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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2017;140(5):051201-051201-10. doi:10.1115/1.4038124.

Enabling high overall pressure ratios (OPR), wave rotors, and piston concepts (PCs) seem to be solutions surpassing gas turbine efficiency. Therefore, a comparison of a wave rotor and three PCs relative to a reference gas turbine is offered. The PPCs include a Wankel, a two-stroke reciprocating engine, and a free piston. All concepts are investigated with and without intercooling. An additional combustion chamber (CC) downstream the piston engine is investigated, too. The shaft power chosen corresponds to large civil turbofans. Relative to the reference gas turbine, a maximum efficiency increase of 11.2% for the PCs and 9.8% for the intercooled wave rotor is demonstrated. These improvements are contrasted by a 5.8% increase in the intercooled reference gas turbine and a 4.2% increase due to improved gas turbine component efficiencies. Intercooling the higher component efficiency gas turbine leads to a 9.8% efficiency increase. Furthermore, the study demonstrates the high difference between intercooler and piston engine weight and a conflict between PC efficiency and chamber volume, highlighting the need for extreme lightweight design in any piston engine solution. Improving piston engine technology parameters is demonstrated to lead to higher efficiency, but not to a chamber volume reduction. Heat loss in the piston engines is identified as the major efficiency limiter.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2017;140(5):051501-051501-11. doi:10.1115/1.4038127.

Over the last years, aero-engines are progressively evolving toward design concepts that permit improvements in terms of engine safety, fuel economy, and pollutant emissions. With the aim of satisfying the strict NOx reduction targets imposed by ICAO-CAEP, lean burn technology is one of the most promising solutions even if it must face safety concerns and technical issues. Hence, a depth insight on lean burn combustion is required, and computational fluid dynamics can be a useful tool for this purpose. In this work, a comparison in large eddy simulation (LES) framework of two widely employed combustion approaches like the artificially thickened flame (ATF) and the flamelet generated manifold (FGM) is performed using ANSYS fluent v16.2. Two literature test cases with increasing complexity in terms of geometry, flow field, and operating conditions are considered. First, capabilities of FGM are evaluated on a single swirler burner operating at ambient pressure with a standard pressure atomizer for spray injection. Then, a second test case, operated at 4 bar, is simulated. Here, kerosene fuel is burned after an injection through a prefilming airblast atomizer within a corotating double swirler. Obtained comparisons with experimental results show different capabilities of ATF and FGM in modeling the partially premixed behavior of the flame and provide an overview of the main strengths and limitations of the modeling strategies under investigation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):051502-051502-9. doi:10.1115/1.4038128.

Lean premixed combustion is prevailing in gas turbines to minimize nitrogen oxide emissions. However, this technology bears the risk of flame flashback and thermoacoustic instabilities. Thermoacoustic instabilities induce velocity oscillations at the burner exit which, in turn, can trigger flame flashback. This article presents an experimental study at ambient conditions on the effect of longitudinal acoustic excitation on flashback in the boundary layer of a channel burner. The acoustic excitation simulates the effect of thermoacoustic instabilities. Flashback limits are determined for different excitation frequencies characterizing intermediate frequency dynamics in typical gas turbine combustors (100–350 Hz). The excitation amplitude is varied from 0% to 36% of the burner bulk flow velocity. For increasing excitation amplitude, the risk of flame flashback increases. This effect is strongest at low frequencies. For increasing excitation frequency, the influence of the velocity oscillations decreases as the flame has less time to follow the changes in bulk flow velocity. Two different flashback regimes can be distinguished based on excitation amplitude. For low excitation amplitudes, flashback conditions are reached if the minimum flow velocity in the excitation cycle falls below the flashback limit of unexcited unconfined flames. For higher excitation amplitudes, where the flame starts to periodically enter the burner duct, flashback is initiated if the maximum flow velocity in the excitation cycle is lower than the flashback limit of confined flames. Consequently, flashback limits of confined flames should also be considered in the design of gas turbine burners as a worst case scenario.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):051503-051503-10. doi:10.1115/1.4038153.

Radiative heat transfer is studied numerically for reacting swirling flow in an industrial gas turbine burner operating at a pressure of 15 bar. The reacting field characteristics are computed by Reynolds-averaged Navier–Stokes (RANS) equations using the k-ϵ model with the partially stirred reactor (PaSR) combustion model. The GRI-Mech 2.11 mechanism, which includes nitrogen chemistry, is used to demonstrate the ability of reducing NOx emissions of the combustion system. A photon Monte Carlo (PMC) method coupled with a line-by-line (LBL) spectral model is employed to accurately account for the radiation effects. Optically thin (OT) and PMC–gray models are also employed to show the differences between the simplest radiative calculation models and the most accurate radiative calculation model, i.e., PMC–LBL, for the gas turbine burner. It was found that radiation does not significantly alter the temperature level as well as CO2 and H2O concentrations. However, it has significant impacts on the NOx levels at downstream locations.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;140(5):051701-051701-10. doi:10.1115/1.4038125.

After the renewed interest in supercritical carbon dioxide cycles, a large number of cycle layouts have been proposed in literature. These works, which are essentially theoretical, consider different operating conditions and modeling assumptions, and thus, the results are not comparable. There are also works that aim to provide a fair comparison between different cycles in order to assess which one is most efficient. These analyses are very interesting but, usually, they combine thermodynamic and technical restrictions, which make it difficult to draw solid and general conclusions with regard to which the cycle of choice in the future should be. With this background, the present work provides a systematic thermodynamic analysis of 12 supercritical carbon dioxide cycles under similar working conditions, with and without technical restriction in terms of pressure and/or temperature. This yields very interesting conclusions regarding the most interesting cycles in the literature. Also, useful recommendations are extracted from the parametric analysis with respect to the directions that must be followed when searching for more efficient cycles. The analysis is based on efficiency and specific work diagrams with respect to pressure ratio and turbine inlet temperature in order to enhance its applicability to plant designs driven by fuel economy and/or footprint.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):051702-051702-10. doi:10.1115/1.4038152.

This paper presents the development, implementation, and validation of a simplified dynamic modeling approach to describe solid oxide fuel cell gas turbine (SOFC/GT) hybrid systems (HSs) in three real emulator test rigs installed at University of Genoa (Italy), German Aerospace Center (DLR, Germany), and National Energy Technology Laboratory (NETL, USA), respectively. The proposed modeling approach is based on an experience-based simplification of the physical problem to reduce model computational efforts with minimal expense of accuracy. Traditional high fidelity dynamic modeling requires specialized skills and significant computational resources. This innovative approach, on the other hand, can be easily adapted to different plant configurations, predicting the most relevant dynamic phenomena with a reduced number of states: such a feature will allow, in the near future, the model deployment for monitoring purposes or advanced control scheme applications (e.g., model predictive control). The three target systems are briefly introduced and dynamic situations analyzed for model tuning, first, and validation, then. Relevance is given to peculiar transients where the model shows its reliability and its weakness. Assumptions introduced during model definition for the three different test rigs are discussed and compared. The model captured significant dynamic behavior in all analyzed systems (in particular those regarding the GT) and showed influence of signal noise on some of the SOFC computed outputs.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2017;140(5):051901-051901-9. doi:10.1115/1.4038248.

New experimental data are provided for full-coverage effusion cooling and impingement array cooling, as applied simultaneously onto the respective external and internal surfaces of a single instrumented test plate. For the effusion cooled surface, presented are spatially resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients. For the impingement cooled surface, presented are spatially resolved distributions of surface Nusselt numbers. Impingement jet arrays at different jet Reynolds numbers, from 7930 to 18,000, are employed. Experimental data are given for spanwise and streamwise impingement hole spacing such that coolant jet hole centerlines are located midway between individual effusion hole entrances. For the effusion cooling, streamwise hole spacing and spanwise hole spacing (normalized by effusion hole diameter) are 15 and 4, respectively. Effusion hole angle is 25 deg, and effusion plate thickness is 3.0 effusion hole diameters. In regard to the impingement cooled cold-side surface of the effusion plate, associated surface Nusselt number variations provide evidence that impingement jets are turned and redirected as they cross the impingement passage, just prior to the entrance of coolant into individual effusion holes. In regard to the effusion cooled hot-side surface of the effusion plate, when compared at particular values of injectant and mainstream Reynolds numbers, streamwise location x/de and blowing ratio BR, significantly increased thermal protection is provided when the effusion coolant is provided by an array of impingement cooling jets (compared to a cross flow channel supply arrangement).

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2017;140(5):052401-052401-11. doi:10.1115/1.4038155.

The reliability of gas turbine (GT) health state monitoring and forecasting depends on the quality of sensor measurements directly taken from the unit. Outlier detection techniques have acquired a major importance, as they are capable of removing anomalous measurements and improve data quality. To this purpose, statistical parametric methodologies are widely employed thanks to the limited knowledge of the specific unit required to perform the analysis. The backward and forward moving window (BFMW) k–σ methodology proved its effectiveness in a previous study performed by the authors, to also manage dynamic time series, i.e., during a transient. However, the estimators used by the k–σ methodology are usually characterized by low statistical robustness and resistance. This paper aims at evaluating the benefits of implementing robust statistical estimators for the BFMW framework. Three different approaches are considered in this paper. The first methodology, k-MAD, replaces mean and standard deviation (SD) of the k–σ methodology with median and mean absolute deviation (MAD), respectively. The second methodology, σ-MAD, is a novel hybrid scheme combining the k–σ and the k-MAD methodologies for the backward and the forward windows, respectively. Finally, the biweight methodology implements biweight mean and biweight SD as location and dispersion estimators. First, the parameters of these methodologies are tuned and the respective performance is compared by means of simulated data. Different scenarios are considered to evaluate statistical efficiency, robustness, and resistance. Subsequently, the performance of these methodologies is further investigated by injecting outliers in field datasets taken on selected Siemens GTs. Results prove that all the investigated methodologies are suitable for outlier identification. Advantages and drawbacks of each methodology allow the identification of different scenarios in which their application can be most effective.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;140(5):052501-052501-9. doi:10.1115/1.4038121.

The present work advances experimental results and analytical predictions on the dynamic performance of an integral squeeze film damper (ISFD) for application in a high-speed super-critical CO2 (sCO2) expander. The test campaign focused on conducting controlled orbital motion mechanical impedance testing aimed at extracting stiffness and damping coefficients for varying end seal clearances, excitation frequencies, and vibration amplitudes. In addition to the measurement of stiffness and damping, the testing revealed the onset of cavitation for the ISFD. Results show damping behavior that is constant with vibratory velocity for each end seal clearance case until the onset of cavitation/air ingestion, while the direct stiffness measurement was shown to be linear. Measurable added inertia coefficients were also identified. The predictive model uses an isothermal finite element method to solve for dynamic pressures for an incompressible fluid using a modified Reynolds equation accounting for fluid inertia effects. The predictions revealed good correlation for experimentally measured direct damping, but resulted in grossly overpredicted inertia coefficients when compared to experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):052502-052502-8. doi:10.1115/1.4038136.

Varying clearance, rotor-following seals are a key technology for meeting the demands of increased machine flexibility for conventional power units. These seals follow the rotor through hydrodynamic or hydrostatic mechanisms. Forward-facing step (FFS) and Rayleigh step designs are known to produce positive fluid stiffness. However, there is very limited modeling or experimental data available on the hydrostatic fluid forces generated from either design. A quasi-one-dimensional (1D) method has been developed to describe both designs and validated using test data. Tests have shown that the FFS and the Rayleigh step design are both capable of producing positive film stiffness and there is little difference in hydrostatic force generation between the two designs. This means any additional hydrodynamic features in the Rayleigh step design should have a limited effect on hydrostatic fluid stiffness. The analytical model is capable of modeling both the inertial fluid forces and the viscous fluid losses, and the predictions are in good agreement with the test data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):052503-052503-7. doi:10.1115/1.4038183.

This paper deals with the estimation of forcing functions on a mistuned bladed rotor from measurements of harmonic response via Kalman filter (KF) in time domain. An unique feature of this approach is that the number of estimated variables can be far greater than the number of measurements. The robustness of this method to measurement errors is shown. It is also shown that direct prediction of amplitude and phase of sinusoidal force vector from input/output frequency response function has a large amount of errors in the presence of unavoidable measurement noise. Numerical examples contain both frequency mistuning and geometric mistuning.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):052504-052504-8. doi:10.1115/1.4038154.

The component mode synthesis (CMS) based on the Craig–Bampton (CB) method has two strong limitations that appear when the number of the interface degrees-of-freedom (DOFs) is large. First, the reduced-order model (ROM) obtained is overweighed by many unnecessary DOF. Second, the reduction step may become extremely time consuming. Several interface reduction (IR) techniques addressed successfully the former problem, while the latter remains open. In this paper, we tackle this latter problem through a simple IR technique based on an a-priory choice of the interface modes. An efficient representation of the interface displacement field is achieved adopting a set of orthogonal basis functions determined by the interface geometry. The proposed method is compared with other existing IR methods on a case study regarding a rotor blade of an axial compressor.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;140(5):052601-052601-12. doi:10.1115/1.4038122.

One of the main design decisions in the development of low-speed axial fans is the right choice of the blade loading versus rotational speed, since a target pressure rise could either be achieved with a slow spinning fan and high blade loading or a fast spinning fan with less flow turning in the blade passages. Both the blade loading and the fan speed have an influence on the fan performance and the fan acoustics, and there is a need to find the optimum choice in order to maximize efficiency while minimizing noise emissions. This paper addresses this problem by investigating five different fans with the same pressure rise but different rotational speeds in the design point (DP). In the first part of the numerical study, the fan design is described and steady-state Reynolds-averaged Navier–Stokes (RANS) simulations are conducted in order to identify the performance of the fans in the DP and in off-design conditions. The investigations show the existence of an optimum in rotational speed regarding fan efficiency and identify a flow separation on the hub causing a deflection of the outflow in radial direction as the main loss source for slow spinning fans with high blade loadings. Subsequently, large eddy simulations (LES) along with the acoustic analogy of Ffowcs Williams and Hawkings (FW–H) are performed in the DP to identify the main noise sources and to determine the far-field acoustics. The identification of the noise sources within the fans in the near-field is performed with the help of the power spectral density (PSD) of the pressure. In the far-field, the sound power level (SWL) is computed using different parts of the fan surface as FW–H sources. Both methods show the same trends regarding noise emissions and allow for a localization of the noise sources. The flow separation on the hub is one of the main noise sources along with the tip vortex with an increase in its strength toward lower rotational speeds and higher loading. Furthermore, a horseshoe vortex detaching from the rotor leading edge and impinging on the pressure side as well as the turbulent boundary layer on the suction side represent significant noise sources. In the present investigation, the maximum in efficiency coincides with the minimum in noise emissions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;140(5):052602-052602-9. doi:10.1115/1.4038137.

The wet compressor (WC) has become a reliable way to reduce gas emissions and increase gas turbine efficiency. However, fuel source diversification in the short and medium terms presents a challenge for gas turbine operators to know how the WC will respond to changes in fuel composition. For this study, we assessed the operational data of two thermal power generators, with outputs of 610 MW and 300 MW, in Colombia. The purpose was to determine the maximum amount of water that can be added into a gas turbine with a WC system, as well as how the NOx/CO emissions vary due to changes in fuel composition. The combustion properties of different gaseous hydrocarbon mixtures at wet conditions did not vary significantly from each other—except for the laminar burning velocity. It was found that the fuel/air equivalence ratio in the turbine reduced with lower CH4 content in the fuel. Less water can be added to the turbine with leaner combustion; the water/fuel ratio was decreased over the range of 1.4–0.4 for the studied case. The limit is mainly due to a reduction in flame temperature and major risk of lean blowout (LBO) or dynamic instabilities. A hybrid reaction mechanism was created from GRI-MECH 3.0 and NGIII to model hydrocarbons up to C5 with NOx formation. The model was validated with experimental results published previously in literature. Finally, the effect of atmospheric water in the premixed combustion was analyzed and explained.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;140(5):052801-052801-10. doi:10.1115/1.4038024.

Turbochargers reduce fuel consumption and CO2 emissions from heavy-duty internal combustion engines by enabling downsizing and downspeeding through greater power density. This requires greater pressure ratios and thus air systems with multiple stages and interconnecting ducting, all subject to tight packaging constraints. This paper considers the aerodynamic optimization of the exhaust side of a two-stage air system for a Caterpillar 4.4 l heavy-duty diesel engine, focusing on the high pressure turbine (HPT) wheel and interstage duct (ISD). Using current production designs as a baseline, a genetic algorithm (GA)-based aerodynamic optimization process was carried out separately for the wheel and duct components to evaluate seven key operating points. While efficiency was a clear choice of cost function for turbine wheel optimization, different objectives were explored for ISD optimization to assess their impact. Optimized designs are influenced by the engine operating point, so each design was evaluated at every other engine operating point, to determine which should be carried forward. Prototypes of the best compromise high pressure turbine wheel and ISD designs were manufactured and tested against the baseline to validate computational fluid dynamics (CFD) predictions. The best performing high pressure turbine design was predicted to show an efficiency improvement of 2.15% points, for on-design operation. Meanwhile, the optimized ISD contributed a 0.2% and 0.5% point efficiency increase for the HPT and low pressure turbine (LPT), respectively.

Commentary by Dr. Valentin Fuster

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