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

J. Eng. Gas Turbines Power. 2018;140(7):071501-071501-13. doi:10.1115/1.4038460.

It has been recognized in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines. Instances of damage, surge, and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that solid ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. Development of analytical and empirical models of the ice crystal icing phenomenon is necessary for both future engine design and this-generation engine certification. A comprehensive model will require the integration of a number of aerodynamic, thermodynamic, and mechanical components. This paper studies one such component, specifically the thermodynamic and mechanical processes experienced by ice particles impinging on a warm surface. Results are presented from an experimental campaign using a heated and instrumented flat plate. The plate was installed in the Altitude Icing Wind Tunnel (AIWT) at the National Research Council of Canada (NRC). This facility is capable of replicating ice crystal conditions at altitudes up to 9 km and Mach numbers up to 0.55. The heated plate is designed to measure the heat flux from a surface at temperatures representative of the early core compressor, under varying convective and icing heat loads. Heat transfer enhancement was observed to rise approximately linearly with both total water content (TWC) and particle diameter over the ranges tested. A Stokes number greater than unity proved to be a useful parameter in determining whether heat transfer enhancement would occur. A particle energy parameter was used to estimate the likelihood of fragmentation. Results showed that when particles were both ballistic and likely to fragment, heat transfer enhancement was independent of both Mach and Reynolds numbers over the ranges tested.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071502-071502-9. doi:10.1115/1.4038523.

To understand the physics of volcanic ash impact on gas turbine hot-components and develop much-needed tools for engine design and fleet management, the behaviors of volcanic ash in a gas turbine combustor and nozzle guide vanes (NGV) have been numerically investigated. High-fidelity numerical models are generated, and volcanic ash sample, physical, and thermal properties are identified. A simple critical particle viscosity—critical wall temperature model is proposed and implemented in all simulations to account for ash particles bouncing off or sticking on metal walls. The results indicate that due to the particle inertia and combustor geometry, the volcanic ash concentration in the NGV cooling passage generally increases with ash size and density, and is less sensitive to inlet velocity. It can reach three times as high as that at the air inlet for the engine conditions and ash properties investigated. More importantly, a large number of the ash particles entering the NGV cooling chamber are trapped in the cooling flow passage for all four turbine inlet temperature conditions. This may reveal another volcanic ash damage mechanism originated from engine cooling flow passage. Finally, some suggestions are recommended for further research and development in this challenging field. To the best of our knowledge, it is the first study on detailed ash behaviors inside practical gas turbine hot-components in the open literature.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2018;140(7):071701-071701-10. doi:10.1115/1.4038362.

This work presents an exergy analysis and performance assessment of three recuperative thermodynamic cycles for gas turbine applications. The first configuration is the conventional recuperative (CR) cycle in which a heat exchanger is placed after the power turbine (PT). In the second configuration, referred as alternative recuperative (AR) cycle, a heat exchanger is placed between the high pressure and the PT, while in the third configuration, referred as staged heat recovery (SHR) cycle, two heat exchangers are employed, the primary one between the high and PTs and the secondary at the exhaust, downstream the PT. The first part of this work is focused on a detailed exergetic analysis on conceptual gas turbine cycles for a wide range of heat exchanger performance parameters. The second part focuses on the implementation of recuperative cycles in aero engines, focused on the MTU-developed intercooled recuperative aero (IRA) engine concept, which is based on a conventional recuperation approach. Exergy analysis is applied on specifically developed IRA engine derivatives using both alternative and SHR recuperation concepts to quantify energy exploitation and exergy destruction per cycle and component, showing the amount of exergy that is left unexploited, which should be targeted in future optimization actions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071702-071702-8. doi:10.1115/1.4038476.

For the concentrating solar power (CSP) applications, the supercritical carbon dioxide (s-CO2) power cycle is beneficial in many aspects, including high cycle efficiencies, reduced component sizing, and potential for the dry cooling option. More research is involved in improving this technology to realize the s-CO2 cycle as a candidate to replace the conventional power conversion systems for CSP applications. In this study, an isothermal compressor, a turbomachine which undergoes the compression process at constant temperature to minimize compression work, is applied to the s-CO2 power cycle layout. To investigate the cycle performance changes of adopting the novel technology, a framework for defining the efficiency of the isothermal compressor is revised and suggested. This study demonstrates how the compression work for the isothermal compressor is reduced, up to 50%, compared to that of the conventional compressor under varying compressor inlet conditions. Furthermore, the simple recuperated and recompression Brayton cycle layouts using s-CO2 as a working fluid are evaluated for the CSP applications. Results show that for compressor inlet temperatures (CIT) near the critical point, the recompression Brayton cycle using an isothermal compressor has 0.2–1.0% point higher cycle thermal efficiency compared to its reference cycle. For higher CIT values, the recompression cycle using an isothermal compressor can perform above 50% in thermal efficiency for a wider range of CIT than the reference cycle. Adopting an isothermal compressor in the s-CO2 layout can imply larger heat exchange area for the compressor which requires further development.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(7):072501-072501-13. doi:10.1115/1.4038542.

A finite element (FE) model of the rotor tester of an aero-engine, having a thin-walled casing structure, mounted with the way of an actual engine, is developed to simulate the intrinsic vibration characteristics under actual engine-mounting condition. First, a modal experiment of the rotor tester for the whole aero-engine is conducted, and the FE model is modified and validated based on the modal experimental results. Second, the first three orders of natural frequencies and the modal shapes are evaluated using the modified FE model under three different types of mounting stiffness, namely, a fixed mounting boundary, a free mounting boundary, and a flexible mounting boundary. Subsequently, the influences of the mounting stiffness on the coupling vibration of the rotor and stator are studied via a new rotor–stator coupling factor, which is proposed in this study. The results show that the higher the rotor–stator coupling degree of the modal shape, the greater the influence of the mounting condition on the modal shape. Moreover, the influence of the mounting stiffness on the rotor–stator coupling degree is nonlinear. The coupling phenomena of the rotor and stator exist in many modal shapes of actual large turbofan engines, and the effect of mounting stiffness on the rotor–stator coupling cannot be ignored. Hence, the mounting stiffness needs to be considered carefully while modeling the whole aero-engine and simulating the dynamic characteristics of the whole aero-engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072502-072502-10. doi:10.1115/1.4038549.

Achieving an optimal design of journal bearings is a very challenging effort due to the many input and output variables involved, including rotordynamic and tribological responses. This paper demonstrates the use of a multivariate response modeling approach based on response surface design of experiments (RSDOE) to design tilting pad bearings. It is shown that an optimal configuration can be achieved in the early stages of the design process while substantially reducing the amount of calculations. To refine the multivariate response model, statistical significance of the factors was assessed by examining the test's p-value. The effect coefficient calculation complemented the statistical hypothesis testing as an overall quantitative measure of the strength of factors, namely; main effects, quadratic effects, and interactions between variables. This provided insight into the potential nonlinearity of the phenomena. Once arriving at an optimized design, a sensitivity analysis was performed to identify the input variables whose variabilities have the greatest influence on the mean of a given response. Finally, an analysis of percent contribution of each input variable standard deviation to the actual response standard deviation was performed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072503-072503-10. doi:10.1115/1.4038603.

High-speed foil bearings are currently used in increasingly demanding, high performance applications. The application under consideration is a 120 krpm natural gas turboexpander-compressor, which requires 38 mm (1.5 in.) foil journal bearings with high stiffness and load capacity to help enhance rotordynamic stability. This paper describes the development of the foil bearing for this application and includes measured stiffness and damping coefficients recorded on a high-speed dynamic bearing test rig. The dynamic test data were taken for several different foil bearing configurations with varying spring-element foil thicknesses, number of spring-element foils, and bearing shim thickness. All three parameters have a direct impact on bearing clearance. The influence of these different parameters on measured stiffness and damping coefficients and thermal performance of the bearings are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072504-072504-9. doi:10.1115/1.4038458.

Engine designers require accurate predictions of ingestion (or ingress) principally caused by circumferential pressure asymmetry in the mainstream annulus. Cooling air systems provide purge flow designed to limit metal temperatures and protect vulnerable components from the hot gases which would otherwise be entrained into disk cavities through clearances between rotating and static disks. Rim seals are fitted at the periphery of these disks to minimize purge. The mixing between the efflux of purge (or egress) and the mainstream gases near the hub end-wall results in a deterioration of aerodynamic performance. This paper presents experimental results using a turbine test rig with wheel-spaces upstream and downstream of a rotor disk. Ingress and egress was quantified using a CO2 concentration probe, with seeding injected into the upstream and downstream sealing flows. The probe measurements have identified an outer region in the wheel-space and confirmed the expected flow structure. For the first time, asymmetric variations of concentration have been shown to penetrate through the seal clearance and the outer portion of the wheel-space between the disks. For a given flow coefficient in the annulus, the concentration profiles were invariant with rotational Reynolds number. The measurements also reveal that the egress provides a film-cooling benefit on the vane and rotor platforms. Further, these measurements provide unprecedented insight into the flow interaction and provide quantitative data for computational fluid dynamics (CFD) validation, which should help to reduce the use of purge and improve engine efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072505-072505-10. doi:10.1115/1.4038613.

Current efforts to model multistage turbomachinery systems rely on calculating independent constraint modes for each degree-of-freedom (DOF) on the boundary between stages. While this approach works, it is computationally expensive to calculate all the required constraint modes. This paper presents a new way to calculate a reduced set of constraint modes referred to as Fourier constraint modes (FCMs). These FCMs greatly reduce the number of computations required to construct a multistage reduced order model (ROM). The FCM method can also be integrated readily with the component mode mistuning (CMM) method to handle small mistuning and the pristine rogue interface modal expansion (PRIME) method to handle large and/or geometric mistuning. These methods all use sector-level models and calculations, which make them very efficient. This paper demonstrates the efficiency of the FCM method on a multistage system that is tuned and, for the first time, creates a multistage ROM with large mistuning using only sector-level quantities and calculations. The results of the multistage ROM for the tuned and large mistuning cases are compared with full finite element results and are found in good agreement.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072506-072506-8. doi:10.1115/1.4038880.

Fracture of blades is usually catastrophic and creates serious damages in the turbomachines. Blades are subjected to high centrifugal force, oscillating stresses, and high temperature which makes their life limited. Therefore, blades should be checked and replaced at specified intervals or utilize a health monitoring method for them. Crack detection by nondestructive tests can only be performed during machine overhaul which is not suitable for monitoring purposes. Blade tip timing (BTT) method as a noncontact monitoring technique is spreading for health monitoring of the turbine blades. One of the main challenges of BTT method is identification of vibration parameters from one per revolution samples which is quite below Nyquist sampling rate. In this study, a new method for derivation of blade asynchronous vibration parameters from BTT data is proposed. The proposed method requires only two BTT sensors and applies least mean square algorithm to identify frequency and amplitude of blade vibration. These parameters can be further used as blade health indicators to predict defect growth in the blades. Robustness of the proposed method against measurement noise which is an important factor has been examined by numerical simulation. An experimental test was conducted on a bladed disk to show efficiency of the proposed method.

Topics: Vibration , Blades , Sensors
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072507-072507-10. doi:10.1115/1.4038361.

In gas turbines, rim seals are fitted at the periphery of stator and rotor discs to minimize the purge flow required to seal the wheel-space between the discs. Ingestion (or ingress) of hot mainstream gases through rim seals is a threat to the operating life and integrity of highly stressed components, particularly in the first-stage turbine. Egress of sealing flow from the first-stage can be re-ingested in downstream stages. This paper presents experimental results using a 1.5-stage test facility designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disk. Re-ingestion was quantified using measurements of CO2 concentration, with seeding injected into the upstream and downstream sealing flows. Here, a theoretical mixing model has been developed from first principles and validated by the experimental measurements. For the first time, a method to quantify the mass fraction of the fluid carried over from upstream egress into downstream ingress has been presented and measured; it was shown that this fraction increased as the downstream sealing flow rate increased. The upstream purge was shown to not significantly disturb the fluid dynamics but only partially mixes with the annulus flow near the downstream seal, with the ingested fluid emanating from the boundary layer on the blade platform. From the analogy between heat and mass transfer, the measured mass-concentration flux is equivalent to an enthalpy flux, and this re-ingestion could significantly reduce the adverse effect of ingress in the downstream wheel-space. Radial traverses using a concentration probe in and around the rim seal clearances provide insight into the complex interaction between the egress, ingress and mainstream flows.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(7):072801-072801-12. doi:10.1115/1.4038543.

Modern diesel engines equip the exhaust gas recirculation (EGR) system because it can suppress NOx emissions effectively. However, since a large amount of exhaust gas might cause the degradation of drivability, the control strategy of EGR system is crucial. The conventional control structure of the EGR system uses the mass air flow (MAF) as a control indicator, and its set-point is determined from the well-calibrated look-up table (LUT). However, this control structure cannot guarantee the optimal engine performance during acceleration operating conditions because the MAF set-point is calibrated at steady operating conditions. In order to optimize the engine performance with regard to NOx emission and drivability, an optimization algorithm in a function of the intake oxygen fraction (IOF) is proposed because the IOF directly affects the combustion and engine emissions. Using the NOx and drivability models, the cost function for the performance optimization is designed and the optimal value of the IOF is determined. Then, the MAF set-point is adjusted to trace the optimal IOF under engine acceleration conditions. The proposed algorithm is validated through scheduled engine speeds and loads to simulate the extra-urban driving cycle of the European driving cycle. As validation results, the MAF is controlled to trace the optimal IOF from the optimization method. Consequently, the NOx emission is substantially reduced during acceleration operating conditions without the degradation of drivability.

Commentary by Dr. Valentin Fuster

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