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

J. Eng. Gas Turbines Power. 2017;139(6):061501-061501-9. doi:10.1115/1.4035201.

Thermoacoustic instabilities pose a major threat to modern gas turbines. The use of acoustic dampers, like Helmholtz resonators, has proven useful for the mitigation of such instabilities. However, assessing the effect of acoustic dampers on thermoacoustic modes in annular combustion chambers remains an intricate task. This results from the implicit nature of the thermoacoustic Helmholtz equation associated with the high number of possible parameter values for the positioning of the dampers and their impedance design. In the present work, the principal challenges of the effective placement and the design of the impedance of acoustic dampers in annular chambers are discussed. This includes the choice of an appropriate objective function for the optimization, the combinatorial challenges when dealing with different possible damper arrangements, and the numerical complexities when using the thermoacoustic Helmholtz equation to approach this issue. As a key aspect, the paper proposes a new adjoint-based approach to tackle these problems. The new algorithm establishes algebraic models that predict the effect of acoustic dampers on the growth rates of the thermoacoustic modes. The theory is exemplified on the basis of a generic annular combustor model with 12 burners.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):061502-061502-7. doi:10.1115/1.4035231.

In this experimental study, we investigate the impact of transverse acoustic velocity fluctuations on flame response to axial velocity fluctuations. Such a scenario where a flame is under the influence of a 2D acoustic field occurs in annular gas turbine combustors during thermoacoustic instability. A generic premixed swirl flame is exposed to simultaneous transverse and axial acoustic forcing. The amplitude of axial forcing was kept constant, while the amplitude and relative phase (with respect to axial forcing) of the transverse forcing was systematically varied. Results obtained indicate that transverse velocity affects flame response, and that both the magnitude of transverse velocity and its phase with respect to axial forcing are important factors.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):061503-061503-9. doi:10.1115/1.4035660.

This paper presents experimental study on self-excited combustion instability characteristics of premixed low-swirl flames in a multinozzle can combustor with counterswirl and coswirl arrays. Experiments were carried out over a wide range of inlet velocity from 4 m/s to 15.5 m/s and equivalence ratio from 0.5 to 0.85. Phase-locked OH planar laser-induced fluorescence was employed to measure flame shape and identify heat release rate. Four operation regions: stable combustion region, unstable combustion region, flashback region, and extinguish region are observed for both array burners. The amplitude of pressure fluctuation for counterswirl arrangement is less than the coswirl array, and the stable operating window of the counterswirl array is wider. In the unstable combustion region, the counterswirl flame triggers the 2L mode of the combustion system, while the coswirl flame incites three longitudinal modes with the highest amplitude near 3L. Rayleigh index distribution reveals neighboring flame interaction results in thermoacoustic coupling for multinozzle flames. Additionally, for the counterswirl array, thermoacoustic couplings also exit in the flame base region and shear region while, for the coswirl array, the instability driving zones also locate at the lip region and the tail of center flame which is totally different with counterswirl flame.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2017;139(6):061701-061701-9. doi:10.1115/1.4035289.

A simulation environment allowing steady state and transient modeling is used for assessing several gas turbine based cycles proposed for solar hybridization. First, representative open cycle gas turbine configurations, namely, (a) single shaft (SS), (b) recuperated single-shaft, (c) twin shaft (TS), and (d) two-spool three-shaft, intercooled, recuperated, are evaluated. The importance of design point selection in terms of solar share value is highlighted. Solar steam injection gas turbine cycle (STIG) alternatives, namely, solar steam only and solar/fuel gas steam, are then assessed. Finally, the concept of a dual fluid receiver (DFR) for exploiting the rejected solar power by producing steam during sunny hours with high irradiation is demonstrated. The effects of hybridization on performance and operability are established and evaluated. Solarization effect on performance is estimated in terms of annual produced power and fossil fuel savings. The results indicate that the spool arrangement affects the suitability of a gas turbine for hybridization. Recuperated configurations performed better for the design constrains imposed by current technology solar parts. Solar steam injection is a promising solution for retrofitted fuel-only and conventional STIG engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):061702-061702-9. doi:10.1115/1.4035646.

The dynamic behavior of a solid oxide fuel cell gas turbine hybrid system (SOFC/GT) from both open and closed loop transients in response to sudden changes in fuel composition was experimentally investigated. A pilot-scale (200–700 kW) hybrid facility available at the U.S. Department of Energy, National Energy Technology Laboratory was used to perform the experiments using a combination of numerical models and actual equipment. In the open loop configuration, the turbine speed was driven by the thermal effluent fed into the gas turbine system, where the thermal effluent was determined by the feedforward fuel cell control system. However, in the closed loop configuration, a load-based speed control system was used to maintain the turbine speed constant at 40,500 rpm by adjusting the load on the turbine, in addition to the implementation of the fuel cell system control. The open loop transient response showed that the impacts of fuel composition changes on key process variables, such as fuel cell thermal effluent, turbine speed, and cathode feed stream conditions, in the SOFC/GT systems were propagated over the course of the test, except for the cathode inlet temperature. The trajectories of the aforementioned variables are discussed in this paper to better understand the resulting mitigation/propagation behaviors. This will help lead to the development of novel control strategies to mitigate the negative impacts experienced during fuel composition transients of SOFC/GT systems.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2017;139(6):061901-061901-9. doi:10.1115/1.4035229.

Heat transfer effects play a significant role in assessing the performance of automotive turbochargers. Thermal effects are becoming increasingly relevant due to reduced machine sizes and increased exhaust gas temperatures. In this work, a study of the individual energy flows is conducted by simulation of a complete turbocharger comprising compressor (dC = 51 mm), turbine, and bearing housing using conjugate heat transfer. Special focus is given to the analysis of the various heat flows occurring in the machine aiming to identify the major heat transfer paths and their sensitivity with respect to varying operating conditions. Cooling of the bearing housing is shown to be a powerful thermal isolator mitigating the heat transferred to the compressor by up to 60%. Moreover, the rotating speed largely dictates the amount of heat transfer in the compressor and the direction of the heat flow: Whereas at low speeds (22% of max. speed), 117 W are introduced into the fluid and 338 W are being discharged from the fluid at maximum speed. At high speed operation, the heat transfer is shown to be insignificant compared to the aerodynamic work. At low speeds, however, it can reach up to 35% of the aerodynamic work. While the turbine inlet temperature largely governs the overall heat that is lost from the exhaust gas passing the turbine (from 630 W at 300 °C up to 3.72 kW at 1050 °C), only a minor effect on the compressor heat transfer is detected.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):061902-061902-9. doi:10.1115/1.4035324.

Gas foil bearings (GFBs) have many noticeable advantages over the conventional rigid gas bearings, such as frictional damping of the compliance structure and tolerance to the rotor misalignment, so they have been successfully adopted as the key element that makes possible oil-free turbomachinery. As the adoption of the GFB increases, one of the critical elements for its successful implementation is thermal management. Even though heat generation inside the GFB is small due to the low viscosity of the lubricant, many researchers have reported that the system might fail without an appropriate cooling mechanism. The objective of the current research is to demonstrate the reliability of GFBs installed in the hot section of a micro-gas turbine (MGT). For the cooling of the GFBs, we designed a secondary flow passage and thermohydrodynamic (THD) analysis has been done for temperature prediction. In the analysis, the 3D THD model for the radial GFB extended to include the surrounding structure, such as the plenum, chamber, and the rotor in the solution domain by solving global mass and energy balance equations. In the MGT, the pressurized air discharged from the compressor wheel was used as the cooling air source, and it was injected into the plenum between two radial GFBs. We monitored the pressure and temperature of the cooling air along the secondary flow passage during the MGT operation. No thermal instability occurred up to the maximum operation speed of 43,000 rpm. The test results also showed that the pressure drop between the main reservoir and the plenum increases with an increasing operation speed, which indicated an increased cooling air flow into the plenum. The plenum and bearing sleeve temperature was maintained close to the cooling air source temperature for the entire speed due to a sufficient cooling air flow into the bearing. In addition, the direct injection of the cooling air from the main stream lowered the bearing sleeve temperature by 5–20 °C over the injection through the reservoirs. The predicted plenum and bearing sleeve temperatures with the developed THD model show good agreement with the test data.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2017;139(6):062501-062501-12. doi:10.1115/1.4035209.

Accurate structural modeling of blisk mistuning is critical for the analysis of forced response in turbomachinery. Apart from intentional mistuning, mistuning can be due to the manufacturing tolerances, corrosion, foreign object damage, and in-service wear in general. It has been shown in past studies that mistuning can increase the risk of blade failure due to energy localization. For weak blade to blade coupling, this localization has been shown to be critical and higher amplitudes of vibration are expected in few blades. This paper presents a comparison of three reduced order models (ROMs) for the structural modeling of blisks. Two of the models assume cyclic symmetry, while the third model is free of this assumption. The performance of the reduced order models for cases with small and large amount of mistuning will be examined. The benefits and drawbacks of each reduction method will be discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062502-062502-7. doi:10.1115/1.4035323.

Root causes of several recent crack failures in turbine units are attributed to oscillation and interaction between generator of turbine unit and devices on the grid, where torsional vibration of the rotor bearing system is observed and identified as an important cause. Exploring vibrational (lateral, torsional, and axial) features in the cracked rotor system with torsional excitation (TE) present can provide a novel view in crack detection and isolation. This work presents dynamic analysis of a cracked rotor system in a steam turbine unit (a typical rotor system with multiple rotors, multiple supports, and oscillating loads), and the vibrational features of the cracked rotor system with comparisons to typical features in monitored vibration data. The results show that coupled vibration in both lateral and torsional components is an effective indicator for cracks in the presence of torsional excitation. Also, vibration characteristics evaluated in different locations of the rotor system are beneficial for fault detection and isolation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062503-062503-10. doi:10.1115/1.4035286.

In the field of turbomachinery, great efforts are made to enhance computational tools to obtain reliable predictions of the vibrational behavior of friction-damped bladed disks. As a trade-off between computational burden and level of simplification, numerous methods were developed to reduce the nonlinear systems dimension. Using component mode synthesis methods (CMS), one is capable to describe the systems motion by interface and modal coordinates. Subsequently or alternatively, the dynamic compliance matrix can be evaluated efficiently by means of modal superposition to avoid the inversion of the dynamic stiffness matrix. Only the equations corresponding to the degrees-of-freedom (DOF) subject to localized nonlinear contact forces need to be solved simultaneously, whereas the solution of the linear DOF is obtained by exploiting the algebraic character of the set of equations. In this paper, an approach is presented to account for rotational speed-dependent stiffness in the subset of nonlinear DOF without the need to re-evaluate the associated eigenvalue problem (EVP) when rotational speed is changed. This is done by means of a Taylor series expansion of the eigenvalues and eigenvectors used for the modal superposition to reconstruct the dynamic compliance matrix. In the context of forced response predictions of friction-damped blisks, the expansion is performed up to different order for a simplified blisk model with nonlinear contact interfaces. The results are compared to the solution obtained by direct evaluation of the EVP at selected rotational speeds and the solution when dynamic compliance matrix is built up by direct inversion of the dynamic stiffness matrix. Finally, the proposed methods computational performance is analyzed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062504-062504-11. doi:10.1115/1.4035658.

This paper systematically presents a complete leakage comparison for various types of wear experienced by labyrinth seals. Labyrinth seals used in turbine engines are designed to work at a clearance during steady-state engine operations. The tooth tip rubs the stator and wears either itself or the stator surface during transient operations, depending on the material properties of the tooth and stator. Any type of wear that increases clearance or deforms the tooth tip will cause permanent and unpredictable leakage degradation. This negatively affects the engine's overall efficiency, durability, and life. The teeth have been reported to wear into a mushroom profile or into a rounded profile. A rub-groove on the opposing surface may form in several shapes. Based on a literature survey, five rub-groove shapes are considered in this work. They are rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this work, leakage degradation due to wear is numerically quantified for both mushroomed and rounded tooth wear profiles. It also includes analyses on rounded teeth with the formation of five rub-groove shapes. All parameters are analyzed at various operating conditions (clearance, pressure ratio, number of teeth, and rotor speed). Computational fluid dynamics (CFD) analyses are carried out by employing compressible turbulent flow in a 2D axisymmetrical coordinate system. CFD analyses show that the following tooth-wear conditions affect leakage from least to greatest: unworn, rounded, and mushroomed. These are for an unworn flat stator. It is also observed that rub-groove shapes considerably affect the leakage depending on the clearance. Leakage increases with the following groove profiles: triangular, rectangular, acute trapezoidal, isosceles trapezoidal, and elliptical. The results show that any type of labyrinth seal wear has significant effects on leakage. Therefore, leakage degradation due to wear should be considered during the engine design phase.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062505-062505-12. doi:10.1115/1.4035358.

Stability and synchronous-response predictions, which were presented by Childs et al. (2014, “A Lateral Rotordynamics Primer on Electric Submersible Pumps (ESPs) for Deep Subsea Applications,” 43th International Pump Users Symposium, Texas A&M University, College Station, TX, pp. 1–18), are re-evaluated to include the effect of the fluid between the pump housing and well casing. Conclusions are made based on these new findings. The same two-line rotor–housing model is used to model the pump's rotor and its housing. The model dimensions are based on direct measurements of an ESP. The pump rotor and pump housing are only connected together at each stage by reaction forces and moments from seals and bushings. The rotor model is pinned to the housing at the rotor's ends. The housing model is pinned to ground at its ends. Synchronous response predictions are presented for: (1) relative rotor–housing motion and (2) housing velocity-response amplitudes. When handling viscosity of water, the rotor–housing model is predicted to be stable at new (centered) 1× clearances but rapidly becomes unstable with enlarged clearances (2× and 3×), primarily due to rapidly dropping rotor–housing natural frequencies. The impact of introducing effective swirl brakes for the stages' wear ring seals was investigated for a pump running at 3600 rpm. Their predicted impact on stability and synchronous response was: (1) Onset speeds of instabilities (OSIs) were elevated well above running speed and (2) synchronous response amplitudes were reduced modestly. Housing-response amplitudes varied considerably with the choice of housing-termination locations. For a pump rotor length of Lr, varying the lengths of a centered housing over 1.5 Lr, 2 Lr, and 3 Lr changes the housing's natural frequency. This natural frequency can coincide with the running speed with proper termination conditions. If the running speed and natural frequency coincide, large housing vibration amplitudes associated with resonance would exceed most vibration regulations; however, relative rotor–stator response amplitudes were a small fraction of clearances for all the cases. When handling emulsions at markedly higher viscosities, with a pump speed of 3600 rpm and new centered clearances, the predicted OSI was below 300 rpm. The OSI rapidly increased as the seals were displaced eccentrically, quickly elevating the first rotor–stator natural frequency above 1800 rpm and the OSI above 3600 rpm. With the model stabilized at 0.2 eccentricity, the synchronous relative rotor–housing amplitudes were a small fraction of seal clearances. Swirl brakes were not predicted to be effective in elevating pump OSIs for high viscosity fluids with new clearances; however, they became effective as clearances were increased. An ESP housing can contact the well casing in many possible scenarios (axial locations, contact-area length or girth, etc.). A midspan, point radial contact was examined and modeled as a stiff-spring connection from the housing to ground. For both water and oil–water emulsions, a stiff housing-to-casing contact produced major elliptical housing motion (versus circular motion without contact). However, it had a comparably minor impact on relative rotor–housing response amplitudes or rotordynamic stability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062506-062506-10. doi:10.1115/1.4035400.

Calculation of the clearances between the blades and casing of the high-pressure-compressor rotors in aeroengines involves calculating the radial growth of the corotating compressor disks. This requires the calculation of the thermal growth of the disks, which in turn requires knowledge of their temperatures and of the Nusselt numbers and the flow structure in the cavity between the disks. The authors have recently published a theoretical model of the buoyancy-induced flow in rotating cavities, and approximate solutions were obtained for laminar Ekman-layer flow on the disks; the equation for the Nusselt numbers, which includes two empirical constants, depends strongly on the Grashof number and on the radial distribution of disk temperature. In this paper, Nusselt numbers and disk temperatures predicted by the buoyancy model are compared with values obtained from published experimental data. For most of the 19 test cases, with Grashof numbers up to nearly 1012, mainly good agreement was achieved between the theoretical and experimental distributions of Nusselt numbers and disk temperatures. This suggests that, owing to Coriolis effects, the laminar model of buoyancy-induced rotating flow could be valid even at the high Grashof numbers found in the compressor rotors of aeroengines. As predicted by the model, for a constant Grashof number increasing the rotational Reynolds number can cause a decrease in the Nusselt number. This is the first time a theoretical model (rather than computational fluid dynamics (CFD)) has been used to predict the temperatures of a compressor disk, and the model takes only seconds to predict disk temperatures that would take days or even weeks to predict using CFD. More experimental data is required if the model is to be used by the designers of compressor rotors, and suggestions for future research are given in the paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062507-062507-9. doi:10.1115/1.4035447.

In recent years, theoretical and experimental efforts have transformed the conventional tilting-pad journal bearing (TPJB) into a smart mechatronic machine element. The application of electromechanical elements into rotating systems makes feasible the generation of controllable forces over the rotor as a function of a suitable control signal. The servovalve input signal and the radial injection pressure are the two main parameters responsible for dynamically modifying the journal oil film pressure and generating active fluid film forces in controllable fluid film bearings. Such fluid film forces, resulting from a strong coupling between hydrodynamic, hydrostatic, and controllable lubrication regimes, can be used either to control or to excite rotor lateral vibrations. If “noninvasive” forces are generated via lubricant fluid film, “in situ” parameter identification can be carried out, enabling evaluation of the mechanical condition of the rotating machine. Using the lubricant fluid film as a “noninvasive calibrated shaker” is troublesome, once several transfer functions among mechanical, hydraulic, and electronic components become necessary. In this framework, the main original contribution of this paper is to show experimentally that the knowledge about the several transfer functions can be bypassed by using output-only identification techniques. This paper links controllable (active) lubrication techniques with operational modal analysis, allowing for in situ parameter identification in rotordynamics, i.e., estimation of damping ratio and natural frequencies. The experimental analysis is carried out on a rigid rotor-level system supported by one single pair of pads. The estimation of damping and natural frequencies is performed using classical experimental modal analysis (EMA) and operational modal analysis (OMA). Very good agreements between the two experimental approaches are found. Maximum values of the main input parameters, namely, servovalve voltage and radial injection pressure, are experimentally found with the objective of defining ranges of noninvasive perturbation forces.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062508-062508-12. doi:10.1115/1.4035602.

A new modeling approach has been developed that explicitly accounts for expected turbulent eddy length scales in cavity zones. It uses a hybrid approach with Poisson and Hamilton–Jacobi differential equations. These are used to set turbulent length scales to sensible expected values. For complex rim-seal and shroud cavity designs, the method sets an expected length scale based on local cavity width which accurately accounts for the large-scale wakelike flow structures that have been observed in these zones. The method is used to generate length scale fields for three complex rim-seal geometries. Good convergence properties are found, and a smooth transition of length scale between zones is observed. The approach is integrated with the popular Menter shear stress transport (SST) Reynolds-averaged Navier–Stokes (RANS) turbulence model and reduces to the standard Menter model in the mainstream flow. For validation of the model, a transonic deep cavity simulation is performed. Overall, the Poisson–Hamilton–Jacobi model shows significant quantitative and qualitative improvement over the standard Menter and k–ε two-equation turbulence models. In some instances, it is comparable or more accurate than high-fidelity large eddy simulation (LES). In its current development, the approach has been extended through the use of an initial stage of length scale estimation using a Poisson equation. This essentially reduces the need for user objectivity. A key aspect of the approach is that the length scale is automatically set by the model. Notably, the current method is readily implementable in an unstructured, parallel processing computational framework.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2017;139(6):062601-062601-9. doi:10.1115/1.4035283.

A tool to create parametric aerodynamic shapes using intuitive design variables based on class shape transformation (CST) curves is presented. To enable this, a system has been developed which accepts arbitrary constraints and automatically derives the analytical expressions which describe the corresponding class shape transformation curves. Parametric geometry definitions for fan cowl and intake aero-lines were developed using the generalized method. Computational fluid dynamics (CFD) analysis of the fan cowl shows that despite the simple geometry definition, its performance characteristics are close to what would be expected of a finished design. The intake geometry was generated in a similar way and met the typical performance metrics for conventional intakes. This demonstrates the usefulness of the tool to quickly and robustly produce parametric aero-lines with good aerodynamic properties, using relatively simple intuitive design variables.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062602-062602-13. doi:10.1115/1.4035230.

Compared to the conventional axisymmetric dual throat nozzle, the axisymmetric divergent dual throat nozzle (ADDTN) can offer larger thrust vector angles. However, the starting problem maybe exists in the ADDTN and results in a huge thrust loss. In this paper, the ADDTN starting problem has been studied by steady and unsteady numerical simulations. The effects of nozzle geometric parameters on internal nozzle performance have been discussed in detail, including cavity divergence angle, cavity convergence angle, cavity length, expansion ratio, rounding radius at the nozzle throat, and rounding radius at the cavity bottom. And, the shock oscillation phenomenon is found inside the recessed cavity in some high-expansion ratio configurations. In addition, a bypass is proposed in this study to solve the ADDTN starting problem. The main numerical simulation results show that the expansion ratio is the most sensitive parameter affecting the starting characteristic of ADDTN, followed by the cavity divergence angle and the cavity length. And, among these parameters, the parameters of cavity convergence angle and rounding radius at the cavity bottom contribute the least to the starting problem. Besides, the ADDTN configurations of large rounding radius at the nozzle throat tend to start.

Topics: Nozzles , Cavities
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062603-062603-12. doi:10.1115/1.4035284.

Turbochargers are key components of engine air-paths that must be carefully considered during the development process. The combination of fluid, mechanical, and thermal phenomenon make the turbocharger a highly dynamic and nonlinear modeling challenge. The aim of this study is to quantify the dynamic response of the turbocharger system across a frequency spectrum from 0.003 Hz to 500 Hz, i.e., for exhaust gas pulsation in steady state, load steps, and cold start drive cycles, to validate the assumption of quasi-steady assumptions for particular modeling problems. A waste-gated turbine was modeled using the dual orifice approach, a lumped capacitance heat transfer model, and novel, physics-based pneumatic actuator mechanism model. Each submodel has been validated individually against the experimental measurements. The turbine inlet pressure and temperature and the waste-gate actuator pressure were perturbed across the full frequency range both individually and simultaneously in separate numerical investigations. The dynamic responses of turbine housing temperature, turbocharger rotor speed, waste-gate opening, mass flow, and gas temperatures/pressures were all investigated. The mass flow parameter exhibits significant dynamic behavior above 100 Hz, illustrating that the quasi-steady assumption is invalid in this frequency range. The waste-gate actuator system showed quasi-steady behavior below 10 Hz, while the mechanical inertia of the turbine attenuated fluctuations in shaft speed for frequencies between 0.1 and 10 Hz. The thermal inertia of the turbocharger housing meant that housing temperature variations were supressed at frequencies above 0.01 Hz. The results have been used to illustrate the importance of model parameters for three transient simulation scenarios (cold start, load step, and pulsating exhaust flow).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062604-062604-10. doi:10.1115/1.4035287.

The turbocharger is a significant noise source in large diesel engines, such as those used in container vessels. Its main noise source is the radial compressor, where improvements in silencers and turbocharger insulation have led to a considerable reduction of compressor inlet noise emission over the past few years. As a result, compressor outlet noise is now becoming increasingly significant for large engines. Recently, an in-house compressor testbed was upgraded by adding an acoustic modal measurement system (MSMS) that allows detailed investigation of modal sound fields inside the piping. This forms part of an updated compressor acoustic qualification procedure. This paper is an in-depth treatise of the characteristics of this modal measurement system. The calculation approach for the modal decomposition and a simplified alternative that assumes axial propagation, as well as relevant considerations, such as spatial resolution, averaging, and the use of multiple reference sensors, are addressed. Various measurement parameters, such as repeatability, measurement time, required temperature stability, pressure scaling, flow noise and their impact on measurement uncertainty were investigated. A successful validation of the modal sound measurement system with a well-known modal sound field at the compressor inlet is also presented. Finally, the characteristics of the modal sound fields of the compressor outlet of a typical modern turbocharger are discussed. Modal decompositions at the first two blade passing frequencies (BPFs) are presented for selected operating points (OPs). The response of total sound power levels (PWLs) to compressor speed along the operating line (OL) is examined by means of both the present and the simplified algorithm. A sensitivity analysis shows the impact of volume flow and rotational speed on the modal sound distribution.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Vehicular and Small Turbomachines

J. Eng. Gas Turbines Power. 2017;139(6):062701-062701-8. doi:10.1115/1.4035396.

The effect of cathode airflow variation on the dynamics of a fuel cell gas turbine hybrid system was evaluated using a cyber-physical emulator. The coupling between cathode airflow and other parameters, such as turbine speed or pressure, was analyzed comparing the results at fixed and variable speed. In particular, attention was focused on fuel cell temperatures and gradients: cathode airflow, which is generally employed for thermal management of the stack, was varied by manipulating a cold-air bypass. A significant difference was observed in the two cases in terms of turbine inlet, exhaust gas, cathode inlet, and average cell temperatures. When the turbine speed was held constant, a change in cathode airflow resulted in a strong variation in cathode inlet temperature, while average cell temperature was not significantly affected. The opposite behavior was observed at variable speed. The system dynamics were analyzed in detail in order to explain this difference. Open-loop response was analyzed in this work for its essential role in system identification. However, a significant difference was observed between fixed and variable speed cases, because of the high coupling between turbine speed and cathode airflow. These results can give a helpful insight of system dynamics and control requirements. Cold-air valve bypass position also showed a strong effect on surge margin and pressure dynamics in both cases.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062702-062702-10. doi:10.1115/1.4035648.

Ceramic turbines can reduce fuel consumption by increasing turbine inlet temperatures (TIT). The need for heat-resistant materials like ceramics is particularly acute for small turbomachines for which efficiencies are limited by the use of uncooled metal turbine as complex cooling schemes are impractical and costly. Efforts to introduce ceramics in the turbine rotor were made between the 1960s and the 1990s by gas turbines and automotive manufacturers in the U.S., Europe, and Japan. While significant progress was made, a suitable level of reliability still cannot be achieved as the brittleness of ceramics leads to crack propagation in the blades loaded in tension and catastrophic failure. The inside-out ceramic turbine (ICT) is a design alternative specific to ceramics that loads the blades in compression by using an outer, air-cooled composite rim that sustains the centrifugal loads. This paper provides an analytical model based on the Brayton cycle to compute the system-level performance of microturbines using an ICT. Loss submodels specific to ICT architectures are developed to account for: (1) composite rim drag, (2) composite rim cooling, (3) leakage through rotating seals, and (4) expansion heat losses. The thermodynamic core model is validated against three state-of-the-art, non-inside-out, microturbines. Based on a Monte Carlo simulation that takes into account the modeling uncertainties, the model predicts a cycle efficiency of 45±1% for a 240 kW ICT-based microturbine, leading to a predicted reduction in fuel consumption of 20% over current all-metal microturbines.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2017;139(6):062801-062801-12. doi:10.1115/1.4035282.

There are increasing pressures upon the automotive industry to reduce harmful emissions as well as meeting the key objective of enhanced fuel efficiency, while improving or retaining the engine output power. The losses in an internal combustion (IC) engine can be divided into thermal and parasitic as well as due to gas leakage because of untoward compression ring motions. Frictional losses are particularly of concern at low engine speeds, assuming a greater share of the overall losses. Piston–cylinder system accounts for nearly half of all the frictional losses. Loss of sealing functionality of the ring pack can also contribute significantly to power losses as well as exacerbating harmful emissions. The dynamics of compression ring is inexorably linked to its tribological performance, a link which has not been made in many reported analyses. A fundamental understanding of the interplay between the top compression ring three-dimensional elastodynamic behavior, its sealing function and contribution to the overall frictional losses is long overdue. This paper provides a comprehensive integrated transient elastotribodynamic analysis of the compression ring to cylinder liner and its retaining piston groove lands' conjunctions, an approach not hitherto reported in the literature. The methodology presented aims to aid the piston ring design evaluation processes. Realistic engine running conditions are used which constitute international drive cycle testing conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2017;139(6):062802-062802-8. doi:10.1115/1.4035395.

This paper proposes a real-time combustion control algorithm using reconstructed in-cylinder pressure traces by principal component analysis (PCA). The PCA method reconstructs the in-cylinder pressure traces using the principal components of the in-cylinder pressure traces. It was shown that using only five principal components, we were able to reconstruct the in-cylinder pressure traces within 1% root mean squared percent error. Furthermore, the reconstructed in-cylinder pressure traces were validated to effectively reduce the cycle-to-cycle variations caused by the noise signals. As a result, the standard deviation of MFB50 which was calculated from the reconstructed in-cylinder pressure was reduced by 45%. Furthermore, this combustion parameter was applied to a real-time combustion control. Since variations of the control variables for the real-time combustion control were reduced, the control performances were enhanced.

Commentary by Dr. Valentin Fuster

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