bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the Rotordynamic Response of a Centrifugal Compressor Subject to Aerodynamic Loads due to Rotating Stall
Authors: Biliotti Davide , Bianchini Alessandro , Vannini Giuseppe , Belardini Elisabetta , Giachi Marco , Tapinassi Libero , Ferrari Lorenzo , Ferrara Giovanni
Published: September 2014

In the current industrial research on centrifugal compressors, manufacturers are showing increasing interest in the extension of the minimum stable flow limit in order to improve the operability of each unit. The aerodynamic performance of a compressor stage is indeed often limited before surge by the occurrence of diffuser rotating stall. This phenomenon generally causes an increase of the radial vibrations, which, however, is not always connected with a remarkable performance detriment.

In case the operating curve has been limited by a mechanical criterion, i.e. based on the onset of induced vibrations, an investigation on the evolution of the aerodynamic phenomenon when the flow rate is further reduced can provide some useful information. In particular, the identification of the real thermodynamic limit of the system could allow one to verify if the new load condition could be tolerated by the rotordynamic system in terms of radial vibrations.

Within this context, recent works showed that the aerodynamic loads due to a vaneless diffuser rotating stall can be estimated by means of test-rig experimental data of the most critical stage. Moreover, by including these data into a rotordynamic model of the whole machine, the expected vibration levels in real operating conditions can be satisfactorily predicted.

To this purpose, a wide-range analysis was carried out on a large industrial database of impellers operating in presence of diffuser rotating stall; the analysis highlighted specific ranges for the resultant rotating force in terms of intensity and excitation frequency. Moving from these results, rotordynamic analyses have been performed on a specific case study to assess the final impact of these aerodynamic excitations.
A Transient CFD Simulation of the Flow in a Test Rig of an Aeroengine Bearing Chamber
Authors: Adeniyi Akinola A. , Morvan Hervé P. , Simmons Kathy A.
Published: September 2014

In this paper, we present results for the application of an Eulerian-Lagrangian technique to the transient simulation of an oil film formation on the walls of an aeroengine bearing chamber. The flow of oil in an aeroengine bearing chamber consists of high speed oil droplets interacting with the bearing structures and flowing oil film. The situation in the chamber is highly rotational and consisting of sheared flow of air over oil. The bearing chamber may also be located in the vicinity of the combustion chamber. The oil provides lubrication and cooling of the hot structures. Modelling the flow in the bearing chamber is therefore complex. The Volume of Fluid (VoF) technique offers a potential platform to model droplet-film interaction; however, it requires fine mesh details to capture the flow to the droplet level. Such detailed resolution would not be practical for the complete chamber geometry because of the prohibitively expensive computational overhead requirements. A Lagrangian formulation is therefore proposed to represent the droplets as source terms in the Navier-Stokes equation while the film is represented using VoF. This effectively reduces the need to resolve the droplets explicitly. The predicted film formation pattern compares with experimental results.
Some Guidelines for the Experimental Characterization of Vaneless Diffuser Rotating Stall in Stages of Industrial Centrifugal Compressors
Authors: Bianchini Alessandro , Biliotti Davide , Giachi Marco , Belardini Elisabetta , Tapinassi Libero , Ferrari Lorenzo , Ferrara Giovanni
Published: September 2014

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors, as it is often connected with the onset of detrimental subsynchronous vibrations which can prevent the machine from operating beyond this limit.

With particular reference to the vaneless diffuser stall, much research has been directed at investigating the physics of the phenomenon, the influence of the main design parameters and the prediction of the stall inception. Few of them, however, focused attention on the evaluation of the aerodynamic unbalance due to the induced pressure field in the diffuser, which, however, could provide a valuable contribution to both the identification of the actual operating conditions and the enhancement of the compressor operating range by a suitable choice of the control strategy.

Although advanced experimental techniques have been successfully applied to the recognition of the stall pattern in a vaneless diffuser, the most suitable solution for a wider application in industrial test-models is based on dynamic pressure measurements by means of a reduced number of probes. Within this context, a procedure to transpose pressure measurements into the spatial pressure distribution was developed and validated on a wide set of industrial test-models.

In this work, the main guidelines of the procedure are presented and discussed, with particular reference to signals analysis and manipulation as well as sensors positioning.

Moreover, the prospects of using a higher number of sensors is analyzed and compared to standard solutions using a limited probes number.
Pore-Level Numerical Simulation of Open-Cell Metal Foams With Application to Aero Engine Separators
Authors: de Carvalho Thiago Piazera , Morvan Hervé P. , Hargreaves David
Published: September 2014

In aero engines, the oil and air interaction within bearing chambers creates a complex two-phase flow. Since most aero engines use a close-loop oil system and releasing oil out is not acceptable, oil-air separation is essential. The oil originates from the engine transmission, the majority of which is scavenged out from the oil pump. The remainder exits via the air vents, where it goes to an air oil separator called a breather. In metal-foam-style breathers separation occurs by two physical processes. Firstly the largest droplets are centrifuged against the separator walls. Secondly, smaller droplets, which tend to follow the main air path, pass through the metal foam where they ideally should impact and coalesce on the material filaments and drift radially outwards, by the action of centrifugal forces. Although these devices have high separation efficiency, it is important to understand how these systems work to continue to improve separation and droplet capture.

One approach to evaluate separation effectiveness is by means of Computational Fluid Dynamics. Numerical studies on breathers are quite scarce and have always employed simplified porous media approaches where a momentum sink is added into the momentum equations in order to account for the viscous and/or inertial losses due to the porous zone [1]. Furthermore, there have been no attempts that the authors know of to model the oil flow inside the porous medium of such devices.

Normally, breathers employ a high porosity open-cell metal foam as the porous medium. The aim of this study is to perform a pore-level numerical simulation on a representative elementary volume (REV) of the metal foam with the purpose of determining its transport properties. The pore scale topology is represented firstly by an idealized geometry, namely the Weaire-Phelan cell [2]. The pressure drop and permeability are determined by the solution of the Navier-Stokes equations. Additionally, structural properties such as porosity, specific surface area and pore diameter are calculated. The same procedure is then applied to a 3D digital representation of a metallic foam sample generated by X-ray tomography scans [3]. Both geometries are compared against each other and experimental data for validation. Preliminary simulations with the X-ray scanned model have tended to under predict the pressure drop when compared to in-house experimental data. Additionally, the few existing studies on flow in metal foams have tended to consider laminar flow; this is not the case here and this also raises the question that Reynolds-averaged turbulence models might not be well suited to flows at such small scales, which this paper considers.
Time-Resolved Heat Transfer and Surface Pressure Measurements for a Fully-Cooled Transonic Turbine Stage
Authors: Nickol Jeremy B. , Mathison Randall M. , Malak Malak F. , Rana Rajiv , Liu Jong S.
Published: September 2014

The flow field in axial gas turbines is driven by strong unsteady interactions between stationary and moving components. While time-averaged measurements can highlight many important flow features, developing a deeper understanding of the complicated flows present in high-speed turbomachinery requires time-accurate measurements that capture this unsteady behavior. Towards this end, time-accurate measurements are presented for a fully cooled transonic high-pressure turbine stage operating at design-corrected conditions. The turbine is run in a short-duration blowdown facility with uniform, radial, and hot streak vane-inlet temperature profiles as well as various amounts of cooling flow. High frequency response surface-pressure and heat-flux instrumentation installed in the rotating blade row, stator vane row, and stationary outer shroud provide detailed measurements of the flow behavior for this stage. Previous papers by Haldeman et al. [1, 2] have reported the time-averaged results from this experiment, but this paper focuses on the strong unsteady phenomena that are observed. Heat-flux measurements from double-sided heat-flux gauges cover three span-wise locations on the blade pressure and suction surfaces. In addition, there are two instrumented blades with the cooling holes blocked to isolate the effect of just blade cooling. The stage can be run with the vane and blade cooling flow either on or off. High-frequency pressure measurements provide a picture of the unsteady aerodynamics on the vane and blade airfoil surfaces, as well as inside the serpentine coolant supply passages of the blade. A time-accurate CFD simulation is also run to predict the blade surface pressure and heat-flux, and comparisons between prediction and measurement are given.

It is found that unsteady variations in heat-flux and pressure are stronger at low to mid-span and weaker at high span, likely due to the impact of secondary flows such as the tip leakage flow. Away from the tip, it is seen that the unsteady fluctuations in pressure and heat-flux are mostly in-phase with each other on the suction side, but there is some deviation on the pressure side. The flow field is ultimately shown to be highly three-dimensional, as the movement of high heat transfer regions can be traced in both the chord and span-wise directions. These measurements provide a unique picture of the unsteady flow physics of a rotating turbine, and efforts to better understand and model these time-varying flows have the potential to change the way we think about even the time-averaged flow characteristics.
Measurements of Stretch Statistics at Flame Leading Points for High Hydrogen Content Fuels
Authors: Marshall Andrew , Lundrigan Julia , Venkateswaran Prabhakar , Seitzman Jerry , Lieuwen Tim
Published: September 2014

Fuel composition has a strong influence on the turbulent flame speed, even at very high turbulence intensities. An important implication of this result is that the turbulent flame speed cannot be extrapolated from one fuel to the next using only the laminar flame speed and turbulence intensity as scaling variables. This paper presents curvature and tangential strain rate statistics of premixed turbulent flames for high hydrogen content fuels. Global (unconditioned) stretch statistics are presented as well as measurements conditioned on the leading points of the flame front. These measurements are motivated by previous experimental and theoretical work that suggests the turbulent flame speed is controlled by the flame front characteristics at these points. The data were acquired with high speed particle image velocimetry (PIV) in a low swirl burner (LSB). We attained measurements for several H2:CO mixtures over a range of mean flow velocities and turbulence intensities. The results show that fuel composition has a systematic, yet weak effect on curvatures and tangential strain rates at the leading points. Instead, stretch statistics at the leading points are more strongly influenced by mean flow velocity and turbulence level. It has been argued that the increased turbulent flame speeds seen with increasing hydrogen content are the result of increasing flame stretch rates, and therefore SL,max values, at the flame leading points. However, the differences observed with changing fuel compositions are not significant enough to support this hypothesis. Additional analysis is needed to understand the physical mechanisms through which the turbulent flame speed is altered by fuel composition effects.
Calculation of Flow Instability Inception in High Speed Axial Compressors
Authors: Liu Xiaohua , Zhou Yanpei , Sun Xiaofeng , Sun Dakun
Published: September 2014

This paper applies a theoretical model developed recently to calculate the flow instability inception point in axial high speed compressors system. After the mean flow field is computed by steady CFD simulation, a body force approach, which is a function of flow field data and comprises of one inviscid part and the other viscid part, is taken to duplicate the physical sources of flow turning and loss. Further by applying appropriate boundary conditions and spectral collocation method, a group of homogeneous equations will yield from which the stability equation can be derived. The singular value decomposition method is adopted over a series of fine grid points in frequency domain, and the onset point of flow instability can be judged by the imaginary part of the resultant eigenvalue. The first assessment is to check the applicability of the present model on calculating the stall margin of one single stage transonic compressors at 85% rotational speed. The reasonable prediction accuracy validates that this model can provide an unambiguous judgment on stall inception without numerous requirements of empirical relations of loss and deviation angle. It could possibly be employed to check over-computed stall margin during the design phase of new high speed fan/compressors. The following validation case is conducted to study the nontrivial role of tip clearance in rotating stall, and a parameter study is performed to investigate the effects of end wall body force coefficient on stall onset point calculation. It is verified that the present model could qualitatively predict the reduced stall margin by assuming a simplified body force model which represents the response of a large tip clearance on the unsteady flow field.
Investigation of 3D Blading and Non-Axisymmetric Hub Endwall Contouring to a Dual-Stage Counter-Rotating Compressor in Multistage Environment
Authors: Zhang Peng , Liu Bo , Zhang Guochen , Na Zhenzhe
Published: September 2014

3D blading technique is an effective way to mitigate flow separation and improve the performance of turbomachinery. Non-axisymmetric endwall contouring technique is widely used to reduce transverse secondary flows near the endwall region in turbines, while the application of non-axisymmetric endwall contouring to compressors is rare.

The investigation rig is the Northwestern Polytechnical University dual-stage counter-rotating compressor. In order to further improve the aerodynamic performance of the compressor, 3D blading optimization of the two rotors and outlet guide vane (OGV) was done in multistage environment at near stall condition. After 3D blading optimization, the radial secondary flows of the two rotors and the separation vortex at the tip of OGV are obviously reduced. However, the transverse secondary flows at the hub endwalls of the two rotors are scarcely improved. Based on the 3D blading optimization, non-axisymmetric hub endwall contouring optimization of the two rotors was carried out afterwards and the circumferential secondary flow losses of the two rotors are reduced effectively. The efficiency of the counter-rotating compressor on optimization point increases and the aerodynamic performance is improved largely after 3D blading and non-axisymmetric hub endwall contouring optimization.
A Simple Format for the Definition of Safety Factors for LCF
Authors: Beretta S. , Foletti S. , Sanguineti A.
Published: September 2014

Turbine components are heavy duty mechanical parts that are usually designed onto safe-life approach, where the low-cycle fatigue analysis is based onto design life curves with suitable probabilistic life margins. However, in order to design for a given reliability, the definition of the design curve should not only include the life variability but also the scatter of applied load. Unfortunately, in the literature there are few indications which only refer to safety factors under HCF, without any specific discussion of the case of LCF.

In this paper, we firstly define a simple lognormal format for calculation of reliability for a given fatigue life under LCF conditions. Then, the partial safety factor to be applied to ε–Nf curve are then obtained for different COV’s of the applied load. The so defined design ε–Nfcurve for a rotor steel is then adopted for a complex analysis of a notched component under LCF, whose life has been estimated by an elasto-plastic analysis.
Thermal Design and Performance Optimization of the ORC System for the Waste Heat Recovery in Refining Petroleum Industry
Authors: Li Yan , Song Jian , Gu Chunwei
Published: September 2014

As the increase of the energy consumption and the deterioration of environment, a carbon tax will be imposed in China to reduce carbon emissions strictly and the industrial waste heat recovery has been getting more attention. The Organic Rankine Cycle (ORC) system has been proven to be a promising solution for the utilization of the low-grade heat sources.

There are five waste heat sources from a 1.2 million ton reforming and extraction unit in Shijiazhuang Refining & Chemical Company of China. The temperatures of the waste heat sources are 98∼80°C, 104∼80°C, 147∼80°C, 205∼80°C and 205∼80°C, and the heat loads are 6.5MW, 11.5MW, 8.6MW, 3.8MW and 2.2MW, respectively. This paper studies the thermal design and performance optimization of a comprehensive utilization system for these waste heat sources, using ORC technology. The selection of suitable organic fluid is studied and the working parameters are designed and optimized with the application of the first law and the second law of thermodynamics. When the ORC systems are designed separately for the recovery of five waste heat sources, and the total power output is 3338.89kW with different organic working fluids. However this kind of designs leads to a very complex recovery system which needs large investment and space occupation. To reduce the overall system complexity, a single ORC system is proposed to recover all five heat sources, and the total amount of output power will only be 2813.02kW, due to the large exergy loss. With the above results shown, and for the purpose of simple system with large power output, this paper further studies the dual ORC systems heat recovery plan, with R245fa as the top cycle working fluid and R141b as the bottom cycle working fluid. The total amount of power output to 3353.27kW. The dual systems with single working fluid heat recovery plan is also studied, and with R141b as the working fluid for both the top cycle and the bottom cycle, the total amount of power output is 3325.03kW, and the heat recovery system is simple and compact, with good economical benefit.
Runge-Kutta/Implicit Scheme for the Solution of Time Spectral Method
Authors: Ma Can , Su Xinrong , Gou Jinlan , Yuan Xin
Published: September 2014

This paper investigates the Runge-Kutta implicit scheme applied to the solution of the time spectral method for periodic unsteady flow simulation. Several explicit and implicit time integration schemes including the Runge-Kutta scheme, Block-Jacobi SSOR (symmetric successive over relaxation)scheme and Block-Jacobi Runge-Kutta/Implicit scheme are implemented into an in-house code and applied to the time marching solution of the time spectral method. The time integration is coupled with Full Approximation Storage (FAS) type multi-grid method for convergence acceleration. The in-house code is based on the finite volume method and solves the RANS (Reynolds Averaged Navier-Stokes) equations on multi-block structured mesh. For spatial discretization the 3rd/5th order WENO (weighted essentially nonoscillatory) upwind scheme is used for reconstruction and the convective flux is computed with Roe approximate Riemann solver. The widely used one-equation Spalart-Allmaras turbulence model is used in the simulations. The time integration schemes for the solution of the time spectral method are tested with two different compressor cascades with periodically oscillating inlet boundary conditions. The first case is a low speed compressor stator with inlet flow angle varying with time. The second case is a high speed compressor rotor with inlet boundary condition profile to simulation the influence of upstream wakes. The results show that for moderate frequencies and wave mode numbers, the Block-Jacobi Runge-Kutta/Implicit scheme shows favorable convergence behavior compared to the other schemes. However, for extremely high frequencies and wave mode numbers such as in the simulation of high rotating speed compressors, the advantage of the Block-Jacobi Runge-Kutta/Implicit scheme over the explicit Runge-Kutta scheme is totally lost.
Experimental Study on Hot Lubricating Oil Anti-Icing System of Aero-Engine Strut
Authors: Dong Wei , Zhu Jianjun , Chen Yong , Zhou Zhixiang
Published: September 2014

This paper presents an experimental study on the performance of hot oil anti-icing system of aero-engine strut in icing wind tunnel. The experiments are performed in YBF-02 icing wind tunnel. The YBF-02 icing wind tunnel has a test section of 0.28m width and 0.18m height. With the limitation of model blocking, airspeed up to 200m/s can be achieved. A refrigeration system can provide the test-section with temperature from −25°C to 0°C. The experimental model is a full-scale inlet strut of a turboshaft engine. The hot lubricating oil flows through the internal passage of the strut and heats the strut to prevent the strut from ice accretion on its surface. In experiments, the hot oil was heated by an electric heater. The temperature and mass flow of the oil are controlled by Programmable logic controller (PLC) system. The temperature distributions on the strut surface are measured by T type thermocouples. The icing environment parameters in icing wind tunnel for engine strut are: the air total temperature is −10°C,-5°C; the airspeed is 40m/s; the liquid water content ranges from 0.5g/m3 to 2.0g/ m3; the median volume diameter (MVD) of the super-cooled droplet is 20μ m. The surface temperatures of the strut and the ice shape accreted on the strut are measured and photographed. The transient temperature variation of the strut surface is recorded in hot oil anti-icing tests. The characteristics of ice shape and temperature variation on the strut surface with a hot oil anti-icing system are analyzed. The effects of temperature and mass flow of hot oil on the performance of the anti-icing system under different icing conditions are studied.
Development and Integration of Rain Ingestion Effects in Engine Performance Simulations
Authors: Roumeliotis I. , Alexiou A. , Aretakis N. , Sieros G. , Mathioudakis K.
Published: September 2014

Rain ingestion can significantly affect the performance and operability of gas turbine aero-engines. In order to study and understand rain ingestion phenomena at engine level, a performance model is required that integrates component models capable of simulating the physics of rain ingestion. The current work provides, for the first time in the open literature, information about the set-up of a mixed-fidelity engine model suitable for rain ingestion simulation and corresponding overall engine performance results. Such a model can initially support an analysis of rain ingestion during the pre-design phase of engine development. Once components and engine models are validated and calibrated versus experimental data, they can then be used to support certification tests, the extrapolation of ground test results to altitude conditions, the evaluation of control or engine hardware improvements and eventually the investigation of in-flight events.

In the present paper, component models of various levels of fidelity are firstly described. These models account for the scoop effect at engine inlet, the fan effect and the effects of water presence in the operation and performance of the compressors and the combustor. Phenomena such as velocity slip between the liquid and gaseous phases, droplet break-up, droplet-surface interaction, droplet and film evaporation as well as compressor stages re-matching due to evaporation are included in the calculations. Water ingestion influences the operation of the components and their matching, so in order to simulate rain ingestion at engine level a suitable multi-fidelity engine model has been developed in the PROOSIS simulation platform. The engine model’s architecture is discussed and a generic high bypass turbofan is selected as a demonstration test case engine.

The analysis of rain ingestion effects on engine performance and operability is performed for the worst case scenario, with respect to the water quantity entering the engine. The results indicate that rain ingestion has a strong negative effect on high-pressure compressor surge margin, fuel consumption and combustor efficiency, while more than half of the water entering the core is expected to remain unevaporated and reach the combustor in the form of film.
Numerical Studies on Effect of Channel Orientation in a Rotating Smooth Wedge-Shaped Cooling Channel
Authors: Srinivasan Balamurugan , Dhamarla Anand , Jayamurugan Chandiran , Rajan Amarnath Balu
Published: September 2014

The increasing demands of better efficiency of modern advanced gas turbine require higher turbine inlet temperatures, which gives great challenges to turbine blade designers. However, the temperature limits of turbine blade material are not high enough to ensure its survival in such incredible operating temperature. Hence, both internal and external cooling approaches have been developed and widely used in today’s turbine blade. To internal cooling problems, a variety of cooling enhancement approaches, such as impingement and turbulators, are employed in order to meet the different needs in leading, middle and trailing region. One of the most critical parts in turbine blade is trailing edge where it is hard to cool due to its narrow shape. Pin-fins are widely used to cool the trailing edge of rotor and stator blades of gas turbine engine. Pin-fins offer significant heat transfer enhancement, they are relatively easy to fabricate and offer structural support to the hollow trailing edge region. The flow physics in a pin-fin roughened channel is very complicated and three-dimensional.

In this work, we have studied the effect of channel orientation on heat transfer in a rotating wedge-shaped cooling channel using numerical methods. Qiu [1] studied experimentally heat transfer effects of 5 different angles of wedge shaped channel orientation for the inlet Reynolds number (5100 to 21000) and rotational speed (zero to 1000 rpm), which results in the inlet Rotation number variation from 0 to 0.68. They observed that compared to the non-rotating condition, there is about 35% overall heat transfer enhancement under highest rotation number. The above said results are validated using current studies with Computational Fluid Dynamics (CFD) revealed that rotation increases significantly the heat transfer coefficient on the trailing surface and reduces the heat transfer coefficient on the leading surface. This is due to the higher velocities associated with the converging geometry near trailing surface.
Optimization Framework Using Surrogate Model for Aerodynamically Improved 3D Turbine Blade Design
Authors: Lee Sanga , Lee Saeil , Kim Kyu-Hong , Lee Dong-Ho , Kang Young-Seok , Rhee Dong-Ho
Published: September 2014

In simple optimization problem, direct searching methods are most accurate and practical enough. However, for more complicated problem which contains many design variables and demands high computational costs, surrogate model methods are recommendable instead of direct searching methods. In this case, surrogate models should have reliability for not only accuracy of the optimum value but also globalness of the solution. In this paper, the Kriging method was used to construct surrogate model for finding aerodynamically improved three dimensional single stage turbine. At first, nozzle was optimized coupled with base rotor blade. And then rotor was optimized with the optimized nozzle vane in order. Kriging method is well known for its good describability of nonlinear design space. For this reason, Kriging method is appropriate for describing the turbine design space, which has complicated physical phenomena and demands many design variables for finding optimum three dimensional blade shapes. To construct airfoil shape, Prichard topology was used. The blade was divided into 3 sections and each section has 9 design variables. Considering computational cost, some design variables were picked up by using sensitivity analysis. For selecting experimental point, D-optimal method, which scatters each experimental points to have maximum dispersion, was used. Model validation was done by comparing estimated values of random points by Kriging model with evaluated values by computation. The constructed surrogate model was refined repeatedly until it reaches convergence criteria, by supplying additional experimental points. When the surrogate model satisfies the reliability condition and developed enough, finding optimum point and its validation was followed by. If any variable was located on the boundary of design space, the design space was shifted in order to avoid the boundary of the design space. This process was also repeated until finding appropriate design space. As a result, the optimized design has more complicated blade shapes than that of the baseline design but has higher aerodynamic efficiency than the baseline turbine stage.
A Numerical Study on the Influence of Hole Aspect Ratio on the Performance Characteristics of a Hole-Pattern Seal
Authors: Migliorini Patrick J. , Untaroiu Alexandrina , Wood Houston G.
Published: September 2014

In turbomachinery, annular seals are used to reduce leakage between regions of high and low pressure. Many configurations of annular seals have been developed and studied in the literature including plain, labyrinth, pocket-damper, honeycomb, and hole-pattern. In machines experiencing stability issues, honeycomb and hole-pattern type seals have been used to replace labyrinth seals. Bulk-flow models are typically used to predict the leakage and dynamic coefficients of hole-pattern seals, relying on empirically derived friction factor coefficients. Previous experimental studies have shown that, for hole-pattern seals, the leakage and stator friction factor are strongly influenced by hole-depth. However, this behavior is not a monotonic function of hole-depth, a fact that might reduce confidence in future bulk-flow model predictions if not properly accounted for. A recent numerical study has highlighted the role of vortex formation in the holes which has a strong influence on the flow in the clearance region. Depending on the shape of the vortex, the flow in the hole can act much like a pinch valve, reducing the effective clearance of the jet flow.

In this paper, computational fluid dynamics simulations of several hole-pattern seal configurations have been performed to study the effect of hole-aspect ratio (depth versus diameter) on the leakage and friction factors. The Reynolds Averaged Navier Stokes (RANS) equations with k-ε turbulence model were solved using ANSYS CFX. It was found that the shape of the hole influences the vortex formation within the hole, effecting the jet flow in the clearance region and the seal leakage. The results show that the leakage is heavily dependent on the hole diameter in addition to the hole depth. The relationship between the friction factors and the geometry of the seal was found to be non-monotonic. It is therefore difficult to develop a friction factor model that will accurately encompass all configurations and it is recommended that friction factor data be interpolated from experimental or numerical results.
Investigations on Three-Dimensional Coupled Flow of Secondary Air System and Main Flow Passages in a Micro Gas Turbine
Authors: Zeng Yuntao , Liu Jianjun
Published: September 2014

The secondary air system is important to the safe operation of a gas turbine. An appropriately designed secondary air system should be able to seal the rotor-stator rim, deliver coolant for the cooling of blades and disks and balance the axial force by using the least amount of leakage air, so that the penalty on the gas turbine performance can be minimal. A three-dimensional CFD analysis of the coupled flow between the secondary air system and the main passages of compressor and turbine in a micro gas turbine was presented in this paper. The full computational domain was divided into two subsystems. The first sub-system includes one blade passage of both the centrifugal compressor impeller and the inlet guide vane, one period (same as the impeller period) of the impeller back cavity and the shaft seal. The second sub-system includes the compressor discharge leakage gap, the guiding hole for the secondary air, the collecting cavity behind the back bearing seal, two balance holes connecting the shaft seal behind the impeller and the collecting cavity, one period of turbine rotor surrounding fluid path including radial labyrinth seal, turbine rotor/stator cavity, one turbine rotor flow passage and one period of turbine rotor back cavity. The two sub-systems are connected through the shaft seal at the impeller back cavity and the balance holes. Computations were carried out separately for the two sub-systems and iterated until the mass flow rate and the averaged static pressure at the sub-system interface are balanced. Computations for two working conditions of full load and idle were performed. The results show that at both working conditions the impeller back cavity has negative leakage mass flow rate. The turbine stator-rotor rim is well sealed. There is no hot gas ingestion from the turbine main passage to the secondary air system at both working conditions. The axial forces are under feasible level.
Endwall Contouring Optimization in a High Pressure Turbine Vane With Consideration of Rim Seal Flow
Authors: Hu Shuzhen , Luo Hualing
Published: September 2014

In this paper, non-axisymmetric endwall contouring optimizations in a high pressure turbine second vane passage were performed with and without including the rim seal flow. Aerodynamic performances of two contoured endwalls were studied and compared with the planar endwall case. The optimization was carried out with a commercial software package NUMECA Fine/Design 3D. A genetic algorithm was chosen as the optimization method. The objective was to minimize the total pressure loss in the high pressure turbine vane.

Without the rim seal flow, the contoured endwall decreased the strength of the passage vortex, and reduced the total pressure loss coefficient up to 10.4%. However, the benefit achieved by this contoured endwall was lost when the rim seal flow was included by adding a under platform section in the computation model. With the existence of the rim seal flow, the flow field near the endwall was significantly changed. After recontouring the endwall using the optimization procedure with including rim seal flow, a 9.8% decrease in total pressure loss coefficient was achieved with the reduction of the vortex strength.
Numerical Investigation on the Stage Matching in a Multi-Stage Centrifugal Compressor
Authors: Zhang Pengfei , Wang Zhiheng , Duan Wenlong , Xi Guang
Published: September 2014

The performance of a multi-stage centrifugal compressor is commonly generated by stacking the individual stage’s performance in the industry applications, which makes the proper stage matching vital in getting favorable performance of the whole machine. Taking a two-stage centrifugal refrigeration compressor as the research object, this paper numerically investigates the two key problems in the stage matching, i.e. 1) The accuracy of overall performance generated by stacking the individual stages’ performance; 2) Effects of the inlet and outlet flow distortions on the individual stage performance and the axial thrust. It is found that the overall performance of the multi-stage centrifugal compressor predicted by the stage-stacking method generally agrees well with that predicted by the CFD analysis at the small and moderate flow rates, and the difference is getting bigger at the large flow rate. The numerical investigations also show that the effect of inlet distortion on the isentropic efficiency and the total pressure ratio exist mainly at the large flow rate. The radial distortion of the inflow has little influence on the performance, while the circumferential distortion of the inflow affects remarkably. The outlet distortion also has an important influence on the performance of upstream stage. The effect of flow distortions at the inlet and outlet on the impeller thrust is presented as well.
Oil Whip Elimination Using Fuzzy Logic Controller
Authors: Dimitri A. S. , Mahfoud J. , El-Shafei A.
Published: September 2014

Oil whip is a self-excited sub-synchronous vibration which limits the range of operating speed of Journal Bearings (JB). JB have wide range of applications due to their high loading capacity, simple geometry, and lubrication. When the speed of rotation increases, the oil whip instability is excited with a frequency corresponding to the rotor critical speed which causes excessive undesirable vibration.

A solution for this instability is implemented through this paper. The control action is implemented through a new integrated bearing device. The bearing consists of JB and Electromagnetic Actuator (EMA). The oil whip control action is applied through the EMA.

A fuzzy logic control algorithm is developed and experimentally applied to a rotor test rig. The fuzzy logic controller is suitable to deal with problems of uncertainties and non-linearity.

The experimental results show the ability of the developed fuzzy logic controller to eliminate the oil whip instability when applied to a test rig which simulates industrial rotor through an integrated bearing prototype which was designed and manufactured.
A Simple Corner Correction Technique for Transient Thermal Measurement
Authors: Chen W. , Jiang H. , Zhang Q. , He L.
Published: September 2014

The 1D conduction analytical and semi-analytical approaches for a semi-infinite domain have been widely adopted in the data processing of transient thermal experiments. The convective heat transfer coefficient (HTC) calculated by the 1D approach contains large errors when lateral conduction effects are significant, especially near a corner of solid domain. The problems could be addressed by alternative full 3D numerical conduction analyses, which tend to be complex as well as requiring extra experimental efforts to obtain the full thermal boundary conditions around corners, in addition to an access to a 3D conduction solver (CFD or FEA).

In the present work, a simple and effective method is developed to correct such errors in the near-corner region without using any commercial tool. The present approach is based on the recognition that a temperature time trace in a 2D situation is the result of the accumulated heat conductions from the normal and lateral directions respectively and summatively. An equivalent semi-infinite 1D conduction temperature trace for a correct HTC can be generated by reconstructing and removing the lateral conduction effect at each discrete time step. This simple new correction procedure enables the standard 1D conduction analysis to be properly used to get the correct HTC, completely analytically without needing any aid of CFD or FEA solutions.

Two test cases of practical interest with turbine blade tip heat transfer and film cooling are used for validation and demonstration. It has been consistently shown that the errors of the conventional 1D conduction analysis in the near corner regions can be greatly reduced by the new corner correction method. The demonstrated validity, the simplicity and robustness of the present method makes it a good candidate for future applications in transient thermal experimental studies.
Automatic Design Optimisation of Profiled Endwalls Including Real Geometrical Effects to Minimize Turbine Secondary Flows
Authors: Shahpar Shahrokh , Caloni Stefano , de Prieëlle Laurens
Published: September 2014

This paper presents a novel optimisation methodology based on both Adjoint sensitivity analysis and trust-based dynamic response surface modelling to improve performance of a modern turbine of a large civil aero-engine in the presence of high-fidelity geometry configurations. The system has been applied to the non-axisymmetric hub and tip endwall optimisation of a high-pressure turbine stage making use of multi-row 3D simulations, parametric modelling and rapid meshing of real geometry features such as rim seals and modelling of film cooling flows.

It has been shown in previous papers that improvements gained using simplified models of the stage are lost when applying the high-fidelity geometry configuration. New results presented in this paper indicate that controlling the purge flow that exits the disc space through the rim seal at the hub of the main annulus is more significant than the reduction of secondary flows in the main passage. For a given rim sealing mass flow rate and whirl velocity, the non-axisymmetric endwalls are optimised such that the detrimental impact of the sealing flow on the turbine performance is reduced, and hence the stage efficiency is significantly increased. The traditional optimisation approaches based on evolutionary methods or even sequential modifications for defining the endwalls shape are computationally demanding. Since, turbomachinery industry continuously strive to reduce the design cycle time, in particular when high fidelity 3D CFD is used, the main body of this paper outlines the novel methods developed to produce a practical design in a very aggressively short design cycle time.
Numerical Study of Impingement and Film Composite Cooling on Blade Leading Edge
Authors: Liu Zhao , Ye Lv , Feng Zhenping
Published: September 2014

In this paper a numerical study is performed to simulate the impingement and film composite cooling on the first stage rotor blade of GE-E3 engine high pressure turbine. A commercial CFD software CFX11.0 with a 3D RANS approach is adopted in the study. Firstly, by comparing with available experimental data, the relative performance of four turbulence models for numerical impingement and film composite cooling is studied, including the standard k-ε model, the RNG k-ε model, the standard k-ω model and the Shear-Stress Transport k-ω model. The Shear-Stress Transport k-ω model is chosen for the numerical study as it shows the best simulation accuracy. Then the simulations consist of five different density ratios (1.16∼4.81) and seven different blowing ratios (0.5∼3.0). The results indicate that the cooling effectiveness on pressure side is lower than that on the suction side. The cooling effectiveness increases with the increase of blowing ratio in the study range, but decreases with the increase of density ratio. On the target surface, the average Nusselt number, the circumferential averaged Nusselt number and its peak value increase with the increasing in blowing ratio, but decrease with the increase of density ratio.
Numerical Investigation on the Flow Characteristics of a Supercritical CO2 Centrifugal Compressor
Authors: Zhao Hang , Deng Qinghua , Zheng Kuankuan , Zhang Hanzhen , Feng Zhenping
Published: September 2014

Supercritical CO2 closed-loop Brayton cycles offer the potential of better economical and practical efficiency due to its compact size and smaller compression work as compared with some traditional working fluids cycles, in which compressor is the key component. In this paper, the aerodynamic design and impeller aerodynamic optimization were conducted for a single stage centrifugal compressor with a combined vaneless and vaned diffuser, operating with CO2 slightly above the vapor-liquid critical point. The NIST REFPROP database was used for the computation of supercritical CO2 properties in design analysis and numerical investigation. The flow characteristics of the supercritical CO2 compressor were investigated by NUMECA FINE/Turbo. In order to weaken the low pressure regions, a vaneless diffuser was applied in this design, which would control and reduce the distribution differences of fluid thermodynamic states and increase fluid static pressure. The results indicate that there are no obvious low pressure regions occurring close to the leading edge of vaned diffuser. So it is observed in the design process that the vaneless diffuser could improve the aerodynamic performance of supercritical CO2 compressor.

Compared with the operating conditions of the compressor only under centrifugal force, the pressure load from the aerodynamic analysis and the centrifugal load due to high speed of rotation were considered in the study of the stress and deformation of the structure of impeller by ANSYS/Mechanical. It can be concluded that supercritical CO2 provides unique properties for the compressor working process, which have a significant influence on finite element modeling in structural analysis. For the present design the maximum von Mises stress and total deformation are shown much smaller than the maximum allowable values, and thus the compressor could work in a wide range of operating conditions.
Friction and Heat Transfer in a Rotating Rib-Roughened Square U-Duct Under High Rotation Numbers
Authors: Wu Xuewang , Tao Zhi , Qiu Lu , Tian Shuqing , Li Yang
Published: September 2014

Experimental investigations have been conducted on a rotating two-pass square channel, in which staggered ribs (attack angle of 45 degree) are roughened on both leading and trailing surfaces. The hydraulic diameter of the channel is 24 mm, and the pitch-to-height ratio and diameter-to-height ratios of the ribs are both 10:1. Reynolds number and rotational speed range from 20000 to 40000 and zero to 1000 rpm respectively. Since the absolute pressure in this channel is increased above 5 atm, the maximum rotation number reaches to 1.025. Regional averaged heat transfer coefficients are measured by classical copper plate technique. Pressure drops are measured by newly designed rotating pressure measurements module. Data are compared to that obtained in rotating smooth U-duct. It shows that the ribbed U-duct achieves enhanced regional heat transfer performances than the smooth case under stationary and rotating conditions at almost all locations except the turn region which has no ribs placed in. In the first passage of the ribbed case, the trends of stream-wise heat transfer distribution on both leading and trailing surfaces are altered compared to the counterparts in smooth case at rotation number range of 0–1.025. Besides, different from the smooth case in which the critical rotation number on heat transfer in the first leading passage decreases as X/D increases, the trend of critical rotation number in the ribbed case is not clear. Moreover, various phenomena reveal that the inserting ribs can offset the effect of rotation on heat transfer. The trends of friction factor and thermal performance as a function of rotation number in ribbed case are totally different to smooth case and they both achieve optimized value at Ro = 0.6.