bluuebunny/crossref_metadata_2025 huggingface dataset:
The Comparative Study Between Swirl and Impingement of Mist/Air Cooling on Blade Leading Edge
Authors: Jiang Yuting , Zheng Qun , Liu Bo , Gao Jie , Zhang Hai
Published: August 2015

A comparative study of the flow field and heat transfer characteristics between swirl and impingement of mist/air cooling on blade leading edge is carried out to find better cooling configuration for phase transition cooling. The Eulerian-Lagrangian particle tracking technique is used to investigate the mist/air cooling. Comparisons are made between these two cooling forms in such aspects as vortex structure, heat transfer enhancement, pressure loss, and thermal uniformity with and without mist injection. The influences of mist ratio and Reynolds numbers on these parameters are studied in this paper. Results show that the heat transfer is enhanced, pressure loss and the thermal uniformity is improved by the swirl flow created by vortex impingement. The heat transfer performance increases by about 46.2% and 51.9% for impingement and swirl cooling with 8% mist injection, and the pressure loss coefficient increases by 19%. The difference of heat transfer coefficient between swirl and impingement cooling with and without mist injection at high Reynolds number is larger than that at low Reynolds number. In addition, heat transfer non-uniform coefficient of swirl cooling is about 15% lower than impingement cooling.
Computational and Experimental Study of Heat Transfer in Rotating Ribbed Radial Turbine Cooling Passages
Authors: Pearce Robert , Ireland Peter , He Li , McGilvray Matthew , Romero Eduardo
Published: August 2015

This study investigates the effect of rotation on the Nusselt number distribution within ribbed radial turbine cooling passages representative of systems used in current jet engines. The results are unusual in that the cooling passage length to diameter ratio is engine representative and full distributions of local Nusselt number have been measured using the transient liquid crystal method. The results are compared to RANS CFD simulations and the level of agreement discussed in detail. A triple-pass serpentine passage is investigated, which includes 45° filleted rib-turbulators and 180° curved bends. The first two passes have an aspect ratio of 1:4 which are radially inward and outward respectively, with the final pass being radially outward with an aspect ratio of 1:2. The Reynolds, Rotation and Buoyancy numbers are all representative of a passage within a HP turbine blade of a gas turbine engine at 97000/108000, 0.081/0.088 and 0.052/0.035 respectively for the 1:4/1:2 aspect ratio passages. CFD simulations are found to give good predictions under stationary conditions however significant differences are observed when rotation is introduced. The Nusselt number distributions depend strongly on both rotation and upstream flow conditions created by the specific geometry.
First and Second Law Analysis of Intercooled Turbofan Engine
Authors: Zhao Xin , Thulin Oskar , Grönstedt Tomas
Published: August 2015

Although the benefits of intercooling for aero engine applications have been realized and discussed in many publications, quantitative details are still relatively limited. In order to strengthen the understanding of aero engine intercooling, detailed performance data on optimized intercooled turbofan engines are provided. Analysis is conducted using an exergy breakdown, i.e. quantifying the losses into a common currency by applying a combined use of the first and second law of thermodynamics.

Optimal intercooled geared turbofan engines for a long range mission are established with CFD based two-pass cross flow tubular intercooler correlations. By means of a separate variable nozzle, the amount of intercooler coolant air can be optimized to different flight conditions. Exergy analysis is used to assess how irreversibility is varying over the flight mission, allowing for a more clear explanation and interpretation of the benefits.

The optimal intercooled geared turbofan engine provides a 4.5% fuel burn benefit over a non-intercooled geared reference engine. The optimum is constrained by the last stage compressor blade height. To further explore the potential of intercooling the constraint limiting the axial compressor last stage blade height is relaxed by introducing an axial radial high pressure compressor. The axial-radial high pressure ratio configuration allows for an ultra-high overall pressure ratio. With an optimal top-of-climb overall pressure ratio of 140, the configuration provides a 5.3% fuel burn benefit over the geared reference engine.

The irreversibilities of the intercooler are broken down into its components to analyze the difference between the ultra-high overall pressure ratio axial-radial configuration and the purely axial configuration. An intercooler conceptual design method is used to predict pressure loss heat transfer and weight for the different overall pressure ratios. Exergy analysis combined with results from the intercooler and engine conceptual design are used to support the conclusion that the optimal pressure ratio split exponent stays relatively independent of the overall engine pressure ratio.
Uncertainty Quantification and Stochastic Variations of Renewable Fuels
Authors: Montomoli F. , Insinna M. , Cappelletti A. , Salvadori S.
Published: August 2015

Renewable fuels have been successfully used in gas turbine combustion chambers and the layout of the chamber does not require major interventions if the composition is known. However, the variation in the composition in renewable fuels is higher than in fossil ones and it is stochastic. In principle, this variation affects the stability of the combustion, the emissions and the temperature distribution.

The combustion chamber tested in this work has been designed to reproduce the temperature distribution of MT1 test case and modelled using reactive CFD simulations. The fuel is an ideal natural gas with a random mix of methane and hydrogen.

In order to account the stochastic variation of the fuel composition, a probabilistic analysis is carried out with two sampling methods: a Monte Carlo simulation with meta-models and a Probabilistic Collocation Method. The two methodologies show similar results in terms of mean value and standard deviation.

The paper proves that is possible to predict the mean value of temperature and emissions in a modern chamber and their associated standard deviation by applying an uncertainty quantification methodology. One of the major drawbacks of the composition change is the maximum temperature variation at the exit that can reduce the life of the downstream turbine. The variation in the emissions seems less important and all the major differences in the composition are mixed out before the combustion chamber exit.
Numerical Investigation of Centrifugal Compressor Tip Clearance
Authors: Jaatinen-Värri Ahti , Turunen-Saaresti Teemu , Grönman Aki , Backman Jari , Tiainen Jonna
Published: August 2015

In this paper, the effect of the axial tip clearance on the performance and flow fields of a centrifugal compressor is studied numerically. The compressor is equipped with a pinched vane-less diffuser. Six different axial clearances were modelled and the relative axial tip clearance was varied from 0.027 to 0.154. The tip clearance was changed by transferring the shroud in the axial direction. The modelled results are compared to measured results obtained in previous projects.

The results indicate that the effect of tip clearance to the impeller performance is linear, even though the clearance is increased to such amounts which are seldom used in traditional compressors. The clearance has significant effect on the flow fields. Even though the mass flow averaged tangential velocity remained constant with the increasing clearance, the velocity fields show increase in tangential velocity in the passage wake between the suction side of the splitter blade and pressure side of the full blade and decreased in the other blade passage with increasing clearance. Radial velocity contours showed that the backflow region near the shroud increased in size when the clearance is increased. The growth was stronger on the suction side of the splitter blade.

Even though there is quite a difference in the stage efficiency between the highest and lowest clearance, the difference in velocity triangles at the impeller exit is small. This should be taken into a consideration when compressors with high relative axial tip clearances are designed.
Experimental and Numerical Investigations of the Solidity Effect on a Linear Compressor Cascade
Authors: Sans J. , Brouckaert J.-F. , Hiernaux S.
Published: August 2015

The solidity in a compressor is defined as the ratio of the aerodynamic chord over the peripheral distance between two adjacent blades, the pitch. The choice of this parameter represents a crucial step in the whole design process. Most of the studies addressing this issue are based on low-speed compressor cascade correlations. In that prospect, aiming at updating those correlation data as well as improving the physical understanding of the solidity effect on compressor performance, both experimental and numerical high-speed cascade investigations have been carried out at the von Karman Institute. The profile is a state-of-the-art controlled diffusion blade, representative of a low pressure compressor stator mid-span profile. The performance in terms of total pressure losses and deviation have been measured in the high-speed C3 cascade facility for three different solidities at six incidences and two Mach numbers. Based on the experimental results, a numerical linear cascade model has been built and computations have been run with FINE/Turbo at the same conditions as the measurements. The quality of the numerical predictions is discussed over the whole incidence range and, in particular, big discrepancies are highlighted at off-design incidences. Focusing on the solidity effects at mid-span, both experimental and numerical results are compared with existing correlations. The establishment of updated correlations for such controlled diffusion profile is addressed for both deviation and total pressure losses and at both optimum and off-design conditions.
Analysis of the Flow Field in Optimized Axial Fans
Authors: Bamberger Konrad , Carolus Thomas
Published: August 2015

Three design strategies for low pressure axial fans are compared. Benchmark fans are designed with the blade element momentum (BEM) method. All common validity limits are respected. Optimized fans are designed according to design guidelines obtained in earlier studies by the authors of this work. Here, “optimal” always means maximum possible total-to-static efficiency at exact fulfillment of a specific design point. One typical difference between the benchmark and optimized fans is that the optimization yields considerably smaller hub-to-tip ratios. The two design methodologies are combined for a third series of fans. These fans are also designed with the BEM method, but with the same hub size and blade number as obtained from the optimization. As a consequence of the smaller hub size, the aforementioned validity limits are violated. All three design strategies are applied at three distinct design points which are supposed to outbid the bounds of axial fans according to Cordier’s diagram. The nine resulting fans are simulated by means of steady CFD (Reynolds-averaged Navier Stokes method, RANS). Quality assurance is considered by a grid independence study and a comparison with transient simulations (SAS method).

The aerodynamic comparison reveals the weaknesses of the BEM designs which suffer from high exit losses (if designed with common validity limits) or from high hydraulic losses (if designed with small hub). Additionally, BEM designs with small hubs still have unnecessary exit losses which originate from an uneven distribution of flow velocity downstream of the fan. In contrast, the optimized designs enable a small hub without increased losses resulting in a total-to-static efficiency improvement by 2–14 percentage points depending on the design point. The flow fields of all fans are analyzed in detail to find reasons for the superiority of the optimized designs. It is found that optimized fans benefit from an evenly distributed meridional velocity profile downstream of the fan. Reasons are given why such a flow field is hardly achievable by BEM designs. Further advantages of optimized fans are found in the blade shape near the hub which strictly avoids flow separations which in standard designs often compromise the efficiency in the complete blade channel.
Concept for a Technology Assistance System to Analyze and Evaluate Materials and Tools for Milling
Authors: Auerbach T. , Gierlings S. , Veselovac D. , Seidner R. , Kamps S. , Klocke F.
Published: August 2015

Turbine engine manufacturers permanently aim to improve the efficiency of their products. This is often accompanied by the development of new materials which have to be introduced to manufacturing. As a consequence, engineers responsible for machining process development are regularly confronted with the question, how to identify the optimal machining conditions in order to deal with the new constraints. Nowadays, the effort and success of such identification processes are to a significant degree depending on technology expert skills and experiences. From the process planning perspective, however, this circumstance is characterized by a significant degree of uncertainty.

This article presents an innovative concept for a technology assistance system (TAS) for milling. The TAS supports the operator to determine optimal machining conditions by autonomously evaluating machinability criteria such as cutting force, tool wear or surface roughness for certain work piece material/ tool combinations. This includes the planning and organization of milling experiments, its standardized and automated execution as well as the generation of surrogate models to describe the machinability criteria for a given parameter range, serving as input for a future optimization. All functionalities of the TAS are conceptually described and first results achieved using a prototype solution are introduced and discussed.
Development of Fuel Control System for More Electric Engine
Authors: Seki Naoki , Morioka Noriko , Oyori Hitoshi , Yamamoto Yasuhiko
Published: August 2015

This paper describes the experimental rig test result and an investigation into issues of system stability and pressure oscillation transmission in the MEE (More Electric Engine) fuel system. This system employs an electric motor-driven pump and directly meters the fuel flow based on the motor rotating speed. The MEE is a system architecture concept for the aircraft turbine engine that reduces fuel consumption and environmental load while improving safety, reliability and maintainability. The improvements were demonstrated by conducting a feasibility study of MEE system for small sized turbofan engine [5, 6]. The authors also conducted an experimental rig test showing capabilities in terms of fuel-metering range, accuracy and response [7]. The capability of the feedback loop control under the engine start condition was shown by the result, but meanwhile, pressure oscillation under the higher fuel flow condition was also observed. The authors repeated the rig test to investigate its root cause. This paper describes the study, which investigates the characteristics of the MEE fuel system and seeks stable control methods under conditions of higher pressure fluctuation, higher instrumentation noise or applying worn gear pump. The paper also describes the study of the pressure oscillation transmission from pump to engine combustor, which may damage the engine combustor and structures. As a result of these studies, a novel control method for the MEE fuel system is proposed, with improved oscillation stability.
Prediction of Heat Transfer Distribution Over the Surface of Nonfilm-Cooled Nozzle Guide Vane in a Transonic Annular Cascade
Authors: Ragab Kasem E. , El-Gabry Lamyaa
Published: August 2015

Having gas turbine components that can withstand high temperatures is a key factor in improving turbine efficiency; therefore, a deeper understanding of the heat transfer phenomena associated with the flow of hot gases over Nozzle Guide Vanes (NGVs) is crucial for proper vane design and implementation of adequate cooling schemes. In this study, the heat transfer distribution over the surface of a nonfilm-cooled NGV in a transonic annular cascade (Mexit=0.89, Reexit=2.6×106) is investigated numerically using a three-dimensional computational fluid dynamics (CFD) model and compared to results from a 2-D Boundary Layer (BL) code (TEXSTAN). The CFD model has been built and analyzed using a finite volume based commercial code (ANSYS CFX). Although the industrial turbine vane is film cooled, the analysis presented will be for the uncooled vane. In order to validate the CFD model against experimental data, a study is carried out on the NASA C3X vane; a CFD model of the C3X vane was built and several modeling parameters are varied in order to obtain good agreement with the experimental data. In addition, the numerical results are compared to those of other analytical and numerical simulations of the C3X vane. The methods found to yield the best agreement for the C3X are implemented in the modeling of the industrial NGV.
A Method to Reduce the Energy Localization in Mistuned Bladed Disks by Application-Specific Blade Pattern Arrangement
Authors: Hohl Andreas , Wallaschek Jörg
Published: August 2015

The focus of the paper is the analysis of mistuning, that is small deviations of the blade properties e.g. due to manufacturing tolerances. The resonant amplitudes of turbine blades are very sensitive to these deviations which can lead to significantly increased vibratory response of some blades with increased risk of high cycle fatigue.

The main part of the paper discusses methods that can be used to find blade patterns which are insensitive to energy localization. The sensitivity of the forced response against harmonic mistuning, that is a harmonic alignment of the blades respective to the mistuning factors of the single blades, is examined. A previously developed reduced order model is used to efficiently conduct Monte Carlo Simulations (MCS). Especially the influence of the variance of the harmonically mistuned blade patterns is discussed. On the basis of this analysis rules are developed to suppress the energy localization. The rules are mainly focused on the alignment of the blades around the bladed disk. The approach also takes advantage of the special properties of harmonic mistuning patterns. An assignment of the blades to insensitive harmonic mistuning patterns with a specific variance and number of periods is used to reduce the maximum of the amplification factor of the forced response. A similar approach uses intentional mistuning patterns with different blade types which are aligned harmonically and are insensitive to an additional stochastic mistuning. In case of specific combinations of the dependent parameters, especially the variance of the mistuning factors and the number of periods, the energy localization can be reduced considerably.
Experimental Investigation of a Leaf Seal Prototype at Engine-Representative Speeds and Pressures
Authors: Pekris Michael J. , Franceschini Gervas , Jahn Ingo H. J. , Gillespie David R. H.
Published: August 2015

The application of compliant filament seals to jet engine secondary air systems has been shown to yield significant improvements in specific fuel consumption and improved emissions. One such technology, the leaf seal, provides comparable leakage performance to the brush seal but offers higher axial rigidity, significantly reduced radial stiffness and improved compliance with the rotor. Investigations were carried out on the Engine Seal Test Facility at the University of Oxford into the behavior of a leaf seal prototype at high running speeds. The effects of pressure, speed and cover plate geometry on leakage and torque are quantified. Early publications on leaf seals showed that air-riding at the contact interface might be achieved. Results are presented which appear to confirm that air-riding is taking place. Consideration is given to a possible mechanism for torque reduction at high rotational speeds.
Measured Rotordynamic and Leakage Characteristics of a Tooth-on-Rotor Labyrinth Seal With Comparisons to a Tooth-on-Stator Labyrinth Seal and Predictions
Authors: Arthur Stephen P. , Childs Dara W.
Published: August 2015

Rotordynamic and leakage data are presented for a see-through tooth-on-rotor (TOR) labyrinth seal with comparisons to a see-through tooth-on-stator (TOS) labyrinth seal. Measurements for both seals are also compared to predictions from XLLaby. Both seals have identical diameter and can be considered as relatively long labyrinth seals. The TOR seal has a length-to-diameter ratio of 0.62, whereas the TOS seal is longer and has a length-to-diameter ratio of 0.75. Both seals also differ by number of teeth, tooth height, and tooth cavity length. TOR labyrinth tests were carried out at an inlet pressure of 70 bar-a (1,015 psia), pressure ratios of 0.4, 0.5, and 0.6, rotor speeds up to 20,200 rpm, a radial clearance of 0.1 mm (4 mils), and three preswirl ratios. For comparison, TOS labyrinth tests were run at identical conditions as the TOR tests but for only one positive preswirl ratio.

TOR labyrinth measurements show a pronounced dependence of rotordynamic coefficients on rotor speed, especially when compared to prior documented TOS labyrinth seal tests run at a radial clearance of 0.2 mm (8mils). The TOR labyrinth cross-coupled stiffness is higher in magnitude and increases at a higher rate than that of the TOS labyrinth across all test speeds. However, the TOR labyrinth effective damping was determined to be greater due to higher measurements of direct damping. Measured leakage rates for the TOR labyrinth were approximately 5–10% less than the TOS labyrinth. XLLaby underpredicted the rotordynamic coefficients for both seals. However, as with measurements, it predicted the TOR labyrinth to have higher effective damping than the TOS labyrinth.
A Review of Wind Turbine Polar Data and its Effect on Fatigue Loads Simulation Accuracy
Authors: Lennie Matthew , Pechlivanoglou Georgios , Marten David , Nayeri Christian Navid , Paschereit Oliver
Published: August 2015

To certify a Wind Turbine the standard processes set out by the GL guidelines and the IEC61400 demand a large number of simulations in order to justify the safe operation of the machine in all reasonably probable scenarios. The result of this rather demanding process is that the simulations rely on lower fidelity methods such as the Blade Element Momentum (BEM) method. The BEM method relies on a number of simplified inputs including the coefficient of lift and drag polar data (usually referred to as polars). These polars are usually either measured experimentally, generated using tools such as XFoil or, in some cases obtained using 2D CFD. It is typical to then modify these polars in order to make them suitable for aeroelastic simulations. Some of these modifications include 360° angle of attack extrapolation methods and polar modifications to account for 3D effects. Many of these modifications can be perceived to be a black art due to the manual selection of coefficients. The polars can misrepresent reality for many reasons, for example, inflow turbulence can affect measurements obtained in wind tunnels. Furthermore, on real wind turbine blades leading edge erosion can reduce performance. Simulated polars can even vary significantly due to the choice of turbulence models. Stack these effects on top of the uncertainties caused by yaw error, pitch error and dynamic stall and one can clearly see an operating environment hostile to accurate simulations. Colloquial evidence suggests that experienced designers would account for all of these sources of errors methodically, however, this is not reflected by the certification process. A review of experimental data and literature was performed to identify some of the inaccuracies in wind turbine polars. Significant variations were found between a range of 2D polar techniques and wind tunnel measurements. A sensitivity study was conducted using the aeroelastic simulation code FAST (National Renewable Energy Laboratory) with lift and drag polars sourced using different methods. The results were post-processed to give comparisons the rotor blade fatigue damage; variations in accumulated damages reached levels of 164%. This variation is not disastrous but is certainly enough to motivate a new approach for certifying the aerodynamic performance of wind turbines. Such an approach would simply see the source of polar data and all post-processing steps documented and included in the checks performed by certification bodies.
Implementation, Optimization and Validation of a Nonlinear Lifting Line Free Vortex Wake Module Within the Wind Turbine Simulation Code QBlade
Authors: Marten David , Lennie Matthew , Pechlivanoglou Georgios , Nayeri Christian Navid , Paschereit Christian Oliver
Published: August 2015

The development of the next generation of large multi-megawatt wind turbines presents exceptional challenges to the applied aerodynamic design tools. Because their operation is often outside the validated range of current state of the art momentum balance models, there is a demand for more sophisticated, but still computationally efficient simulation methods. In contrast to the Blade Element Momentum Method (BEM) the Lifting Line Theory (LLT) models the wake explicitly by a shedding of vortex rings. The wake model of freely convecting vortex rings induces a time-accurate velocity field, as opposed to the annular averaged induction that is computed from the momentum balance, with computational costs being magnitudes smaller than those of a full CFD simulation. The open source code QBlade, developed at the Berlin Institute of Technology, was recently extended with a Lifting Line - Free Vortex Wake algorithm. The main motivation for the implementation of a LLT algorithm into QBlade is to replace the unsteady BEM code AeroDyn in the coupling to FAST to achieve a more accurate representation of the unsteady aerodynamics and to gain more information on the evolving rotor wake and flow-field structure. Therefore, optimization for computational efficiency was a priority during the integration and the provisions that were taken will be presented in short. The implemented LLT algorithm is thoroughly validated against other benchmark BEM, LLT and panel method codes and experimental data from the MEXICO and NREL Phase VI tests campaigns. By integration of a validated LLT code within QBlade and its database, the setup and simulation of LLT simulations is greatly facilitated. Simulations can be run from already existing rotor models without any additional input. Example use cases envisaged for the LLT code include; providing an estimate of the error margin of lower fidelity codes i.e. unsteady BEM, or providing a baseline solution to check the soundness of higher fidelity CFD simulations or experimental results.
T100 Micro Gas Turbine Converted to Full Humid Air Operation: A Thermodynamic Performance Analysis
Authors: De Paepe Ward , Montero Carrero Marina , Bram Svend , Contino Francesco
Published: August 2015

Waste heat recovery has become more and more important for the profitability of small-scale Combined Heat and Power (CHP) plants like micro Gas Turbines (mGTs). Adding a saturation tower to the mGT unit is such a waste heat recovery route. The cycle includes the saturation tower after the compressor to humidify the compressed air. Simulations show that this cycle, known as the micro Humid Air Turbine (mHAT), increases mGT electric efficiency by 7% relatively (2% absolute), improving the general economic performance. The mHAT concept with saturation tower was however never tested experimentally. To show the potential of the cycle, the Turbec T100 mGT of the University of Brussels was converted into a mHAT cycle by adding a spray saturation tower to the system. In this paper, we present the results of several water injection tests in the T100 mGT at part and nearly nominal load. The water injection experiments resulted in stable mGT operation at reduced rotational speed and pressure ratio and increased electric efficiency. Experimental results showed a reduced fuel mass flow rate by 4.3% and a relative electric efficiency increase of 4.8% for the different experiments. In addition, the impact of the water on the other turbine parameters has been studied.
Transient Simulations of a T100 Micro Gas Turbine Converted Into a Micro Humid Air Turbine
Authors: Montero Carrero Marina , Ferrari Mario Luigi , De Paepe Ward , Parente Alessandro , Bram Svend , Contino Francesco
Published: August 2015

Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP) thanks to their overall energy efficiencies of 80% (30% electrical + 50% thermal) and the advantages they offer with respect to internal combustion engines. The main limitation of mGTs lies in their rather low electrical efficiency: whenever there is no heat demand, the exhaust gases are directly blown off and the efficiency of the unit is reduced to 30%. Operation in such conditions is generally not economical and can eventually lead to shutdown of the machine. To address this issue, the mGT cycle can be modified so that in moments of low heat demand the heat in the exhaust gases is used to warm up water which is then re-injected in the cycle, thereby increasing the electrical efficiency. The introduction of a saturation tower allows for water injection in mGTs: the resulting cycle is known as a micro Humid Air Turbine (mHAT).

The static performance of the mGT Turbec T100 working as an mHAT has been characterised through previous numerical and experimental work at Vrije Universiteit Brussel (VUB). However, the dynamic behaviour of such a complex system is key to protect the components during transient operation. Thus, we have modelled the Turbec T100 mHAT with the TRANSEO tool in order to simulate how the cycle performs when the demanded power output fluctuates. Steady-state results showed that when operating with water injection, the electrical efficiency of the unit is incremented by 3.4% absolute. The transient analysis revealed that power increase ramps higher than 4.2 kW/s or power decrease ramps lower than 3.5 kW/s (absolute value) lead to oscillations which enter the unstable operation region of the compressor. Since power ramps in the controller of the Turbec T100 mGT are limited to 2kW/s, it should be safe to vary the power output of the T100 mHAT when operating with water injection.
Development of High Order LES Solver for Heat Transfer Applications Based on the Open Source OpenFOAM Framework
Authors: Mangani Luca , Launchbury David Roos , Casartelli Ernesto , Romanelli Giulio
Published: August 2015

The computation of heat transfer phenomena in gas turbines plays a key role in the continuous quest to increase performance and life of both component and machine.

In order to assess different cooling approaches computational fluid dynamics (CFD) is a fundamental tool. Until now the task has often been carried out with RANS simulations, mainly due to the relatively short computational time. The clear drawback of this approach is in terms of accuracy, especially in those situations where averaged turbulence-structures are not able to capture the flow physics, thus under or overestimating the local heat transfer.

The present work shows the development of a new explicit high-order incompressible solver for time-dependent flows based on the open source C++ Toolbox OpenFOAM framework. As such, the solver is enabled to compute the spatially filtered Navier-Stokes equations applied in large eddy simulations for incompressible flows.

An overview of the development methods is provided, presenting numerical and algorithmic details.

The solver is verified using the method of manufactured solutions, and a series of numerical experiments is performed to show third-order accuracy in time and low temporal error levels. Typical cooling devices in turbomachinery applications are then investigated, such as the flow over a turbulator geometry involving heated walls and a film cooling application. The performance of various sub-grid-scale models are tested, such as static Smagorinsky, dynamic Lagrangian, dynamic one-equation turbulence models, dynamic Smagorinsky, WALE and sigma-model. Good results were obtained in all cases with variations among the individual models.
Characterization of a Single-Nozzle FLOX® Model Combustor Using kHz Laser Diagnostics
Authors: Yin Zhiyao , Nau Patrick , Boxx Isaac , Meier Wolfgang
Published: August 2015

A single-nozzle FLOX® model combustor was used to produce a confined, premixed CH4-air flame with an equivalence ratio of ϕ = 0.74 and a jet exit velocity of vjet = 150m/s with a preheat temperature of T0=300°C. For the first time for this combustor, surface thermometry was performed on the chamber walls. In addition, particle imaging velocimetry (PIV) and planar laser-induced fluorescence of hydroxyl radical (OH PLIF) were acquired simultaneously in this flame at 5 kHz repetition rate. The interface between burnt and unburnt gas mixture were identified from instantaneous OH PLIF images and were compared with corresponding PIV results for flame-turbulence interaction analysis. Combustion instabilities were analyzed via proper orthogonal decomposition and phase-averaged flow field and OH distribution. A pronounced flapping motion of the jet was identified and its impact on the recirculation of hot burnt gas was characterized.
A Simplified Analytical Approach for Calculating the Start-Up Time of Industrial Steam Turbines for Optimal and Fast Start-Up Procedures
Authors: Beer Wolfgang , Propp Lukas , Voelker Lutz
Published: August 2015

New flexible operational regimes with fast start-ups and fast-changing load cycles for steam turbines require calculation procedures for determining optimal start-up times in order not to exceed the limits of thermal stress for the steam turbine parts.

This work presents a start-up time calculation for various kinds of industrial steam turbines. An analytical approach for estimating the optimal thermal load of a turbine from quasi-steady or steady condition is developed. The geometry of the respective turbine components, the changing of the steam parameters and heat transfer effects during the start-up procedure are taken into account while observing the respective material properties and stress limits. The temperature distributions of the respective turbine parts are calculated with a one-dimensional numerical algorithm of Fourier’s heat conduction equation. Three-dimensional influences of the geometry and of the the heat flux are considered analytically by adjusting the numerical solutions of elementary bodies (e.g. one-dimensional plate). The start-up time calculation is performed in small time steps to guarantee the stability of the numerical solution. The unsteady stress analysis for the start-up procedure does not uniquely identify one critical component. The calculation must be repeated for each time step to identify the component which limits the start-up gradient. Other boundary conditions, such as restricted speed ranges of the rotor with minimum transients and time for synchronization with the electrical grid, are considered by the model too and can further limit the start-up gradient and lead to slower start-up procedures. The one-dimensional calculation models were verified with a three-dimensional FEA of the casing and a two axis symmetrical FEA of the rotor. The results for the temperature distribution are presented and compared to the one-dimensional results.

The final result of the analytical approach for an optimized start-up time calculation is verified with two typical start-up calculations, one for a generator drive steam turbine and one for a mechanical-drive steam turbine.
The Delta Phi Method of Evaluating Overall Film Cooling Performance
Authors: Rutledge James L. , Polanka Marc D. , Bogard David G.
Published: August 2015

Film cooling designs are often evaluated experimentally and characterized in terms of their spatial distributions of adiabatic effectiveness, η, which is the nondimensionalized form of the adiabatic wall temperature, Taw. Additionally, film cooling may alter the convective heat transfer coefficient with the possibility of an increase in h that offsets the benefits of reduced Taw. It is therefore necessary to combine these two effects to give some measure of the benefit of film cooling. The most frequently used method is the net heat flux reduction, which gives the fractional reduction in heat flux that accompanies film cooling for the hypothetical case of constant wall temperature. NHFR is imperfect in part due to the fact that this assumption does not account for the primary purpose of film cooling — to reduce the metal temperature to an acceptable level. In the present work we present an alternative method of evaluating film cooling performance that yields the reduction in metal temperature, or in the nondimensional sense, an increase in ϕ that would be predicted with film cooling. This Δϕ approach is then applied using experimentally obtained η and h/h0 values on a simulated turbine blade leading edge region. The delta-phi approach agrees well with the legacy NHFR technique in terms of the binary question of whether the film cooling is beneficial or detrimental, but provides greater insight into the temperature reduction that a film cooling design would provide an actual turbine component. For example, instead of giving an area-averaged NHFR = 0.67 (indicating a 67% reduction in heat flux through film cooling) on the leading edge region with M = 0.5, the Δϕ approach indicates an increase in ϕ of 0.061 (or a 61 K surface temperature decrease with a notional value of T∞ - Tc = 1000 K). Alternatively, the technique may be applied to predict the maximum allowable increase in T∞ against which a film cooling scheme could protect.
Comparison of the Three-Dimensional Boundary Layer on Flat Versus Contoured Turbine Endwalls
Authors: Lynch Stephen P. , Thole Karen A.
Published: August 2015

The boundary layer on the endwall of an axial turbomachine passage is influenced by streamwise and cross-stream pressure gradients, as well as a large streamwise vortex, that develop in the passage. These influences distort the structure of the boundary layer and result in heat transfer and friction coefficients that differ significantly from simple two-dimensional boundary layers. Three-dimensional contouring of the endwall has been shown to reduce the strength of the large passage vortex and reduce endwall heat transfer, but the mechanisms of the reductions on the structure of the endwall boundary layer are not well understood.

This study describes three-component measurements of mean and fluctuating velocities in the passage of a turbine blade obtained with a laser Doppler velocimeter. Friction coefficients obtained with the oil film interferometry method were compared to measured heat transfer coefficients. In the passage, the strength of the large passage vortex was reduced with contouring. Regions where heat transfer was increased by endwall contouring corresponded to elevated turbulence levels compared to the flat endwall, but the variation in boundary layer skew across the passage was reduced with contouring.
Large Eddy Simulation of Evaporating Spray Jets
Authors: Deevi Sri Vallabha , Mathew Joseph
Published: August 2015

Large Eddy Simulations (LES) of an evaporating spray jet is performed using an explicit filtering approach for carrier phase equations. Droplets are treated as representative point particles without any modeling of particle sub-grid-scale (SGS) evolution. The simulation of a recent benchmark dilute acetone spray experiment showed that close quantitative agreement could be obtained of the downstream self-preserving turbulent state. Several other simulations are performed to understand the effect of inflow fluctuation level, evaporation and droplet size on the relaxation to the self-preserving state.
Analysis on Turbine Cooling Air Bleeding of Intercooled Recuperated Turbofan Engine
Authors: Gong Hao , Wang Zhanxue , Zhou Li , Zhang Xiaobo , Wang Jingkai
Published: August 2015

In order to further improve the intercooled recuperated turbofan engine (IRT) performance, the possible high pressure turbine (HPT) cooling air bleeding schemes were analyzed. There are two HPT cooling air extraction sections, i.e. the high pressure compressor exit (forward to the recuperator cold section inlet) and the combustion chamber inlet (back from the recuperator cold section outlet). The analysis results indicate that, bleeding the HPT cooling air from the combustion chamber inlet has the potential to reduce the engine specific fuel consumption. And to determine the most suitable HPT cooling air bleeding scheme, effects of allowable turbine blade metal temperature, turbine cooling technology level, engine weight addition, different intercooler and recuperator effectiveness should be taken into account.
Axial Compressor Blade Failure, Design Modification, and its Validation
Authors: Hulme C. J. , Fiebiger S. W. , Szwedowicz J.
Published: August 2015

Utilising a comprehensive design tool suite and in-house knowledge, the root cause analysis of High Cycle Fatigue (HCF) failures on the first stage compressor blade in a gas turbine (GT) engine is presented in this paper.

Based on the experience gained from the root cause analysis more reliable lifetime prediction for compressor blade design is possible. Tip timing measurements of blades in service have been evaluated to obtain valuable information about the real vibratory stress levels of the first stage compressor blades, to validate design methods, and to identify the failure mechanism. Metallurgical evaluation and testing of the failed parts were also used as part of the investigation. During the failure investigations the impacts of important damage mechanisms, not identified in the original design process, were determined. These damage mechanisms included, water droplet erosion of blades due to impingement on highly stressed regions of airfoil, mistuning within manufactured blade sets, and a very strong transient aerodynamic loading.

Considering the damage mechanisms, improved design methodologies have been used to design robust compressor blade sets. These have been validated in successful engine tests. Subsequent continued successful operation of new blade designs has been recorded in engines during an extended validation period.
Experimental and Numerical Investigation of Partial Admission of a Radial Turbocharger Turbine for Improved Off-Design Operation
Authors: Ilievski Mirko , Heidinger Frederic , Fuhrer Christopher , Schatz Markus , Vogt Damian M. , Challand Sebastian
Published: August 2015

A new partial admission concept for turbocharger turbine operation at off-design is designed and investigated numerically and experimentally in a turbocharger test rig. This new concept is called MEDUSA (Multiple Exhaust Duct with Source Adjustment) and is based on a partial admission turbine system consisting of several separate flow channels that connect the cylinder of the engine and individual nozzle segments of the turbine. The turbine flow is adjusted by the chosen admission rate according to the available exhaust enthalpy. In the present study, a reference turbocharger has been equipped with a four segment partial admission system instead of its conventional Waste-Gate scroll.

The numerical results indicate similar loss mechanisms compared to an axial turbine stage, when partial admission is applied. Based on a stator design optimization, a design was chosen for manufacturing and testing on the turbocharger test rig. Despite the fact that the turbocharger efficiency at partial admission for the MEDUSA-system drops, a higher turbine expansion ratio can be achieved to obtain the same compressor operating point. These results indicate an enhanced use of the available exhaust enthalpy at part-load conditions, thus improving the turbocharger performance at low end torque engine speeds.
Admissible Roughness in Gas Turbines
Authors: Goodhand Martin N.
Published: August 2015

Roughness below a certain, admissible level will not increase an aerofoil’s skin friction and thus will not impact on engine performance. In this paper a simple model is developed that demonstrates how this admissible roughness height changes through the engine. The model is determined by combining existing analytical models and is backed up by computational validation where necessary.

It is shown that, given a fixed inlet/exit stagnation temperature and pressure to a blade row, the admissible roughness height is only a weak function of chord Reynolds number and Mach number for typical gas turbine blades. The aerofoil geometry/duty is also shown to have little impact. This allows the model to give a general picture of where roughness matters, irrespective of the exact details of the flow conditions/blade geometries.

The model shows that the admissible roughness height decreases as the stagnation pressure increases. The lowest admissible roughness level occurs at the high pressure end of the compressor at sea-level; the admissible roughness increases as the stagnation pressure drops towards the front of the compressor. Turbines are also most sensitive to roughness close to the combustor, but even there the admissible roughness will be around three times greater than at the rear of the compressor.
The Limitations of “Ra” to Describe Surface Roughness
Authors: Goodhand Martin N. , Walton Karl , Blunt Liam , Lung Hang W. , Miller Robert J. , Marsden Reg
Published: August 2015

Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centreline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ.

To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case, was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested.

The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.