Turbomachinery Applications Research Articles

Closed-loop active flow control of a non-steady flow field in a highly-loaded compressor cascade

The provision of secure compressor operation under circumstances of a pulsed detonation engine is crucial for the success of pressure gaining combustion processes for turbo machinery applications. This paper discusses active flow control as a possible solution to approach this challenge. The presented experiments were conducted on a highly loaded low speed linear compressor stator cascade operated at \(Re=600{,}000\) and \(Ma =0.07\). A choking-device which was located in the wake of the cascade simulated the non-steady outflow condition that is expected under the conditions of pressure gaining combustion. In the discussed experiments, the choking-device generated a periodic disturbance to every passage at a typical Strouhal number of \(Sr =0.03\). The flow structures of the non-steady flow field were strongly correlated to the working-phase of the choking-device. In this paper, an iterative learning controller was used to find an optimized actuation trajectory that was used for closed-loop sidewall-actuation to control the corner separation in the non-steady flow field. The iterative learning controller took advantage of the periodicity of the disturbance to calculate a non-steady actuation trajectory that optimally suppressed the impact of the choking-device on the flow. The active flow control effect was evaluated by means of static pressure rise using five hole probe measurements in the wake of one passage.

Unsteady Adjoint of Pressure Loss for a Fundamental Transonic Turbine Vane

High-fidelity simulations, e.g., large eddy simulation (LES), are often needed for accurately predicting pressure losses due to wake mixing and boundary layer development in turbomachinery applications. An unsteady adjoint of high-fidelity simulations is useful for design optimization in such aerodynamic applications. In this paper, we present unsteady adjoint solutions using a large eddy simulation model for an inlet guide vane from von Karman Institute (VKI) using aerothermal objectives. The unsteady adjoint method is effective in capturing the gradient for a short time interval aerothermal objective, whereas the method provides diverging gradients for long time-averaged thermal objectives. As the boundary layer on the suction side near the trailing edge of the vane is turbulent, it poses a challenge for the adjoint solver. The chaotic dynamics cause the adjoint solution to diverge exponentially from the trailing edge region when solved backward in time. This results in the corruption of the sensitivities obtained from the adjoint solutions. An energy analysis of the unsteady compressible Navier–Stokes adjoint equations indicates that adding artificial viscosity to the adjoint equations can dissipate the adjoint energy while potentially maintaining the accuracy of the adjoint sensitivities. Analyzing the growth term of the adjoint energy provides a metric for identifying the regions in the flow where the adjoint term is diverging. Results for the vane obtained from simulations performed on the Titan supercomputer are demonstrated.

Design Optimization and Testing of High-Performance Motors: Evaluating a Compromise Between Quality Design Development and Production Costs of a Halbach-Array PM Slotless Motor

To meet the needs of the ongoing expansion in the gas industry, common requirements such as high reliability, efficiency, and dynamic performance must be satisfied while still keeping costs competitive. This article reports on an industrial research and development (R&D) project to find the best compromise between performance and production cost targets for a special direct-drive turbomachinery application. The development and testing of a 640-kW 10,000-r/min motor prototype characterized by a slotless stator and a Halbach-magnetized, magnetically levitated permanentmagnet (PM) rotor is discussed, including the technology selection, the design optimization processes, the machine manufacturing, and testing results.

Experimental investigation on flow improvement in compressor cascades

There is no general rule in the literature to help choose a correct flow control device for any given case of turbomachinery applications. This suggests individual optimization of flow control devices for each specific case. The objective of this study is to prove experimentally the benefits of passive control methods in improving the compressor performance. This allows to reduce the fuel consumption, leading to energy saving and reduction of atmospheric pollution. Two features have been controlled in this study: flow separation over the blade surfaces and the secondary flow over the cascade endwalls. Vortex generator ribs are tested on the blade suction side for flow reattachment on the blade surfaces, and low-profile vortex generators are tested on the side walls of the compressor cascade against secondary flow losses. Different vortex generator designs are compared for total pressure recovery, flow turning, boundary layer characteristics, and pressure distributions over the endwalls. Copyright © 2016 John Wiley & Sons, Ltd.

Development of a Fast-Response Aerodynamic Pressure Probe Based on a Waveguide Approach

Currently, fast-response aerodynamic probes are widely used for advanced experimental investigations in turbomachinery applications. The most common configuration is a virtual three-hole probe. This solution is a good compromise between probe dimension and accuracy. Several authors have attempted to extend the capabilities of these probes in terms of bandwidth and operating conditions. Even though differences exist between the solutions in the literature, all of the designs involve the positioning of a dynamic pressure sensor close to the measurement point. In general terms, the higher the frequency response, the more the sensor is exposed to the flow. This physical constraint puts a limit on the probe applicability since the measurement conditions have to comply with the maximum allowed operating conditions of the sensor. In other applications, when the conditions are particularly harsh and a direct measurement is not possible, a waveguide probe is commonly used to estimate the local pressure. In this device, the sensor is connected to the measurement point through a transmitting duct which guarantees that the sensor is operating in a less critical condition. Generally, the measurement is performed through a pressure tap and particular attention must be paid to the probe design in order to have an acceptable frequency response function. In this study, the authors conceived, developed, and tested a probe which combines the concept of a fast-response aerodynamic pressure probe with that of a waveguide probe. Such a device exploits the benefits of having the sensor far from the harsh conditions while maintaining the capability to perform an accurate flow measurement.

Aerodynamic Optimization of a Microturbine Inserted in a Magic-Angle Spinning System

The fluid dynamics of a microturbine system that is applied in a device for chemical and biological analysis—a so-called magic-angle spinning (MAS) probe—is investigated. The drive fluid is pressurized air at ambient temperature provided by nozzles aligned on an intake spiral, driving a Pelton-type microturbine. Computational fluid dynamics (CFD) simulations have been performed and compared with fluid dynamics measurements of the MAS system with 1.3 mm rotor diameter for spinning rates between 23 kHz and 67 kHz. The main optimization criteria of the MAS system are rotor speed and turbine stability and not primarily efficiency, which is standard for turbomachinery applications. In the frame of fabrication tolerances, a sensitivity study has been carried out by varying the nozzles diameter and the nozzle position relative to the rotor. The presented fluid dynamics study of the microturbine system includes the analysis of local fluid flow values such as velocity, temperature, pressure, and Mach number, as well as global quantities like forces and driven torque acting on the turbine. Comparison with the experimental results shows good agreement of the microturbine efficiency. Furthermore, the parameter study of the nozzle diameter reveals optimization potential for this high-speed microturbine system employing a smaller nozzle diameter.

Dynamic performance of foil bearings with a quadratic stiffness model

Hydrodynamic foil bearings found various applications in high speed small turbomachinery due to several advantages compared with traditional oil lubricated bearing and rolling element bearings such as high speed, pollution free, high temperature, long service life, etc. Corrugated bump foils made of thin sheet metal is the most widely adopted foil structure used in many commercial foil bearings. However, due to various factors during manufacturing process of the foil structure and mounting methods of the foil structure on to the bearing sleeve, measured stiffness curves of the foil structure show very strong non-linearity. The non-linearity of the foil structure is often neglected in most simulation studies in the past, and single value of bump stiffness combined with appropriate structural loss factor were used in these simulation studies. Based on measured non-linear structural stiffness of entire foil bearing, a quadratic stiffness model of individual bump foil was derived and utilized in transient rotordynamic simulations in time domain by solving Reynolds equation, rotor equations of motion, and bump deflections together. The simulation results using the quadratic bump stiffness model were compared with those using linear bump stiffness models.

Experimental and Numerical Heat Transfer Investigation of an Impinging Jet Array on a Target Plate Roughened by Cubic Micro Pin Fins1

A generic impingement cooling system for turbomachinery application is modeled experimentally and numerically to investigate heat transfer and pressure loss characteristics. The experimental setup consists of an array of 9 × 9 jets impinging on a target plate with cubic micro pin fins. The cubic micro pin fins have an edge length of 0.22 D and enlarge the target area by 150%. Experimentally heat transfer is measured by the transient liquid crystal (TLC) method. The transient method used requires a heated jet impinging on a cold target plate. As reference temperature for the heat transfer coefficient, we use the total jet inlet temperature which is measured via thermocouples in the jet center. The computational fluid dynamics (CFD) model was realized within the software package ANSYS CFX. This model uses a Steady-state 3D Reynolds-averaged Navier–Stokes (RANS) approach and the shear stress transport (SST) turbulence model. Boundary conditions are chosen to mimic the experiments as close as possible. The effects of different jet-to-plate spacing (H/D = 3–5), crossflow schemes, and jet Reynolds number (15,000–35,000) are investigated experimentally and numerically. The results include local Nusselt numbers as well as area and line averaged values. Numerical simulations allow a detailed insight into the fluid mechanics of the problem and complement experimental measurements. A good overall agreement of experimental and numerical behavior for all investigated cases could be reached. Depending on the crossflow scheme, the cubic micro pin fin setup increases the heat flux to about 134–142% compared to a flat target plate. At the same time, the Nusselt number slightly decreases. The micro pin fins increase the pressure loss by not more than 14%. The results show that the numerical model predicts the heat transfer characteristics of the cubic micro pin fins in a satisfactory way.

A New Particle Image Velocimetry Technique for Turbomachinery Applications

Nonintrusive measurement techniques such as particle image velocimetry (PIV) are growing in both capability and utility for turbomachinery applications. However, the restrictive optical access afforded by multistage research compressors typically requires the use of a periscope probe to introduce the laser sheet for measurements in a rotor passage. This paper demonstrates the capability to perform three-dimensional PIV in a multistage compressor without the need for intrusive optical probes and requiring only line-of-sight optical access. The results collected from the embedded second stage of a three-stage axial compressor highlight the rotor tip leakage flow, and PIV measurements are qualitatively compared with high-frequency response piezoresistive pressure measurements to assess the tip leakage flow identification.

Modeling of deviation angle and performance losses in wet steam turbines using GMDH-type neural networks

In the present study group method of data handling (GMDH) type of artificial neural networks are used to model deviation angle (θ), total pressure loss coefficient (ω), and performance loss coefficient (ξ) in wet steam turbines. These parameters are modeled with respect to four input variables, i.e., stagnation pressure (P z ), stagnation temperature (T z ), back pressure (P b), and inflow angle (β). The required input and output data to train the neural networks has been taken from numerical simulations. An AUSM–Van Leer hybrid scheme is used to solve two-phase transonic steam flow numerically. Based on results of the paper, GMDH-type neural networks can successfully model and predict deviation angle, total pressure loss coefficient, and performance loss coefficient in wet steam turbines. Absolute fraction of variance (R 2 ) and root-mean-squared error related to total pressure loss coefficient (ω) are equal to 0.992 and 0.002, respectively. Thus GMDH models have enough accuracy for turbomachinery applications.

An OpenFOAM pressure-based coupled CFD solver for turbulent and compressible flows in turbomachinery applications

ABSTRACTIn this article, a recently developed pressure-based, fully coupled solver capable of predicting fluid flow at all speeds is extended to deal with turbulent flows in a rotating frame of reference and emphasizing turbomachinery applications. The pressure–velocity coupling at the heart of the Navier-Stokes equations is resolved by deriving a pressure equation in a similar fashion to a segregated SIMPLE algorithm but with implicit treatment of the velocity and pressure fields. The resulting system of coupled equations is solved using an algebraic multigrid solver. The above numerical procedures have been implemented within OpenFOAM®, which is an open-source code framework capable of dealing with industrial-scale flow problems. The OpenFOAM-based coupled solver is validated using experimental and numerical data available from reference literature test cases as well as with a segregated solver based on the SIMPLE algorithm. This is done in addition to evaluating its performance by solving an industrial problem. In comparison with the segregated solver, the coupled solver results indicate substantial reduction in computational cost with increased robustness.

Linear stability analysis of finite hydrodynamic journal bearing under turbulent lubrication with coupled-stress fluid

Purpose Several researchers have observed that to satisfy modern day’s need, it is essential to enhance the characteristics of journal bearing, which is used in numerous applications. Moreover, the use of Newtonian fluid as a lubricant is diminishing day by day, and the use of Non-Newtonian fluids is coming more into picture. Furthermore, if turbo-machinery applications are taken into account, then it can be seen that journal bearings are used for high speed applications as well. Thus, neglecting turbulent conditions may lead to erroneous results. Hence, this paper aims to present focuses on studying the stability characteristics of finite hydrodynamic journal bearing under turbulent coupled-stress lubrication. Design/methodology/approach First, the governing equation relevant to the problem is generated. Then, the dynamic analysis is carried out by linear perturbation technique, leading to three perturbed equations, which are again discretized by finite difference method. Finally, these discretized equations are solved with the help of Gauss-Seidel Iteration technique with successive over relaxation scheme. Consequently, the film response coefficients and the stability parameters are evaluated at different parametric conditions. Findings It has been concluded from the study that with increase in value of the coupled-stress parameter, the stability of the journal may increase. Whereas, with increase in Reynolds number, the stability of the journal decreases. On the other hand, stability increases with increasing values of slenderness ratio. Originality/value Researches have been performed to study the dynamic characteristics of journal bearing with non-Newtonian fluid as the lubricant. But in the class of non-Newtonian lubricants, the use of coupled-stress fluid has not yet been properly investigated. So, an attempt has been made to perform the stability analysis of bearings with coupled-stress fluid as the advanced lubricant.

A novel data reduction technique for single slanted hot-wire measurements used to study incompressible compressor tip leakage flows

The single slanted hot-wire technique has been used extensively as a method for measuring three velocity components in turbomachinery applications. The cross-flow orientation of probes with respect to the mean flow in rotating machinery results in detrimental prong interference effects when using multi-wire probes. As a result, the single slanted hot-wire technique is often preferred. Typical data reduction techniques solve a set of nonlinear equations determined by curve fits to calibration data. A new method is proposed which utilizes a look-up table method applied to a simulated triple-wire sensor with application to turbomachinery environments having subsonic, incompressible flows. Specific discussion regarding corrections for temperature and density changes present in a multistage compressor application is included, and additional consideration is given to the experimental error which accompanies each data reduction process. Hot-wire data collected from a three-stage research compressor with two rotor tip clearances are used to compare the look-up table technique with the traditional nonlinear equation method. The look-up table approach yields velocity errors of less than 5 % for test conditions deviating by more than 20 °C from calibration conditions (on par with the nonlinear solver method), while requiring less than 10 % of the computational processing time.

Comparative Experimental Analysis of Load Carrying Capability of Air Foil Thrust Bearing for Different Configuration of Foil Assembly

Foil bearings are self-acting (aerodynamic) hydrodynamic bearings supports lightly loaded shafts with major advantages like physically non-contacting high speed operation using process fluid as medium in hostile environments. However, the lack of load carrying capacity at relatively lower speeds limits their applications in heavy turbo machinery and as such are highly suitable in lightly loaded, high speed turbo machinery like small gas turbines.This paper discusses the design and assessment of dynamic characteristics in terms of load carrying capabilities as a function of speed, gap between the bearing and the runner as well as shape of foils for an air foil thrust bearing. The effects of various bearing parameters like foil thickness, number of foils fixed circumferentially, foil geometry configuration and position of foils along the circumference such that inner edge and outer edge of the same foil are at different height and same height. Characteristics of performance defined essentially in terms of load carrying capabilities and static stiffness have been used for evaluation. Experiments were conducted for sector foils (with inner edge height less than outer edge height) and sector foils (with inner edge height is equal to outer edge height) by varying number of foils. Along with the above two types the load carrying capabilities of air foil thrust bearing using newly conceptualized cascaded foils (Patented) with increased number of wedge films essentially from the point view of increased pressure profile culminating in relatively higher load carrying capabilities as compared to other two types (conventional) Air foil thrust bearing.The experimental results show that the effect of foil assembly, number and improved formation of wedge film which enhances the load carrying capabilities of air foil thrust bearing.Along with the above experimental work an COMSOL model of the thrust bearing have been analysed to know that the pressure distribution profile on the foil which clearly indicates that the pressure developed by the conventional type of foils configuration is less than the newly configured foils in which inner edge height is equal to outer edge along the radial direction i.e the height of the foil with respect to the runner is equal in the radial direction.

The Three-Dimensional Velocity Distribution of Wide Gap Taylor-Couette Flow Modelled by CFD

A numerical investigation is conducted for the flow between two concentric cylinders with a wide gap, relevant to bearing chamber applications. This wide gap configuration has received comparatively less attention than narrow gap journal bearing type geometries. The flow in the gap between an inner rotating cylinder and an outer stationary cylinder has been modelled as an incompressible flow using an implicit finite volume RANS scheme with the realisablek-εmodel. The model flow is above the critical Taylor number at which axisymmetric counterrotating Taylor vortices are formed. The tangential velocity profiles at all axial locations are different from typical journal bearing applications, where the velocity profiles are quasilinear. The predicted results led to two significant findings of impact in rotating machinery operations. Firstly, the axial variation of the tangential velocity gradient induces an axially varying shear stress, resulting in local bands of enhanced work input to the working fluid. This is likely to cause unwanted heat transfer on the surface in high torque turbomachinery applications. Secondly, the radial inflow at the axial end-wall boundaries is likely to promote the transport of debris to the junction between the end-collar and the rotating cylinder, causing the build-up of fouling in the seal.

Density-based solver for all Mach number flows

A density-based solver for turbomachinery application is developed based on the central-upwind scheme of Kurganov using the open source CFD-library OpenFOAM. Preconditioning of Weiss and Smith is utilised to extend the applicability down to the incompressibility limit. Implicit residual averaging, bulk viscosity damping and local time stepping are employed to speed up the simulations. A low-storage four-stage Runge-Kutta scheme and dual time-stepping are used for time integration. The presented solver is validated using measurement data from three well-known reference cases. The inviscid performance is investigated using the circular bump test cases at incompressible, subsonic, transonic and supersonic conditions. The laminar flow around a circular cylinder is analysed and compared to analytical and experimental data. Finally, a low speed centrifugal compressor is simulated. All three cases show a very good agreement with the reference data.

High-order implementation of a non-local transition model in a DG solver for turbomachinery applications

Transition modelling represents a key ingredient for improving the performances of modern turbomachinery, affecting losses and the heat-transfer phenomenon. Different methods have been proposed in the finite volume context to predict the laminar-turbulent transition, but, according to the literature, not yet for high-order methods. In this paper the transition model proposed by Kožulović and Lapworth [1] was considered, which is integral, non-local and based upon experimental correlations, takes into account all the relevant modes of transition, and is well suited for turbomachinery applications. The model was implemented into a parallel high-order accurate Discontinuous Galerkin code, named MIGALE, which allows to solve the Reynolds averaged Navier–Stokes (RANS) equations coupled with the k-ω˜ (ω˜=log(ω)) turbulence model. The objective of this work is to show the capability of high-order DG methods in accurately computing complex transitional flows on coarse and low quality grids. The transitional flow around a flat plate (T3A and T3B configurations) and through the T106A turbine cascade has been computed up to fifth order and compared with available numerical and experimental data.

Experimental Studies on Air Foil Thrust Bearing Load Capabilities Considering the Effect of Foil Configuration

Abstract. Foil bearings are self-acting hydrodynamics bearings used to support lightly loaded high speed rotating machinery. The advantages that they offer to process fluid lubricated machines usingworking fluid as a lubricant (ambient air) physically non-contacting high speed operation. Foil bearings have been considered as an alternative to conventional bearings with the capacity to cater for high-speeds and hostile environment (high temperature). However, the lack of load carrying capacity at relatively lower speeds limits their applications in heavy turbo machinery and as such are highly suitable in lightly loaded, high speed turbo machinery like small gas turbines.This paper discusses the design and assessment of dynamic characteristics in terms of load carrying capabilities as a function of speed, gap between the bearing and the runner as well as shape of foils for an air foil thrust bearing. The effects of various bearing parameters like foil thickness, number of foils fixed circumferentially and along the axis of rotation and with foil geometry configuration. Characteristics of performance defined essentially in terms of load carrying capabilities and static stiffness have been used for evaluation. Experiments were conducted both for angular foils (with inner edge height less than outer edge height) and square foils by varying number of foils. The experimental results shows that the effect of foil configuration enhances the load carrying capabilities of air foil thrust bearing.

Dissipated power and induced velocity fields data of a micro single dielectric barrier discharge plasma actuator for active flow control.

In recent years, single dielectric barrier discharge (SDBD) plasma actuators have gained great interest among all the active flow control devices typically employed in aerospace and turbomachinery applications [1,2]. Compared with the macro SDBDs, the micro single dielectric barrier discharge (MSDBD) actuators showed a higher efficiency in conversion of input electrical power to delivered mechanical power [3,4]. This article provides data regarding the performances of a MSDBD plasma actuator [5,6]. The power dissipation values [5] and the experimental and numerical induced velocity fields [6] are provided. The present data support and enrich the research article entitled “Optimization of micro single dielectric barrier discharge plasma actuator models based on experimental velocity and body force fields” by Pescini et al. [6].

Optimized design of linear cascades for turbomachinery applications

This paper presents a cascade optimization methodology for representing axial turbomachinery profiles of series NACA-65. In geometrical parameterization scheme of the cascade the following are considered as design variables: the stagger angle, cascade pitch, and camber line. For the calculation of the flow, the panel method of Hess and Smith with modifications is used to introduce the viscous effects by calculating the boundary layer. This methodology used a viscous/inviscid interaction and has as advantage the low computational cost and being favorable for the optimization process. The maximization of the relationship between the lift and the drag coefficient was defined as objective function and was added a penalty function, given by the minimum deflection angle of the flow in the cascade. For the optimal solution, a controlled random search algorithm is used. On the other hand, is also constructed a response surface by interpolation of radial basis functions to obtain comparative analysis of the solutions. Results of pressure distributions and aerodynamic coefficients are also obtained for the optimal cascades.