bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Study of the Effect of Honeycomb Tip on Tip Leakage Flow in Turbine Cascade
Authors: Fu Yunfeng , Chen Fu , Chen Cong , Song Yanping
Published: September 2016

A novel leakage flow control strategy with honeycomb seal applied on the tip of the rotor blades in a highly-loaded turbine cascade is proposed. The numerical method is used to study the tip leakage flow in a highly-loaded turbine cascades with flat tip and with honeycomb seal structure, the mechanism of honeycomb tip on inhibiting leakage flow is analyzed, the influence of various relative gap heights is also been investigated. The discussions of the action of the honeycomb-tip structure in reducing leakage flow and improving the turbine efficiency provide the according for control methods of tip leakage flow.

Through the comparative study among three different tip structures of honeycomb tip, honeycomb casing and flat tip, the results show that both honeycomb tip and honeycomb casing inhibit the leakage flow effectively, but honeycomb tip has positive effect on reducing the flow loss in cascade. For the cascade with honeycomb tip, on one hand, the vortices rolled up in the regular hexagon honeycomb cavities dissipate the energy of the tip leakage flow, and the range of influence of the vortices is nearly one third of the tip clearance height. On the other hand, the radial jets caused by the honeycomb obstruct the tip leakage flow like a “pneumatic fence”, resulting in weaker leakage flow and less leakage flow rate. Besides, the honeycomb tip reduces the scale of the leakage vortex, thus the leakage loss also decreases. Compared with the flat tip cascade at the clearance height of 1%H, the honeycomb tip cascade with the same clearance height obtains decrease of the leakage flow rate and leakage flow speed in circumference by 10.16% and 20%. As a result, the leakage vortex in honeycomb tip cascade is undermined, the loss is reduced by nearly 4.43%.

Considering the abradable property of the honeycomb seal that can protect the blade tips from damage, the cascade with honeycomb tip structure can obtain a smaller clearance height and achieve better sealing effect. Compared to cascade with the flat tip at the clearance height of 2%H, the amount of leakage flow using inlet flow in the honeycomb tip cascades decreases by 17.33%, 36.63% and 54.79% at the clearance heights of 2%H, 1.5%H and 1%H, the losses related to the leakage flow is reduced by nearly 5.71%, 14.33% and 25.24%, respectively.
Integrated Simulation Model of Rotor/Propeller Powered Aerobot in Steady Flight
Authors: Zhao Wei , Wang Zhenguo
Published: September 2016

In this paper, a cost-effective integrated simulation model for rotor/propeller driven aerobot is proposed based on the free wake model. It is helpful to provide aerodynamic input for preliminary control law design, especially for conceptual design at the beginning of a project. Apart from the computational efficient, the model proposed has two more advantages, grid-free and containing the aerodynamic interaction. The governing equation is the Laplacian Equation with the assumption of invisid, incompressible flow. The solver used is inspired by the well-known Pseudo-Implicit Predictor Corrector (PIPC) for the rotors. Two validation cases are carried out. Firstly, the simulation results for the Harrington coaxial rotors and the Hamilton coaxial propellers show good agreement with the experiment. The simulation of a 45° swept-back wing also goes well with the corresponding experiment. Before the end, a simulation for a transition state with 0° tilt angle for the Eagle Eye is used to show the aerodynamic interaction, which has more influence on the wings.
Investigation on Pitch-Wise Non-Uniform and Inflecting Inlet Flow of Low-Speed Plane Cascade
Authors: Xue Xingxu , Zhou Xun , Liu Xun , Wen Fengbo
Published: September 2016

The pitch-wise uniformity of inlet flow was proved by experiment in low-speed plane cascade, and simulations were performed for investigation and improvement in this study after that. The deflector, which was connected to the blade strictly, was proved to be the cause of the problem according to the simulation, and unexpected inflection of inlet flow were proved at the same time. It was also proved that a gap, which was placed between the blade and the deflector, could weaken the influence of deflector. However, the leakage that caused by the gap would also lead to some negative effects that attentions must be paid to. Finally, with cascade moved half-pitch along x-axis and one extra blade added, more acceptable result between 1 and 4 relative pitch-wise was showed in one of the improved cases (Case B). Better inlet condition with more periodic pressure distribution and acceptable incident angle were gained.
Low-Leakage Shaft End Seals for Utility-Scale Supercritical CO2 Turboexpanders
Authors: Bidkar Rahul A. , Sevincer Edip , Wang Jifeng , Thatte Azam M. , Mann Andrew , Peter Maxwell , Musgrove Grant , Allison Timothy , Moore Jeffrey
Published: September 2016

Supercritical carbon dioxide (sCO2) power cycles could be a more efficient alternative to steam Rankine cycles for power generation from coal. In this paper, the end seal layout for a nominally 500 MWe sCO2 turbine is presented and the shaft end sealing requirements for such utility-scale sCO2 turbines are discussed. Shaft end leakage from a closed-loop sCO2 cycle and the associated recompression load can result in net cycle efficiency loss of about 0.55% points to 0.65% points for a nominally 500 MWe sCO2 power cycle plant. Low-leakage hydrodynamic face seals are capable of reducing this leakage loss (and net cycle efficiency loss), and are considered a key enabling component technology for achieving 50–52% or greater thermodynamic cycle efficiencies with indirect coal-fired sCO2 power cycles. In this paper, a hydrodynamic face seal concept is presented for end seals on utility-scale sCO2 turbines. A 3D computational fluid dynamics (CFD) model with real gas CO2 properties is developed for studying the physics of the thin fluid film separating the seal stationary ring and the rotor. The results of the 3D CFD model are also compared with the predictions of a Reynolds-equation-based solver. The 3D CFD model results show large viscous shear and the associated windage heating challenge in sCO2 face seals. Following the CFD model, an axisymmetric finite-element analysis (FEA) model is developed for parametric optimization of the face seal cross-section with the goal of minimizing the coning of the stationary ring. A preliminary thermal analysis of the seal is also presented. The fluid, structural and thermal results show that large-diameter (about 24 inch) face seals with small coning or out-of-plane deformations (of the order of 0.0005 inch) are possible. The fluid, structural and thermal results are used to highlight the design challenges in developing large-diameter and high-differential-pressure face seals for the operating conditions of utility-scale sCO2 turbines.
Computational Analysis of Compressor Blade Erosion in a Wet Compression System
Authors: Chidambaram Palani Kumar , Song Tae Hoon , Kim In Won , Lee Kwon Hee , Kim Heuy Dong
Published: September 2016

In a gas turbine engine a wet compression system is installed to increase the net power output. However, this may erode compressor blades due to fine water droplets hitting the surface. In the present work, numerical investigations are carried out to study blade erosion by water droplets. The computational domain consists of a rotating blade jig on which four blades are mounted. This represents the simultaneous experiments being carried out. Sliding mesh method is used to incorporate rotary movement of the blades. Water is injected as spray using an impact-pin nozzle. Experimentally measured spray parameters like the flow rate, particle diameter, etc. are given as initial and boundary conditions in the simulations. Discrete Phase Model (DPM) is used to track the particles in the domain. Droplet parameters like average velocity, diameter and number of droplets hitting each cell on the blade surface are monitored. These parameters are then used to predict the rate of erosion on the surface. In this manuscript, the progresses in the prediction of blade erosion at various blade rotational speed (rpm) is reported.
Development and Evaluation of a Performance Estimation Methodology for Fans Operating Within Non-Homogeneous Inflow
Authors: Schoenweitz D. , Schnell R.
Published: September 2016

The design process of modern aircraft engines extensively makes use of computational fluid dynamics (CFD). During the design process fan, compressor and turbine stages are typically evaluated using single passage CFD-simulations. For the design of an inlet distortion tolerant fan, the inhomogeneity of the inflow in circumferential direction does not allow the application of periodic boundary conditions and hence single passage simulations. Accordingly transient full annulus simulations have to be applied to correctly resolve the underlying physical interactions. The associated computation time for such a high fidelity simulation prohibit its application in today’s design procedures. This paper compares the simulation results of the ingestion of a mild and generic fuselage boundary layer of a blended wing body aircraft into an UHBR engine on the basis of four different simulation methods. The four different simulations include: transient full annulus CFD, Harmonic Balance frequency domain based CFD, steady full annulus CFD (Frozen Rotor) and steady single passage CFD for each blade passage. The results of the simulations are assessed using a newly developed stream tube based post-processing procedure. For the ingestion of the fuselage boundary layer the four numerical setups result in different aerodynamic conditions in the fan stage. For different aerodynamic measures different discrepancies are encountered for the different simulation methods. While the Harmonic Balance method can reproduce most features well, the simpler models overestimate the variations in the different flow features.
Experimental and Numerical Verification of the Core-Flow in a New Low-Pressure Turbine
Authors: Henke Michael , Wein Lars , Kluge Tim , Guendogdu Yavuz , Biester Marc Heinz-Otto , Seume Joerg R.
Published: September 2016

The flow field in modern axial turbines is non-trivial and highly unsteady due to secondary flow and blade row interaction. In recent years, existing design-tools like two-dimensional flow solvers as well as fully three-dimensional CFD methods have been validated for the assumption of a quasi-steady flow field. Since the inevitable unsteadiness of the flow field has a direct impact on unsteady loss generation and work transfer, existing design methods stand in need of validation for local unsteady effects within the flow field. In order to clearly separate end-wall losses from those generated by blade row interaction within the blade passage, a two-dimensional core-flow is essential for the investigation. Hence, a new 1.5-stage high aspect ratio low pressure turbine has been designed to determine the intensity of core-flow blade row interaction for different axial gaps. First, inlet and outlet conditions of the test rig are evaluated with regard to homogeneity of the flow parameters in their radial and circumferential distributions. Secondly, the measurement data gained from rig tests have been applied as boundary conditions to time-averaged numerical computations. The flow field analysis for two different axial gaps focuses on the verification of the core flow. The authors show that the new turbine has been successfully verified using both test data and the numerical predictions, serving as a precondition for the validation of the numerical model for unsteady effects within the core-flow.
Investigation of Ozone Stimulated Combustion in the SGT-800 Burner at Atmospheric Conditions
Authors: Lantz Andreas , Larfeldt Jenny , Ehn Andreas , Zhu Jiajian , Subash Arman Ahamed , Nilsson Elna J. K. , Li Zhongshan , Aldén Marcus
Published: September 2016

The effect of ozone (O3) in a turbulent, swirl-stabilized natural gas/air flame was experimentally investigated at atmospheric pressure conditions using planar laser-induced fluorescence imaging of formaldehyde (CH2O PLIF) and dynamic pressure monitoring. The experiment was performed using a dry low emission (DLE) gas turbine burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. CH2O PLIF imaging was carried out for four different seeding gas compositions and seeding injection channel configurations. Two seeding injection-channels were located around the burner tip while the other two were located along the center axis of the burner at different distances upstream the burner outlet. Four different seeding gas compositions were used: nitrogen (N2), oxygen (O2) and two ozone/oxygen (O3/O2) mixtures with different O3 concentration. The results show that the O3 clearly affects the combustion chemistry. The natural gas/air mixture is preheated before combustion which is shown to kick-start the cold combustion chemistry where O3 is highly involved. The CH2O PLIF signal increases with O3 seeded into the flame which indicates that the pre-combustion activity increases and that the cold chemistry starts to develop further upstream. The small increase of the pressure drop over the burner shows that the flame moves upstream when O3 is seeded into the flame, which confirms the increase in pre-combustion activity.
Influence of Nonlinear Flame Models on Thermoacoustic Instabilities in Combustion Chambers
Authors: Campa Giovanni , Cademartori Roberto
Published: September 2016

Thermoacoustic phenomena observed in gas turbine combustion chambers are usually related to a dependence of the heat release fluctuations to the velocity fluctuations at the injection point. Linear models for this dependence give information about stability, while nonlinear models can predict also limit cycles amplitude and frequency. This nonlinear dependence can be obtained by means of experiments or numerical tools by measuring the flame describing function (FDF). Analytical expressions have been proposed in the years to obtain nonlinear flame models to be introduced into simplified tools. Through these nonlinearities, information about limit cycle amplitudes, bifurcation diagrams and hysteresis can be achieved. The importance of this information is related to the possibility of managing the variation of certain operating parameters to keep the combustor in a safe operating zone. Additionally, from the bistable zone information about the operational margin can be obtained in order to stay far from the unstable condition.

The aim of this work is to describe a simplified procedure to track combustors bifurcation diagrams using nonlinear flame models implemented in a 3D thermoacoustic tool based on the finite element methods (FEM) in the frequency domain. First flame describing functions (FDF) for not swirled and swirled burners from the literature are considered and applied to simplified configurations. Then, FDFs are introduced into an industrial configuration and the bifurcation diagrams are tracked. For both cases, the paper describes the influence of various FDFs parameters and shapes on the predictions of combustors stability boundaries.
Investigation of Different Operation Strategies to Provide Balance Energy With an Industrial CHP Plant Using Dynamic Simulation
Authors: Kahlert Steffen , Spliethoff Hartmut
Published: September 2016

Intermittency of renewable electricity generation poses a challenge to thermal power plants. While power plants in the public sector see a decrease in operating hours, the utilization of industrial power plants is mostly unaffected because process steam has to be provided. This study investigates to what extent the load of a CHP plant can be reduced while maintaining a reliable process steam supply. A dynamic process model of an industrial combined CHP plant is developed and validated with operational data. The model contains a gas turbine, a single pressure HRSG with supplementary firing and an extraction condensing steam turbine. Technical limitations of the gas turbine, the supplementary firing and the steam turbine constrain the load range of the plant. In consideration of these constraints, different operation strategies are performed at variable loads using dynamic simulation. A simulation study shows feasible load changes in 5 min for provision of secondary control reserve. The load change capability of the combined cycle plant under consideration is mainly restricted by the water-steam cycle. It is shown that both the low pressure control valve of the extraction steam turbine and the high pressure bypass control valve are suitable to ensure the process steam supply during the load change. The controllability of the steam turbine load and the process stability are sufficient as long as the supplementary is not reaching the limits of the operating range.
Evaluation of Centrifugal Compressor Performance Models Using Large Eddy Simulation Data
Authors: Sundström Elias , Kerres Bertrand , Mihăescu Mihai
Published: September 2016

Zero-dimensional (0D) compressor performance models, which consist of several sub-models for different loss terms, are useful tools in early design stages. In this paper, one typical model for centrifugal compressors is evaluated by comparing the loss-terms predicted by the model to data extracted from experimentally validated Large-Eddy-Simulation. The simulations were run on a truck-sized turbocharger compressor with a ported shroud and a vaneless diffuser. Four operating points are considered: One mass flow at design conditions and one mass flow close to surge, on two speedlines. The performance prediction models evaluated are impeller incidence loss, impeller skin friction loss, diffuser skin friction loss, and the tip clearance loss. Results show that the total losses are well-predicted by the model at design conditions. Friction losses are approximately independent of mass flow in the LES data, while the 0D model assumes a quadratic increase. The assumption of constant tip clearance loss is validated by the LES data, and the impeller incidence loss model also fits the data well. Due to the ported shroud, most of the losses as calculated by entropy increase occur through isobaric mixing at the impeller inlet.
Influence of the Inter-Stage Coupling Flexibility on the Dynamics of Multi-Stage Rotors
Authors: Nyssen Florence , Golinval Jean-Claude
Published: September 2016

In this work, a characterization of the inter-stage coupling in a one-piece multi-stage bladed structure is performed. More particularly, the effect of the inter-stage coupling flexibility on the mode-shapes is evaluated. To this purpose, the MAC matrix between the mono-stage and multi-stage modes is computed for different drum Young’s modulus of the inter-stage coupling. In parallel, the strain energy located in the blades and in the connecting structure is computed for different levels of inter-stage coupling. This enables to establish a criterion to determine when a multi-stage finite element analysis is necessary instead of only computing the different mono-stage models separately. This criterion is based on the localization of the energy in the structure. Numerical analyses are performed on a two-stages academic bladed structure.
Design Optimization of Profiled Endwall With Consideration of Cooling and Rim Seal Flow Effects
Authors: Tang Huimin , Liu Shuaiqiang , Luo Hualing
Published: September 2016

Profiled endwall is an effective method to improve aerodynamic performance of turbine. This approach has been widely studied in the past decade on many engines. When automatic design optimisation is considered, most of the researches are usually based on the assumption of a simplified simulation model without considering cooling and rim seal flows. However, many researchers find out that some of the benefits achieved by optimization procedure are lost when applying the high-fidelity geometry configuration. Previously, an optimization procedure has been implemented by integrating the in-house geometry manipulator, a commercial three-dimensional CFD flow solver and the optimization driver, IsightTM. This optimization procedure has been executed [12] to design profiled endwalls for a turbine cascade and a one-and-half stage axial turbine. Improvements of the turbine performance have been achieved. As the profiled endwall is applied to a high pressure turbine, the problems of cooling and rim seal flows should be addressed. In this work, the effects of rim seal flow and cooling on the flow field of two-stage high pressure turbine have been presented. Three optimization runs are performed to design the profiled endwall of Rotor-One with different optimization model to consider the effects of rim flow and cooling separately. It is found that the rim seal flow has a significant impact on the flow field. The cooling is able to change the operation condition greatly, but barely affects the secondary flow in the turbine. The influences of the profiled endwalls on the flow field in turbine and cavities have been analyzed in detail. A significant reduction of secondary flows and corresponding increase of performance are achieved when taking account of the rim flows into the optimization. The traditional optimization mechanism of profiled endwall is to reduce the cross passage gradient, which has great influence on the strength of the secondary flow. However, with considering the rim seal flows, the profiled endwall improves the turbine performance mainly by controlling the path of rim seal flow. Then the optimization procedure with consideration of rim seal flow has also been applied to the design of the profiled endwall for Stator Two.
Thermo-Fluid Dynamic Analysis on the Compression Process of Liquid-Gas Two-Phase
Authors: Mohan Abhay , Chidambaram Palani Kumar , Suryan Abhilash , Kim Heuy Dong
Published: September 2016

In a typical compressor - turbine cycle, the majority of the output power from the turbine is consumed for the operation of the compressor. In order to obtain higher output from turbines, certain techniques are employed to reduce compressor work. It is known that the compressor work increases continually with the increase in temperature of the operating fluid. One of the techniques to achieve reduction of temperature of the fluid is through the inter-cooling between the compressor stages. But this may lead to the decrease in the overall efficiency of the cycle. Another concept is to utilize the high enthalpy of vaporization of water. One of these techniques is known as wet compression. Here water droplets are introduced into the compressor and the fluid mixture is compressed. The droplets absorb the heat from the surroundings and evaporate, and thus reduces the temperature of the operating fluid. This in turn decreases the compression work. Also in order to maintain the O/F ratio of gas turbine, the mass flow rate of the fuel will also increase. All the above mentioned factors thus increase the net power output of the turbine engine.

For the current study, a cylinder-piston system containing air involving fine droplets of water is modeled as a simple representation of the wet compression process. The compression process is achieved by the movement of the piston. Thermodynamic properties such as pressure and temperature are investigated in detail for different parameters such as rates of compression, droplet mass and sizes. Analytical equations are derived and validated using the classical D-square law. These equations are used in order to track the change in fluid properties during the compression process and its deviation from dry air compression. The results thus obtained are discussed in terms of the rates of compression, absolute and relative humidities. Corresponding thermodynamic curves are generated which are seen to deviate significantly from the dry isentropic curves. It is observed that smaller diameter droplets, slower speeds of compression and higher amount of overspray percentages lead to lower compressor work.
Exhaust Gas Recirculation on Humidified Flexible Micro Gas Turbines for Carbon Capture Applications
Authors: De Paepe Ward , Montero Carrero Marina , Giorgetti Simone , Parente Alessandro , Bram Svend , Contino Francesco
Published: September 2016

From all fossil fuels, natural gas has the lowest carbon to hydrogen ratio, which enables Gas Turbines (GTs) running on natural gas to produce electricity with the lowest CO2 emissions per produced kWh. These lower emissions have pushed power production towards natural gas. However, if we want to move towards a carbon clean power production, the carbon in the exhaust must be captured. This leads to a major challenge since the low CO2 concentration in the exhaust of a GT makes carbon capture much more expensive compared to coal fired power production.

The CO2 concentration can be increased by performing Exhaust Gas Recirculation (EGR). However, EGR on GT cycles negatively affects the efficiency. Using the concept of Humid Air Turbine (HAT), we investigate whether the efficiency losses can be compensated by introducing water in the cycle. This paper presents this novel approach by showing the impact of EGR on a flexible humidified micro Gas Turbine (mGT). It is based on results of simulations performed in Aspen® using the Turbec T100 mGT as reference case. Both the dry and wet operation of the Turbec T100 were simulated and validated with experimental results. For improved carbon capture, EGR was simulated in both the dry and the humidified mGT cycle. Simulation results indicate that EGR has no effect on the thermodynamic performance of the mGT and its components (compressor, turbine and recuperator), however efficiency is reduced significantly (up to 3.8% relative at nominal power output) because of additional losses to the fan blower installed to ensure the EGR. Additionally, the cycle performance strongly depends on the degree of cooling of the EGR stream before injection in the compressor inlet. Nevertheless, the simulation results also reveal that mGT humidification increases the total cycle efficiency, entirely compensating the EGR induced losses. Humidifying the mGT with EGR even leads to a higher electric efficiency than the standard mGT cycle, unlocking the idea of carbon capture in mGTs.
FLOvane: A New Approach for HP Vane Design
Authors: Wang Dingxi , Ornano Francesco , Li Yan Sheng , Wells Roger , Hjalmarsson Christer , Hedlund Lars , Povey Thomas
Published: September 2016

The University of Oxford has been working for a number of years on a conceptually new philosophy for HP vane design [1].

The desired function of the HP vane is to accept flow from a — generally — annular intake, and turn and accelerate it with minimum loss and heat transfer, and with due consideration to mechanical, manufacturing and integration issues. The conventional design process is well-defined: annulus line definition; through-flow analysis; 2D airfoil section definition; vane stack definition; lean/sweep treatments; secondary flow contouring and fillet design; vane and platform cooling system design; integrated aero/thermal/capacity optimization. The process is based on optimizing 2D airfoil stacks to control secondary flows. More recent research (last 20 years) has led to better understanding of airfoil lean, airfoil sweep, leading edge treatments and end-wall profiling. There may be some iterative optimization in which the airfoil sections themselves are modified, but these treatments (as implied by the terminology) are essentially modifications to a 2D airfoil stack which interrupts the upstream annular (most aero-engines) or can-partitioned (most land-based gas turbines) duct.

It is proposed that a more natural design starting point for admitting and accelerating flow with minimum loss (etc.) is a trumpet-shaped flow-path which gradually turns to the desired angle. This approach rejects many traditional concepts in aerodynamics (the sacrosanct nature of the airfoil, concepts of good and bad lift distributions, concepts of vane and platform, etc.), and focuses our attention on the design of a highly-lofted flow-path.

The purpose of this paper is to show that there may be merit in reappraising the starting point for HP vane design, and in investigating Fully Lofted Oval vane (FLOvane) forms of the type described. The corresponding author has not yet heard a good argument against the proposed design philosophy. Despite significantly diverging from conventional wisdom in terms of both process and final geometric form, the following advantages are claimed: improved aerodynamic loss characteristics, reduced heat load, improved cooling performance, improved thermal-mechanical life, improved stage/engine efficiency.

The claimed advantages are justified with detailed numerical analyses of two current industrial gas turbines, compared back-to-back with modified turbines incorporating unoptimised FLOvane designs. Significant performance improvements were observed when FLOvane designs were used.
Investigation of Shock-Wave/Boundary-Layer Interaction on Aeroelastic Stability: Towards Active Control
Authors: Rendu Quentin , Rozenberg Yannick , Aubert Stéphane , Ferrand Pascal
Published: September 2016

In order to predict oscillating loads on a structure, time-linearized methods are fast enough to be routinely used in design and optimization steps of a turbomachine stage. In this work, frequency-domain time-linearized Navier-Stokes computations are proposed to predict the unsteady separated flow generated by an oscillating bump in a transonic nozzle. We also investigate the interaction of backward traveling pressure waves and moving surface on the unsteady behavior of a strong shock-wave with separated boundary-layer. This case is representative of transonic stall flutter of a compressor blade submitted to downstream stator potential effects. The influence of frequency is first investigated on a generic oscillating bump to identify the most unstable configuration. Introducing back pressure fluctuations, we then show that the aeroelastic stability of the system depends on the phase-shift between the fluctuations and the bump motion. Finally, we propose to actively control the instability by generating backward traveling pressure waves at prescribed amplitude, frequency and phase.
Determination of Laminar Flame Speed and Markstein Numbers Deduced From Turbulent Flames Using the Bomb Method
Authors: Baust Tobias , Habisreuther Peter , Zarzalis Nikolaos
Published: September 2016

The flame speed is a central element not only in theoretical combustion and basic research but also a key parameter for several combustion models. Various methods to define and measure laminar flame speed have been applied. One method that is being relevant to determine flame speed and flame stretch (quantified by Markstein numbers) under high pressure and high temperature conditions is the bomb method, where an unsteady spherical expanding flame is investigated in a closed vessel. Especially for higher pressures, instabilities occur in the flame front of spherically expanding flames, due to the decreased flame thickness and diffusion processes. These so called cellular structures increase the flame surface and therefore the laminar flame speed cannot be determined in the usual manner. As high pressures are common under engine conditions, there is a need to be still able to determine the flame speed within these pressure ranges. By exposing the flame to a turbulent flow field, the vortex interactions are mainly responsible for the deformation of the flame surface. This fact can be used to deduce the laminar flame speed from a turbulent flame. In the present work, a turbulent flame speed model to determine unstretched laminar flame speed and Markstein numbers is introduced and validated. For this purpose the flame is exposed to a well-known turbulent flow field that is nearly homogenous and isotropic. By estimating the flame surface, the laminar flame speed can be calculated, using a turbulent stretch model as well as the flamelet assumption in an implicit approach. A high speed 2D laser imaging technique is used to capture the flame propagation. The most significant issue of this method is to correctly identify the surface and the volume of the flame, because solely a cross section of the flame surface can be visualized in the laser sheet. In case of spherical flames, the radius of the cross section corresponds to the flame surface and volume whereas a single radius is not sufficient to quantify a turbulent flame. The idea is to use the circumference of the cross section in a power law approach to quantify the cellular structures as a mean. The implicit model has been validated for methane and hydrogen flame speed at various equivalence ratios at atmospheric and elevated pressure. The results are in a good agreement with laminar determined flame speed and also the Markstein number is obtained qualitatively correct. Although the flame surface and volume cannot be determined directly by 2D measurement techniques, a model has been developed to determine laminar unstretched flame speed and Markstein numbers by investigating unsteady propagating flames in a turbulent flow field.
An Investigation on Aerodynamics Loss Mechanism of Squealer Tips of a High Pressure Turbine Blade Using URANS
Authors: Jung Jin-sol , Kwon Okey , Son Changmin
Published: September 2016

The flow leaking over the tip of a high pressure turbine blade generates significant aerodynamic losses as it mixes with the mainstream flow. This study investigates the effect of blade tip geometries on turbine performance with both steady RANS and unsteady URANS analyses. Five different squealer geometries for a high pressure turbine blade have been examined: squealer on pressure side, squealer on suction side, cavity squealer, cavity squealer with pressure side cutback, and cavity squealer with suction side cutback. With the case of the cavity squealer, three different squealer wall thickness are investigated for the wall thickness (w) of 1x, 2x and 4x of the tip gap (G). The unsteady flow analyses using CFX have been conducted to investigate unsteady characteristics of the tip leakage flow and its influence on turbine performances. Through the comparison between URANS analyses, detailed vortex and wake structures are identified and studied at different fidelities. It is found that the over tip leakage flow loss is affected by the tip suction side geometry rather than that of the pressure side geometry. The unsteady results have contributed to resolve the fundamentals of vortex structures and aerodynamic loss mechanisms in a high pressure turbine stage.
Endwall Loss in Turbine Cascades
Authors: Coull John D.
Published: September 2016

Prior to the detailed design of components, turbomachinery engineers must guide a mean-line or throughflow design towards an optimum configuration. This process requires a combination of informed judgement and low-order correlations for the principle sources of loss. With these requirements in mind, this paper examines the impact of key design parameters on endwall loss in turbines, a problem which remains poorly understood.

The analysis focuses on parametric computational studies of linear cascades, which represent a simplified model of real-engine flow. The designs are nominally representative of the Low Pressure Turbine blades of an aero engine. The paper examines the variations across the design space in a quantitative manner, examining the impact of flow angles, blade thickness and suction surface aerodynamics.
Efficient Multi-Objective Optimization of Labyrinth Seal Leakage in Steam Turbines Based on Hybrid Surrogate Models
Authors: Cremanns Kevin , Roos Dirk , Hecker Simon , Dumstorff Peter , Almstedt Henning , Musch Christian
Published: September 2016

The demand for energy is increasingly covered through renewable energy sources. As a consequence, conventional power plants need to respond to power fluctuations in the grid much more frequently than in the past. Additionally, steam turbine components are expected to deal with high loads due to this new kind of energy management. Changes in steam temperature caused by rapid load changes or fast starts lead to high levels of thermal stress in the turbine components. Therefore, todays energy market requires highly efficient power plants which can be operated under flexible conditions. In order to meet the current and future market requirements, turbine components are optimized with respect to multi-dimensional target functions. The development of steam turbine components is a complex process involving different engineering disciplines and time-consuming calculations. Currently, optimization is used most frequently for subtasks within the individual discipline. For a holistic approach, highly efficient calculation methods, which are able to deal with high dimensional and multidisciplinary systems, are needed. One approach to solve this problem is the usage of surrogate models using mathematical methods e.g. polynomial regression or the more sophisticated Kriging. With proper training, these methods can deliver results which are nearly as accurate as the full model calculations themselves in a fraction of time.

Surrogate models have to face different requirements: the underlying outputs can be, for example, highly non-linear, noisy or discontinuous. In addition, the surrogate models need to be constructed out of a large number of variables, where often only a few parameters are important. In order to achieve good prognosis quality only the most important parameters should be used to create the surrogate models. Unimportant parameters do not improve the prognosis quality but generate additional noise to the approximation result. Another challenge is to achieve good results with as little design information as possible. This is important because in practice the necessary information is usually only obtained by very time-consuming simulations.

This paper presents an efficient optimization procedure using a self-developed hybrid surrogate model consisting of moving least squares and anisotropic Kriging. With its maximized prognosis quality, it is capable of handling the challenges mentioned above. This enables time-efficient optimization. Additionally, a preceding sensitivity analysis identifies the most important parameters regarding the objectives. This leads to a fast convergence of the optimization and a more accurate surrogate model. An example of this method is shown for the optimization of a labyrinth shaft seal used in steam turbines. Within the optimization the opposed objectives of minimizing leakage mass flow and decreasing total enthalpy increase due to friction are considered.
A Turbofan-Engine Nacelle Shape Design and Optimization Method for Natural Laminar Flow Control
Authors: Li Songyang , Zhong Yongjian
Published: September 2016

A correctly profiled engine nacelle can delay the transition in the boundary layer and allow laminar flow to extend back, resulting in a substantial drag reduction. Therefore, the laminar flow nacelle has lower fuel consumption than current turbulent designs. In this paper, aerodynamic shape optimization of natural laminar flow nacelle has been studied by using a novel nacelle shape design method and transition prediction with CFD. First, the 2D longitudinal profile-line of nacelle is optimized, in order to extend its laminar region and achieve minimum drag coefficient within the design space. Second, the optimized longitudinal profile-line is then circumferentially stacked to construct the 3D nacelle aerodynamic shape. At last, the aerodynamic improvement of the new shape is evaluated by 3D CFD simulation. A nacelle geometry generator has been developed where the deflection angle (related to the curvature) along the cord is controlled by using Non-Uniform Rational B-Splines. It is then analytically integrated to obtain the longitudinal profile-line. And also a leading edge matching function is involved in the generator. This technique improves the smoothness of nacelle profile-line, which ensures the curvature and slope of curvature to be continuous all over the nacelle surface. The pressure distribution over the nacelle surface has been improved with no spikes in Mach number. A transition model coupling with shear stress transport turbulent model is used in solving Navier-Stokes equations for transition prediction. An optimization system has been established in combination with the geometry generator, the transition prediction model with CFD, a Kriging surrogate model and a Multi-Island Genetic Algorithm. As a result, the aerodynamic improvement, with one profile-line optimized, is obvious against the original nacelle shape by CFD validation in 3D simulation. The optimized nacelle can achieve a laminar flow up to 23% and its drag coefficient has reduced by 6.5%. It is indicated that the optimization system is applicable in nacelle aerodynamic shape design.
Parametric Bird Strike Study of a Transonic Rotor Using Isogeometric Analysis
Authors: Duckitt Sam , Bisagni Chiara , Shahpar Shahrokh
Published: September 2016

This paper investigates the use of isogeometric analysis (IGA) to study high velocity impact on a transonic compressor rotor resulting from a bird strike. An approach is developed for creating volumetric NURBS blade models which are suitable for IGA. A newly implemented 3D solid NURBS element within the development version of LS-Dyna is validated against finite elements for the NASA rotor 37 under a steady centrifugal load. The smoothed particle hydrodynamics (SPH) method is then used to simulate impact from a bird strike. As a preliminary assessment for multi-disciplinary optimisation (MDO), with the objective to improve aerodynamic performance whilst satisfying mechanical constraints from impact, a number of different blade designs are created by modifying the NURBS control points directly. Hence the control points used in analysis can also be used in the design space. This approach eliminates the need for re-meshing, highlighting the advantages that IGA can bring to design optimisation, since without filtering, moving finite element nodes can result in non-smooth geometries. NURBS parametrisations are also more efficient resulting in fewer design variables, thereby accelerating the optimisation process. The effect of blade sweep, lean, twist and thickness on the impact response are investigated. The results in this paper show the promise that IGA holds in this field but some limitations of the current LS-Dyna implementation are also discussed.
Comparison of Flame Transfer Functions Acquired by Chemiluminescence and Density Fluctuation
Authors: Peterleithner Johannes , Basso Riccardo , Heitmeir Franz , Woisetschläger Jakob , Schlüßler Raimund , Czarske Jürgen , Fischer Andreas
Published: September 2016

The goal of this study was to measure the Flame Transfer Function of a perfectly and a partially premixed turbulent flame by means of Laser Interferometric Vibrometry. For the first time, this technique is used to detect integral heat release fluctuations. The results were compared to classical OH*-chemiluminescence measurements. Effects of equivalence ratio waves and vortex rollup were found within those flames and were then investigated by means of time resolved planar CH*/OH*-chemiluminescence and Frequency modulated Doppler global velocimetry. This work is motivated by the difficulties chemiluminescence encounters when faced with partially premixed flames including equivalence ratio waves and flame stretching. LIV, recording the time derivative of the density fluctuations as line-of-sight data, is not affected by these flame properties.
Nonlinear Hydrodynamics of a Bluff-Body Stabilized Turbulent Premixed Flame
Authors: Lee C. Y. , Cant R. S.
Published: September 2016

Bluff-body stabilized turbulent premixed flames can experience hydrodynamic instability caused by the interaction of the flame with small-scale vortices in the separated shear layer around the recirculation region, as well as with the large-scale coherent structures in the far-wake. A globally hydrodynamically unstable system, for example one which involves vortex shedding, can exhibit limit-cycle behaviour due to the coupling between pressure oscillation and velocity fluctuations. In this work, the hydrodynamic behaviour of a bluff-body stabilized turbulent premixed propane/air flame in a model jet-engine afterburner is investigated using Computational Fluid Dynamics (CFD). A URANS approach was found to be appropriate for the range of frequencies considered in this study. Combustion is modelled using a modified flame surface density (FSD) approach. The observed self-excited hydrodynamic oscillations are analyzed using a nonlinear dynamical framework which is capable of capturing elaborate nonlinear behaviour including quasiperiodicity and chaos. The results from the CFD are first validated using available experimental data. The velocity at the inlet is gradually increased from 14 m/s to 33 m/s and the global flame structure is observed. With increasing inlet velocity, the flame first transitions from steady state to an oscillating state with a symmetrical flame structure, and eventually to an asymmetrical flame structure at higher velocities. The flame is essentially steady in the lower range of velocities considered before transitioning to a limit cycle oscillation after a critical velocity is exceeded.

A doubling in the frequency of the hydrodynamic oscillation is also observed at intermediate values of inlet velocity. This investigation demonstrates that turbulent premixed reacting flows can exhibit strong hydrodynamic oscillation. An understanding of such behaviour can assist in developing methods to control flow instabilities and therefore help in suppressing thermoacoustic oscillation.
A Second Turbulent Regime When a Fully Developed Axial Turbulent Flow Enters a Rotating Pipe
Authors: Cloos Ferdinand-J. , Zimmermann Anna-L. , Pelz Peter F.
Published: September 2016

When a fluid enters a rotating circular pipe a swirl boundary layer with thickness of δ̃s appears at the wall and interacts with the axial momentum boundary layer with thickness of δ̃. We investigate a turbulent flow applying Laser-Doppler-Anemometry to measure the circumferential velocity profile at the inlet of the rotating pipe. The measured swirl boundary layer thickness follows a power law taking Reynolds number and flow number into account. A combination of high Reynolds number, high flow number and axial position causes a transition of the swirl boundary layer development in the turbulent regime. At this combination, the swirl boundary layer thickness as well as the turbulence intensity increase and the latter yields a self-similarity. The circumferential velocity profile changes to a new presented self-similarity as well. We define the transition inlet length, where the transition appears and a stability map for the two regimes is given for the case of a fully developed axial turbulent flow enters the rotating pipe.