Shroud Exit Research Articles

Aerodynamic Investigation of Shrouded Rotors with Dual Exit Channels

The growing need for rotary-wing aerial vehicles with high-speed forward movement and dependable hover performance is critical in various applications. Shrouded rotors enhance aerodynamic performance, create more overall thrust with the same power consumption as open rotors, and have a more uniform induced velocity flow field. This paper presents a parametric computational investigation centered on the hypothesis, that dividing the shroud exit channel into convergent inner and divergent outer channels would enhance flow uniformity, reducing power losses, and preventing airflow separation from the main shroud’s inner walls. The validity of this hypothesized concept is demonstrated through extensive Computational Fluid Dynamic (CFD) simulations. The paper includes a case analysis utilizing experimental data from a highly- maneuverable drone, named Navig8, equipped with a 9-inch shrouded propeller where various shrouded configurations are examined using Computational Fluid Dynamics. Results typically show an increase in total thrust with the incorporation of an inner shroud for a given power.

Rotating Instabilities in Shrouded Low Pressure Turbine at Design and Off-Design Conditions

Abstract The present study aims to analyze the rotating instabilities that may occur inside shroud cavities above rotors of low-pressure turbine configurations. Unsteady simulations have been carried out at design and off-design conditions on two configurations, one being a multi-stage configuration. Unsteady flow structures, uncorrelated from blade passing frequencies and depending on operating points, are identified in every rotor tip shroud cavity. Similarities regarding flow patterns and interactions with the main flow are observed: hot spots of gas having different azimuthal extend come from the tip shroud cavity and rotate at the interface with the main flow path. The influence on the main flow and the origin of these instabilities are discussed. The study at off-design operating points deepens the analysis and allows the identification of physical parameters which drive the instability.

Parametric design analysis of elliptical shroud profile

<abstract> <p>Parametric design analysis for Eccentric Rotated Ellipsoid (ERE) shroud profile is conducted whereas the design model is validated experimentally. A relation between shroud inlet, length and exit diameter is established, different ratios related to the wind turbine diameter are introduced, and solution for different ERE family curves that passes on the inlet, throat, and exit points is studied. The performance of the ERE shroud is studied under different wind velocities ranging from 5–10 m/s.</p> <p>The method used in creating the shroud profile is by solving the ERE curve equations to generate large family of solutions. The system is modeled as axisymmetric system utilizing commercial software package. The effect of the parameters; shroud length, exit diameter, inlet diameter, turbine position with respect to the shroud throat, and wind velocity are studied. An optimum case for each shroud length, exit diameter and location of the shroud with respect to the wind turbine throat axis are achieved.</p> <p>The simulation results show an increase in the average wind velocity by 1.63 times of the inlet velocity. This leads to a great improvement in the wind turbine output power by 4.3 times of bare turbine. One of the achieved optimum solutions for the shroud curves has been prototyped for experimental validation. The prototype has been manufactured using 3D printing technology which provides high accuracy in building the exact shape of shroud design curve. The results show very good agreement with the experimental results.</p></abstract>

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part I: Design Concept and Typical Performance of a Swirl Breaker

In high and intermediate pressure (HIP) steam turbines with shrouded blades, it is well known that shroud leakage losses contribute significantly to overall losses. Shroud leakage flow with a large tangential velocity creates a significant aerodynamic loss due to mixing with the mainstream flow. In order to reduce this mixing loss, two distinct ideas for rotor shroud exit cavity geometries were investigated using computational fluid dynamics (CFD) analyses and experimental tests. One idea was an axial fin placed from the shroud downstream casing to reduce the axial cavity gap, and the other was a swirl breaker placed in the rotor shroud exit cavity to reduce the tangential velocity of the leakage flow. In addition to the conventional cavity geometry, three types of shroud exit cavity geometries were designed, manufactured, and tested using a 1.5-stage air model turbine with medium aspect ratio blading. Test results showed that the axial fin and the swirl breaker raised turbine stage efficiency by 0.2% and 0.7%, respectively. The proposed swirl breaker was judged to be an effective way to achieve highly efficient steam turbines because it not only reduces the mixing losses but also improves the incidence angle distribution onto the downstream blade row. This study is presented in two papers. The basic design concept and typical performance of the proposed swirl breaker are presented in this part I, and the effect of axial distance between a swirl breaker and rotor shroud on efficiency improvement is discussed in part II [8].

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part II: Effect of Axial Distance Between a Swirl Breaker and a Rotor Shroud on Efficiency Improvement

The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I: Design Concept and Typical Performance of a Swirl Breaker.” When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and the rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In Part II, five cases of geometry with different axial distances between the swirl breaker and the rotor shroud, which covered a range for the stage axial distance of actual high and intermediate pressure (HIP) steam turbines, were investigated using a single-rotor computational fluid dynamics (CFD) analysis and verification tests in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and the rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and the rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the measured maximum pressure fluctuation in the swirl breaker cavity was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.

Turbine Aerodynamic Low-Frequency Oscillation and Noise Reduction Using Partial Shrouds

The design of highly efficient low-pressure gas turbines in compliance with strict emission regulations for pollutants and noise, as well as long component lifetime, requires the detailed knowledge of potential excitations in the flow. The analysis of fast response measurement data from a 1.5-stage low-pressure axial turbine in the frequency domain shows multiple relevant excitation and noise sources in the flowfield, in addition to the primary rotor–stator interaction. Especially nonsynchronous low-frequency modes at 10% of the blade-passing frequency, occurring in the tip shroud exit cavity, are found to communicate with the main flow and are amplified through the downstream stator row by a flow instability in the interaction area of the passage vortex and boundary layer. Full-annular unsteady computational fluid dynamics simulations are carried out in addition to the experiments to probe the origin of the cavity modes. They predict the rotor–stator interaction well, but they struggle to resolve low-frequency oscillations in the cavities. Out of the four tested configurations, a shroud trailing-edge cutback is the most efficient option to prevent the formation of the cavity modes. The tradeoff for a low-frequency fluctuation reduction of 12 dB using a trailing-edge partial shroud is a total–total stage efficiency drop of 0.7%.

Tip-Shroud Cutbacks in a Low-Pressure Gas Turbine Stage

This paper presents an experimental and computational study of tip-shroud modifications in a low-pressure gas turbine. Partial shrouds are a viable option to reduce the static stresses in high-speed stages compared to full shrouds while still benefiting from superior aerodynamic efficiency compared to unshrouded blades. Three different tip-shroud platform cutbacks have been tested experimentally in a low-aspect-ratio 1.5-stage low-pressure axial turbine and are compared to a full-shroud baseline. A detailed analysis of the fluid dynamics is carried out to provide a starting point for shroud optimizations. The leading- and trailing-edge platforms are cut back separately to isolate the effect of each modification. The final rotor then features a combined partial shroud on both leading and trailing edge, with the same material reduction as the shroud leading-edge cutback. The time-resolved flowfield and pressure measurements are accompanied by three-dimensional, unsteady Reynolds-averaged Navier–Stokes simulations, which provide insight into the over-tip leakage flows and their interaction with the main flow both at cavity inlet and outlet. The aerodynamic losses with the trailing-edge cutback are reduced compared to the baseline, but the resulting underturning from the interaction with the shroud exit cavity makes it inferior in terms of efficiency compared to the leading-edge cutback. The isolated shroud leading-edge cutback shows the best tradeoff between stress reduction and aerodynamic efficiency penalty (0.7%).

Effect of shroud on the performance of horizontal axis hydrokinetic turbines

The idea of wind turbine power enhancement by means of a shroud has been investigated for decades with no commercial success. However, due to some beneficial aspects, shrouds are poised to serve the hydrokinetic turbine industry. We report on the experimental measurements performed in a water tunnel on a 19.8cm diameter horizontal axis hydrokinetic model turbine utilizing two shrouds. Power and thrust coefficients of the model turbine with and without the shrouds investigated are obtained over the range of the power curve. A maximum power enhancement of 91% over the unshrouded turbine is obtained with one of the shrouds. Maximum power coefficients are recalculated based on the maximum area of shroud and are found to be comparable to that of the unshrouded turbine. At flow speed of 1m/s, an extended blade that generates the same power as the shrouded turbine is found to have a diameter equal to the diameter of the shroud exit. This aspect is critical as the use of shrouds contrary to much of the literature is not to increase the hydrokinetic turbine efficiency, but to gain advantages in the powertrain design, and have the turbine power less impacted by changes in the flow direction.

Comparison of leakage performance and fluid-induced force of turbine tip labyrinth seal and a new kind of radial annular seal

To minimize fluid leakage loss and fluid-induced force of traditional turbine tip seals, a new kind of radial annular rim seal (RARS) is proposed in this paper. Comparing with the conventional labyrinth rim seal (LRS), the fluid leakage direction is modified from the axial to the radial direction. The flow resistance increases, and the flow-induced force is greatly reduced. A complete three-dimensional CFD model including both the rim seal and the rotor blade row was employed to analyze the inherent characteristics of the fluid flow in the whole passage. The calculated results show that the leakage flux of the RARS is about 0.03% lower than that of the LRS. The calculated results also show that the tangential force acting on the blade wheel is much smaller than that acting on the shroud in the present conditions. The rotating speed has a significant influence on the tangential force and a relatively smaller influence on the radial force. Both forces increase linearly with the increasing speed. The radial force acting on the rotating part with the LRS is about 7–11 times larger than that with the RARS, and the tangential force with the RARS is approximately 0.9 times smaller than that with the LRS. Finally, the effect of the rim seal leakage on the main flow was studied. An improvement in flow angle and efficiency at the exit of rotor blade row is calculated by reducing the leakage jet velocity and the aerodynamic mixing losses in the shroud exit cavity. Furthermore, an eccentric blade wheel tends to make a great difference of the yaw angle distribution in the circumferential direction due to the nonuniform clearance.

Finite-wavelength scattering of incident vorticity and acoustic waves at a shrouded-jet exit

As the vortical disturbances of a shrouded jet pass the sharp edge of the shroud exit some of the energy is scattered into acoustic waves. Scattering into upstream-propagating acoustic modes is a potential mechanism for closing the resonance loop in the ‘howling’ resonances that have been observed in various shrouded jet configurations over the years. A model is developed for this interaction at the shroud exit. The jet is represented as a uniform flow separated by a cylindrical vortex sheet from a concentric co-flow within the cylindrical shroud. A second vortex sheet separates the co-flow from an ambient flow outside the shroud, downstream of its exit. The Wiener–Hopf technique is used to compute reflectivities at the shroud exit. For some conditions it appears that the reflection of finite-wavelength hydrodynamic vorticity modes on the vortex sheet defining the jet could be sufficient to reinforce the shroud acoustic modes to facilitate resonance. The analysis also gives the reflectivities for the shroud acoustic modes, which would also be important in establishing resonance conditions. Interestingly, it is also predicted that the shroud exit can be ‘transparent’ for ranges of Mach numbers, with no reflection into any upstream-propagating acoustic mode. This is phenomenologically consistent with observations that indicate a peculiar sensitivity of resonances of this kind to, say, jet Mach number.

Control of Shroud Leakage Loss by Reducing Circumferential Mixing

Shroud leakage flow undergoes little change in the tangential velocity as it passes over the shroud. Mixing due to the difference in tangential velocity between the main stream flow and the leakage flow creates a significant proportion of the total loss associated with shroud leakage flow. The unturned leakage flow also causes negative incidence and intensifies the secondary flows in the downstream blade row. This paper describes the experimental results of a concept to turn the rotor shroud leakage flow in the direction of the main blade passage flow in order to reduce the aerodynamic mixing losses. A three-stage air model turbine with low aspect ratio blading was used in this study. A series of different stationary turning vane geometries placed into the rotor shroud exit cavity downstream of each rotor blade row was tested. A significant improvement in flow angle and loss in the downstream stator blade rows was measured together with an increase in turbine brake efficiency of 0.4 %.

Base Pressure and Noise Produced by the Abrupt Expansion of Air in a Cylindrical Duct

In the investigation described, the base pressure resulting from the abrupt expansion of an air jet from a circular nozzle into a concentric cylindrical duct or shroud has been measured. Stagnation pressure ratios of the forcing jet to atmospheric of up to six were used with shrouds of various lengths and diameters. As the primary or forcing jet pressure is increased and then decreased, the jet flow attaches and separates from the shroud wall and a hysteresis effect is exhibited by the pressure at the base of the shroud. With an attached flow, the base pressure attains a minimum value which depends mainly on the duct to nozzle area ratio and on the geometry of the nozzle, lower base pressures being obtained with convergent-divergent nozzles. When the jet pressure was increased beyond that required to attain the minimum value of the base pressure, it was observed that the ratio of the forcing jet pressure to base pressure remained constant. Noise measurements of the flow from the nozzle and shroud are presented. The plot of overall noise showed a minimum value of noise at a jet pressure approximately equal to that required to produce the minimum base pressure. As the pressure was increased beyond this value, the noise from the shroud was essentially that of a subsonic jet until the flow at the shroud exit became choked. The excessive overall noise observed at low forcing pressures was due to the emission of powerful discrete tones from an organ-pipe resonance within the ducts.