow pump during the startup period.