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

J. Eng. Gas Turbines Power. 2015;138(1):011501-011501-9. doi:10.1115/1.4031144.

A numerical investigation of a low NOx partially premixed fuel nozzle for heavy-duty gas turbine applications is presented in this paper. Availability of results from a recent test campaign on the same fuel nozzle architecture allowed the exhaustive comparison study presented in this work. At first, an assessment of the turbulent combustion model was carried out, with a critical investigation of the expected turbulent combustion regimes in the system and taking into account the partially premixed nature of the flame due to the presence of diffusion type pilot flames. In particular, the fluent partially premixed combustion model and a flamelet approach are used to simulate the flame. The laminar flamelet database is generated using the flamelet generated manifold (FGM) chemistry reduction technique. Species and temperature are parameterized by mixture fraction and progress variable. Comparisons with calculations with partially premixed model and the steady diffusion flamelet (SDF) database are made for the baseline configuration in order to discuss possible gains associated with the introduced dimension in the FGM database (reaction progress), which makes it possible to account for nonequilibrium effects. Numerical characterization of the baseline nozzle has been carried out in terms of NOx. Computed values for both the baseline and some alternative premixer designs have been then compared with experimental measurements on the reactive test rig at different operating conditions and different split ratios between main and pilot fuel. Numerical results allowed pointing out the fundamental NOx formation processes, both in terms of spatial distribution within the flame and in terms of different formation mechanisms. The obtained knowledge would allow further improvement of fuel nozzle design.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011502-011502-9. doi:10.1115/1.4031179.

This paper uses detailed computational fluid dynamics (CFD) modeling with the kiva-chemkin code to investigate the influence of injection timing, combustion phasing, and operating conditions on combustion instability. Using detailed CFD simulations, a large design of experiments (DOE) is performed with small perturbations in the intake and fueling conditions. A response surface model (RSM) is then fit to the DOE results to predict cycle-to-cycle combustion instability. Injection timing had significant tradeoffs between engine efficiency, emissions, and combustion instability. Near top dead center (TDC) injection timing can significantly reduce combustion instability, but the emissions and efficiency drop close to conventional diesel combustion levels. The fuel split between the two direct injection (DI) injections has very little effect on combustion instability. Increasing exhaust gas recirculation (EGR) rate, while making adjustments to maintain combustion phasing, can significantly reduce peak pressure rise rate (PPRR) variation until the engine is on the verge of misfiring. Combustion phasing has a very large impact on combustion instability. More advanced phasing is much more stable, but produces high PPRRs, higher NOx levels, and can be less efficient due to increased heat transfer losses. The results of this study identify operating parameters that can significantly improve the combustion stability of dual-fuel reactivity-controlled compression ignition (RCCI) engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011503-011503-12. doi:10.1115/1.4031239.

An experimental study is presented on the interaction of flashback originating from flame propagation in the boundary layer (1), from combustion driven vortex breakdown (2) and from low bulk flow velocity (3). In the investigations, an aerodynamically stabilized swirl burner operated with hydrogen–air mixtures at ambient pressure and with air preheat was employed, which previously had been optimized regarding its aerodynamics and its flashback limit. The focus of the present paper is the detailed characterization of the observed flashback phenomena with simultaneous high speed (HS) particle image velocimetry (PIV)/Mie imaging, delivering the velocity field and the propagation of the flame front in the mid plane, in combination with line-of-sight integrated OH*-chemiluminescence detection revealing the flame envelope and with ionization probes which provide quantitative information on the flame motion near the mixing tube wall during flashback. The results are used to improve the operational safety of the system beyond the previously reached limits. This is achieved by tailoring the radial velocity and fuel profiles near the burner exit. With these measures, the resistance against flashback in the center as well as in the near wall region is becoming high enough to make turbulent flame propagation the prevailing flashback mechanism. Even at stoichiometric and preheated conditions this allows safe operation of the burner down to very low velocities of approximately 1/3 of the typical flow velocities in gas turbine burners. In that range, the high turbulent burning velocity of hydrogen approaches the low bulk flow speed and, finally, the flame begins to propagate upstream once turbulent flame propagation becomes faster than the annular core flow. This leads to the conclusions that finally the ultimate limit for the flashback safety was reached with a configuration, which has a swirl number of approximately 0.45 and delivers NOx emissions near the theoretical limit for infinite mixing quality, and that high fuel reactivity does not necessarily rule out large burners with aerodynamic flame stabilization by swirling flows.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011504-011504-11. doi:10.1115/1.4031224.

Several recent cavitation models for the analysis of two-phase flows in diesel injectors with single- and two-fluid modeling approaches have been evaluated, including the Saha–Abu-Ramandan–Li (SAL), Schnerr–Sauer (SS), and Zwart–Gerber–Belamri (ZGB) models. The SAL model is a single-fluid model, while the other two models have been implemented with both single- and two-fluid approaches. Numerical predictions are compared with experimental results available in literature, qualitatively with experimental images of two-phase flow in an optically accessible nozzle, and quantitatively with measured mass flow rates and velocity profiles. It is found that at low injection pressure differentials there can be considerable discrepancy in the predictions of the vapor distribution from the three models considered. This discrepancy is reduced as the injection pressure differential is increased. Implementation of the SS and ZGB models with single- and two- fluid approaches yields noticeable differences in the results because of the relative velocity between the two phases, with two-fluid approach providing better agreement with experimental results. The performance of the SS and ZGB models implemented with the two-fluid approach is comparable with the SAL single-fluid model, but with significantly more computational time. Overall, the SAL single-fluid model performs comparatively better with respect to the other two models.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011505-011505-10. doi:10.1115/1.4031180.

Nitrogen oxides (NOx) emissions from diesel engines can profoundly be suppressed if a portion of exhaust gases is cooled through a heat exchanger known as exhaust gas recirculation (EGR) cooler and returned to the intake of the combustion chamber. One major hurdle though for the efficient performance of EGR coolers is the deposition of various species, i.e., particulate matter (PM) on the surface of EGR coolers. In this study, a model is proposed for the deposition and removal of soot particles carried by the exhaust gases in a tubular cooler. The model takes thermophoresis into account as the primary deposition mechanism. Several removal mechanisms of incident particle impact, shear force, and rolling moment (RM) have rigorously been examined to obtain the critical velocity that is the maximum velocity at which the particulate fouling can profoundly be suppressed. The results show that the dominant removal mechanism changes from one to another based particle size and gas velocity. Based on particle mass and energy conservation equations, a model for the fouling resistance has also been developed which shows satisfactory agreement when compared with the fouling experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011506-011506-9. doi:10.1115/1.4031226.

Alternative fuels for aviation are now a reality. These fuels not only reduce reliance on conventional petroleum-based fuels as the primary propulsion source, but also offer promise for environmental sustainability. While these alternative fuels meet the aviation fuels standards and their overall properties resemble those of the conventional fuel, they are expected to demonstrate different exhaust emissions characteristics because of the inherent variations in their chemical composition resulting from the variations involved in the processing of these fuels. This paper presents the results of back-to-back comparison of emissions characterization tests that were performed using three alternative aviation fuels in a GE CF-700-2D-2 engine core. The fuels used were an unblended synthetic kerosene fuel with aromatics (SKA), an unblended Fischer–Tropsch (FT) synthetic paraffinic kerosene (SPK) and a semisynthetic 50–50 blend of Jet A-1 and hydroprocessed SPK. Results indicate that while there is little dissimilarity in the gaseous emissions profiles from these alternative fuels, there is however a significant difference in the particulate matter emissions from these fuels. These differences are primarily attributed to the variations in the aromatic and hydrogen contents in the fuels with some contributions from the hydrogen-to-carbon ratio of the fuels.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2015;138(1):011701-011701-10. doi:10.1115/1.4031004.

In this paper, an innovative system combining a heat pump (HP) and an organic Rankine cycle (ORC) process is proposed. This system is integrated with a solar roof, which is used as a thermal source to provide heat in winter months (HP mode) and electricity in summer months (ORC mode) when an excess irradiation is available on the solar roof. The main advantage of the proposed unit is its similarity with a traditional HP: the HP/ORC unit only requires the addition of a pump and four-way valves compared to a simple HP, which can be achieved at a low cost. A methodology for the optimal sizing and design of the system is proposed, based on the optimization of both continuous parameters such as heat exchanger size or discrete variables such as working fluid. The methodology is based on yearly simulations, aimed at optimizing the system performance (the net yearly power generation) over its whole operating range instead of just nominal sizing operating conditions. The simulations allow evaluating the amount of thermal energy and electricity generated throughout the year, yielding a net electric power output of 3496 kWh throughout the year.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011702-011702-10. doi:10.1115/1.4031145.

Maritime transportation is a significant contributor to SOx, NOx, and particle matter (PM) emissions, and to a lesser extent, of CO2. Recently, new regulations are being enforced in special geographical areas to limit the amount of emissions from the ships. This fact, together with the high fuel prices, is driving the marine industry toward the improvement of the energy efficiency of ships. Although more sophisticated and complex engine designs can improve significantly of the energy systems on ships, waste heat recovery arises as the most effective technique for the reduction of the energy consumption. In this sense, it is estimated that around 50% of the total energy from the fuel consumed in a ship is wasted and rejected through liquid and gas streams. The primary heat sources for waste heat recovery are the engine exhaust and coolant. In this work, we present a study on the integration of an organic Rankine cycle (ORC) in an existing ship, for the recovery of the main and auxiliary engines (AE) exhaust heat. Experimental data from the engines on the cruise ship M/S Birka Stockholm were logged during a port-to-port cruise from Stockholm to Mariehamn, over a period of 4 weeks. The ship has four main engines (ME) Wärtsilä 5850 kW for propulsion, and four AE 2760 kW which are used for electrical generation. Six engine load conditions were identified depending on the ship's speed. The speed range from 12 to 14 kn was considered as the design condition for the ORC, as it was present during more than 34% of the time. In this study, the average values of the engines exhaust temperatures and mass flow rates, for each load case, were used as inputs for a model of an ORC. The main parameters of the ORC, including working fluid and turbine configuration, were optimized based on the criteria of maximum net power output and compactness of the installation components. Results from the study showed that an ORC with internal regeneration using benzene as working fluid would yield the greatest average net power output over the operating time. For this situation, the power production of the ORC would represent about 22% of the total electricity consumption on board. These data confirmed the ORC as a feasible and promising technology for the reduction of fuel consumption and CO2 emissions of existing ships.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):011703-011703-8. doi:10.1115/1.4031182.

Bechtel Marine Propulsion Corporation (BMPC) is testing a supercritical carbon dioxide (sCO2) Brayton system at the Bettis Atomic Power Laboratory. The integrated system test (IST) is a simple recuperated closed Brayton cycle with a variable-speed turbine-driven compressor and a constant-speed turbine-driven generator using sCO2 as the working fluid designed to output 100 kWe. The main focus of the IST is to demonstrate operational, control, and performance characteristics of an sCO2 Brayton power cycle over a wide range of conditions. Therefore, the IST was designed to operate in a configuration and at conditions that support demonstrating the controllability of the closed sCO2 Brayton cycle. Operating at high system efficiency and meeting a specified efficiency target are not requirements of the IST. However, efficiency is a primary driver for many commercial applications of sCO2 power cycles. This paper uses operational data to evaluate component off-nominal performance and predict that design system operation would be achievable.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Marine

J. Eng. Gas Turbines Power. 2015;138(1):012201-012201-9. doi:10.1115/1.4031170.

Coolant is one of the important factors affecting the overall performance of the intercooler for the intercooled (IC) cycle marine gas turbine. Conventional coolants, such as water and ethylene glycol, have lower thermal conductivity which can hinder the development of highly effective compact intercooler. Nanofluids that consist of nanoparticles and base fluids have superior properties like extensively higher thermal conductivity and heat transfer performance compared to those of base fluids. This paper focuses on the application of two different water-based nanofluids containing aluminum oxide (Al2O3) and copper (Cu) nanoparticles in IC cycle marine gas turbine intercooler. The effectiveness-number of transfer unit method is used to evaluate the flow and heat transfer performance of intercooler, and the thermophysical properties of nanofluids are obtained from literature. Then, the effects of some important parameters, such as nanoparticle volume concentration, coolant Reynolds number, coolant inlet temperature, and gas side operating parameters on the flow and heat transfer performance of intercooler, are discussed in detail. The results demonstrate that nanofluids have excellent heat transfer performance and need lower pumping power in comparison with base fluids under different gas turbine operating conditions. Under the same heat transfer, Cu–water nanofluids can reduce more pumping power than Al2O3–water nanofluids. It is also concluded that the overall performance of intercooler can be enhanced when increasing the nanoparticle volume concentration and coolant Reynolds number and decreasing the coolant inlet temperature.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2015;138(1):012301-012301-8. doi:10.1115/1.4031142.

The bearing system of a turbocharger has to keep the rotor in the specified position and thus has to withstand the rotor forces that result from turbocharger operation. Hence, its components need to be designed in consideration of the bearing loads that have to be expected. The applied bearing system design also has significant influence on the overall efficiency of the turbocharger and impacts the performance of the combustion engine. It has to ideally fulfill the trade-off between bearing friction and load capacity. For example, the achievable engine’s low end-torque is reduced, if the bearing system produces more friction losses than inherently unavoidable for safe and durable operation because a higher portion of available turbine power needs to be employed to compensate bearing losses instead of providing boost pressure. Moreover, also transient turbocharger rotor speed up can be compromised and hence the response of the turbocharged combustion engine to a load step becomes less performant than it could be. Besides the radial bearings, the thrust bearing is a component that needs certain attention. It can already contribute to approximately 30% of the overall bearing friction, even if no load is applied and this portion further increases under thrust load. It has to withstand the net thrust load of the rotor under all operating conditions resulting from the superimposed aerodynamic forces that the compressor and the turbine wheel produce. A challenge for the determination of the thrust forces appearing on engine is the nonsteady loading under pulsating conditions. The thrust force will alternate with the pulse frequency over an engine cycle, which is caused by both the engine exhaust gas pressure pulses on the turbine stage and—to a smaller amount—the nonsteady compressor operation due to the reciprocating operation of the cylinders. The conducted experimental investigations on the axial rotor motion as well as the thrust force alternations under on-engine conditions employ a specially prepared compressor lock nut in combination with an eddy-current sensor. The second derivative of this signal can be used to estimate the occurring thrust force changes. Moreover, a modified thrust bearing—equipped with strain gauges—was used to cross check the results from position measurement and thrust force modeling. All experimental results are compared with an analytical thrust force model that relies on the simultaneously measured, crank angle resolved pressure signals before and after the compressor and turbine stage. The results give insight into the axial turbocharger rotor oscillations occurring during an engine cycle for several engine operating points. Furthermore, they allow a judgment of the accuracy of thrust force modeling approaches that are based on measured pressures.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2015;138(1):012501-012501-6. doi:10.1115/1.4031155.

Testing and simulation of aero-engine spectra with dwell times are reported in this paper. The modeling concept used is built on linear elastic fracture mechanics (LEFM) and provides a history-dependent evolution description of dwell damage and its interaction with cyclic load. The simulations have been carried out for three spectra: (1) cyclic loads, (2) combined sustained load and cyclic loads, and (3) slow load ramps and cyclic loads, all for surface cracks at 550 °C for Inconel 718. All simulations show reasonable good agreement with experimental results. Prediction of multiple tests of several batches is also provided to show statistical scatter.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012502-012502-7. doi:10.1115/1.4031157.

In this paper, scatter in crack growth for dwell time loadings in combination with overloads has been investigated. Multiple tests were performed for surface cracks at 550 °C in the commonly used high temperature material Inconel 718. The test specimens originate from two different batches which also provide for a discussion of how material properties affect the dwell time damage and overload impact. In combination with these tests, an investigation of the microstructure was also carried out, which shows how it influences the growth rate. The results from this study show that, in order to take overloads into consideration when analyzing spectrum loadings containing dwell times, one needs a substantial amount of material data available as the scatter seen from one batch to the other are of significant proportions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012503-012503-7. doi:10.1115/1.4031158.

Thermomechanical fatigue (TMF) crack growth modeling has been conducted on Inconel 718 with dwell time at maximum load. A history dependent damage model taking dwell damage into account, developed under isothermal conditions, has been extended for TMF conditions. Parameter determination for the model is carried out on isothermal load controlled tests at 550–650 °C for surface cracks, which later have been used to extrapolate parameters used for TMF crack growth. Further, validation of the developed model is conducted on a notched specimen subjected to strain control at 50–550 °C. Satisfying results are gained within reasonable scatter level compared for test and simulated number of cycles to failure.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012504-012504-11. doi:10.1115/1.4031203.

A novel nested compression spring gas foil bearing (NSFB), which used a series of nested compression springs as compliant supporting structure, was proposed and designed. NSFBs can be easily manufactured and are able to provide a support with high stiffness, high damping, and tunable structural characterization in turbomachinery. An analytical model, which considered the effects of interaction and friction between adjacent springs, was established to predict the structural characterization of the compliant structure. Static and dynamic tests were conducted to analyze the structural performance of NSFBs. The predicted hysteresis loop of the compliant structure corresponded well with the measured results from the pull–push tests. A static test result comparison between an NSFB and a bump-type gas foil bearing (BFB) showed that the NSFB had a larger loss factor, which implied its superior damping performance. The effects of spring number and axial preload between adjacent springs on bearing performance were investigated. The static and dynamic loss factors of bearings with nested structures (47 and 39 springs) were similar to each other, but greater than the loss factor of bearings without nested structures (31 springs). The estimated static and dynamic loss factors of bearings with axial preload were significantly improved compared with bearings without axial preload.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012505-012505-8. doi:10.1115/1.4031238.

High speed rotors supported on bump-type foil bearings (BFBs) often suffer from large subsynchronous whirl motions. Mechanically preloading BFBs through shimming is a common, low cost practice that shows improvements in rotordynamic stability. However, there is an absence of empirical information related to the force coefficients (structural and rotordynamic) of shimmed BFBs. This paper details a concerted study toward assessing the effect of shimming on a first generation BFB (L = 38.1 mm and D = 36.5 mm). Three metal shims, 120 deg apart, are glued to the inner surface of the bearing cartridge and facing the underside of the bump foil strip. The shim sets are of identical thickness, either 30 μm or 50 μm. In static load tests, a bearing with shims shows a (nonlinear) structural stiffness larger than for the bearing without shims. Torque measurements during shaft acceleration also demonstrate a shimmed BFB has a larger friction coefficient. For a static load of 14.3 kPa, dynamic loads with a frequency sweep from 250 Hz to 450 Hz are exerted on the BFB, without and with shims, to estimate its rotordynamic force coefficients while operating at ∼50 krpm (833 Hz). Similar measurements are conducted without shaft rotation. Results are presented for the original BFB (without shims) and the two shimmed BFB configurations. The direct stiffnesses of the BFB, shimmed or not, increase with excitation frequency, thus evidencing a mild hardening effect. The BFB stiffness and damping coefficients decrease slightly for operation with rotor speed as opposed to the coefficients when the shaft is stationary. For frequencies above 300 Hz, the direct damping coefficients of the BFB with 50 μm thick shims are ∼30% larger than the coefficients of the original bearing. The bearing structural loss factor, a measure of its ability to dissipate mechanical energy, is derived from the direct stiffness and damping coefficients. The BFB with 50 μm thick shims has a 25% larger loss factor—average from test data collected at 300 Hz to 400 Hz—than the original BFB. Further measurements of rotor motions while the shaft accelerates to ∼50 krpm demonstrate the shimmed BFB (thickest shim set) effectively removes subsynchronous whirl motions amplitudes that were conspicuous when operating with the original bearing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012506-012506-7. doi:10.1115/1.4031240.

Oxidation performance of thermal barrier coatings (TBCs) deposited by the axial suspension plasma spraying (ASPS) method was evaluated under isothermal and cyclic conditions with a peak temperature of 1080 °C. The TBC systems are based on two nickel-based superalloy substrates (CMSX-4 and IN738LC), platinum aluminide bond coat (BC), and yttria-stabilized zirconia (8YSZ) top coat (TC) of either vertically cracked (VC) or columnar structure. Samples with IN738LC substrate exhibited longer isothermal oxidation lives whereas the ones with CMSX-4 substrate showed greater cyclic oxidation lives. Outward diffusion of W and Ta in TBC systems containing CMSX-4 was found to have progressed to the interface between thermally grown oxide (TGO) and TC; this has contributed to the reduced isothermal oxidation life. The longer cyclic oxidation lives of TBC systems with CMSX-4 were attributed to less coefficient and thermal expansion (CTE) mismatch between coating layers (reduced strain energy) and better creep resistance of diffusional BC on CMSX-4, hence less TGO rumpling. TBC systems with columnar YSZ had longer isothermal oxidation lives while those with VC YSZ seemed to result in longer cyclic lives.

Topics: oxidation , Coatings
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012507-012507-10. doi:10.1115/1.4031343.

The ability to quantify leakage flow and windage heating for labyrinth seals with honeycomb lands is critical in understanding gas turbine engine system performance and predicting its component life. Variety of labyrinth seal configurations (number of teeth, stepped or straight, honeycomb cell size) are in use in gas turbines, and for each configuration, there are many geometric factors that can impact a seal's leakage and windage characteristics. This paper describes the development of a numerical methodology aimed at studying the effect of honeycomb lands on leakage and windage heating. Specifically, a three-dimensional computational fluid dynamics (CFD) model is developed utilizing commercial finite volume-based software incorporating the renormalization group (RNG) k-ε turbulence model with modified Schmidt number. The modified turbulence model is benchmarked and fine-tuned based on several experiments. Using this model, a broad parametric study is conducted by varying honeycomb cell size, pressure ratio (PR), and radial clearance for a four-tooth straight-through labyrinth seal. The results show good agreement with available experimental data. They further indicate that larger honeycomb cells predict higher seal leakage and windage heating at tighter clearances compared to smaller honeycomb cells and smooth lands. However, at open seal clearances larger honeycomb cells have lower leakage compared to smaller honeycomb cells.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2015;138(1):012601-012601-7. doi:10.1115/1.4031223.

Accounting for the impact of uncaptured particles that cause compressor fouling and subsequently performance degradation when a filter system is in place is often ignored when evaluating the performance of filtration systems. Too often, the emphasis is on capture efficiency and the corresponding differential pressure loss, which are important aspects, however only constitutes a part of the overall impact on the engine performance. The main aim of this study is a first step to quantify the loss that is attributed to compressor fouling by the uncaptured particles, identify a threshold point for which further increase in pressure losses (increasing capture efficiency) no longer yields further increases in fouling levels, and subsequently investigate these respective losses and total losses in a reference high efficiency system (HES) and a hypothetical low efficiency system. Corrected operational data from a 268 MW gas turbine engine were used to evaluate the levels of degradation in the engine at different power settings. With the measured filter media pressure loss during operation and turbomatch (an in-house gas turbine performance simulation software), the impact of power reduction due to pressure loss of the filter was accounted for in the total estimated losses due to engine degradation. That of fouling was calculated based on applicable assumptions, while deducting the loss due to filtration systems from the total loss due to degradation. The study shows the inverse relationship between fouling effects and filter pressure losses as expected. More importantly, it indicates that the higher efficiency system performs better than the low efficiency system, notwithstanding the more dominant impact of higher differential pressure losses. It was also observed that the threshold where fouling effects are zero or negligible is around 800 Pa at high power setting and 600 Pa at lower power setting. In general, for all forms of the degradation using the engine data and simulation software, it is observed that at lower power settings, the impact on the engine is a lot more severe in a single-shaft constant speed operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012602-012602-9. doi:10.1115/1.4031204.

This paper aims to present an integrated rotorcraft (RC) multidisciplinary simulation framework, deployed for the comprehensive assessment of combined RC–powerplant systems at mission level. The proposed methodology comprises a wide-range of individual modeling theories applicable to RC performance and flight dynamics, as well as the gas turbine engine performance. The overall methodology has been deployed to conduct a preliminary tradeoff study for a reference simple cycle (SC) and conceptual regenerative twin-engine-light (TEL) and twin-engine-medium (TEM) RC configurations, modeled after the Airbus Helicopters Bo105 and Aérospatiale SA330 models, simulated under the representative mission scenarios. The installed engines corresponding to both reference RC are notionally modified by incorporating a heat exchanger (HE), enabling heat transfer between the exhaust gas and the compressor delivery air to the combustion chamber. This process of preheating the compressor delivery air prior to combustion chamber leads to a lower fuel input requirements compared to the reference SC engine. The benefits arising from the adoption of the on-board HE are first presented by conducting part-load performance analysis against the reference SC engine. The acquired results suggest substantial reduction in specific fuel consumption (SFC) for a major part of the operating power range with respect to both RC configurations. The study is further extended to quantify mission fuel burn (MFB) saving limit by conducting an extensive HE tradeoff analyses at mission level. The optimum fuel burn saving limit resulting from the incorporation of on-board HEs is identified within realistically defined missions, corresponding to modern RC operations. The acquired results from the mission analyses tradeoff study suggest that the suboptimum regenerated RC configurations are capable of achieving significant reduction in MFB, while simultaneously maintaining the respective airworthiness requirements in terms of one-engine-inoperative. The proposed methodology can effectively be regarded as an enabling technology for the comprehensive assessment of conventional and conceptual RC–powerplant systems at mission level.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012603-012603-14. doi:10.1115/1.4031205.

In literature, there are some studies related to the fouling phenomena in transonic compressors, but, in industrial applications (heavy-duty compressor, pumping stations, etc.) the subsonic compressors are widespread. It is of great interest to the manufacturer to discover the fouling phenomenon related to this type of compressor. This paper presents three-dimensional numerical simulations of the microparticle ingestion on a subsonic axial compressor rotor carried out by means of a commercial computational fluid dynamic code. Particle trajectory simulations use a stochastic Lagrangian tracking method that solves the equations of motion separate from the continuous phase. The number of particles, sizes, and concentrations are specified in order to perform a quantitative analysis of the particle impact on the blade surface. In this paper, the particle impact pattern and the kinematic characteristics (velocity and angle) of the impact are shown. Both of the blade zones affected by particle impact and the blade zones affected by particle deposition are analyzed. The particle deposition is established by using the quantity called sticking probability (SP). The SP links the kinematic characteristics of particle impact on the blade with fouling phenomenon. The results show that microparticles tend to follow the flow by impacting at full span with a higher impact concentration on the leading edge (LE). The suction side (SS) is affected only close to the LE and, at the hub, close to the trailing edge (TE). Particular fluid-dynamic phenomena such as separation, stagnation, and tip leakage vortex strongly influence the impact location of the particles. The kinematic analysis showed a high tendency of particle adhesion on the SS, especially for smaller particles for which the fluid dynamic phenomena play a key role regarding particle impact velocity and angle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2015;138(1):012604-012604-10. doi:10.1115/1.4031206.

Solid particle ingestion is one of the principal degradation mechanisms in the compressor section of heavy-duty gas turbines. Usually, foulants in the ppm range, not captured by the air filtration system, i.e., (0–2) μm cause deposits on blading and result in a severe performance drop of the compressor. It is of great interest to the industry to determine which areas of the compressor airfoils are interested by these contaminants as a function of the location of the power unit. The aim of this work is the estimation of the actual deposits on the blade surface in terms of location and quantity. The size of the particles, their concentrations, and the filtration efficiency are specified in order to perform a realistic quantitative analysis of the fouling phenomena in an axial compressor. This study combines, for the first time, the impact/adhesion characteristic of the particles obtained through a computational fluid dynamics (CFD) and the real size distribution of the contaminants in the air swallowed by the compressor. The blade zones affected by the deposits are clearly reported by using easy-to-use contaminant maps realized on the blade surface in terms of contaminant mass. The analysis showed that particular fluid-dynamic phenomena such as separation, shock waves, and tip leakage vortex strongly influence the pattern deposition. The combination of the smaller particles (0.15 μm) and the larger ones (1.50 μm) determines the highest amounts of deposits on the leading edge (LE) of the compressor airfoil. From these analyses, some guidelines for proper installation and management of the power plant (in terms of filtration systems and washing strategies) can be drawn.

Commentary by Dr. Valentin Fuster

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