bluuebunny/crossref_metadata_2025 huggingface dataset:
Absolute/Convective Instability Transition in a Backward Facing Step Combustor: Fundamental Mechanism and Influence of Density Gradient
Authors: Manoharan Kiran , Hemchandra Santosh
Published: September 2014

Hydrodynamic instabilities of the flow field in lean premixed gas turbine combustors can generate velocity perturbations that wrinkle and distort the flame sheet over length scales that are smaller than the flame length. The resultant heat release oscillations can then potentially result in combustion instability. Thus, it is essential to understand the hydrodynamic instability characteristics of the combustor flow field in order to understand its overall influence on combustion instability characteristics. To this end, this paper elucidates the role of fluctuating vorticity production from a linear hydrodynamic stability analysis as the key mechanism promoting absolute/convective instability transitions in shear layers occurring in the flow behind a backward facing step. These results are obtained within the framework of an inviscid, incompressible, local temporal and spatio-temporal stability analysis. Vorticity fluctuations in this limit result from interaction between two competing mechanisms — (1) production from interaction between velocity perturbations and the base flow vorticity gradient and (2) baroclinic torque in the presence of base flow density gradients. This interaction has a significant effect on hydrodynamic instability characteristics when the base flow density and velocity gradients are co-located. Regions in the space of parameters characterizing the base flow velocity profile, i.e. shear layer thickness and ratio of forward to reverse flow velocity, corresponding to convective and absolute instability are identified. The implications of the present results on prior observations of flow instability in other flows such as heated jets and bluff-body stabilized flames is discussed.
Numerical Investigation on Impact of a Flush-Mounted Airintake Exit Distortion to the Rear Fan Performance
Authors: Liu Lei , Chen Fu , Chen Huanlong , Song Yanping
Published: September 2014

For the sake of investigating impact of inlet distortion on the fan stage performance, numerical simulation of the whole structure of a flush-mounted S-shaped inlet and the rear fan stage was conducted in this paper. The single fan stage with uniform air admission was researched at the same time for comparison. Considering substantial boundary layer ingesting, a scheme of suction control imposed at the first bend of the inlet was also carried out. The results show that the total pressure ratio as well as the efficiency of the fan stage decreases dramatically and the choked mass flow has a reduction about 1.20% as compare with the uniform air condition. With suction control, aerodynamic performance of the fan stage improves slightly, the choked mass flow and total pressure ratio at the maximum isentropic efficiency point increase about 0.28% and 0.25% respectively, and the stable operation range is extended. With effect of rotating rotor, the significant low energy region at bottom of the airintake exit decrease continually as it travels downstream to the rotor and covers nearly three flow passages at the front-edge rotor blade, moreover, the high-energy fluid mixes with the low-energy fluid.
The Analysis of Axial Momentum of the Rotor Tip Flows for Axial Compressors With Circumferential Grooves
Authors: Nan Xi , Lin Feng , Wang Sichen , Liu Le , Ma Ning , Chen Jingyi
Published: September 2014

A new control volume analysis is developed in this paper aiming at assessing the circumferential grooves effectiveness on stability improvement. The underlying mechanism for this approach is based on the hypothesis that the spike stall precursors can be triggered by the forward spillage of the rotor tip leakage flow and the onset condition of such a spillage is determined by the axial momentum balance within the rotor tip region. Control volumes are defined to quantify the axial momentum balance of the whole region where the grooves influence the flow at the rotor tip. The distribution curve of the cumulative axial momentum along the axial chord indicates that the grooves change the rotor tip loading and increase the stability, which is useful to assess the different grooved casings. As an example, multiple-groove configurations for a transonic rotor are analyzed. The results verified the cumulative axial momentum distribution for different grooved casings are in accordance with the stall margin extension variations. Another example is to prescreen the best double-groove configurations for a low speed compressor. By using this current approach, a double-groove configuration was selected and validated by experiments. These examples demonstrate the current approach has great potential in helping pre-screen circumferential grooves. As an important issue, peak efficiency changed by the grooves is discussed in the last section. Entropy production is quantitatively compared with and without grooves.
Numerical Analysis of Oil Anti-Icing System Performance of Turboshaft Engine Inlet Strut
Authors: Dong Wei , Zhu Jianjun , Chen Yong , Zheng Mei
Published: September 2014

A numerical computation method of simulating the temperature and heat transfer of the anti-icing strut under icing condition is presented. The flow field around the strut is obtained by using CFD software Fluent. The trajectories of super-cooled water droplets and the collection-efficiency are calculated by Lagrangian approach after the computation of the air flow field. The coupling effects of heat transfer and mass transfer are considered in the temperature computation of the strut. The thermal model considers the mass balance of water and energy balance on the surface of the strut. The comparison of computation results with experiment results is given to assess the validity of present computational method in this paper. Good agreement between the numerical predictions and the experimental results is obtained for the surface temperatures. The effects of temperature and mass flow of hot oil on the temperature of the inlet strut surface under different icing conditions are studied using numerical simulations. Some recommendations are given for an optimized design of hot oil anti-icing system.
Investigation of Inlet Bent-Pipe’s Effect on a Turbocharger Compressor: Unsteadiness of Mass Glow and Location of Main Flow Loss
Authors: Zhao Ben , Lao Dazhong , Hu Liangjun , Yang Ce
Published: September 2014

As inlet bent-pipes are often used in vehicle turbocharger compressors, understanding of the inlet bent-pipe’s effect on centrifugal compressor and the mechanism of flow loss is important to improve the turbocharger compressor performance. Experiment and numerical simulation were carried out on a centrifugal compressor with straight pipe and bent-pipe. Numerical simulated compressor performance was compared with test data obtained from compressor flow bench test and to validate the numerical method. The analysis is mainly based on numerical results and it indicated that the inlet bent pipe induces a serious distortion at impeller inlet and increases the risk of blade high-cycle fatigue. The level of distortion changes to be more serious as the operation point of compressor varies from design point to high load. When an impeller channel enters distortion, the larger pressure difference between inlet and outlet makes this channel get clogged and the flow rate through this channel is reduced. At design point the bent pipe mainly influences the flow loss in the inlet pipe and impeller while at high load more serious flow distortion induced by bent-pipe is able to extend its stronger effect to the downstream diffuser and volute.
Prediction of 3D Unsteady Flow and Heat Transfer in Rotating Cavity by Discontinuous Galerkin Method and Transition Model
Authors: Tan Qinxue , Ren Jing , Jiang Hongde
Published: September 2014

Rotating cavities with axial throughflow are found inside the compressor rotors of turbo-machines. The flow pattern and heat transfer in the cavities are known as sophisticated problems. Because of the numerical errors and model errors, as well as the stiffness introduced by low-Ma number, prediction of 3D unsteady flow and heat transfer in rotating cavity is still a challenge for modern CFD technology.

An in-house 3D unsteady CFD code was developed in this study. The discontinuous galerkin method, which can fulfill any high-order accuracy on the unstructured grid, was applied to reduce the discretization errors. The SST-γ-Reθ transition model proposed by Menter was applied to reduce the model errors. To overcome the stiffness and achieve good convergence characteristics and solution quality, the preconditioning matrix technique combined with DG method was introduced for low-Ma number viscous flow. First, natural convection of air in a square cavity was studied to test the code. The feasibility and credibility, of applying the DG method and the preconditioned matrix technique for buoyancy–induced Rayleigh-Bénard like flow, were further verified. Second, the 3D compressible flow field in a rotating cavity was investigated numerically using the FV method, DG method and laminar/SA/SST-transition turbulence model. It is demonstrated that the whole flow structure of all calculated cases was similar after comparing the calculated results with the available experimental data. But, the transition turbulence model fitted the experimental data better. On the other hand, the performance of high-order method was much better for both the rotating cavity flow and natural convection, in terms of heat transfer.

To better understand this phenomenon, an accuracy analysis of heat flux using DG method and FV method was performed. It showed the DG method could realize arbitrary precision of viscous stress and heat fluxes on irregular unstructured grids, while the FV method could only realize the first-order accuracy of the heat fluxes at the boundary faces and may exhibit erroneous behaviors. It also demonstrated that the high-order accuracy of gradients was needed to decrease errors of heat fluxes and viscous stresses, and that DG method was a promising method.
The Dissipation Function-Based Efficiency for Turbomachinery: Part 1 — The Efficiency of a Cooled Turbine Row
Authors: Cha Chong M.
Published: September 2014

The effect of coolant addition or “mixing loss” on aerodynamic performance is formulated for the turbine, where mixing takes place between gas streams of different compositions as well as static temperatures. To do this, a second law efficiency measure is applied to a generalization of the one-dimensional mixing problem between a main gas stream and a single coolant feed, first introduced and studied by Hartsel [1] for the turbine application. Hartsel’s 1972 model for mass-transfer cooling loss still remains the standard for estimating mixing loss in today’s turbines. The present generalization includes losses due to the additional contributions of “compositional mixing” (mixing between unlike compositions of the main and coolant streams) as well as the effect of chemical reaction between the two streams. Scaling of the present dissipation function-based loss model to the mainstream Mach number and relative cooling massflow and static temperature is given. Limitations of the constant specific heats assumptions and the impact of fuel-to-air ratio are also quantified.
Sensitivity Analysis of Multiharmonic Self-Excited Limit-Cycle Vibrations in Gas-Turbine Engine Structures With Nonlinear Contact Interfaces
Authors: Petrov E. P.
Published: September 2014

An efficient frequency-domain method is developed to calculate the sensitivity of multiharmonic amplitudes of the limit-cycle oscillations and their primary frequency to variation of the nonlinear contact interface parameters and aerodynamic characteristics. The method allows highly accurate and fast determination of the sensitivity coefficients together with the calculation of the limit-cycle amplitudes and frequencies using realistic large-scale finite element models of structures. The nonlinear contact interactions analysed include: friction contacts, unilateral interaction along normal direction at a contact interface, closing and opening clearances and interferences, cubic spring nonlinearity.

The capabilities of the method are demonstrated on a set of test cases.
Parametric Dynamics of Mistuned Bladed Disk
Authors: Wang Peiyi , Li Lin
Published: September 2014

The mistuning of bladed disk comes from manufacturing tolerances and in-service wear and tear. Consequently the cyclic symmetry has been destroyed by mistuning, even small mistuning levels could result in drastic changes in the dynamics of bladed disks. Specifically, mistuning can cause mode localization and an increase of the maximum forced response. It has been known that frequency veering, modal localization and magnification of response are three most classical dynamic properties of bladed disk. However few researches has focused on the relationships between dynamic characters and design parameters, because the proper variation ranges of the design parameters are difficult to be determined. The aim of this paper is to investigate the relationship between designed parameters and dynamic properties of mistuned bladed disk. Based on a lumped parametric model of bladed disk and utilizing parameterized eigenvalue solution, a reasonable range of designed parameter corresponding to specific nodal diameter index was provided. The numerical results showed that the curves of the gap of frequency veering versus coupling strength or blade stiffness have bowel-style. It was also found that there exists a quasi-saddle-surface while the vibration amplification factor varies with coupling strength and mistuning strength. The quasi-saddle-surface reveals that the existence of threshold of vibration amplification factor depends on the value of coupling strength. The result means that a proper choice of combination of coupling strength and mistuning strength could lead to a suppression of mistuned vibration amplification.
Hydrodynamics and Sonic Flow Transition in Dry Gas Seals
Authors: Thatte Azam , Zheng Xiaoqing
Published: September 2014

Dry gas seals (DGS) are widely used in turbomachinery applications. They are recently being also recommended for sealing novel super critical CO2 turbomachinery space. However, these seals can render interesting behavior under certain operating conditions which needs to be carefully monitored so that intended level of dynamic characteristics can be achieved. The ability of these seals to maintain low leakage by riding at small clearances makes them an attractive solution where secondary flows need to be minimized. To understand the significance of some of the key design features of these seals, in this work an analysis on a gas lubricated spiral groove dry gas seal is presented. Equations in polar coordinates governing the compressible flow through the DGS gap and a numerical method to solve such non-linear partial differential equation is presented. The resulting sets of equations are solved for hydrodynamic pressure distribution and the axial separation force and the film stiffness at the rotor-stator interface is calculated. A detailed study on key spiral groove features is then performed to investigate the effect of spiral angle, groove depth, groove pitch and dam width ratio on the hydrodynamic pressure generation capacity, film stiffness and hence on overall performance of the DGS. Another important phenomenon that can occur in DGS under high operating pressure is the sonic transition. It is shown that choked flow under such conditions can take place over the dam section of the seal which manifests itself into large local pressure and temperature variations and can result into dynamic instabilities.
Applications of Explicit Algebraic Reynolds Stress Models to Transonic Turbulence Flow Simulations Based on Discontinuous Galerkin Methods
Authors: Hao Zengrong , Qiu Hang , Ren Xiaodong , Gu Chunwei
Published: September 2014

Two applications of the non-linear eddy-viscosity model EARSM are presented in the simulations of transonic turbulent flow involving shock waves and other related complex features. The simulations are implemented applying an in-house CFD program based on the unstructured discontinuous Galerkin method, an alternative discretization method of the classical finite volume one to precisely capture the flow features. A series of turbulence feature variables in boundary layers are comparatively observed and analyzed. For the first case of transonic flow over a bump, the redistribution effect of Reynolds stress components rooted in the non-linear constitution relation promotes streamwise turbulence fluctuation and suppresses the normal one in boundary layer, comparing with the traditional linear constitution relation, especially when passing the shocks. The production magnitudes of the turbulence shear stress and kinetic energy for the non-linear model show slightly more sensitive to perturbations, such as the occurrence of shock front or compression corner, than the linear one. For the second case of a transonic turbine vane, similar redistribution effect of the non-linear model is also verified on suction surface around the strong shock. The straightforward redistribution effect is absent on pressure surface around middle part of the vane with favorable pressure gradients. There the non-linear model evaluates higher magnitudes of streamwise, normal and shear Reynolds stress components than the linear one, thus resulting locally stronger heat convection and higher surface temperature.
Investigation on the Effect of Asymmetric Vane Spacing on the Reduction of Rotor Blade Vibration
Authors: Niu Yonghong , Hou Anping , Zhang Mingming , Sun Tianrui , Wang Rui , Guo Haoyan
Published: September 2014

A transient method to analyze blade forced response under stator-rotor wake influence is proposed in dealing with asymmetric aerodynamic load. The vibration response of the blades is calculated in the fluid-structure coupled manner for the asymmetric vane spacing. Four different types of modification are adopted in the investigation. The reduction effect on the vibration stress due to the asymmetric vane spacing is examined by comparing the response characteristics of frequency and amplitude. Though the asymmetric vane spacing does not much affect the performance of the turbine, the results show that the proper asymmetric vane spacing can decrease the levels of the excitation force at specific frequencies to control the downstream blade forced response. The stress amplitude at the vane passing frequency is decreased by 51% in the most desirable modification in the study. After investigating the vibration characteristics of the blades under the wake excitation from upstream, the mechanism of the vibration reduction due to the asymmetric vane spacing is analyzed.
Influence of Tip Shroud Cavity Detailing on Turbine Blade Forcing Calculations
Authors: Gezork Tobias , Mayorca María A. , Groth Pieter , Vogt Damian M. , Fransson Torsten
Published: September 2014

Forced response in turbomachinery refers to the vibration of a component due to an excitation originating from another component. Obstacles, such as struts and blade rows in the upstream and downstream flow path of a turbomachine engine lead to engine order (EO) excitations. To be able to predict the severity of these excitations, both aerodynamic and structural calculations are performed. There is a risk of critical high cycle fatigue (HCF) failure when the force acts at a resonance frequency.

Customarily, forcing calculations exclude detailing features, such as leakage flows. The current investigation uses a two stage subsonic model steam turbine configuration with shrouded rotor blades to demonstrate the influence of neglecting flow through seal cavities for blade forcing predictions. Upstream and down-stream vanes are the excitation sources on the rotor blade. Calculation results are compared for a configuration including and excluding the tip shroud cavity. Computed data is compared to available pressure data from tests in the model turbine.

The investigation shows for the first blade passing excitation at design point that the axial and circumferential rotor forcing change by +22%, respectively +4% when including the tip shroud cavity for the investigated configuration. The change in forcing arises from the interaction of the leakage flow with the main stream flow. For highly loaded designs this can be of importance if there is a critical excitation.
Stability of a Cracked Rotor Subjected to Parametric Excitation
Authors: Sawicki Jerzy , Kulesza Zbigniew
Published: September 2014

It is well known that parametric vibrations may appear during the rotation of a rotor with a cracked shaft. The vibrations occur due to periodic stiffness changes being the result of crack breathing. A parametrically excited system may exhibit parametric resonances and anti-resonances affecting the stability of the system. In most cases the destabilizing effect due to parametric resonances is studied. Anti-resonant cases seem to be uninteresting. However, the anti-resonances have an interesting property of introducing additional artificial damping to the system, thus improving its stability and reducing the vibration amplitude. Apart from different control applications, this stabilizing effect may be interesting for its probable ability to indicate the shaft crack. The possible application of the additional damping introduced by parametric excitation for the shaft crack detection is analyzed in the present paper.

The approach is demonstrated with a mathematical model of a rotor with a cracked shaft. The stability analysis of the rotor is performed analytically by employing the averaging method. Stability boundaries for different frequencies of the parametric excitation and for different crack depths are derived. The results of this analysis are checked numerically by means of the Floquet’s theory. Next, possible applications of the parametric excitation for the shaft crack detection are validated numerically. The results demonstrate the potential of the suggested approach, yet further analytical and experimental studies are required.
Gross Error Detection Based on Serial Elimination: Applications to an Industrial Gas Turbine
Authors: Martini Alessio , Coco Dario , Sorce Alessandro , Traverso Alberto , Levorato Paolo
Published: September 2014

Gross Error Detection (GED) is a technique used to identify possible systematic errors in measurements and validate data for a further diagnostic phase. It is always applied along with Data Reconciliation (DR), a technique to improve the accuracy of process data by adjusting the measured values to fit the process equations describing the physical phenomena. They have been applied for a long time to chemical plants with balance equations (mass and composition) and recently extended to industrial power plants [1,2,3]. In this paper a well-known GED technique based on serial elimination has been applied in a gas turbine plant operating in a combined cycle power plant. In a first analysis errors have been imposed manually in the field data to understand the minimum error amplitude avoiding the smearing effect: at the beginning a single gross error has been imposed on the fuel flow rate and on the compressor discharge temperature respectively, then multiple gross errors have been imposed simultaneously on the same measurements. The single gross error tests showed a high capacity of detection and localization, while the multiple gross error analysis highlighted the problems due to the smearing effect (the minimum error intensity to detect and locate errors increased with respect to the single error case). In a second analysis the GED technique has been used to detect and locate a gross error among the three sensors measuring the compressor discharge temperature. The main objective was to analyze the ineffectiveness in error detection and localization of using the mean for redundant measurements.
Experimental Analysis of Rotating Instability Using Hot Wire Triggered PIV and Wavelet Transform
Authors: Hermle Georg , Lawerenz Martin
Published: September 2014

The authors present initial results of hot wire anemometry triggered PIV measurements of periodic effects in an annular compressor cascade. The investigations are focused on the unsteady effect of a rotating instability (RI) and are based on PIV measurements performed within the passage of a highly loaded annular compressor cascade. Simultaneously, hot wire measurements were conducted within the same passage. Post-processing the hot wire signal by means of the wavelet transform obtained the time resolved frequency spectrum as well as the time resolved phase angles. Such spectra allow filtering the PIV results to select only the results that feature the investigated effect of PI. The corresponding phase angles of the wavelet transform enable grouping the selected PIV results in phase classes. The average of each class gives the statistical mean flow field for the corresponding phase angle. Afterwards, the statistical mean period of the RI can be displayed by discrete ensemble averaged PIV results.

The presented method improves the possible temporal resolution of PIV systems beyond their technical sampling rate. In the case presented, the statistical method leads to a temporal resolution corresponding to a sampling rate of 1.5 kHz, whereas the effective sampling rate of the PIV system used is 4 Hz. The possible resolution in time is only limited by the number of phase classes and accordingly by the number of PIV measurements.
Dependence of External Heat Transfer Coefficient and Aerodynamics on Wall Temperature for 3-D Turbine Blade Passage
Authors: Maffulli R. , He L.
Published: September 2014

The dependency of convective heat transfer coefficient (HTC) on wall temperature has been recognized in some previous works but existing corrections are confined to either empirically based correlations or based on a boundary layer approach. A recent study by the present authors on a 2D configuration highlights upstream flow history has a strong impact on HTC for a non-adiabatic blade surface, and such an effect cannot be adequately corrected by the use of existing empirical correlations. A boundary layer based approach may be used in a 2D case for the correction as attempted previously. However, it is strongly argued that a boundary layer based method would become very difficult, if not impossible, to apply for complex 3D flows as those in endwall and secondary flow regions of a turbine blade passage.

The present work is aimed to examine how the HTC and main 3D passage aerodynamic features of interest may be affected by the wall temperature condition. A systematic computational study has been firstly carried out for a 3D NGV passage. The impacts of wall temperature on the secondary flows, trailing edge shock waves and the passage flow capacity are discussed, underlining the connection and interactions between the wall temperature and the external aerodynamics of the 3D passage. The local errors in HTC in these 3D flow regions can be as high as 30–40% if the wall temperature dependence is not corrected.

The effort is then directed to a new 3-point non-linear correction method. The benefit of the 3-point method in reducing errors in HTC is clearly demonstrated. A further study illustrates that the new method also offers much enhanced robustness in the HTC procedure, particularly relevant when the wall thermal condition is shown to influence the laminar-turbulent transition as exhibited by two well-established transition models adopted in the present work.
Endwall Film Cooling Using the Staggered Combustor-Turbine Gap Leakage Flow
Authors: Zhang Yang , Yuan Xin
Published: September 2014

A key technology of gas turbine performance improvement was the increase in the turbine inlet temperature, which brought high thermal loads to the Nozzle Guide Vane (NGV) components. Strong pressure gradients in the NGVs and the complex secondary flow field had made thermal protection more challenging. As for the endwall surface near the pressure side gill region, the relatively higher local pressure and cross flow apparently decreased the film-cooling effectiveness. The aim of this investigation was to evaluate a new design, improving the film-cooling performance in a cooling blind area with upstream staggered slot, simulating the combustor-turbine leakage gap flow. The test cascades model was manufactured according to the GE-E3 nozzle guide vane scaled model, with a scale ratio of 2.2. The experiment was performed under the inlet Mach number 0.1 and the Reynolds number 3.5×105 based on an axial chord length of 78 mm. The staggered slots were positioned upstream of the cascades to simulate the combustor-turbine gap leakage. The Pressure Sensitive Painting (PSP) technique was used to detect the film cooling effectiveness distribution on the endwall surface.

Through the investigation, the following results could be achieved: 1) the film-cooling effectiveness on the endwall surface downstream the slot and along the pitchwise direction increased, with the highest parameter at Z/Pitch = 0.6; 2) a larger cooled region developed towards the suction side as the blowing ratio increased; 3) the advantage of the staggered slot was apparent on the endwall surface near the inlet area, while the coolant film was obviously weakened along the axial chord at a low blowing ratio. The influence of the staggered slots could only be detected in the downstream area of the endwall surface at the higher blowing ratio.
Thermo-Economic Optimization of an Integrated Solar Combined Cycle
Authors: Fusaro Francesco , Conte Enrico , Baykal Sevket , Pattacini Marco
Published: September 2014

If compared to a conventional combined cycle, the optimal design of an Integrated Solar Combined Cycle (ISCC) requires the consideration of additional design parameters, such as the optimal integration point of the solar steam import in the combined cycle, the solar steam generator rating or the size of the solar field. Moreover, to identify the optimal design over the entire operating profile, the trade-off between amount of solar steam generation capacity and the (over)sizing of key components like HRSG, steam turbine, and condenser needs to be considered.

Alstom is prepared to integrate its proprietary technologies in an ISCC. Taking on the above challenge while providing the best possible value to customers calls for advanced methodologies for the selection of the design parameters.

This paper presents a methodology for the initial design of an ISCC, which through determination of the plant thermo-economic performance for a given set of technical and economical boundary conditions, as well as under consideration of the plant’s operating profile, allows identifying the ISCC design with the highest return on investment.

The additional capabilities implemented in the latest development of Alstom’s in-house plant optimization toolbox show that choice of the operating regimes has a strong effect on customer investment strategy.

This confirms that availability of an optimization tool allowing a systematic identification of the right ISCC configuration for the given project specific constraints, is vital.

This tool development handles the critical design parameters with the required accuracy, thus enabling to match the ISCC design with the operational, economical, and physical boundaries of an ISCC project. Furthermore, the tool allows to carry out sensitivity studies and to prioritize development efforts.
Investigation of Wrapping Ribs Onto Smooth Walls for Mid-Chord Internal Cooling Passages
Authors: McGilvray Matthew , Gillespie David , Ryley Josh
Published: September 2014

Internal cooling passage performance is highly influenced by the geometric features placed on the walls to promote high heat transfer. This paper investigates the effects of wrapping the rib turbulators onto either inter passage wall of a filleted, aspect ratio 1:3 passage. Full passage Nusselt number distributions and friction factor along the passage from stationary experiments are presented as a function of Reynolds number. Accompanying numerical calculations applying the realizable k-ε turbulence model are presented to assess RANS calculations ability to predict heat transfer distribitions. These generally show heat transfer similar trends and distributions. However, average heat transfer levels are underpredicted especially at low Reynolds numbers. Comparisons of the rib geometries with non-wall wrapped configurations are made in terms of both Nusselt number distributions and averaged Nusselt number. As expected, adding passage surface area using ribs results in higher total heat transfer and higher pressure losses, both up to 50% compared to baseline rib configuration. Wrapping the ribs onto the trailing edge passage wall results in similar total heat transfer on the ribbed wall with a smaller associated pressure loss than when the ribs are wrapped onto the leading edge of the passage. Application of different turbulence models (realizable k-ε, k-ω SST and RSM) showed difference in Nusselt number distribution, interrogation of the flow solutions reveal differences in mixing and turbulent kinetic energy production.
Experimental and Numerical Investigations on Basic Characteristics of High-Performance Abradable-Aero Hybrid Seal
Authors: Kuwamura Yoshihiro , Matsumoto Kazuyuki , Uehara Hidekazu , Ooyama Hiroharu , Tanaka Yoshinori , Nishimoto Shin
Published: September 2014

As key technologies to improve the performance of steam turbines, various types of high performance seal, such as active clearance control (ACC) seals and leaf seals [1], have been developed by Mitsubishi Heavy Industries, LTD (MHI). In recent years, a new seal concept using an aerodynamic approach called “aero seal” has also been developed, which remarkably reduces the leakage flow while maintaining fin clearances. Furthermore, more robust and higher performance sealing technology called “abradable-aero hybrid seal” which combines the aero seal concept with the abradable seal technology was proposed.

The main concept of the aero seal is to control and utilize the vortex structure in the cavities of the labyrinth seal. In the cavities of the aero seal, the locally-controlled flow on the upstream side of the fin tip causes a strong contraction of the leakage flow and reduces the discharge coefficient significantly. This concept allows for a remarkably reduced leakage flow while maintaining fin clearances. Moreover, in order to achieve more robust and higher performance by minimizing the fin clearances, the abradable seal technology was applied to the aero seal concept. However, when the abradable seal is applied, the grooves may be formed on the wall surface of the abradable material due to rubbing of the fin into the abradable material. This situation leads to concern that the groove breaks the effective vortex structure of aero seal and causes negative effects on the seal performance.

In this paper, the improved aero seal configuration consisting of slant fins was proposed and it was verified that the reduction in the discharge coefficient of improved aero seal is up to 40% compared to the conventional labyrinth seal. Furthermore, more robust and higher performance sealing technology called “abradable-aero hybrid seal” was proposed and basic characteristics such as the effects of the presence of grooves, the axial position of the fin and seal clearance on the leakage mass flow and the vortex structure were parametrically investigated both experimentally and numerically. In the experiments, not only leakage mass flow measurements but also PIV measurements were carried out in order to visualize the flow patterns in the cavity of the abradable-aero hybrid seal. From the results, it was confirmed that the effective vortex structures were formed even with grooves at various fin positions and the leakage flow can be stably reduced over 40% in a wide range of axial position and reduced by 50% at the optimum position.
Impact of Cooling Injection on Transonic Over-Tip Leakage Flow and Squealer Aerothermal Design Optimization
Authors: Wang Z. , Zhang Q. , Liu Y. , He L.
Published: September 2014

The present study has numerically investigated the effect of cooling injection on the Over-Tip-Leakage (OTL) flow and squealer designs in a transonic flow regime. A series of quasi-3D analyses were carried out to understand the cooling jet - OTL flow interaction at various hole locations and inclination angles. The results indicate that the rankings between flat and squealer tip designs might be altered by the addition of cooling injection. Full 3D conjugate analyses demonstrate that partially replacing the squealer cavity with a flat shape in the transonic flow regime would offer a much improved coolablility without paying extra aerodynamic penalty.
Performance Comparison of Twisted Low Solidity Vaned Diffusers With Different Shapes
Authors: Venkateswara Rao Pothuri , Venkata Ramana Murty Govindaraju , Venkata Rao Ganapathiraju
Published: September 2014

Computational Fluid Dynamic (CFD) analysis is conducted to improve the diffuser performance on an industrial centrifugal compressor stage. The present work is concentrated on twisted vaned diffusers(TVD) with different shapes like constant thickness flat plate (FP), cambered curved arc plate (CCAP), uncambered aerofoil (NACA 0010) and cambered aerofoil (NACA 2410). For constant thickness plates, the leading and trailing edges have been rounded to have a smooth entry and exit. The aerofoil diffuser vane is formed from a standard section profile with minor modifications at the trailing edge. The twist is given to the diffuser blade by rotation at its leading edge resulting in different stagger angles from hub to shroud. The analysis was conducted at an impeller tip Mach number of 0.35. The overall stage performance is evaluated in terms of head coefficient, stage efficiency, power coefficient of the stage and static pressure recovery coefficient of diffuser with varying flow coefficients. The TVD NACA 2410 provides best performance for the chosen compressor stage.
Optimal Design of the Volute for a Turbocharger Radial Flow Compressor
Authors: Mojaddam Mohammad , Hajilouy-Benisi Ali , Movahhedy Mohammad Reza
Published: September 2014

In this research the design methods of radial flow compressor volutes are reviewed and the main criterions in volute primary designs are recognized and most effective ones are selected. The effective parameters i.e. spiral cross section area, circumferential area distribution, exit cone and tongue area of the compressor volute are parametrically studied to identifythe optimum values.

A numerical model is prepared and verified through experimental data which are obtained from the designed turbocharger test rig. Different volutes are modeled and numerically evaluated using the same impeller and vane-less diffuser. For each model, the volute total pressure ratio, static pressure recovery and total pressure loss coefficients and the radial force on the impeller are calculated for different mass flow rates at design point and off-design conditions. The volute which shows better performanceand causes lower the net radial force on the impeller, at desiredmass flow rates is selected as an optimal one.

The results show the volute design approach differences at the design point and off-design conditions. Improving the pressure ratio and reducing total pressure loss at design point, may result inthe worse conditions at off-design conditions as well as increasing radial force on the impeller.
High Efficiency SOFC Power Cycles With Indirect Natural Gas Reforming and CO2 Capture
Authors: Campanari Stefano , Gazzani Matteo
Published: September 2014

Driven by the search for the highest theoretical efficiency, several studies have investigated in the last years the adoption of fuel cells in the field of power production from natural gas with CO2 capture. Most of the proposed power cycles rely on high temperature fuel cells, namely Solid Oxide Fuel Cells (SOFC) and Molten Carbonate Fuel Cells (MCFC), based on the concept of hybrid fuel cell plus gas turbine cycles. Accordingly, high temperature fuel cells are integrated with a simple or modified Brayton cycle. As far as SOFC are concerned, two main plant solutions can be identified depending on the integration with the natural gas reforming/shift section: (i) systems where natural gas is — partially or totally — internally reformed in the fuel cell and (ii) systems where natural gas is reformed before the fuel cell and the cell is fed with a high hydrogen syngas. In both cases, CO2 can be separated downstream the fuel cell via a range of available technologies, e.g. chemical or physical separation processes, oxy-combustion and cryogenic methods.

Following a literature review on very promising plant configurations, this work investigates the advantages and limits of adopting an external natural gas conversion section with respect to the plant efficiency. As a reference plant we considered a power cycle proposed by Adams and Barton [8], whose performance is the highest found in literature for SOFC-based power cycles, with 82% LHV electrical efficiency. It is based on a pre-reforming concept where fuel is reformed ahead the SOFC which thus works with a high hydrogen content fuel. This plant was firstly reproduced considering all the ideal assumptions proposed by the original authors. As second step, the simulations were focused on revising the power cycle, implementing a complete set of assumptions about component losses and more conservative operating conditions about fuel cell voltage, heat exchangers minimum temperature differences, maximum steam temperature, turbomachinery efficiency, component pressure losses and other adjustments.

Considering the consequent modifications with respect to the original layout, the net electric efficiency changes to around 66% LHV with nearly complete (95%+) CO2 capture, a still remarkable but less attractive value, while requiring a very complex and demanding heat exchangers network. Detailed results are presented in terms of energy and material balances of the proposed cycles. All the simulations have been carried out with the proprietary code GS, developed by the GECOS group at Politecnico di Milano.
Effect of Endwall Boundary Layer Thickness on Losses in a LPT Cascade With Unsteady Wakes
Authors: Steurer Anika , Benton Stuart I. , Bons Jeffrey P.
Published: September 2014

The effects of inlet endwall boundary layer thickness and up-stream unsteady wakes are investigated experimentally in a low-speed linear cascade. The examined airfoil is the front-loaded L2F, a high-lift low-pressure turbine profile with high resistance to separation even in the low Reynolds number regime. Cases are documented with and without incoming wakes for two inlet endwall boundary layers of different thickness at a Reynolds number of Re = 30,000. Periodic incoming wakes are simulated with moving bars upstream of the cascade. The inlet endwall boundary layer is conditioned with a two-part splitter plate, one part downstream and one part upstream of the wake generator. By the documentation of pressure distributions on the blades, velocity profiles in the cascade inlet as well as total pressure loss and phase-locked velocity data in the outlet, this work attempts to show that varying the inlet endwall boundary layer thickness combined with the effect of incoming wakes has significant influence on the performance of blades with relatively low aspect ratio in cascade experiments. Depending on boundary layer thickness, wakes are shown to have either a stronger impact on midspan or on endwall performance. Time-resolved velocity and vorticity plots additionally show the motion of the vortex and loss core at the blade trailing edge during the event of wake passing.
Conjugate Heat Transfer Analysis in an Actual Gas Turbine Rotor Blade in Comparison With Pyrometer Data
Authors: Tsukamoto Kazuhiro , Horiuchi Yasuhiro , Sugimura Kazuyuki , Higuchi Shinichi
Published: September 2014

Conjugate Heat Transfer (CHT) was analyzed in a first stage rotor blade in an actual gas turbine. The main objectives of this research were to simulate and validate improvements to the accuracy of predicting temperature on the surfaces of rotor blades in a gas turbine and compare these with experimental results.

This simulation was carried out under similar conditions to those during gas turbine operation. Computational grids were generated based on CAD data obtained from the rotor blades with fully resolved rib turbulators and pin fins for both fluid and solid domains during CHT analysis. A tetrahedral mesh with prism layers was used and the y+ of the first mesh adjacent to the wall was kept at less than 1.0 over the whole surface. Thermal barrier coating was modeled by adding thermal resistance at the fluid-solid interfaces. Inlet boundary conditions for the external- and internal-gas-flow regions were defined based on one-dimensional analysis and measured results. Steady Reynolds-averaged Navier-Stokes simulation was carried out using the Shear Stress Transport (SST) turbulence model. The simulated results were compared with measured data obtained from a pyrometer and thermocouple.

The temperature distributions predicted from CHT analysis agreed with those obtained from an experiment near the leading edge of the rotor blades. However, the temperature distribution at the center of the pressure side had a difference of 50 K with that obtained from the experimental data. The heat transfer coefficients on the surfaces of the blades were almost equal to those on the pressure side. Thus, we considered that the internal cooling flows contributed more to temperature distributions on the surfaces of the blades rather than the external gas flows. The main stream in the internal cooling flow passages leaned toward one side of the walls and the temperatures on this side became lower than those obtained from the experimental results. Therefore, we suspect CHT analysis underestimated the mixing effect generated by the rib turbulators. It is important to solve the complex flow phenomena in internal cooling passages to better predict the accuracy of temperature distributions on the surfaces of blades.
Analysis of 3 Dimensional Turbine Flow by Using Mode Decomposition Techniques
Authors: Fjällman Johan , Mihaescu Mihai , Fuchs Laszlo
Published: September 2014

Today one of the most popular ways of lowering the fuel consumption and emissions of the Internal Combustion Engine (ICE) is by downsizing the engine. Downsizing means that the swept volumes of the cylinders are decreased; this lowers the frictional and thermal losses. By combining the downsizing with a well matched turbocharger system the performance is preserved while the advantages are retained. Since more and more of the development work is being performed by simulations there is an increasing need for more accurate methods. These methods are more complex and require more resources than the simpler, faster and more robust models used today. In this study Large Eddy Simulations (LES) of the unsteady flow in a radial turbine designed for a gasoline ICE has been performed and analyzed. The flow inside the turbine is highly 3 dimensional, pulsating and characterized by secondary flow motions and high curvatures. All these are reasons for which the method of choice should be LES. LES is able to resolve a large range of scales and capture the flow dynamics. The considered case concerns a non-pulsating flow condition but with engine like mass flow and temperature. Post-processing tools based on Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to analyse the large amount of LES based flow data. The POD method is used to investigate the energy content of the dominant, large structures present in the flow. The DMD method on the other hand is used to reveal the flow structures responsible for specific frequencies found in the flow field. Preliminary data show a fair agreement between experimental data and LES results in terms of predicting the turbine performance parameters.