Hub Surface Research Articles

Design and Experimental Assessment of Bladeless Turbines for Axial Inlet Supersonic Flows

Abstract Supersonic inlet flow is usually considered to be detrimental to the performance of turbine systems. A new class of bladeless turbines was developed, which allows for power extraction from supersonic axial inflows without swirl with minimal maintenance cost. This is achieved through a wavy hub surface that promotes shocks and expansion fans and hence generates torque. In a first step, a baseline bladeless turbine is designed and the power extraction is analyzed. The bladeless turbine surface is parametrized in matlab and subsequently imported into Hexpress (Numeca) to mesh an unstructured grid. The first layer thickness is kept below one for all the simulations. The unstructured mesh is loaded into cfd++ (Metacomp) to solve the three dimensional steady Reynolds-averaged Navier–Stokes (RANS) equations. Second, the work extraction principle is broken down from a three-dimensional unsteady problem into a two-dimensional steady phenomenon. An experimental campaign is outlined and test details are discussed. Finally, after the experimental characterization, the operational envelope and scaling of the bladeless turbine are described for several reduced mass flows, reduced speeds, and geometrical features of the turbine (amplitude of the wavy surface, helix angle, hub radius, and length of the axial turbine).

Adjoint-based shape sensitivity of multi-row turbomachinery

The performance sensitivity of a low-pressure turbine stator-rotor stage of a commercial jet engine to its blades and hub shapes is analyzed. The derivatives of various metrics, such as isentropic efficiency, total pressure ratio, total pressure loss, and loss coefficient are computed using an adjoint solver capable of handling multi-row analyses. The importance of considering the coupled stator-rotor stage is highlighted from the computed sensitivities, which reveal that performing individual stator or rotor row component optimization may lead to an unexpected performance loss of the whole stage. The obtained coupled sensitivities are then used in an endwall contouring application, consisting of two Hicks-Henne bumps applied on the hub surface of the rotor to maximize stage efficiency, at locations selected where the impact is found to be the highest from the adjoint-based sensitivity analysis. The performance gain obtained with the bumps is correlated to the changes in the flow field, in particular with the effect on secondary flows.

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.

Integrated lifting line/surface panel method for optimal propeller design with consideration of hub effect

An optimal marine propeller design method is proposed, which integrates the lifting line and surface panel method and is characterized by the use of the surface panel method to take the hub effect into consideration. By developing an integrated approach instead of an iterative method for the calculation of the interaction between the hub and the designed blades, the hub effects on the optimal circulation can be accounted for throughout the theoretical design procedure. This new integrated method provides a fast and accurate enough method to model the straight forward hub surface, in the optimal propeller design. A systematic design procedure from the basic design inputs to the blade geometry determination is performed and the designed propellers are validated by the surface panel method and the RANS method. The design and analysis cases are considered by different approaches with comparison and validation. And a comparative study including different hub geometries is also performed to reveal the mechanism of the hub effect on the distributions of the propeller optimal loads.

Study of Flow Instability in a Miniature Centrifugal Pump Based on Energy Gradient Method

Flow instability in a miniature centrifugal pump is numerically simulated with the RANS equations and the SST k-ω turbulence model. The energy gradient method is adopted to analyze the flow instability at design load and two off-design loads, and the results are compared with those analyzed by Q-criterion. The regions with large magnitude of energy gradient function (K) indicate pronounced turbulent intensity and poor flow stability. Internal flow stability is investigated in details for both the near blade surfaces region and the impeller passages. To study the mechanism of energy gradient method, internal flow parameters such as the velocity and total pressure, the transverse gradient of total mechanical energy and the work done by shear stresses are investigated respectively. The results show that the energy dissipation reaches its maximum around the leading edge of suction surface. The value of the energy gradient function K presents a different magnitude for the near blade surfaces region and the impeller passages, and the K in the impeller passage is much larger. Regions with maximum of the work done by shear stresses are concentrated on the suction surface, regions with large transverse gradient of total mechanical energy is concentrated on the hub surfaces or shroud surfaces. It is further found that the K can reflect the influence of the outer boundaries of vortex on the flow near blade surface.

Effect of blade tip leakage flow on erosion of a radial inflow turbine for compressed air energy storage system

The erosion caused by sand particles significantly influences the safety of radial inflow turbine in Compressed Air Energy Storage (CAES) system which operated in desert area. In present study, effects of tip leakage flow on erosion of a CAES radial inflow turbine are investigated at different tip clearances and total pressure ratios. A CFD model coupling Tabakoff and Grant erosion model is utilized for the analysis. Results illustrate that the trailing edge of stator, the leading edge of rotor blades, the shroud and hub surfaces near inlet of rotor cause more severe erosion at each tip clearance. Therefore, wear-resistant coating should be adopted. The severe erosion can also be found at blade tip of rotor. However, it is gradually suppressed with the increase of tip clearance. With the increase of total pressure ratio, the region with higher erosion rate on the shroud of rotor gradually extends to downstream. However, the erosion rate at leading edge of rotor blades is decreased obviously. In conclusion, the erosion effect can be alleviated remarkably when the tip clearance size is of 2% and total pressure ratio is more than 1.84, while the isentropic efficiency is almost kept the same.

Smooth flank milling tool path generation for blade surfaces considering geometric constraints

In practical machining, flank milling path generation methods of blade surfaces should concern machine axes’ motions to improve machined surface quality, as well as the gouging-avoidance with the hub surface and the geometric deviation constraints. The smoothness of the tool orientation in the part coordinate system cannot assure the smooth rotary axis motions of a five-axis machine tool, since the conversion of tool orientation from the part coordinate system to the machine coordinate system is nonlinear. A flank milling path is represented with the cutter reference point trajectory and the displacement curves of the two rotary axes in this research, so that the machine axes’ motions along the tool path can be directly smoothed when generating the path. The point-to-surface distance function is adopted to evaluate the geometric deviation, and an algorithm based on the differential evolution algorithm is presented to solve the distance function robustly. The sum of the squares of the first, second, and third derivatives of the cutter reference point trajectory and the two rotary axes’ displacement curves are adopted as the smoothness indicators. Then, a flank milling path generation model for blade surface smoothing the machine axes’ motions and considering the hub surface gouging-avoidance and geometric deviations constraints is developed. Numerical examples and machining experiments for five-axis flank milling of a blade surface are given to confirm the validity and effectiveness of the proposed approach.

Loss Reduction in a 1.5 Stage Axial Turbine by Computer-Driven Stator Hub Contouring

Improvements in stage isentropic efficiency and reductions in total pressure loss are sought in a 1.5 stage axial turbine. This is representative of power generation equipment used in thermal power cycles, which delivers about 80% of the 20 × 1012 kWh world-wide electricity. Component-level improvements are therefore timely and important toward achieving carbon dioxide global emission targets. Secondary flow loss reduction is sought by applying a nonaxisymmetric endwall design to the turbine stator hub. A guide groove directs the pressure side branch of the horseshoe vortex away from the airfoil suction side, using a parametric endwall hub surface, which is defined as to obtain first-order smooth boundary connections to the remainder of the passage geometry. This delays the onset of the passage vortex and reduces its associated loss. The Automatic Process and Optimization Workbench (apow) generates a Kriging surrogate model from a set of Reynolds-averaged Navier–Stokes simulations, which is used to optimize the hub surface. The three-dimensional steady Reynolds-averaged Navier–Stokes model with an axisymmetric hub is validated against reference experimental measurements from the Rheinisch-Westfälische Technische Hochschule (RWTH) Aachen. Comparative computational fluid dynamics (CFD) predictions with an optimized nonaxisymmetric hub show a decrease in the total pressure loss coefficient and an increase in the isentropic stage efficiency at and off design conditions.

Drag reduction within radial turbine rotor passages using riblets

In this paper, reducing the friction losses in a radial inflow turbine rotor surface by adding engineered features (riblets) is explored. Initially, computational fluid dynamics analysis was used to study the operating mechanism of riblets and to test their ability to reduce drag within the rotor passage when running the turbine at the design point. Thereafter, riblets with different heights and spacing have been implemented at the rotor hub to study the effect of riblets geometry and arrangement on the drag reduction, which leads to determine the riblet geometry where the maximum benefit on turbine performance can be achieved. The effect of riblets on boundary layer development and on the secondary flow generation within the rotor passage has been examined. It was found that the introduction of riblets could reduce the wall shear stress at the hub surface, and on the other hand, they contribute to increasing the stream-wise vorticity within the rotor passage. The maximum wall shear reduction was achieved with riblet with relative height hrel = 2.5% equivalent to 19.3 wall units, while the maximum performance happens when using riblets with hrel = 1.5% equivalent to 11.8 wall units as the later contributes less in secondary flow generation within the passage. For riblets with height more than 19.3 wall units, the overall effect is negative, as they cause an increase in drag and give rise to secondary flow leading to lower turbine performance.

The Interactions of Turbine Hot Streak and Leakage Flow from the Inter-Platform Gap

ABSTRACTThe paper presents the effects of the leakage flow from the inter-platform gap on the migration of hot streak in a first stage turbine by three-dimensional unsteady Reynolds-Averaged Navier-Stokes simulations. Five circumferential positions of the hot streak are considered. The comparisons between the results with/without slot leakage show significant differences. The leakage changes the hot streak in the vane passage significantly and it protects the vane suction surface trailing from the hot gas. The leakage also changes the secondary flow and results in forming a new couple of vortices in the vane passage. In general, rotor passage, the hot gas usually gathers in the rotor hub and the pressure surface. In the present study, the leakage coolant from upstream slot is entrained to the unsteady rotor secondary flows and transported toward the rotor hub and pressure surface effectively. The cooling effect is related to the relative circumferential positions between the hot streaks and slot. When the hot streak is positioned at the slot suction side, the time-averaged temperature reduction on the blade leading edge can be more than twice of that for the hot streak at the slot pressure side.

Optimization of rough transonic axial compressor

The influence of wall roughness on the performance of the axial transonic compressor stage was investigated with different values of roughness added to the blade, hub and shroud sections. The dimensionless sand-grain roughness model was used to capture the roughness effect and the results indicated that the increment of both end wall and blade surface roughness caused the deterioration of compressor stage performance. The sensitivity analysis method was used to distinguish which section mostly contributes to the whole performance degradation. Approximately a 95.31% degradation of the compressor peak efficiency came from the induced blade roughness, 3.58% from the hub surface roughness and only 1.08% from the casing surface. The present study also investigated how the optimized design of compressor blades was affected by considering a surface roughness effect representative of in-service use. Two optimization strategies were proposed to improve the compressor efficiency and total pressure ratio by changing the distributions of the blade angles along the chord. The first strategy considered the compressor surface to be hydraulically smooth and the consequent Pareto Front designs were degraded by increasing the level of surface roughness with the second approach considering the surface roughness from the outset of optimization. The optimization result showed that the degraded compressors from the first strategy was still among the best performing Pareto Front designs in terms of adiabatic efficiency and pressure ratio when compared to the second approach. This means that the roughness effect can be regarded as an additional factor and be considered in the end of the design process for single-stage compressors.

선형 터빈 익렬에서 열전달 수치해석 및 실험과 비교

Numerical analysis of heat transfer on the linear turbine endwall is conducted. Then, the results of Nusselt number are compared with those of equivalent heat transfer experiment. The generated mesh of numerical model has the averaged y+ is lower than 1 on the hub surface. From numerical investigation with two different turbulence model(SST and Reynolds stress model). The result gives resonable agreement with the experiment result from literature in vortex free region. The numerical results have higher Nu than experiment at stagnation point of leading edge due to conduction error. The passage vortex is described better by numerical analysis than experiment. In trailing edge, both boundary layer profile and Nu of numerical result have different trend from experimental one. Except high vortex region and trailing edge, numerical analysis has resonable agreement with the experimental result.

Effect of end-wall riblets on radial turbine performance

This paper presents a detailed study of the impact of manufacturing residual riblets at the rotor hub surface of a radial inflow turbine on the flow within the rotor passages and their contribution to drag reduction. Numerical analysis has been used to study the effects of those features at design point conditions. Riblets with different height and spacing have been examined to determine the riblet geometry where the maximum drag reduction is achieved. The relative height of the riblets to rotor inlet blade height was introduced to generalise the results. At the end of this study the results were compared with the available data in literature. It was found that the introduction of riblets could reduce the wall shear stress at the hub surface, while they contribute to increasing the streamwise vorticity within the rotor passage. For the geometries tested, the minimum drag was achieved using riblets with relative height hrel = 2.5% equivalent to 19.3 wall units. The results revealed that the spacing between riblets have a minor effect on their performance, this is due to the size of the streamwise vortex above the hub surface which will be discussed in this work.

A real-time visual and control system for screw holes location and shielding of automobile wheel hubs

In this paper, machine vision technique is used to solve screw holes shielding problems during spray painting in wheel hub making industry. The shape and geometric structure of automobile wheel hub are studied; the wheel hub image is captured from the assembly line; image processing method is used to recognise the holes on the wheel hub surface; the diameters, positions and some other parameters are calculated; a 2D motion control method is used to locate the central position of screw holes; a compact shielding device is developed to cover the holes by special balls. This method can be used for all types of wheel hubs, and the labour costs can be reduced. The productive process is more efficient and automatic.

A real-time visual and control system for screw holes location and shielding of automobile wheel hubs

In this paper, machine vision technique is used to solve screw holes shielding problems during spray painting in wheel hub making industry. The shape and geometric structure of automobile wheel hub are studied; the wheel hub image is captured from the assembly line; image processing method is used to recognise the holes on the wheel hub surface; the diameters, positions and some other parameters are calculated; a 2D motion control method is used to locate the central position of screw holes; a compact shielding device is developed to cover the holes by special balls. This method can be used for all types of wheel hubs, and the labour costs can be reduced. The productive process is more efficient and automatic.

Effect of end-wall roughness on performance of transonic axial compressor

The influence of end-wall roughness on the performance of the axial compressor stage was investigated with different values of roughness added to the hub and shroud surfaces of transonic compressor stage, NASA Stage 35. Firstly, the numerical code was validated against the experimental data, which were available from the open literature. Afterwards the model was applied to simulate the effect of end-wall roughness with different amplitudes of dimensionless sand-grain roughness height. Numerical results indicated that the increment of end-wall roughness caused the deterioration of compressor stage performance. To understand the mechanism behind, the distributions of loadings, losses and the detail flow situations near end-wall region were analyzed and discussed. The results show that the overall performance drop is mainly due to the thickened end-wall boundary layer. Near the hub region, the rough hub surface induces larger corner stall and the shock wave moves upstream. Meanwhile, the casing roughness leads to the slight increment of tip leakage mass flow and extension of blockage in the circumferential direction.

A nonlinear and semi-analytical actuator disk method accounting for general hub shapes. Part 1. Open rotor

The paper presents a newly developed method for the analysis of the flow around open rotors characterised by hubs of general shape. The exact and implicit solution of the axysimmetric, inviscid and incompressible flow is represented as the superposition of infinite ring vortices properly arranged along the hub surface and the rotor wake. The solution is made explicit through a semi-analytical and iterative procedure. The proposed semi-analytical approach can deal with hubs of arbitrary shape and with quite general rotor load distributions. The method strongly couples the flow induced by the rotor and the hub. Moreover, the contraction/divergence and the rotation of the wake can be fully taken into account. The results of the semi-analytical method are also compared with those obtained with a widely diffused actuator disk model based on computational fluid dynamics (CFD) techniques. Finally, in comparison with more advanced methods, such as those relying on a CFD approach, this method is characterised by an extremely reduced computational cost. The computer code is freely available on contacting the authors.

Finite element simulation of flange rolling on the L63 brass workpiece

The paper develops a mathematical model of the flange cold rolling process on the cylindrical workpiece made of L63 alloy. It analyzes stressed and strained states of the workpiece during three stages of a flanged piece forming. It is found that at the first stage the base area of the formed cone is most problematic in terms of possible workpiece destruction. Risks at the second and the final stage of flange forming are in the flange end surface and cylindrical hub surface of the rolling component.

Fault detection in heavy duty wheels by advanced vibration processing techniques and lumped parameter modeling

Heavy duty wheels are used in applications such as automatic vehicles and are mainly composed of a polyurethane tread glued to a cast iron hub. In the manufacturing process, the adhesive application between tread and hub is a critical assembly phase, since it is completely made by an operator and a contamination of the bond area may happen. Furthermore, the presence of rust on the hub surface can contribute to worsen the adherence interface, reducing the operating life. In this scenario, a quality control procedure for fault detection to be used at the end of the manufacturing process has been developed. This procedure is based on vibration processing techniques and takes advantages of the results of a lumped parameter model. Indicators based on cyclostationarity can be considered as key parameters to be adopted in a monitoring test station at the end of the production line due to their not deterministic characteristics.

The Research of Hydrodynamic Performance for Ducted Propeller with PBCF

The Propeller Boss Cap Fins (PBCF) is often used to ordinary propeller, a good energy-saving effect being obtained. In order to study the energy-saving mechanism of ducted propeller with PBCF, in this paper, the FLUENT has been taken to simulate the distribution of thrust coefficient, torque coefficient, blade pressure and velocity vector of hub surface at different advance coefficients. By contrasting the results of numerical simulation of hydrodynamic performance of ducted propeller between with fins and without fins, we know that at the low advance coefficient, the ducted propeller with fins will increase the thrust coefficient and decrease the torque coefficient; rising the open water propeller efficiency, improving the efficiency under the premise of the efficiency increase by duct. The existence of fins has changed velocity distribution of water around the hub and made the water that flowed around the propeller hub with propeller rotation direction flow to propeller tail along the fins not gather in the cub, so it weakened the hub vortex.