Rotor-stator Cavity Research Articles

Swirl Flow and Heat Transfer in a Rotor-Stator Cavity with Consideration of the Inlet Seal Thermal Deformation Effect

In the typical structure of a turboshaft aero-engine, the mass flow of the cooling air in the rotor-stator cavity is controlled by the inlet seal labyrinth. This study focused on the swirl flow and heat transfer characteristics in a rotor-stator cavity with considerations of the inlet seal thermal deformation effect. A numerical framework was established by integrating conjugate heat transfer (CHT) analysis and structural finite element method (FEM) analysis to clarify the two-way aero-thermo-elasto coupling interaction among elastic deformation, leakage flow, and heat transfer. Simulation results showed that the actual hot-running clearance was non-uniform along the axial direction due to the temperature gradient and inconsistent structural stiffness. Compared with the cold-built clearance (CC), the minimum tip clearance of the actual non-uniform hot-running clearance (ANHC) was reduced by 37–40%, which caused an increase of swirl ratio at the labyrinth outlet by 5.3–6.9%, a reduction of the Nusselt number by up to 69%. The nominal uniform hot-running clearance (NUHC) was defined as the average labyrinth tip clearance. The Nusselt number of the rotating disk under the ANHC was up to 81% smaller than that under the NUHC. Finally, a clearance compensation method was proposed to increase the coolant flow and decrease the metal temperature.

Flow and Performance of the Disk Cavity of a Marine Gas Turbine at Varying Nozzle Pressure and Low Rotation Speeds: A Numerical Investigation

In marine gas turbines, variations in rotational speed occur all the time. To ensure adequate cooling effects on the turbine blades, the valves need to be adjusted to change the pressure upstream of the pre-swirl nozzle. Changing such pressure will have significant effects on the local or overall parameters, such as core swirl ratio, temperature, flow rate coefficient, moment coefficient, axial thrust coefficient, etc. In this paper, we studied the flow characteristics within the pre-swirl system of a marine gas turbine at low rotational speed by varying the pressure at the pre-swirl nozzle. The corresponding global Reynolds number ranged from Re = 2.3793 × 105 to 9.5172 × 105. The flow in the rotor-stator cavities was analyzed to find the effects of nozzle pressure on the radial velocity, core swirl ratio, and pressure. According to the simulation results, we introduced a new leakage flow term into the formulary in the references to calculate the values of K between the inner seal and the pre-swirl nozzle. The matching characteristics between the pre-swirl nozzle and the inclined receiving hole was predicted. Performance of the pre-swirl system was also analyzed, such as the pressure drop, through-flow capacity, and cooling effects. After that, the moment coefficient and the axial thrust coefficient were predicted. This study provides some reference for designers to better design the pre-swirl system.

Prediction of flows in enclosed rotor–stator cavities

A robust and fully predictive mathematical representation has been derived to predict the swirl ratio, pressure, and torque in enclosed rotor–stator cavities. For the flow regime of interest, turbulent boundary layers form on the rotor and stator, between which there is an inviscid fluid core. Utilizing the momentum-integral equations with suitable assumptions and the continuity of mass equation, governing equations are derived for the flow behavior, with the assumption that the swirl ratio is dependent on the radius. Predictive initial conditions are formulated, and a robust, computationally inexpensive numerical technique is implemented and predictions are shown to be in reasonable agreement with existing experimental data. The swirl ratio was proven to be highly dependent on the radius, with the distribution modified by the shroud and hub conditions. Additionally, the effects of two geometrical parameters were studied, namely, the gap ratio and non-dimensional inner radius. As the gap ratio increases from 0.05 to 0.5, the swirl ratio at the outer radius decreases from 0.56 to 0.35, leading to an increase in torque coefficient from 0.38 to 0.44. The swirl ratios for different inner radii differ at the inner part of the cavity while converging to a single value at the outer radius. The presented model can easily be integrated into the design code for gas turbine engines, contributing to the design of more efficient engines.

Experimental investigations on flow and heat transfer characteristics in a high-speed rotor–stator disk cavity with axial throughflow

The rotor–stator disk cavity system is of great importance for turbine blade cooling and preventing gas invasion in the aircraft engine. This study presents an experimental investigation on a high-speed rotor–stator disk cavity with axial throughflow. The aims are to experimentally measure and reveal the pressure characteristics, windage heating, swirl characteristics, and relative temperatures at a high rotational speed of 0–9000 rpm with mass flow rates of 0.05–0.15 kg/s. Especially, a new measurement idea was proposed and tested that relative temperature in the rotor system can be obtained by measuring absolute parameters in the stator system. Experiment results reveal that rotational speed and inlet mass flow are the main parameters to determine flow characteristics in the rotor–stator cavity. The increase of rotational speed will enhance the effect of centrifugal boosting, windage heating, and swirl characteristics. But increasing inlet mass flow rate will lead to the opposite effect. When the airflow is in the heat balance state, relative total temperature and adiabatic temperature can be acquired by measuring absolute temperature and swirl ratio at the same radius. The maximum deviation is below 2 K. The results can be beneficial to the optimization design of rotor–stator disk cavity in the gas turbine engines.

Effect of windage heating on a micro high-speed rotor-stator cavity

The main objective of this work is to investigate the effect of windage heating on the micro high-speed rotor-stator cavity. The influences of centrifugal force and spacing on the windage heating are analyzed with and without the change of gap ratio respectively. The results demonstrate that there is no difference in the flow structure between micro and large-scale rotor-stator cavities at the same rotational Reynolds number and gap ratio. However, the windage heating induced by the larger centrifugal force and smaller spacing brings the different heat transfer laws for the micro rotor-stator cavity. The larger centrifugal force weakens the local heat transfer near the rotor periphery. Such influence can be strengthened at higher rotational Reynolds numbers and lower rotor excess temperatures. Besides, the smaller spacing further enhances the windage heating and reduces the average heat transfer especially under the condition of lower gap ratio. The findings of this work contribute to the design of rotor-stator cavity utilized in the micro rotating machinery.

Flow Characteristics and Optimization Design of the Stator–Rotor Cavity of the Full Tubular Pump

The full tubular pump device is taken as the research object in this article. This research method adopts the numerical simulation technology based on the SST (Shear-Stress-Transport) k-ω turbulence model to explore the internal flow characteristics of the stator–rotor cavity of the full tubular pump and optimize the stator–rotor clearance structure. The research shows that under the design conditions, compared with the axial flow pump, the torque increases by 47.91 N·m at the stator–rotor cavity structure and the efficiency decreases by about 20%. The torque at the rotor clearance of the full tubular pump accounts for about 50% of the torque at the rotor. Since there is a large area of backflow on both sides of the cavity, and there is a vortex structure on the inlet side of the cavity, it shows that the rotor structure and its area greatly affect the operating efficiency of the pump device. With the reduction in the rotor force area, the clearance length, and the outer diameter of the disc, the operating efficiency of the pump device gradually increases. Under the design conditions, the optimized model has a maximum efficiency increase of 14.04% and the torque at the cavity rotor is reduced by 39.25 N·m. The results show that the operating efficiency of the full tubular pump is closely related to its rotor structure area, and the force area of the rotor structure needs to be controlled in the actual design process.

Swirl Ratio Prediction Model in Rotor-Stator Cavity With Superposed Outward Radial Throughflow Based on Region Partition

Abstract The Rotor-Stator cavity (R-S cavity) is a prototype model in many engineering applications such as gas turbine secondary air systems. The flow characteristics of the R-S cavity are relatively complex considering the rotation effect. A radial through flow is usually superposed in the R-S cavity, further complicating the fluid motion. The flow inside an R-S cavity with a superposed radial throughflow can be divided into four regions based on flow characteristics: a source region, a rotor entrainment layer, a rotating core, and a mixing region. In the present work, a one-dimensional (1-D) radial swirl ratio predictive model is built and verified based on computational fluid dynamics (CFD) results in the rotor entrainment layer and rotating core region. A swirl ratio gradient governing equation is deduced at first. The equation involves two scale factors CS and CR which are related to the stator and rotor friction correspondingly. The governing equation in the rotor entrainment layer is further simplified by neglecting the stator friction factor CS where the rotor friction prevails. Then, based on the discretized governing equation, CR and CS are determined via approximation with CFD results. Correlations between CR, CS, and nondimensional radial through flowrate cw are determined and verified. The obtained correlations and the discretized governing equation together form the complete swirl ratio, predictive model. The model accuracy is described by cross-correlation coefficients, which show a good agreement. The 1-D model is then implemented to different rotating speed cases, based on which the model portability is discussed.

Turbomachine Operation with Magnetic Bearings in Supercritical Carbon Dioxide Environment

In the sCO2-HeRo project, the Chair of Turbomachinery at the University of Duisburg-Essen developed, built and tested a turbomachine with an integral design in which the compressor, generator and turbine are housed in a single hermetic casing. However, ball bearings limited operation because their lubricants were incompatible with supercritical CO2 (sCO2) and they had to operate in gaseous CO2 instead. To overcome this problem, the turbomachine was redesigned built and tested in the sCO2-4-NPP project. Instead of ball bearings, magnetic bearings are now used to operate the turbomachine with the entire rotor in sCO2. This paper presents the revised design, focusing on the usage of magnetic bearings. It also investigates whether the sCO2 limits the operating range. Test runs show that increasing the density and rotational speed results in greater deflection of the rotor and greater forces on the bearings. Measurements are also analyzed with respect to influence of the density increase on the destabilizing forces in the rotor–stator cavities. The conclusion is that for the operation of the turbomachine, the control parameters of the magnetic bearings must be adjusted not only to the rotor speed, but also to the fluid density. This enabled successful operation of the turbomachine, which reached a speed of about 40,000 rpm during initial tests in CO2.

An Improved One-Dimensional Flow Model for Side Chambers of Centrifugal Pumps Considering the Blade Slip Factor

Abstract The blade slip factor significantly influences the prediction accuracy of the self-closure one-dimensional flow model for side chambers of centrifugal pumps. Wiesner's and Stodola's slip factors, which are used to formulate the blade outlet pressure and served as the boundary condition for the model, are examined, which is an improvement of the previous study (Gu et al., 2020, “A Pressure Model for Open Rotor–Stator Cavities: An Application to an Adjustable-Speed Centrifugal Pump With Experimental Validation,” ASME J. Fluids Eng., 142(10), p. 101301). Both computational fluid dynamics (CFD) simulations and experiments for the centrifugal pump are conducted to support the improvement. A good agreement exists between the performance at the best efficiency points (BEPs) of different rotating speeds obtained by simulations and experiments. Through the CFD analysis, the flow in the impeller remarkably deviates from the blade-congruent flow, especially in the quasi-triangular regions downstream of the throats. Meanwhile, the reason for pressure over-predictions of the side chamber one-dimensional flow model that embeds Wiesner's slip factor (FMW) is that Wiesner's expression underestimates the impeller flow deflection and overestimates pressure boundary. By contrast, the side chamber one-dimensional flow model with Stodola's slip factor (FMS) is closer to CFD in terms of relative flow angle and chamber inlet pressure. Compared with the side chamber pressure measurements, the accuracy of FMS is upgraded approximately by 3.5% than FMW. At the BEPs of different rotating speeds, FMS generates lower shroud thrust coefficients but slightly greater volumetric efficiencies than FMW. This work provides a simple approach to better calculate flow characteristics in the side chambers.

Unsteady numerical investigation on gas ingestion into the rotor-stator disk cavities of a 1.5-stage turbine

AbstractTo investigate the downstream rim seal gas ingestion characteristics of a 1.5-stage turbine, the URANS equations were solved numerically using the SST turbulence model. The effects of different purge flow rates and the second vane on the ingestion characteristics of the aft cavity and the nonuniform fluctuations of the main gas path pressure are analysed. The results showed that the aft cavity is affected by the combined effects of the blade and the second vane, and the potential field at the leading edge of the second vane greatly influence the airflow variation in the aft cavity, which enhances the ingress of the mainstream into the wheel-space. The front purge flow weakens the egress between the suction side of the blade and the suction side of the second vane. The potential field at the leading edge of the second vane suppresses the nonuniform distribution of airflow in the aft cavity caused by the rotational effect of the blade.

Influence of Blade Fracture on the Flow of Rotor-Stator Systems with Centrifugal Superposed Flow

Rotor-stator cavities are often found in turbomachinery; they supply cold air that is bled from the compressor to the turbine blades. The pressure of the outlet of a rotor-stator cavity is axisymmetric under normal circumstances. However, its pressure would be non-axisymmetric in the event of blade fracture. The impact of blade fracture on a rotor-stator cavity with centrifugal superposed flow is studied in this paper. The Euler number E, the rotational Reynolds number Reφ, and the low-pressure zone range θ are investigated and, for the first time, with the non-axisymmetrical boundary conditions employing numerical simulation. The results of the numerical calculations show that after turbine blade fracture, the velocity is more affected in the downstream region at a high radius, especially when the Reφ is large. As for the distribution of the mass flow rate, there may be a critical θc at which the other blades are least affected. The θc would increase as the Reφ or the E increase, and the θc≅0.2 when Cw=10,137, Reφ=5.12×105, and 0.2≤E≤0.4. In addition, the thrust coefficient increases as the E or the θ increases, and the increase in the thrust coefficient does not exceed 4% when the E=0.2 and the θ=0.1 in this paper. However, the moment coefficient on the rotating shaft is almost independent of the E and the θ. An increase in the Reφ will reduce the effect of turbine blade fracture on the thrust and moment coefficients, when the Reφ is small.

Numerical simulation analysis of flow characteristics in the cavity of the rotor-stator system

ABSTRACT Investigating the flow characteristics in the cavity by changing the rotor profile of the rotor-stator system is of considerable significance, this paper simplifies clearance flow to rotor-stator cavity flow. The numerical simulation results are compared with the experimental results of the windage torque of the rotor-stator system, and the results show that the accuracy of the numerical calculation meets the requirements. The inlet flow rate and outlet gap were unchanged, and the flow in the concave cavity was most similar to the original model. The torque increased slightly and was more entrained, while the flow pattern of the convex cavity was more chaotic, and various forms of vortices appeared in the rotor boundary layer. The interference of the rotor and stator disks and the viscous effect of the fluid in the cavity are the main reasons for the generation of torque, and the Reynolds stress near the rotor wall also contributes partly to the torque. After data processing, an interesting phenomenon was observed. In the boundary layer of the rotor, the radial flow in which the inertial force plays a major role is transformed into a circular motion dominated by the rotation of the rotor.

Experimental study on the aerodynamic influence of purge flow in a turbine cascade with different mainstream incidence angles

Demand for high reliability and long life of modern turbine requires that turbine components should be cooled adequately. The cooling flow purged into the rotor-stator disk cavity will inevitably interact with the mainstream. The current paper mainly focuses on the aerodynamic influence of cooling flow on the secondary flows in the mainstream. Both particle image velocimetry and blade wall pressure measurement were utilized to study the flow field within the turbine blade passage under different mainstream incidence angles and purge flow rates. The purge flow was found to promote the development of the passage vortex by inducing vortices which can enhance the vorticity of the passage vortex. In addition, the mainstream incidence angle also has an impact on the development of the passage vortex through affecting the blade loading and the horseshoe vortex. Furthermore, the transient results demonstrate that the time-averaged vortex is the superposition of large number of transient vortices, and the purge flow causes more transient vortices with large size and high strength.

Experimental Investigation on Flow Characteristics in a Turbine Rotor-Stator Cavity with Inlet at Low Radius

An experimental investigation has been designed and performed to determine the fluid flow characteristics in a rotor-stator cavity with large gap ratio and inlet at low radius. In this investigation, the particle image velocimetry (PIV) technique is employed for the velocity measurements of airflow in the rotor-stator cavity. Local swirl ratios are calculated from these velocity distributions. A range of parameter combinations of interest in fluid flow characteristics is covered, including combinations of variations in axial location, rotational Reynolds number (denoted as Reφ), and inlet nondimensional mass flow rate (denoted as CW). The results indicate that the flow in the rotor-stator cavity is still dominated by circumferential motion. The velocity is basically unchanged along the circumference. However, the velocity increases with the increase of radial position. In different axial positions, the velocity distribution is different. The closer to the rotor, the more uneven the flow in the rotor-stator cavity is. The disturbance of the rotor rotating motion to the fluid mainly occurs in the high radius region, which should be paid more attention to in the process of reforming the flow field characteristics. To the circumferential velocity of low radius fluid, the Reφ is in direct proportion, while the CW is in inverse. The swirl ratio in the core region is very close to that in the high radius near the wall region of the rotor, which can be inferred to be caused by the backflow in the rotor-stator cavity.

Swirl Flow and Heat Transfer in a Rotor-Stator Cavity with Consideration of Inlet Seal Thermal Deformation Effect

Swirl Flow and Heat Transfer in a Rotor-Stator Cavity with Consideration of Inlet Seal Thermal Deformation Effect

Effect of Rotational Speed Variation on the Flow Characteristics in the Rotor-Stator System Cavity

In this paper, to study the effect of dynamic and static interference of clearance flow in fluid machinery caused by changes in rotational speed, the model was simplified to a rotor-stator system cavity flow. Investigating the flow characteristics in the cavity by changing the rotor speed of the rotor-stator system is of considerable significance. ANSYS-CFX was applied to numerically simulate the test model and the results were compared with the experimental results of the windage torque of the rotor-stator system. The inlet flow rate and geometric model remained unchanged. With an increase in the rotating Reynolds number, the shear stress on the rotor wall gradually increased, and the maximum gradient was within l* < 0.15. In addition to the shear stress, the tangential Reynolds stress Rrθ contributed partly to the torque on the rotor wall. The swirling vortex formed by entrainment in the cavity of the rotor-stator system tended to separate at ReΦ= 3.53 × 106. As the rotating Reynolds number continued to increase, the secondary vortex finally separated completely. The strength of the vortex in the rotor turbulent boundary layer decreased with an increase in the rotating speed, but the number of vortex cores increased with the increase of speed. Depending on the application of the fluid machine, controlling the rotating speed within a reasonable range can effectively improve the characteristics of the clearance flow.

Erratum: Hu et al. Numerical Investigation of Flow and Heat Transfer in a Rotor-Stator Cavity with Centripetal Carbon Dioxide Through-Flow. Energies 2019, 12, 2613

The authors would like to make the following explanations and changes to the original paper [...]

Flow and heat transfer characteristics in a pre-swirl rotor-stator cavity

The present study investigates flow and heat transfer characteristics in a rotor-stator pre-swirl system. Considered are rotational Reynolds numbers ReФ ranging from 3.4 × 105 to 6.8 × 105, with jet Reynolds numbers Rew of approximately 8.8 × 104 to 3.81 × 105, and associated turbulent flow parameter λT = Cw/ReФ0.8 varying from 0 to 0.4. Experimentally-measured and numerically-predicted results illustrate the combined and separate influences of jet impingement and rotor entrainment, as they affect and govern flow and heat transfer characteristics within the cavity. These results show that the relative strength of the two effects varies with ReФ and Rew, with similar performance for flow conditions with similar magnitudes of the turbulent flow parameter λT. When 0<λT < 0.27, jet impingement and rotor entrainment both have important influences on flow and heat transfer characteristics. However, when λT > 0.27, the effects of rotor entrainment are less pronounced.

Numerical research on effect of dislocated turbine rims on hot gas ingestion

The flow characteristics in the rim clearances of turbines are sensitive to the turbine rim structures. From this standpoint, rotor–stator cavity models with dislocated clearances are proposed in this study to reveal the effect of dislocated turbine rim lips on hot gas ingestion and the clearance flow characteristics. The results show that the rim clearance flow is unstable, and Kelvin–Helmholtz vortices are induced by the relatively large difference in circumferential velocity of the flow fields inside and outside the rim seal. Moreover, the dislocated turbine rim lips decrease the effectiveness of the rim seal. Ultimately, the responses of the flow characteristics to the dislocated rims are determined to be primarily reflected in the effect of the forward or backward step flow on the Kelvin–Helmholtz vortices in turbine rims.

Aero-thermal optimization of the rim seal cavity to enhance rotor platform thermal protection

Abstract The flow field in the rotor-stator rim cavity is controlled by both the instantaneous distortion caused by rotor and stator airfoils. Naturally one would then assume that the design of optimal rim seal geometries would require full unsteady turbine stage simulations, that would then require an experimental assessment. This manuscript presents a reduced-order approach, neglecting the obvious unsteady interaction effects, to optimize the cavity geometry with a two-dimensional axisymmetric approach, making it possible to explore a wide range of geometries followed by an experimental assessment in a linear cascade. Two parameterization strategies are presented, coupled with a genetic optimizer to maximize thermal protection to the rotor rear platform while minimizing the cooling massflow required. This reduced-order optimization scheme was then assessed at different levels of sophistication to assess the effect of rotation, the vanes, and the rotor geometry with a series of increasing levels of fidelity to the real turbine conditions. Finally, this paper demonstrated the sensitivity of the optimal geometries to variations in the axial gap and purge coolant total pressure.