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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2018;140(6):061201-061201-9. doi:10.1115/1.4038159.

Auxiliary power unit (APU) operators face increasingly stricter airport requirements concerning exhaust gas and noise emission levels. To simultaneously reduce exhaust gas and noise emissions and to satisfy the increasing demand of electric power on board, optimization of the current technology is necessary. Prior to any possible demonstration of optimization potential, detailed data of thermodynamic properties and emissions have to be determined. Therefore, the investigations presented in this paper were conducted at a full-scale APU of an operational aircraft. A Pratt & Whitney (East Hartford, CT) APS3200, commonly installed in the Airbus A320 aircraft family, was used for measurements of the reference data. In order to describe the APS3200, the full spectrum of feasible power load and bleed air mass flow combinations were adjusted during the study. Their effect on different thermodynamic and performance properties, such as exhaust gas temperature, pressure as well as electric and overall efficiency is described. Furthermore, the mass flows of the inlet air, exhaust gas, and fuel input were determined. Additionally, the work reports the exhaust gas emissions regarding the species CO2, CO, and NOx as a function of load point. Moreover, the acoustic noise emissions are presented and discussed. With the provided data, the paper serves as a database for validating numerical simulations and provides a baseline for current APU technology.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(6):061501-061501-10. doi:10.1115/1.4038156.

Thermoacoustic instabilities are a major threat for modern gas turbines. Frequency-domain-based stability methods, such as network models and Helmholtz solvers, are common design tools because they are fast compared to compressible flow computations. They result in an eigenvalue problem, which is nonlinear with respect to the eigenvalue. Thus, the influence of the relevant parameters on mode stability is only given implicitly. Small changes in some model parameters, may have a great impact on stability. The assessment of how parameter uncertainties propagate to system stability is therefore crucial for safe gas turbine operation. This question is addressed by uncertainty quantification. A common strategy for uncertainty quantification in thermoacoustics is risk factor analysis. One general challenge regarding uncertainty quantification is the sheer number of uncertain parameter combinations to be quantified. For instance, uncertain parameters in an annular combustor might be the equivalence ratio, convection times, geometrical parameters, boundary impedances, flame response model parameters, etc. A new and fast way to obtain algebraic parameter models in order to tackle the implicit nature of the problem is using adjoint perturbation theory. This paper aims to further utilize adjoint methods for the quantification of uncertainties. This analytical method avoids the usual random Monte Carlo (MC) simulations, making it particularly attractive for industrial purposes. Using network models and the open-source Helmholtz solver PyHoltz, it is also discussed how to apply the method with standard modeling techniques. The theory is exemplified based on a simple ducted flame and a combustor of EM2C laboratory for which experimental data are available.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061502-061502-10. doi:10.1115/1.4038417.

It has become increasingly cost-effective for the steel industry to invest in the capture of heavily carbonaceous basic oxygen furnace or converter gas, and use it to support the intensive energy demands of the integrated facility, or for surplus energy conversion in power plants. As industry strives for greater efficiency via ever more complex technologies, increased attention is being paid to investigate the complex behavior of by-product syngases. Recent studies have described and evidenced the enhancement of fundamental combustion parameters such as laminar flame speed due to the catalytic influence of H2O on heavily carbonaceous syngas mixtures. Direct formation of CO2 from CO is slow due to its high activation energy, and the presence of disassociated radical hydrogen facilitates chain branching species (such as OH), changing the dominant path for oxidation. The observed catalytic effect is nonmonotonic, with the reduction in flame temperature eventually prevailing, and overall reaction rate quenched. The potential benefits of changes in water loading are explored in terms of delayed lean blow-off (LBO), and primary emission reduction in a premixed turbulent swirling flame, scaled for practical relevance at conditions of elevated temperature (423 K) and pressure (0.1–0.3 MPa). Chemical kinetic models are used initially to characterize the influence that H2O has on the burning characteristics of the fuel blend employed, modeling laminar flame speed and extinction strain rate across an experimental range with H2O vapor fraction increased to eventually diminish the catalytic effect. These modeled predictions are used as a foundation to investigate the experimental flame. OH* chemiluminescence and OH planar laser-induced fluorescence (PLIF) are employed as optical diagnostic techniques to analyze changes in heat release structure resulting from the experimental variation in water loading. A comparison is made with a CH4/air flame and changes in LBO stability limits are quantified, measuring the incremental increase in air flow and again compared against chemical models. The compound benefit of CO and NOx reduction is quantified also, with production first decreasing due to the thermal effect of H2O addition from a reduction in flame temperature, coupled with the potential for further reduction from the change in lean stability limit. Power law correlations have been derived for change in pressure, and equivalent water loading. Hence, the catalytic effect of H2O on reaction pathways and reaction rate predicted and observed for laminar flames are appraised within the challenging environment of turbulent, swirl-stabilized flames at elevated temperature and pressure, characteristic of practical systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061503-061503-10. doi:10.1115/1.4038324.

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061504-061504-6. doi:10.1115/1.4038326.

We report experimental evidence of thermoacoustic bistability in a lab-scale turbulent combustor over a well-defined range of fuel–air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts,” i.e., sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions (PDFs) of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bistable region. The gain and phase between acoustic pressure and heat release rate fluctuations are evaluated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of the corresponding statistics is new and particularly informative. It shows that the system is characterized, in average, by a nearly constant gain and by a drift of the phase as function of the oscillation amplitude. This finding may suggest that the bistability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the nonconstant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061505-061505-10. doi:10.1115/1.4038234.

Due to the high temperature of the flue gas flowing at high velocity and pressure, the wall cooling is extremely important for the liner of a gas turbine engine combustor. The liner material is heat-resistant steel with relatively low heat conductivity. To accommodate outside wall forced air cooling, the liner is designed to be thin, which unfortunately facilitates the possibility of high-amplitude wall vibrations (and failure due to fatigue) in case of pressure fluctuations in the combustor. The latter may occur due to a possible occurrence of a feedback loop between the aerodynamics, the combustion, the acoustics, and the structural vibrations. The structural vibrations act as a source of acoustic emitting the acoustic waves to the confined fluid. This leads to amplification in the acoustic filed and hence the magnitude of instability in the system. The aim of this paper is to explore the mechanism of fluid–structure interaction (FSI) on the LIMOUSINE setup which leads to limit cycle of pressure oscillations (LCO). Computational fluid dynamics (CFD) analysis using a RANS approach is performed to obtain the thermal and mechanical loading of the combustor liner, and finite element model (FEM) renders the temperature, stress distribution, and deformation in the liner. Results are compared to other numerical approaches like zero-way interaction and conjugated heat transfer model (CHT). To recognize the advantage/disadvantage of each method, validation is made with the available measured data for the pressure and vibration signals, showing that the thermoacoustic instabilities are well predicted using the CHT and two-way coupled approaches, while the zero-way interaction model prediction gives the largest discrepancy from experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061506-061506-9. doi:10.1115/1.4038025.

In this work, a single sector lean burn model combustor operating in pilot only mode has been investigated using both experiments and computations with the main objective of analyzing the flame structure and soot formation at conditions relevant to aero-engine applications. Numerical simulations were performed using the large eddy simulation (LES) approach and the conditional moment closure (CMC) combustion model with detailed chemistry and a two-equation model for soot. The CMC model is based on the time-resolved solution of the local flame structure and allows to directly take into account the phenomena associated to molecular mixing and turbulent transport, which are of great importance for the prediction of emissions. The rig investigated in this work, called big optical single sector rig, allows to test real scale lean burn injectors. Experiments, performed at elevated pressure and temperature, corresponding to engine conditions at part load, include planar laser-induced fluorescence of OH (OH-PLIF) and phase Doppler anemometry (PDA) and have been complemented with new laser-induced incandescence (LII) measurements for soot location. The wide range of measurements available allows a comprehensive analysis of the primary combustion region and can be exploited to further assess and validate the LES/CMC approach to capture the flame behavior at engine conditions. It is shown that the LES/CMC approach is able to predict the main characteristics of the flame with a good agreement with the experiment in terms of flame shape, spray characteristics and soot location. Finite-rate chemistry effects appear to be very important in the region close to the injection location leading to the lift-off of the flame. Low levels of soot are observed immediately downstream of the injector exit, where a high amount of vaporized fuel is still present. Further downstream, the fuel vapor disappears quite quickly and an extended region characterized by the presence of pyrolysis products and soot precursors is observed. The strong production of soot precursors together with high soot surface growth rates lead to high values of soot volume fraction in locations consistent with the experiment. Soot oxidation is also very important in the downstream region resulting in a decrease of the soot level at the combustor exit. The results show a very promising capability of the LES/CMC approach to capture the main characteristics of the flame, soot formation, and location at engine relevant conditions. More advanced soot models will be considered in future work in order to improve the quantitative prediction of the soot level.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061507-061507-8. doi:10.1115/1.4038836.

Diesel-natural gas dual fuel engine has gained increasing interesting in recent years because of its excellent power and economy. However, the reliability of the dual fuel engine does not meet the requirements of practical application. The piston maximun temperature (PMT) of the dual fuel engine easily exceeds the security border. In view of this, this paper proposes a method based on the lasso regression to estimate the PMT of the dual fuel engine, so as to real-timely monitor the health state of the dual fuel engine. Specifically, PMTs under some working conditions were offline acquired by the finite element analysis with ANSYS. A model is presented to describe the relationship between the PMT and some indirect engine variables, including NOx emission, excess air coefficient, engine speed, and inlet pressure, and the model parameters are optimized using the lasso regression algorithm, which can be easily implemented by the electronic control unit (ECU). Finally, the model is employed to real-timely estimate the PMT of the dual fuel engine. Experiments reveal that the proposed model produces satisfying predictions with deviations less than 10 °C.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061508-061508-14. doi:10.1115/1.4038416.

Increasingly stringent regulations on NOx emissions are enforced by governments owing to their contribution in the formation of ozone, smog, fine aerosols, acid rains, and nutrient pollution of surface water, which affect human health and the environment. The design of high-efficiency, low-emission combustors achieving these ever-decreasing emission standards requires thermochemical mechanisms of sufficiently high accuracy. Recently, a comprehensive set of experimental data, collected through laser-based diagnostics in atmospheric, jet-wall, stagnation, premixed flames, was published for all isomers of C1–C4 alkane and alcohol fuels. The rapid formation of NO through the flame front via the prompt (Fenimore) route was shown to be strongly coupled to the maximum concentration of the methylidyne radical, [CH]peak, and the flow residence time within the CH layer. A proper description of CH formation is then a prerequisite for accurate predictions of NO concentrations in hydrocarbon–air flames. However, a comparison against the Laser-induced fluorescence (LIF) experimental data of Versailles, P., et al. (2016, “Quantitative CH Measurements in Atmospheric-Pressure, Premixed Flames of C1–C4 Alkanes,” Combust. Flame, 165, pp. 109--124) revealed that (1) modern thermochemical mechanisms are unable to accurately capture the stoichiometric dependence of [CH]peak, and (2) for a given equivalence ratio, the predictions of different mechanisms span over more than an order of magnitude. This paper presents an optimization of the specific rate of a selection of nine elementary reactions included in the San Diego combustion mechanism. A quasi-Newton algorithm is used to minimize an objective function defined as the sum of squares of the relative difference between the numerical and experimental CH–LIF data of Versailles, P., et al. (2016, “Quantitative CH Measurements in Atmospheric-Pressure, Premixed Flames of C1–C4 Alkanes,” Combust. Flame, 165, pp. 109--124), while constraining the specific rates to physically reasonable values. A mechanism properly describing CH formation for lean to rich, C1–C3 alkane–air flames is obtained. This optimized mechanism will enable accurate predictions of prompt-NO formation over a wide range of equivalence ratios and alkane fuels. Suggestions regarding which reactions require further investigations, either through experimental or theoretical assessments of the individual specific rates, are also provided.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):061509-061509-11. doi:10.1115/1.4038521.

This paper studies the differences in spray structure and emissions trends between diesel and biodiesel fuels in a compression ignition engine. A computationally efficient and predictive quasi-dimensional simulation model is combined with fuel-specific physical properties and chemical kinetic mechanisms to predict spray mixing, combustion, and emissions behavior. The results underscore the complex relationships between NOx emissions, operational parameters, and fuel chemistry and provide further evidence of a link between stoichiometry near the flame lift-off length and formation of NOx.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2018;140(6):062101-062101-7. doi:10.1115/1.4038351.

Increasing turbine inlet temperature is important for improving the efficiency of gas turbine engine. Elevated thermal load causes severe oxidation and corrosion for base alloy in turbine airfoils. To survive in this extreme high-temperature and harsh oxidation environment, both outside protection like thermal barrier coatings (TBC) and inside air cooling have been applied to turbine blades. Significantly more protection can be achieved if the cooling channels are embedded near surface, constructed partially by the coating system and partially by the superalloy substrate. However, neither the ceramic coating layer nor the metallic bond coating layer in current TBC system can provide structural support to such internal cooling channels. Development of structural bond coating layers consequently becomes one of the key technologies to achieve this goal. The present study proposed a method to fabricate structural coating layers on top of turbine blades with the aid of additive manufacturing (AM) and oxide dispersion strengthened (ODS) nickel-based alloy. ODS powder comprised of evenly distributed host composite particles (Ni, Al, Cr) with oxide coating layers (Y2O3) was subjected to a direct metal laser sintering (DMLS) process to fabricate a desirable structural coating layer above nickel-based superalloy substrates. Systematic experimental tests were carried out focusing on the interface adhesion, mechanical strength, microstructure, and surface finish of the ODS coating layer. Based on characterization results from indentation tests and microscopy observations, an optimal coating quality was obtained under ∼250 W laser power. The selected samples were then characterized under isothermal conditions of 1200 °C for 2000 h. Scanning electron microscope (SEM) observations and energy-dispersive X-ray spectroscopy (EDX) analysis were conducted in different stages of the oxidation process. Results indicated a formation of Al2O3 scale on top of the ODS coating layer at early stage, which showed long-term stability throughout the oxidation test. The formation of a stable alumina scale is acting as a protective layer to prevent oxygen penetrating the top surface. Spallation of part of nickel oxide and chromium oxide is observed but the thickness of oxide scale is almost no change. In addition, the observed adhesion between ODS coating layer and substrate was tight and stable throughout the entire oxidation test. The present study has provided strong proof that additive manufacturing has the capability to fabricate structural and protective coating layers for turbine airfoils.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2018;140(6):062401-062401-10. doi:10.1115/1.4038472.

In rotating equipment, thrust bearings aid to balance axial loads and control shaft position. In turbomachinery, axial loads depend on shaft speed and pressure rise/drop on the impellers. This paper details a water-lubricated test rig for measurement of the performance of hydrostatic thrust bearings (HTBs). The rig contains two water-lubricated HTBs (105 mm outer diameter (OD)), one is the test bearing and the other a slave bearing. Both bearings face the outer side of thrust collars of a rotor. The paper shows measurements of HTB axial clearance, flow rate, and recess pressure for operation with increasing static load (max. 1.4 bar) and supply pressure (max. 4.14 bar) at a rotor speed of 3 krpm (12 m/s OD speed). Severe angular misalignment, static and dynamic, of the bearing surface against its collar persisted and affected all measurements. The HTB axial clearance increases as the supply pressure increases and decreases quickly as the applied load increases. The reduction in clearance increases the flow resistance across the film lands, thus reducing the through flow rate with an increase in recess pressure. In addition, an estimated bearing axial stiffness increases as the operating clearance decreases and as the supply pressure increases. Predictions from a bulk flow model qualitatively agree with the measurements. Alas they are not accurate enough. The differences likely stem from the inordinate tilts (static and dynamic) as well as the flow condition. The test HTB operates in a flow regime that spans from laminar to incipient turbulent. Quantification of misalignment at all operating conditions is presently a routine practice during operation of the test rig.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(6):062501-062501-9. doi:10.1115/1.4038284.

High loads and bearing life requirements make journal bearings the preferred choice for use in high-power, planetary gearboxes in jet engines. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each other and create complex kinematic conditions. This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Preliminary modeling considerations are discussed. The findings and conclusions are used to create a three-dimensional (3D), two-component computational fluid dynamics (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a nonorbiting representation, rotating about its own axis, with a circumferentially constant, i.e., concentric, lubricating gap. In order to track the phase interface between the oil and the air, the volume of fluid (VoF) method is used. External journal bearing oil flow is simulated with a number of different mesh densities. Two different operating temperatures, representing low and high viscosity oil, are used to assess the effect on the external flow field behavior. In order to achieve the future objective of creating a design tool for routine use, key areas are identified in which further progress is required.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062502-062502-11. doi:10.1115/1.4038474.

Fast response pressure data acquired in a high-speed 1.5-stage turbine hot gas ingestion rig (HGIR) show the existence of pressure oscillation modes in the rim-seal-wheelspace cavity of a high pressure gas turbine stage with purge flow. The experimental results and observations are complemented by computational assessments of pressure oscillation modes associated with the flow in canonical cavity configurations. The cavity modes identified include shallow cavity modes and Helmholtz resonance. The response of the cavity modes to variation in design and operating parameters are assessed. These parameters include cavity aspect ratio (AR), purge flow ratio, and flow direction defined by the ratio of primary tangential to axial velocity. Scaling the cavity modal response based on computational results and available experimental data in terms of the appropriate reduced frequencies appears to indicate the potential presence of a deep cavity mode as well. While the role of cavity modes on hot gas ingestion cannot be clarified based on the current set of data, the unsteady pressure field associated with turbine rim cavity modal response can be expected to drive ingress/egress.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062503-062503-6. doi:10.1115/1.4038524.

We developed and successfully applied a direct simulation Monte Carlo (MC) scheme to quantify the risk of fracture for heavy-duty rotors commonly used in the energy sector. The developed probabilistic fracture mechanics (FM), high-performance computing methodology, and code ProbFM routinely assess relevant modes of operation for a component by performing billions of individual FM simulations. The methodology can be used for new design and life optimization of components, as well as for the risk of failure RoF quantification of in service rotors and their requalifications in conjunction with nondestructive examination techniques, such as ultrasonic testing (UT). The developed probabilistic scheme integrates material data, UT information, duty-cycle data, and finite element analysis (FEA) in order to determine the RoF. The methodology provides an integrative and robust measure of the fitness for service and allows for a save and reliable operation management of heavy-duty rotating equipment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062504-062504-7. doi:10.1115/1.4038551.

Adaptive lubricants involve binary mixture of synthetic oil and dissolved carbon dioxide (CO2). Unlike conventional lubricant oils, the lubricant viscosity not only varies with the temperature within the bearing but also can be directly adjusted through the CO2 concentration in the system. In this study, we consider the synthetic oil to be fully saturated by CO2 to investigate the maximum impacts of adaptive lubricants on the performance of a hybrid journal bearing. The adaptive lubricant analyzed for this study was the polyalkylene glycol (PAG) oil with low concentration of CO2 (<30%). A three-dimensional (3D) computational fluid dynamic (CFD) model of the bearing was developed and validated against the experimental data. The mixture composition and the resultant mixture viscosity were calculated as a function of pressure and temperature using empirical equations. The simulation results revealed that the viscosity distribution within the PAG/CO2-lubricated bearing is determined primarily by the pressure at the low operating speed. When the speed becomes higher, it is the temperature effect that dominates the viscosity distribution within the bearing. Moreover, the PAG/CO2-lubricated bearing can reduce up to 12.8% power loss than the PAG-lubricated bearing due to the low viscosity of PAG/CO2 mixture. More importantly, we have found that the PAG/CO2 can enhance the load capacity up to 19.6% when the bearing is operating at high-speed conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062505-062505-11. doi:10.1115/1.4038773.

The purpose of this paper is to propose an effective strategy for the design of turbine blades with underplatform dampers (UPDs). The strategy involves damper “pre-optimization,” already proposed by the authors, to exclude, before the blades-coupled nonlinear calculation, all those damper configurations leading to low damping performance. This paper continues this effort by applying pre-optimization to determine a damper configuration which will not jam, roll, or detach under any in-plane platform kinematics (i.e., blade bending modes). Once the candidate damper configuration has been found, the damper equilibrium equations are solved by using both the multiharmonic balance method (MHBM) and the direct-time integration (DTI) for the purpose of finding the correct number of Fourier terms to represent displacements and contact forces. It is shown that contrarily to non-preoptimized dampers, which may display an erratic behavior, one harmonic term together with the static term ensures accurate results. These findings are confirmed by a state-of-the-art code for the calculation of the nonlinear forced response of a damper coupled to two blades. Experimental forced response functions (FRF) of the test case with a nominal damper are available for comparison. The comparison of different damper configurations offers a “high-level” validation of the pre-optimization procedure and highlights the strong influence of the blades mode of vibration on the damper effectiveness. It is shown that the pre-optimized damper is not only the most effective but also the one that leads to a faster and more flexible calculation.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(6):062601-062601-7. doi:10.1115/1.4038246.

The overall efficiency and operational behavior of aircraft engines are influenced by the surface finish of the airfoils. During operation, the surface roughness significantly increases due to erosion and deposition processes. The aim of this study is to analyze the influence of roughness on the aerodynamics of the low-pressure turbine (LPT) of a midsized high bypass turbofan. In order to gain a better insight into the operational roughness structures, a sample of new, used, cleaned, and reworked turbine blades and vanes are measured using the confocal laser scanning microscopy technique. The measurement results show local inhomogeneities. The roughness distributions measured are then converted into their equivalent sand grain roughness ks,eq to permit an evaluation of the impact on aerodynamic losses. The numerical study is performed using the computational fluid dynamics (CFD)-solver turbomachinery research aerodynamics computational environment (TRACE) which was validated before with the existing data from rig experiments. It is observed that the influence of the surface roughness on the turbine efficiency is significant at take-off but negligible at cruise. A detailed analysis on the aerodynamics at take-off shows that very rough airfoils lead to higher profile and secondary loss. Due to the higher disturbances present in flows circulating over rough walls, the transition occurs earlier, and the momentum thickness increases in the turbulent boundary layer. The service-induced roughness structures cause an efficiency drop in the LPT of ηT=0.16% compared to new parts. A gas path analysis showed that this results in an increased fuel flow of Δm˙f=+0.06% and an exhaust gas temperature (EGT) rise of ΔEGT=+1.2K for fixed engine pressure ratio which is equivalent to roughly 4% of the typical EGT margin of a fully refurbished engine. This result stresses the importance of roughness-induced loss in LPTs.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062602-062602-10. doi:10.1115/1.4038482.

The long length of subsea electric submersible pumps (ESPs) requires a large amount of annular seals. Loading caused by gravity and housing curvature changes the static equilibrium position (SEP) of the rotor in these seals. This analysis predicts the SEP due to gravity and/or well curvature loading. The analysis also displays the rotordynamics around the SEP. A static and rotordynamic analysis is presented for a previously studied ESP model. This study differs by first finding the SEP and then performing a rotordynamic analysis about the SEP. Predictions are shown in a horizontal and a vertical orientation. In these two configurations, viscosities and clearances are varied through four cases: 1X 1cP, 3X 1cP, 1X 30cP, and 3X 30cP. In a horizontal, straight-housing position, the model includes gravity and buoyancy on the shaft. At 1cP-1X and 1cP-3X, the horizontal statics, show a moderate eccentricity ratio for the shaft with respect to the housing. With 30cP-1X, the predicted static eccentricity ratio is low at 0.08. With 30cP-3X, the predicted eccentricity ratio increases to 0.33. Predictions for a vertical case of the same model are also presented. The curvature of the housing is varied in the Y–Z plane until rub or close-to-wall rub is expected. The curvature needed for a rub with a 1X 1cP fluid is 7.5 deg of curvature. Curvature has little impact on stability. With both 1X 30cP and 3X 30cP, the maximum curvature for a static rub is over 25 deg of curvature. Both 1X 30cP and 3X 30cP remain unstable with increasing curvature.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062603-062603-8. doi:10.1115/1.4038552.

Rankine and Brayton cycles are common energy conversion cycles and constitute the basis of a significant proportion of global electricity production. Even a seemingly marginal improvement in the efficiency of these cycles can considerably decrease the annual use of primary energy sources and bring a significant gain in power plant output. Recently, supercritical Brayton cycles using CO2 as the working fluid have attracted much attention, chiefly due to their high efficiency. As with conventional cycles, improving the compressor performance in supercritical cycles is major route to increasing the efficiency of the whole process. This paper numerically investigates the flow field and performance of a supercritical CO2 centrifugal compressor. A thermodynamic look-up table is coupled with the flow solver, and the look-up table is systematically refined to take into account the large variation of thermodynamic properties in the vicinity of the critical point. Effects of different boundary and operating conditions are also discussed. It is shown that the compressor performance is highly sensitive to the look-up table resolution as well as the operating and boundary conditions near the critical point. Additionally, a method to overcome the difficulties of simulation close to the critical point is explained.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062604-062604-9. doi:10.1115/1.4038769.

This study analyzes the fluid dynamic characteristics of an ultrasupercritical (USC) high-pressure turbine with additional steam supplied through an overload valve between the second and third stages. The mixing between the main and admission flows causes complex flow phenomena such as swirl and changes of velocity vectors of the main flow. This causes a pressure drop between the second-stage outlet and third-stage inlet, which could potentially affect the performance of the turbine. First, a single-passage computational analysis, which is usually preferred in predicting the performance of multistage turbomachines, was performed using a simple model of an admission flow path and a single passage (SP) for the second and third stages of the turbine. However, the actual flow in the overload valve is supplied through the admission flow path, which has the shape of a casing that circumferentially surrounds the turbine, after flowing in two directions perpendicular to the turbine axis. This necessitates full-passage computational analyses of the two stages and the flow paths of the admission flow. To achieve this, we implemented a full three-dimensional (3D) geometric model of the admission flow path and conducted a full-passage computational analysis for all the flow paths, including those of the second and third stages of the turbine. The focus of analysis was on the pressure drop due to the admission flow. The results of the single and full-passage analyses were compared, and the effects of two different methods were analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062605-062605-6. doi:10.1115/1.4038619.

Modern high-pressure turbine blades operate at high-speed conditions. The over-tip-leakage (OTL) flow can be high-subsonic or even transonic. From the consideration of problem simplification and cost reduction, the OTL flow has been studied extensively in low-speed experiments. It has been assumed a redesigned low-speed blade profile with a matched blade loading should be sufficient to scale the high-speed OTL flow down to the low-speed condition. In this paper, the validity of this conventional scaling approach is computationally examined. The computational fluid dynamics (CFD) methodology was first validated by experimental data conducted in both high- and low-speed conditions. Detailed analyses on the OTL flows at high- and low-speed conditions indicate that, only matching the loading distribution with a redesigned blade cannot ensure the match of the aerodynamic performance at the low-speed condition with that at the high-speed condition. Specifically, the discrepancy in the peak tip leakage mass flux can be as high as 22%, and the total pressure loss at the low-speed condition is 6% higher than the high-speed case. An improved scaling method is proposed hereof. As an additional dimension variable, the tip clearance can also be “scaled” down from the high-speed to low-speed case to match the cross-tip pressure gradient between pressure and suction surfaces. The similarity in terms of the overall aerodynamic loss and local leakage flow distribution can be improved by adjusting the tip clearance, either uniformly or locally.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(6):062801-062801-14. doi:10.1115/1.4038456.

In this study, a numerical study is performed by KIVA–CHEMKIN code to investigate the effects of biodiesel addition and exhaust gas recirculation (EGR) on diesel engine premixed charge compression ignition (PCCI) combustion, performance, and emission characteristics. The studies are performed for neat diesel fuel and mixture of 10–40% biodiesel addition at 67%, 50%, and 40% EGR. For this purpose, a multichemistry surrogate mechanism using methyl decanoate (MD) and methyl-9-decenoate (MD9D) is used. The main innovation of this work is analyzing the chemical, thermodynamic, and dilution effects of biodiesel addition as well as different EGR ratios on PCCI combustion behavior. The results show that the main effect of EGR on PCCI combustion of biodiesel blend is related to the high temperature heat release (HTHR), and its effect on low temperature heat release (LTHR) is low. With increasing biodiesel addition, the role of the chemical effect is increased compared to the thermodynamic and dilution effects. Rate of production analysis (ROPA) indicate that for the different biodiesel ratios, the effect of reaction nC7H16 + HO2 = C7H15-2 + H2O2 is more effective on the start of combustion (SOC) compared to the other reactions. For a defined biodiesel addition, with decreasing EGR, total (unburned) hydrocarbon (THC) and CO are decreased, while NOx and indicated specific fuel consumption (ISFC) are increased.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062802-062802-10. doi:10.1115/1.4038457.

To predict the vibration characteristics of the crankshaft of the larger marine diesel engine accurately and comprehensively, based on the finite element models of the crankshaft and the engine block reduced by a component mode synthesis (CMS) method as well as extended Reynolds equation and Greenwood-Tripp theory, a mixed thermo-elasto-hydro-dynamic lubrication coupling model between a whole flexible engine block and a rotating flexible crankshaft is set up. According to this strongly coupled nonlinear model, the torsional-axial-lateral three-dimensional (3D) vibration of the crankshaft can be calculated simultaneously. The method is verified through comparison with other computational methods. Also, the vibrations are compared under different support models and whether to consider the effect of temperature. Specific 3D vibrations are displayed, and some stage nonlinear characteristics are shown in 3D direction. The modeling method will contribute to reveal the vibration mechanism and optimize the design of the shafting system.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(6):062803-062803-12. doi:10.1115/1.4038469.

Brush seals are a mature technology that has generated extensive experimental and theoretical work. Theoretical models range from simple correlations with experimental results to advanced numerical approaches coupling the bristles deformation with the flow in the brush. The present work follows this latter path. The bristles of the brush are deformed by the pressure applied by the flow, by the interference with the rotor and with the back plate. The bristles are modeled as linear beams but a nonlinear numerical algorithm deals with the interferences. The brush with its deformed bristles is then considered as an anisotropic porous medium for the leakage flow. Taking into account, the variation of the permeability with the local geometric and flow conditions represents the originality of the present work. The permeability following the principal directions of the bristles is estimated from computational fluid dynamics (CFD) calculations. A representative number of bristles are selected for each principal direction and the CFD analysis domain is delimited by periodicity and symmetry boundary conditions. The parameters of the CFD analysis are the local Reynolds number and the local porosity estimated from the distance between the bristles. The variations of the permeability are thus deduced for each principal direction and for Reynolds numbers and porosities characteristic for brush seal. The leakage flow rates predicted by the present approach are compared with experimental results from the literature. The results depict also the variations of the pressures, of the local Reynolds number, of the permeability, and of the porosity through the entire brush seal.

Commentary by Dr. Valentin Fuster

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