bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Ribbed Surfaces on Profile Losses of Low-Pressure-Turbine Blades
Authors: Dellacasagrande M. , Lengani D. , Simoni D. , Ubaldi M. , Bertini F.
Published: October 2022

In this work, streamwise oriented riblets were installed on a flat plate exposed to an adverse pressure gradient simulating the diffusing part of a low-pressure turbine (LPT) blade and, successively, on the suction side of an LPT cascade operating under unsteady inflow condition. Different riblets heights, spacings and positions have been tested, and complementary measuring techniques have been used to quantify their effects on the boundary layer evolution, and on losses. Experiments carried out on the flat plate allowed the detailed description of the riblet effects on the coherent structures developing within the boundary layer affecting transition, thus providing a rationale for the identification of the optimal riblet geometry once scaled in wall units. For riblet heights equal to about 20 wall-units, a maximum loss reduction of 8% was observed with respect to the smooth case. Otherwise, for larger riblet dimensions, earlier transition occurs due to enhanced boundary layer instability and losses significantly increase. Interestingly, the streamwise extension of the ribbed surfaces with respect to the transition region was found to play a minor role compared with the riblet dimension. The riblet configurations providing the highest reductions of viscous losses on the flat plate were then tested in the LPT blade cascade for different Reynolds numbers. In this case tests have been carried out with impinging upstream wakes. An overall reduction of the profile losses comparable to that observed in the flat plate case has been confirmed also in the unsteady operation of the turbine cascade. Low sensitivity of the profile losses to the riblet streamwise extension was also observed in the cascade application. This confirms that positive effects in terms of loss reduction can be obtained even when the exact transition position is not known a priori.
Analysis of Combustion Performance of Non-Conventional Syngas in mGT Combustor: Assessment of the Impact of the Quality of Synthesis Gas Towards Flame Stability and Emissions
Authors: Bompas Jérémy , Pappa Alessio , De Paepe Ward
Published: October 2022

The current limited availability of conventional fossil fuels, e.g., natural gas, as a result of the active demand in the industrial revival, leading to increasing prices as well as uncertainty and tension on the international market, strengthens the interest on energy source diversification to ensure sufficient sustainable resources for both heat and electricity demand. Non-conventional renewable resources like biogas, syngas, and biofuels are good candidates to achieve these energy mix goals, especially in a decentralized power production context, e.g., when used in micro Gas Turbines (mGTs) in small-scale cogeneration applications. Moreover, they also present the benefit of reducing CO2 emission and, while doing so, helping to move towards global zero emission by 2050. However, given their specific properties, i.e., a lower energy content, a lower overall conversion efficiency, different and altering composition, as well as an important steam fraction when not properly post-treated, a better characterize of these non-conventional energy sources in their combustion behavior is needed.

The aim of our work is thus to identify the combustion behaviour of several characteristic syngases in a typical industrial combustor, the Turbec T100 combustion chamber, designed for operation using natural gas. The performance of the combustor has been analysed using CFD calculations, under different syngas composition with increasing remaining steam fraction, aiming to determine the maximal allowable rest steam fraction, in an attempt to limit syngas post-treatment after production towards enhanced global cycle performance. In particular, the ignition and stability of the flame has been studied under a syngas pilot flame. To study this, velocity and temperature fields are analysed, as well as the specific flue gas composition. While the simulation results using dry syngas show temperature distributions and emission prediction in line with previous observation, the steam dilution effect leads to a clear shift in both temperature and emissions. More specifically, it is found that especially NOx emissions are sensible towards changes in pilot and main flame fuel distribution alterations. These obtained results will serve as benchmark for future characterization for a specific range of diluted inlet conditions of raw syngases, as well as specific studies on the impact of pilot/main flame fuel distribution towards flame stability and emissions control, which will allow to fully exploit their potential in small-scale cogeneration application.
Statistical Rule Extraction for Gas Turbine Trip Prediction
Authors: Bechini Giovanni , Losi Enzo , Manservigi Lucrezia , Pagliarini Giovanni , Sciavicco Guido , Stan Ionel Eduard , Venturini Mauro
Published: October 2022

Gas turbine trip is an operational event that arises when undesirable operating conditions are approached or exceeded, and predicting its onset is a largely unexplored area. The application of novel artificial intelligence methods to this problem is interesting both from the computer science and the engineering point of view, and the results may be relevant in both the academia and the industry. In this paper we consider data gathered from a fleet of Siemens industrial gas turbines in operation that include several thermodynamic variables observed during a long period of operation. To assess the possibility of predicting trip events, we first apply a new, systematic statistical analysis to identify the most important variables, then we use a novel machine learning technique known as temporal decision tree, which differ from canonical decision tree because it allows a native treatment of the temporal component, and has an elegant logical interpretation that eases the post-hoc validation of the results. Finally, we use the learned models to extract statistical rules. As a result, we are able to select the 5 most informative variables, build a predictive model with an average accuracy of 73%, and extract several rules. To our knowledge, this is the first attempt to use such an approach not only in the gas turbine field, but also in the whole industry domain.
Investigating the Nature and Invariance of Field Inversion Based on Transition in a Turbine Cascade
Authors: Bleh Alexander , Backhaus Jan , Morsbach Christian
Published: October 2022

The generation of data-driven turbulence models inherently requires the use of a sufficiently large database of high-fidelity reference data from DNS or LES. For technically relevant flows, such data is usually not readily available. However, in many cases there is a significant amount of experimental data available, though data points are mostly few and sparse. An approach which aims at deriving modelling errors by evaluating deviations from a given reference data set is the field inversion method proposed in [1]. Our aim is to verify this method as a tool which gives insight in the cause and nature of a given model’s inconsistencies. We therefore apply field inversion on the turbine cascade T106C for different reference data setups. We find, that for the investigated case, field inversion proved to qualitatively give the right hints towards the expected model correction, when only few data points were used as reference.
Windage Loss and Flow Characteristics in Impeller Back Clearance of sCO2 Centrifugal Compressor
Authors: Hu Lehao , Deng Qinghua , Li Zhigang , Li Jun , Feng Zhenping
Published: October 2022

In this paper, the windage loss and the flow characteristics in impeller back clearance divided into disk-type and shaft-type clearance for sCO2 compressor are investigated by numerical method. The effects of rotation speed, inlet pressure and disk-type clearance width on the windage loss and flow characteristics are analyzed in detailed. Through detailed numerical verification, realized k-ε turbulence model is finally selected, and all simulations are performed under the condition of the rotation speed of 20 ∼ 60 kr/min and inlet pressure of the 9 ∼ 12 MPa, respectively. The study results show that windage loss in disk-type clearance increases with rotation speed and inlet pressure increasing, and the influences of rotation speed on windage loss are more obvious than that of inlet pressure. Both the flow instability and more vortexes at inlet become main reasons why windage loss increases. The relative deviations of 105% ∼ 217% between Daily’s loss model results and simulated results indicate that loss model is not applicable to predict the windage loss. What more, the windage loss in shaft-type clearance is basically increases linearly with the rotation speed and inlet pressure increasing. The proportion of windage loss in shaft clearance to total loss ranges from 9% to 16%, which indicates the windage loss in shaft-type clearance can be neglected for the preliminary design on sCO2 compressor. In addition, the maximum relative deviation of 2 % for the leakage model can be completely applicable to the design of sCO2 compressor. Disk-type clearance width has significant influence on both loss and flow characteristics. The windage loss in disk-type clearance decreases with the clearance width increasing, and the variation rate of windage loss increases as the clearance width increases. Larger width makes number of vorticities decrease at turning angle and changes flow direction easily, which is favorable to decrease windage loss. Finally, Daily’s loss model is modified and the modified loss model is validated. The maximum relative deviation of 17% for modified loss model is completely accepted, which indicates the modified loss mode can substantially improve accuracy of calculating windage loss in the impeller back clearance of sCO2 centrifugal compressor.
Quantification of Autoignition Risk in Aeroderivative Gas Turbine Premixers Using Incompletely Stirred Reactor and Surrogate Modelling
Authors: Iavarone Salvatore , Gkantonas Savvas , Jella Sandeep , Versailles Philippe , Yousefian Sajjad , Monaghan Rory F. D. , Mastorakos Epaminondas , Bourque Gilles
Published: October 2022

The design and operation of premixers for gas turbines must deal with the possibility of relatively rare events causing dangerous autoignition. Rare autoignition events may occur in the presence of fluctuations of operational parameters, such as temperature and fuel composition, and must be understood and predicted. This work presents a methodology based on Incompletely Stirred Reactor (ISR) and surrogate modelling to increase efficiency and feasibility in premixer design optimisation for rare events. For a representative premixer, a space-filling design is used to sample the variability of three influential operational parameters. An ISR is then reconstructed and solved in a post-processing fashion for each sample, leveraging a well-resolved CFD solution of the non-reacting flow inside the premixer. Via detailed chemistry and reduced computational costs, the evolution of autoignition precursors and temperature, conditioned on a mixture fraction, is tracked, and accurate surrogate models are trained on all samples. The final quantification of the autoignition probability is achieved by querying the surrogate models via Monte Carlo sampling of the random parameters. The approach is fast and reliable so that user-controllable, independent variables can be optimised to maximise system performance while observing a constraint on the allowable probability of autoignition.
Gas Turbine Rotor Damper Lift-Off Study
Authors: Manoharan S. , Abhimanyu S. , Babu S. , Tognarelli Leonardo , Peano Guido
Published: October 2022

Gas turbine rotor system is subjected to vibratory stresses due to excitations caused by various excitation sources at varying energy levels. To dissipate the energy that occurs in the rotating system, damping is induced internally or externally. Damping is achieved by having damper at the appropriate location in the rotor system. The damper needs to have a provision to assemble in the rotor assembly, which is typically a split, facilitating the damper to wind and unwind in circumferential direction. The damper used in the rotating system needs to be axisymmetric with a suitable cross-section, to achieve the required level of damping. As the damper is used at the interior location of the rotor system, it needs to build necessary preloading by itself in such a way that the damper rotates with the rotor system during operation. Damper design study is very vital in deciding the shape of the damper in such a way that it provides efficient damping and minimizing the vibratory stress. At some vibrating condition, damper tends to detach from its supporting feature which is referred to as damper lift-off. The magnitude of this lift-off is expected to be higher at the split regions compared to other circumferential location of the damper, which makes the damper design very complex. When the damper split region passes through the maximum lobe and minimum lobe position of any nodal diameter, the region near the split tends to lift-off from its support and behaves like a simply supported beam and cantilever beam respectively. A careful consideration of this damper lift-off phenomenon in damper design is key towards achieving a robust damper design. Various types of simulation techniques using finite element analysis are followed in performing necessary design assessments. This paper is intended to describe the steps followed in performing damper design study to help achieving design robustness.
Experimental Study on the Surface Geometry of Foam-Metal Casing Treatment
Authors: Wang Yuqing , Sun Dakun , Li Jia , Zhang Jinming , Dong Xu , Sun Xiaofeng
Published: October 2022

Foam-metal casing treatment (FMCT) installed over the rotor tip has been proved to affect the performance of compressors. To develop a design strategy for FMCTs, different parameters of surface geometries (SGs), including the shape, staggering, and denseness, based on a 2mm-depth groove were designed to model the FM surface morphology, which dominates the transverse flow resistance near the end wall region. Eight axial installation locations of the casing treatment were set on the tested low-speed compressor, among which the optimum stall margin improvement (SMI) up to 11.5% with 1% efficiency loss was achieved by a brush-type SG at a near mid-chord axial location. But the acoustic measurements of all the SGs showed nearly no benefit of broadband noise reduction. Accompanied by the investigations into the flow field and pressure unsteadiness, the mechanism of FMCT on compressor stability and noise attenuation were discussed and the design strategy was obtained.
Modeling the Convection of Entropy Waves in Strongly Non-Parallel Turbulent Flows Using a Linearized Framework
Authors: Kaiser Thomas Ludwig , Noiray Nicolas , Male Quentin , Oberleithner Kilian
Published: October 2022

The study investigates two effects, which can lead to a mitigation of entropy waves in a gas turbine combustor: Mean flow shear dispersion and turbulent mixing. Using a transport equation for coherent fluctuations of a passive scalar in combination with a k-ε turbulence model, the advection and turbulent diffusion of entropy waves is modeled. The method is applied to a flow in a duct of rectangular cross section, previously investigated with experimental and numerical means by Weilenmann et al. [1]. We analyze the impact of a steady jet in crossflow (JIC) in the rectangular duct on the mitigation of entropy waves by the mean flow shear dispersion mechanism. First, a comparison to Large Eddy Simulation (LES) results demonstrates the capability of the linearized approach to quantify the decay due to turbulent mixing and mean flow shear dispersion. The results further indicate that the inhomogeneous highly three dimensional flow profile and increased turbulence caused by the JIC significantly mitigates entropy waves due to the enhancement of the mean flow shear dispersion and turbulent mixing in comparison to a base line configuration without the JIC. Second, we investigate in a parameter study the effect of turbulent mixing on the mitigation of the entropy waves. It is shown that the results are highly case dependent. While in some situations an increase in turbulent mixing — as expected — leads to a mitigation of entropy waves, in other situations it may have the opposite effect. We demonstrate that this is the case if turbulent mixing annihilates entropy fluctuations selectively in those regions, which contribute significantly to mean flow shear dispersion.
Three-Dimensional Multi-Objective Design Optimization of a 6.5-Stage Axial Flow Compressor Blades With Lean and Twist
Authors: Lei Fan , Zhang Xiawen , Ju Yaping , Zhang Chuhua
Published: October 2022

With great development in numerical optimization and wide application of computational fluid dynamic methods, the three-dimensional design optimization of multi-stage axial flow compressor will become a routine practice in the future. The common critical issue arisen from the practice is how to carry out the three-dimensional design optimization of multi-stage axial flow compressor at an affordable design cost. To tackle this problem, a three-dimensional design optimization method is developed, in which reference vector guided evolutionary algorithm, the ensemble of surrogate models, adaptive sampling method and computational fluid dynamics (CFD) simulation are integrated, to improve the adiabatic efficiency and broaden the stability margin. The ensemble of surrogate model consists of linear regression model, support vector regression model, Kriging model and radial basis function. The blade lean and twist are set as design variables for all the stators while for the rotors only the blade twist is considered. By taking advantage of these advanced methods, a 6.5-stage axial flow compressor with totally 60 design variables is optimized at design speed-line. CFD calculations show that the adiabatic efficiency at design point is increased by 0.88%. The stall margin is increased from 12.72% to 14.08%, and the choke margin is increased from 20.41% to 21.35%. Flow analysis of the optimal compressor indicates that the blade lean is mainly responsible for suppressing the endwall flow separation while the blade twist for the stage match between adjacent blade rows. This work is of great significance for three-dimensional multi-objective design optimization of multi-stage axial flow compressor.
Influence of Gas-To-Wall Temperature Ratio on the Leakage Flow And Cooling Performance of a Turbine Squealer Tip
Authors: Yan Dongjie , Li Jieling , Xie Wenbo , Wang Zhaoguang , Lu Shaopeng , Jiang Hongmei , Zhang Qiang
Published: October 2022

In high-pressure turbines, there is a large difference in temperature between the mainstream and the turbine blade surface. Most of the turbine blade tip heat transfer researches were conducted under the assumption that the Over-Tip-Leakage (OTL) flow field is independent of the wall thermal condition. Recent numerical and experimental studies have revealed that two-way coupling effect between aerodynamics and heat transfer should not be neglected. The heat transfer coefficient obtained by conventional method is not able to match realistic engine condition accurately. This study investigates the impact of wall thermal boundary condition on tip cooling performance of squealer turbine blades. The RANS CFD result was validated against experimental tip heat transfer data obtained from a highspeed test rig with the effect of high-speed relative casing motion. The aerothermal performance for both uncooled and cooled squealer tips were studied at two different gas-to-wall temperature ratios 1.7 and 1.1, with the mainstream gas temperature held constant at 340 K. It was found that the location and strength of cavity vortices varied with different wall thermal boundary conditions, leading to different signatures in tip heat transfer and cooling performance. It is recommended that experimental heat transfer data and film cooling effectiveness obtained at near adiabatic wall boundary condition should be corrected before their application to the tip cooling design process. It would be more reliable to match the wall-to-gas temperature ratio during tip experimental study.
An Upgraded Chemical Kinetic Mechanism for ISO-Octane Oxidation: Prediction of Polyaromatics Formation in Laminar Counterflow Diffusion Flames
Authors: Ramirez Hernandez Astrid , Kathrotia Trupti , Methling Torsten , Braun-Unkhoff Marina , Riedel Uwe
Published: October 2022

Iso-octane is widely recognized as a prominent candidate to represent the oxidation of iso-alkanes within jet fuel and gasoline surrogates. This work evaluated a chemical kinetic mechanism for iso-octane focusing on the model’s capability to predict the formation of polycyclic aromatic hydrocarbons (PAHs). As the model is intended to be further coupled with soot models, the chemical kinetic mechanism must supply good predictability of the formation and consumption of PAHs considered as major soot precursors. A first validation of the iso-octane sub-model as incorporated within ESTiMatE-Mech, using experimental data from literature, reveals the need to improve the sub-model. Considerable deviations were observed in the prediction of the PAHs, although concentration profiles of major species and fundamental combustion properties such as ignition delay time and laminar flame speed were accurately predicted. Through rate of production and sensitivity analyses of the mechanism, nine reactions were identified to have a strong influence in the (over)prediction of the PAHs. These reactions have been modified based on information gathered from literature resulting in an updated version of the mechanism called ESTiMatE-Mech_mod. Simulation results with this modified mechanism showed that this updated mechanism is now capable of predicting well the targeted PAHs, while retaining the good initial prediction of the major species concentration profiles as well as of laminar flame speeds and ignition delay times.
Experimental Assessment of a Reverse Brayton Cycle Based on Automotive Turbochargers and E-Chargers for Cryogenic Applications
Authors: Serrano José R. , Dolz Vicente , Ponce-Mora Alberto , López-Carrillo Juan A.
Published: October 2022

The utilization of automotive engine components for the development of a reverse Brayton cycle is shown in this study. The well-known characteristics of turbochargers and electrical centrifugal compressors commonly used in the automotive industry is a starting point for developing an experimental cycle configuration of a reverse Brayton refrigeration cycle with the ability to achieve up to 115K after a multi-stage compression with intermediate cooling and a single-stage expansion in a radial turbine using air (R-729) as working fluid. The use of a variable nozzle turbine allows to evaluate the optimum rack position for each operating point, as well as having control over refrigeration capacity. Studies over the minimum cycle pressure at the inlet of the first stage of compression have been performed to assess the possible benefit of creating vacuum or pressurize the working fluid. A 1m3 cooling chamber was attached to the installation to test the ability of cooling different thermal loads working under different operation ways, open or closed cycle, this is, letting the working fluid to interact with the thermal load or cooling it through a heat exchanger. The evaluation of thermodynamic parameters allows to obtain the coefficient of performance (COP) of the installation as well as the efficiency of each component. The analysis of turbomachinery performance will allow identifying weak points to improve cycle performance, and to improve the coupling model between the rotatory machines as well as to adapt the size of the installation to the refrigeration capacity required for different applications.
Experimental Investigation Into the Effect of a Ceramic Matrix Composite Surface on Film Cooling
Authors: Wilkins Peter H. , Lynch Stephen P. , Thole Karen A. , Vincent Tyler , Quach San , Kaufman Eleanor
Published: October 2022

Ceramic matrix composite (CMC) components enable high turbine entry temperatures, which can lead to improved efficiencies in gas turbines. Implementing film cooling over CMC components, similar to how it is employed for conventional metal components, can extend part life and push operating temperatures beyond the temperature capabilities of CMCs alone. However, CMCs have unique surface topology that can influence film cooling performance. Often this topology takes the form of an irregular wavy pattern due to the weave of the fibers that make up the strengthening component of the composite. In this study, shaped 7-7-7 film cooling holes are embedded in a 5 Harness Satin weave pattern representative of a CMC, at two orientations of the pattern. Detailed adiabatic film effectiveness measurements are obtained in a wind tunnel using an infrared camera while near-wall flowfield measurements are obtained with a high-speed particle image velocimetry system. A range of blowing ratios between one and three are investigated at a density ratio of 1.5 and freestream turbulence intensities of 0.5% and 13%. Across the majority of the tested conditions, the CMC surfaces result in lower film cooling performance than a smooth surface. At a freestream turbulence intensity of 0.5%, the adiabatic film effectiveness is moderately insensitive to blowing ratio for both weave orientations. The boundary layer over the CMC surfaces increases the mixing between the coolant and the mainstream through a combination of increased turbulence, reduced near wall velocities, and a thicker boundary layer.
Aero-Thermal Analysis of Non-Axisymmetric and Flat Endwalls Under the Operational and Geometrical Uncertainties
Authors: Tao Zhi , Wang Jie , Song Liming , Li Jun
Published: October 2022

Given the rapidly increasing turbine inlet temperature and aerodynamic loads, turbine endwalls are bearing exceptionally harsh flow and thermal conditions. Therefore, the turbine endwalls must be meticulously and efficiently designed, with special attention to the influences of geometric and operational uncertainties. This is of vital importance to the efficient and reliable operation of gas turbines. In this paper, a generic uncertainty analysis framework was established for different endwall schemes. Operational fluctuations of the mainstream and geometrical tolerances of the purge slot were taken as input uncertainties. The aero-thermal performance of a benchmark flat endwall and a non-axisymmetric endwall contouring (NEC) was statistically analyzed and compared under the propagation of the input uncertainties. Results showed that, despite the impact of the input uncertainties, the NEC endwall remained effective in enhancing the statistical performance of all three criteria. It was noteworthy that the NEC endwall also greatly reduced the sensitivity of the areas dominated by secondary vortices to the input uncertainties. However, its profiled regions were found to be highly sensitive to the input uncertainties. Furthermore, the influential uncertain parameters were also identified and compared for the flat and NEC endwalls. The inlet flow angle was the most significant parameter for the three performance criteria of the flat endwall. However, for the NEC endwall, the importance of the inlet flow angle was significantly reduced. Instead, the mainstream turbulence intensity became the most influential parameter for the aerodynamic and heat transfer performance, and the slot width became the most influential parameter for the film cooling performance. The underlying flow physics was well explained by CFD results.
Experimental Force Coefficients for a Fully-Partitioned Pocket Damper Seal and Comparison to Other Two Seal Types
Authors: Delgado Adolfo , San Andrés Luis , Yang Jing , Thiele Jonathan
Published: October 2022

Labyrinth seals (LS), honeycomb seals (HCS) and hole-pattern seals, pocket damper seals (PDS) and fully-partitioned damper seals (FPDS) serve as balance pistons on the discharge side of barrel type centrifugal compressors. These seals, facing large pressure differentials along with significant changes in gas density, produce sizeable lateral forces that impact compressor stability and its rotordynamic performance. There is extensive archival data on the dynamic forced performance of LS and textured surface seals. However, the experimental database for a FPDS is insufficient, thus a comprehensive comparison of their dynamic force performance against that of textured surface seals is missing. The paper presents experimentally derived rotordynamic force coefficients for a FPDS along with a direct comparison to published data for an HCS and a LS, both similar in size and in operating conditions. The dynamic load tests with the FPDS include operation at a shaft speed (Ω) of 10 krpm (rotor surface speed of 60 m/s) while supplied with air at Pin = 70 bar and Tin =10 °C, and a discharge at Pout = 0.25, 0.5, and 0.65 of Pin. The pre-swirl circumferential velocity at the seal inlet is low, approximately 6% of rotor surface speed. The FPDS having eight pockets × eight axial blades produces a direct stiffness (K) increasing with excitation frequency (ω), though K < 0 for the lowest pressure ratio PR = (Pout/Pin) = 0.25. The cross-coupled stiffness (k) is invariant to frequency whereas the direct damping (C) steadily decreases as ω/Ω → 1, and then increases for larger frequencies. Hence, the effective damping coefficient (Ceff = C – k/ω) is approximately constant for ω < Ω, while increasing for higher frequencies. For the three PRs, the FPDS leakage is roughly up to 26% larger than the HCS leakage, while the 20-blade LS leaks in between both. The comparison of results shows the LS has insignificant forces compared to those from the HCS and FPDS, both producing a comparable Ceff and similar crossover frequencies (Ceff > 0). The HCS produces a large direct stiffness (K), three to four times that from the FPDS. Comparison of CFD predictions for the FPDS against the experimental force coefficients shows significant differences, in particular an over estimation of direct stiffness as the frequency grows along with a lower direct damping for frequencies equal and above the shaft speed. Besides manufacturing considerations, a FPDS is a better alternative to an HCS whose large centering stiffness (K) may affect the compressor critical speed, hence shortening the separation margin with respect to the operating speed.
Investigation on Strain and Stress Principal Axes in Unsteady DNS Turbine Data
Authors: Lengani D. , Simoni D. , De Vincentiis L. , Ðurović K. , Pralits J. , Henningson D. S. , Hanifi A.
Published: October 2022

In the present work, high-fidelity direct numerical simulation (DNS) data has been adopted in conjunction with an extensive post-processing to provide a detailed description of the turbulence characteristics and its production within a low pressure turbine (LPT) cascade blade passage operating with unsteady inflow. Proper orthogonal decomposition is used at first to provide the statistical representation of the flow structures that occur in the blade passage. Different inlet turbulent scales are isolated and a representation of the turbulence produced in the passage is also provided. Principal axes of the Reynolds stress and the strain tensors have been analyzed to provide further insight on the turbulence production. Since each spatial POD mode captures a quota of the Reynolds stress tensor, the POD modes are well suited to provide reduced order models (ROMs) that represent the different scales of turbulence. Namely, four different scales are defined, and the eigenvectors of the stress tensor for each reduced model are discussed. The discussion includes the comparison with the principal axis of the strain rate tensor.

It is shown that the spatial locations where the eigenvectors of the strain and stress tensors are aligned lead to the largest production of turbulent kinetic energy. The deterministic periodic perturbations induced at the inlet by the unsteady incoming wakes lead to the largest production of turbulence in the passage region where the highest strain is detected and where the eigenvectors of the two tensors are aligned. In the suction side boundary layers, the highest production is related to the local maximum of the Reynolds shear stress due to the stochastic perturbations. The deterministic perturbations do not contribute to the production of turbulence in the suction side boundary layer, even though their induced stress is not negligible, because the eigenvector directions have a maximum misalignment.
Techno-Economic Comparison of Several Technologies for the Waste Heat Recovery From Gas Turbine Exhausts
Authors: Ghilardi Alessandra , Frate Guido Francesco , Baccioli Andrea , Ulivieri Dario , Ferrari Lorenzo , Desideri Umberto , Cosi Lorenzo , Amidei Simone , Michelassi Vittorio
Published: October 2022

The waste heat recovery from the gas turbine (GT) exhaust is typical for increasing performance and reducing CO2 emissions in industrial facilities. Nowadays, numerous already operating GT gas turbine plants could be retrofitted and upgraded with a bottoming cycle powered by the GT exhaust gasses. In this case, the standard solution would be to use a water Steam Rankine Cycle (SRC). However, even if this technology the SRC usually yields the best efficiency, other technologies alternatives are often preferred on the lower size scale. Organic Rankine Cycles (ORCs) are the commercial alternatives to SRC Steam Rankine Cycles, but many other alternative cycles exist or can be developed, with potential benefits from safety, technical or economic points of view. This study compares several alternative technologies suited to recover GT gas turbine waste heat, and a detailed cost analysis for each is presented. On this basis, a guideline is proposed for the technology choice considering a wide range of application sizes and temperature levels typical for waste heat recovery from GTs gas turbines. The compared technologies are ORCs, Rankine Cycles (RCs) with water and ammonia mixtures at constant composition, supercritical CO2 cycles (sCO2), sCO2 cycles with mixtures of CO2, and other gasses. As it resulted, ORCs can achieve the lowest levelized cost of energy (32 $/MWh – 46 $/MWh) if flammable fluids can be employed. Otherwise, RCs Rankine cycles with a constant composition mixture of water and ammonia are a promising alternative, reaching a Levelised Cost Of Energy (LCOE) of 36–58 $/MWh.
Off-Design Modelling and Operational Optimization of Trans-Critical Carbon Dioxide Heat Pumps
Authors: Gabrielli Paolo , Singh Siddhant , Sansavini Giovanni , Sanz Garcia Luis , Jacquemoud Emmanuel , Jenny Philipp
Published: October 2022

Industrial heat pumps, and specifically those using carbon dioxide (CO2) as a refrigerant, can play a key role in the de-carbonization of the heating and cooling sector, due to their low global warming potential, toxicity and flammability. However, challenges arise when dealing with the modeling and optimization of CO2 heat pumps under different operating conditions. We address this challenge by presenting a modeling and optimization tool to predict and optimize the operation of heat pumps in off-design conditions. The tool improves on the current state-of-the-art in several ways. First, it describes a novel thermodynamic cycle, which features higher performance than conventional heat pumps. Also, it is based on a mathematical model that describes accurately the behavior of CO2 across a wide range of thermodynamic conditions, especially near its critical region, and takes into account effects of motor-cooling, leakages and performance limits. Furthermore, it maximizes the coefficient of performance (COP) of the heat pump via an accurate and computationally-efficient optimization problem. The capabilities of the model are illustrated by looking at different typical heat pump applications based on real-world projects within the heating and cooling sector. Different case studies are considered, showing how the heat pump is optimally operated during the year to maximize its COP while meeting the varying boundary conditions.
Experiment Study of Pilot Stage Swirler Outlet Angles and Swirl Number on Flame Structures and Flow Field in a Stratified Swirl Combustor
Authors: Zhang Liang , Xue Xin , Yang Qian , Wang Huiru , Li Jibao
Published: October 2022

The stratified swirl combustor is generally used in an advanced aero-engine, in order to achieve a low emission and wide stabilization at the same time. The complex interactions between the pilot stage and main stage can determine the flame structure, flow field, further affecting the emission formation and stabilization. The present study investigates the interaction between pilot flame and flow field of a stratified swirl combustor, by varying the outlet angles and swirl numbers of the pilot stage swirler. Flame structures and flow fields are obtained while only the pilot stage working. The flame structures are represented by the OH* chemiluminescence (OH-CL) image with Abel deconvolution, non-reacting and reacting flow fields are measured by Particle Image Velocimetry (PIV). The results show that heat release from the pilot flame causes rapid expansion of the pilot gas which may significantly change flow field distribution. Two types of flame structures and flow fields are formed with different outlet angles and swirl numbers of pilot stage. The correspondence between the flame structure and flow field is obtained. For large outlet angle or high swirl number, a M-shape flame combining with a typical stratified swirl flow field and converged pilot and main flow jet are observed, including a large main recirculation zone (MRZ), a lip recirculation zone (LRZ), and a corner recirculation zone (CRZ). For small outlet angle and low swirl number, the flame structure presents a lifted inverted U-shape flame, pilot and main stage jet are separated, there are a small pilot recirculation zone (PRZ), a main recirculation zone (MRZ) and a corner recirculation zone (CRZ) in the combustor. The MRZ is broken into two parts by the accelerated of pilot flow, the LRZ is merged into the MRZ.
Characterization of Flame Behavior and Blowout Limits at Different Air Preheating Temperatures in Plasma Assisted Stabilized Combustor
Authors: De Giorgi Maria Grazia , Mehdi Ghazanfar , Bonuso Sara , Shamma Mohamed , Harth Stefan , Trimis Dimosthenis , Zarzalis Nikolaos
Published: October 2022

This study investigates the flame characterization near to lean blowout (LBO) limits at different pre-heating temperatures in lifted swirl methane-air plasma discharge stabilized flame. The combustor was equipped with a needle-ring type plasma actuator driven by a sinusoidal generator at 20 kHz directly coupled with methane-air premixed flame close to the injector exit. The needle electrode connected with high voltage (HV) and the nozzle acted as a grounded electrode. The electrical characterizations of combustion characteristics and LBO measurements have been carried out at different plasma power, air preheating temperatures, and air mass flow rates. Flame shape and intensity in the flame area have been analyzed based on the images acquired by using a high-resolution camera. Acoustic measurements were performed using high sensitivity microphone. Furthermore, post-processing of the microphone data involved the application of Fast Fourier Transform (FFT) and the wavelet decomposition to identify and characterize any periodic phenomena present in the system. It has been perceived that when the plasma is on, LBO limits were extended significantly. The maximum reduction of equivalence ratio at LBO was noticed in terms of percentage (79.4%) at 40% voltage amplitude, T = 373 K, and ma = 6 m3/h. Proper orthogonal decomposition was implemented on the high-speed chemiluminescence images and the frequency analysis of the time coefficients of the modes of the POD permits the identification of the dominant frequency ranges in the different operating conditions. The results were in good agreement with the findings of the FFT and Wavelet analysis on the microphone measured signals.
Rotordynamic Force Coefficient Prediction of an Unshrouded Radial Inflow Expander and Labyrinth Seal Using Transient Computational Fluid Dynamic Analysis
Authors: Kulhanek Chris D. , Delgado Hector A. , Marshall Michael P. , Moore J. Jeffrey , Lillard Joseph , Kim Yunbae
Published: October 2022

The current study uses a transient computational fluid dynamic (CFD) analysis to predict the rotordynamic force coefficients for an unshrouded radial inflow expander and labyrinth wheel seal in a high pressure natural gas turboexpander. Direct and cross-coupled terms are predicted for both the stiffness and damping for the expander wheel and labyrinth seal, including the aerodynamic cross-coupling forces or cross-coupled stiffness coefficients that impact rotordynamic stability. A static eccentric method was applied where a transient simulation where the rotor portion is displaced with a known static eccentricity and the resulting forces allow for the calculation of direct stiffness and cross-coupled stiffness. In addition, a whirling method using transient simulations at various whirling frequencies is applied to solve for the full set of rotordynamic coefficients, including direct and cross-coupled stiffness and damping terms through curve fitting a KCM model for both the expander wheel and labyrinth seal. Wheel force coefficients are presented for several operating conditions, including low, nominal, and high flow regimes. In addition, a parametric study is performed, including hypothetical cases to evaluate sensitivity of wheel tip clearance, rotational speed, gas density on the expander cross-coupled stiffness. The predicted wheel cross-coupled stiffness shows a very strong dependence on the wheel tip clearance compared to the other parameters studied.
To Drill or Not to Drill? - Creep of an Oxide-Oxide Composite With Diamond-Drilled Discharge Holes at Elevated Temperature
Authors: Harkins Megan , Ruggles-Wrenn Marina
Published: October 2022

Ceramic matrix composites (CMCs) are leading contenders for use in aircraft engines. Yet even with their high-temperature capabilities, many CMC components will need cooling. External or film cooling technique of a component requires rows of small holes through the component surface. Effects of multiple small holes on tensile stress-strain and tensile creep resistance of an oxide fiber-oxide matrix composite comprising Nextel™720 alumina-mullite fibers and a porous alumina matrix were evaluated at 1200°C. Gauge section of each test specimen included seventeen (17) 0.5-mm diameter holes. The holes were precision drilled using diamond coated drill bits. The presence of diamond-drilled (DD) holes noticeably degraded tensile strength and modulus. Specimens with DD discharge holes were creep tested in air and steam at 1200°C. Applied creep stress varied from 40 to 130 MPa. Endurance of 100 h at creep stress without failure was defined as creep run-out. The existence of diamond drilled holes lowered creep resistance of the CMC as evidenced by higher steady-state creep strain rates and lessened creep lives. An earlier study at the Air Force Institute of Technology considered the effects of small holes drilled using a CO2 laser on the tensile properties and tensile creep resistance of this material system. Geometry of test specimens and test conditions were the same as in the present effort. The earlier study found that the existence of rows of small laser drilled (LD) holes also considerably lowered the creep resistance of the Nextel™720/alumina CMC. In both cases the reduction in tensile strength and creep resistance are due to damage caused to composite microstructure by fabrication of the holes, i. e. drilling. However, different hole drilling techniques result in different microstructure degradation mechanisms. Changes to the CMC microstructure triggered by these two drilling techniques and implications for mechanical performance and durability are discussed.
Effect of Deflection Angle on Flow and Heat Transfer in a Rotating U-Channel
Authors: Li Honglin , Li Lei , Tang Zhonghao , Zhang Zhenyuan , Yuan Tianyu
Published: October 2022

In this work, the effect of spanwise deflection of a rotating smooth U-channel on flow and heat transfer characteristics is investigated through numerical simulation. Seven deflected U-channel with different angles are adopted, including α = 90°, 60°, 30°, 0°, −30°, −60°, −90°, and three rotation numbers vary from 0 to 0.29. Using verified numerical method, the flow pattern, detailed and overall heat transfer characteristics and flow friction factor are analyzed. Results show that the deflection angle will enhance the heat transfer in turning section and outlet passage, which can be attributed to the enhancement of flow impact and more complex vortex structure. The deflection of U-channel would generate a Coriolis force at turning section and thus interact with the centrifugal force to change the vortices, which makes the U-channel with negative deflection angle has higher heat transfer enhancement and larger movement of peak Nu number. Compared with positive deflection angle, the negative deflection angle has higher thermal performance and the maximum emerges at α = −30° since the U-channel can generate larger heat transfer enhancement with smaller flow loss increase. Besides, the rotation will enhance the local and overall thermal performance of the U-channel, and the effect of deflection angle on the thermal performance of U-channel keep the same at different rotations numbers.
Transition Model Extension for Roughness Effects
Authors: Kožulović Dragan , Juchmann Nemo , Führing Alexander , Bode Christoph
Published: October 2022

Depending on the manufacturing process and the operating conditions, airfoil surface may show many different roughness characteristics, together with a significant influence at the boundary layer development. In this paper, only the influence at the laminar-turbulent transition is considered and modeled by an extension of the γ-ReΘ transition model of Langtry and Menter. Essentially, an additional transport equation for a roughness amplification scalar is used to modify the transition onset and development, as already presented by Dassler et al. in 2012. In the meantime, the calibrated functional relationship for Argr has been released for the publication for the first time in this paper. In addition, the model performance will be demonstrated and discussed on test cases with increasing complexity, including turbomachinery cascades and rigs.

As an input to the model, the equivalent sand-grain roughness is required. In this way, the versatile roughness characteristics of the investigated surface are reduced to only one parameter. The model has been implemented into the CFD software package TRACE of DLR Institute of Propulsion Technology. Only steady flow test cases have been investigated and validated. The transition intermittency is coupled to the two-equation turbulence model of Wilcox. In this model, the roughness influence at the fully turbulent boundary layers is also captured by the variation of the boundary condition for the specific turbulence dissipation rate ω.