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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2018;140(11):111201-111201-13. doi:10.1115/1.4038815.

This paper adapted and extended the preliminary two-step wave rotor design method with another step of experimental validation so that it became a self-validating wave rotor design method with three steps. First, the analytic design based on unsteady pressure wave models was elucidated and adapted to a design function. It was quick and convenient for a first prediction of the wave rotor. Second, the computational fluid dynamics (CFD) simulation was adapted so that it helped to adjust the first prediction. It provided detailed information of the wave rotor inner flow. Thirdly, an experimental method was proposed to complement the validation of the wave rotor design. This experimental method realized tracing the pressure waves and the flows in the wave rotor with measurement on pressure and temperature distributions. The critical point of the experiment is that the essential flow characteristics in the rotor were reflected by the measurements in the static ports. In all, the three steps compensated for each other in a global design procedure, and formed an applicable design method for generic cases.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;140(11):111501-111501-7. doi:10.1115/1.4038475.

In this paper, we present a method to determine the quantitative stability level of a lean-premixed combustor from dynamic pressure data. Specifically, we make use of the autocorrelation function of the dynamic pressure signal acquired in a combustor where a turbulent flame acts as a thermoacoustic driver. In the proposed approach, the unfiltered pressure signal including several modes is analyzed by an algorithm based on Bayesian statistics. For this purpose, a Gibbs sampler is used to calculate parameters like damping rates and eigenfrequencies in the form of probability density functions (PDF) by a Markov-chain Monte Carlo (MCMC) method. The method provides a robust solution algorithm for fitting problems without requiring initial values. A further advantage lies in the nature of the statistical approach since the results can be assessed regarding its quality by means of the PDF and its standard deviation for each of the obtained parameters. First, a simulation of a stochastically forced van-der-Pol oscillator with preset input values is carried out to demonstrate accuracy and robustness of the method. In this context, it is shown that, despite a large amount of uncorrelated background noise, the identified damping rates are in a good agreement with the simulated parameters. Second, this technique is applied to measured pressure data. By doing so, the combustor is initially operated under stable conditions before the thermal power is gradually increased by adjusting the fuel mass flow rate until a limit-cycle oscillation is established. It is found that the obtained damping rates are qualitatively in line with the amplitude levels observed during operation of the combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111502-111502-7. doi:10.1115/1.4039056.

Stringent emissions standards for NOx and carbon monoxide (CO) prompt lean combustor development. With this motivation, combustion stability issues emerge since the desired operating point approaches the lean blowout limit. In this paper, an atmospheric, 15 kW lean premixed prevaporizing-type swirl burner, equipped with a plain jet airblast atomizer, was investigated at various atomizing pressures and combustion air flow rates, using quarls from 0 deg to 60 deg in 15 deg steps. Both the 15 deg and the 30 deg quarls provided a 42% higher lean blowout stability on average in terms of mean mixing tube discharge velocity, compared to the baseline burner. However, the superior stability regime was encumbered by a rapidly increasing CO emission. In parallel, the NOx emission vanished due to the more dilution air and incomplete combustion. The 60 deg quarl provided a moderately extended blowout stability limitation, while the NOx emission slightly increased and the CO emission reduced compared to the baseline burner.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111503-111503-9. doi:10.1115/1.4039760.

Self-excited combustion instabilities in a high pressure, single-element, lean, premixed, natural gas (NG) dump-combustor are investigated. The combustor is designed for optical access and instrumented with high frequency pressure transducers at multiple axial locations. A parametric survey of operating conditions including inlet air temperature and equivalence ratio has been performed, resulting in a wide range of pressure fluctuation amplitudes (p) of the mean chamber pressure (pCH). Two representative cases, flames A and B with p/pCH=23% and p/pCH=12%, respectively, both presenting self-excited instabilities at the fundamental longitudinal (1L) mode of the combustion chamber, are discussed to study the coupling mechanism between flame-vortex interactions and the acoustic field in the chamber. 10 kHz OH*-chemiluminescence imaging was performed to obtain a map of the global heat release distribution. Phase conditioned and Rayleigh index analysis as well as dynamic mode decomposition (DMD) is performed to highlight the contrasting mechanisms that lead to the two distinct instability regimes. Flame interactions with shear layer vortex structures downstream of the backward-facing step of the combustion chamber are found to augment the instability magnitude. Flame A engages strongly in this coupling, whereas flame B is less affected and establishes a lower amplitude limit cycle.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111504-111504-9. doi:10.1115/1.4040286.

The effect of the chamber pressure on combustion of a twin-fluid-atomized spray of straight vegetable oil (VO) in a swirl stabilized combustion system is experimentally studied. A system with high pressure capabilities was developed, and flame and emissions characteristics of VO are investigated at elevated pressures up to about 5 bars, different heat release rates (HRRs), and atomizing air to liquid ratios (ALR) by mass. An image analysis technique was developed to infer flame and soot characteristics from visual images acquired by a digital camera. An increase in the ALR resulted in improved combustion of VO, characterized by blue flames, lower CO and NOx emissions, and minimal soot formation. For a given fuel flow rate, an increase in the chamber pressure resulted in smaller volume flames with lower CO levels but higher NOx emissions. Compared to diesel, as pressure increased, straight VO flames produced lower NOx and more voluminous flames characterized by distributed combustion with less soot formation. Overall, straight VO could be atomized and combusted at elevated pressures using the twin-fluid atomizer of the present study, and the resulting VO flames exhibited less sensitivity to chamber pressure variations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111505-111505-9. doi:10.1115/1.4040007.

Finite-rate chemical effects at gas turbine conditions lead to incomplete combustion and well-known emissions issues. Although a thin flame front is preserved on an average, the instantaneous flame location can vary in thickness and location due to heat losses or imperfect mixing. Postflame phenomena (slow CO oxidation or thermal NO production) can be expected to be significantly influenced by turbulent eddy structures. Since typical gas turbine combustor calculations require insight into flame stabilization as well as pollutant formation, combustion models are required to be sensitive to the instantaneous and local flow conditions. Unfortunately, few models that adequately describe turbulence–chemistry interactions are tractable in the industrial context. A widely used model capable of employing finite-rate chemistry is the eddy dissipation concept (EDC) model of Magnussen. Its application in large eddy simulations (LES) is problematic mainly due to a strong sensitivity to the model constants, which were based on an isotropic cascade analysis in the Reynolds-averaged Navier–Stokes (RANS) context. The objectives of this paper are: (i) to formulate the EDC cascade idea in the context of LES; and (ii) to validate the model using experimental data consisting of velocity (particle image velocimetry (PIV) measurements) and major species (1D Raman measurements), at four axial locations in the near-burner region of a Siemens SGT-100 industrial gas turbine combustor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111506-111506-13. doi:10.1115/1.4040288.

The internal combustion Rankine cycle (ICRC) engine utilizes pure oxygen as the oxidant instead of air during combustion to prevent the generation of nitrogen oxide emissions and lower the cost of CO2 recovery. To control combustion intensity and increase efficiency, water injection technology is implemented as it can increase the in-cylinder working fluid during combustion process. To further enhance the system thermal efficiency, the injected water is heated using coolant and waste heat before being directly injected into combustion chamber. The main challenge of controlling the ICRC engine is the interaction between water injection process and combustion stability. Ion current detection provides a potential solution of real-time detection of in-cylinder combustion status and water injection process simultaneously. In this paper, the characteristics of ion current signal in an ICRC engine were studied. The results indicate the ion current signal is primarily affected by the combination of trapped water vapor injected in the last cycle and in-cylinder combustion intensity. The water vapor contributes to the ionization reactions, which lead to enhanced ion current signals under water cycle. The ion current signal is capable of reflecting the operating conditions of the in-cylinder water injector. The phase of the ion current peak value has a linear relation as the water injection timing is delayed, and ion current detection technology has the potential to detect the combustion phase under different engine loads in an internal combustion Rankine cycle engine.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2018;140(11):111701-111701-10. doi:10.1115/1.4040181.

Integrated gasification humid air turbine (IGHAT) cycle is an advanced power generation system, combining gasification technology and humid air turbine (HAT) cycle. It draws great attention in the energy field considering its high specific power, high efficiency, and low emission. There are only a few HAT cycle plants and IGHAT cycle is still on the theory research stage. Therefore, the study on control strategies of IGHAT cycle has great significance in the future development of this system. A design method of control strategy is proposed for the unknown gas turbine systems. The control strategy design is summarized after IGHAT control strategy and logic is designed based on the dynamic simulation results and the operation experience of gas turbine power station preliminarily. Then, control logic is configured and a virtual control system of IGHAT cycle is established on the Ovation distribution control platform. The model-in-loop control platform is eventually set up based on the interaction between the simulation model and the control system. A case study is implemented on this model-in-loop control platform to demonstrate its feasibility in the practical industry control system. The simulation of the fuel switching control mode and the power control mode is analyzed. The power in IGHAT cycle is increased by 24.12% and 32.47%, respectively, compared to the ones in the simple cycle and the regenerative cycle. And the efficiency of IGHAT cycle is 1.699% higher than that of the regenerative cycle. Low component efficiency caused by off-design performance and low humidity caused by high pressure are the main limits for system performance. The results of case study show the feasibility of the control strategy design method proposed in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111702-111702-11. doi:10.1115/1.4039811.

Approximately 30% of the energy from an internal combustion engine is rejected as heat in the exhaust gases. An inverted Brayton cycle (IBC) is one potential means of recovering some of this energy. When a fuel is burnt, water and CO2 are produced and expelled as part of the exhaust gases. In an IBC, in order to reduce compression work, the exhaust gases are cooled before compression up to ambient pressure. If coolant with a low enough temperature is available, it is possible to condense some of the water out of the exhaust gases, further reducing compressor work. In this study, the condensation of exhaust gas water is studied. The results show that the IBC installed in series on a turbocharged engine can produce an improvement of approximately 5% in brake-specific fuel consumption at the baseline conditions chosen and for a compressor inlet temperature of 310 K. The main factors that influence the work output are heat exchanger pressure drop, turbine expansion ratio, coolant temperature, and turbine inlet temperature. For conditions when condensation is possible, the water content of the exhaust gas has a significant influence on work output. The hydrogen to carbon ratio of the fuel has the most potential to vary the water content and hence the work generated by the system. Finally, a number of uses for the water generated have been presented such as to reduce the additional heat rejection required by the cycle. It can also potentially be used for engine water injection to reduce emissions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):111703-111703-10. doi:10.1115/1.4039828.

A detailed look at the gas turbine (GT) technology over the last three decades clearly shows that an asymptotic limit is reached in thermal efficiency in combined cycle (CC) configuration. There is little reason to expect a technology leap anytime soon to change this appreciably as long as the industry sticks to the classic Brayton cycle configuration and the brute force approach of ever higher turbine inlet temperatures (TITs). The changing landscape of electric power generation, especially integration of fossil fuel fired technologies with renewable generation and wide variation in natural gas prices, suggests that it is time to look at the direction of future development efforts differently. This paper looks at the need of a paradigm shift in GT CC technology using thermodynamic and economic arguments.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(11):112501-112501-8. doi:10.1115/1.4040177.

This work presents the comparison between experimental and theoretical results obtained for three straight annular seals. One of the annular seals has smooth rotor and stator while the others have a textured stator; the textures are equally spaced shallow round holes, with two different depths. The experimental results were obtained on a test rig dedicated to the identification of the dynamic coefficients of high Reynolds bearings and annular seals. The test rig uses hot water (<50 °C) as a working fluid. Dynamic excitations imposed by piezoelectric shakers to the rotor enable the identification of dynamic coefficients via complex impedances. Theoretical results compared with experimental findings were obtained by numerically solving the “bulk flow” equations (film thickness averaged equations dominated by inertia effects). The numerical model was extensively validated for smooth annular seals but is less confident for textured surfaces. The present comparisons between experimental and numerical results enable to estimate the accuracy of the numerical model employed for the textured seals.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112502-112502-11. doi:10.1115/1.4040308.

Secondary air is bled from the compressor in a gas turbine engine to cool turbine components and seal the cavities between stages. Unsealed cavities can lead to hot gas ingestion, which can degrade critical components or, in extreme cases, can be catastrophic to engines. For this study, a 1.5 stage turbine with an engine-realistic rim seal was operated at an engine-relevant axial Reynolds number, rotational Reynolds number, and Mach number. Purge flow was introduced into the interstage cavity through distinct purge holes for two different configurations. This paper compares the two configurations over a range of purge flow rates. Sealing effectiveness measurements, deduced from the use of CO2 as a flow tracer, indicated that the sealing characteristics were improved by increasing the number of uniformly distributed purge holes and improved by increasing levels of purge flow. For the larger number of purge holes, a fully sealed cavity was possible, while for the smaller number of purge holes, a fully sealed cavity was not possible. For this representative cavity model, sealing effectiveness measurements were compared with a well-accepted orifice model derived from simplified cavity models. Sealing effectiveness levels at some locations within the cavity were well-predicted by the orifice model, but due to the complexity of the realistic rim seal and the purge flow delivery, the effectiveness levels at other locations were not well-predicted.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112503-112503-11. doi:10.1115/1.4039834.

Condensing flow induced vibration (CFIV) of the rotor blade is a tough problem for designers of nuclear turbines because nonequilibrium condensing flow excitation (NECFE) is hard to be directly modeled. Generally, in design, NECFE is assumed as equilibrium condensing flow excitation (ECFE), of which the pressure fluctuations caused by phase temperature difference (PTD) between gaseous and liquid are ignored. In this paper, a novel method to calculate the equivalent load of NECFE based on the principle of virtual work was proposed. This method could consider the effects of PTD-induced pressure fluctuations by simulating nonequilibrium condensation with ANSYS cfx, and improve computational efficiency. Once the equivalent NECFE load is determined, CFIV of the rotor blade, which was modeled as a pretwisted asymmetric cantilever beam, can then be predicted by the finite element method (FEM). Additionally, to estimate the effects of PTD-induced pressure fluctuations, comparisons between NECFE and ECFE as well as their induced vibrations were presented. Results show that PTD in nucleation area could change the position and type of shock waves, restructure the pressure distribution, as well as enhance the pressure fluctuations. Compared with ECFE, the frequency ingredients and amplitude of the equivalent NECFE load and its induced vibrations are increased. Specifically, the amplitude of the equivalent NECFE load is increased by 9.38%, 15.34%, and 7.43% in the tangential component, axial component, and torsion moment. The blade vibration responses induced by NECFE are increased by 11.66% and 19.94% in tangential and axial.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(11):112601-112601-18. doi:10.1115/1.4039423.

This work investigates the unsteady pressure fluctuations and inception of vortical flow in a hydraulic turbine during speed-no-load conditions. At speed-no-load (SNL), the available hydraulic energy dissipates to the blades without producing an effective torque. This results in high-amplitude pressure loading and fatigue development, which take a toll on a machine's operating life. The focus of the present study is to experimentally measure and numerically characterize time-dependent pressure amplitudes in the vaneless space, runner and draft tube of a model Francis turbine. To this end, ten pressure sensors, including four miniature sensors mounted in the runner, were integrated into a turbine. The numerical model consists of the entire turbine including Labyrinth seals. Compressible flow was considered for the numerical study to account for the effect of flow compressibility and the reflection of pressure waves. The results clearly showed that the vortical flow in the blade passages induces high-amplitude stochastic fluctuations. A distinct flow pattern in the turbine runner was found. The flow near the blade suction side close to the crown was more chaotic and reversible (pumping), whereas the flow on the blade pressure side close to the band was accelerating (turbine) and directed toward the outlet. Flow separation from the blade leading edge created a vortical flow, which broke up into four parts as it traveled further downstream and created high-energy turbulent eddies. The source of reversible flow was found at the draft tube elbow, where the flow in the center core region moves toward the runner cone. The vortical region located at the inner radius of the elbow gives momentum to the wall-attached flow and is pushed toward the outlet, whereas the flow at the outer radius is pushed toward the runner. The cycle repeats at a frequency of 22.3 Hz, which is four times the runner rotational speed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112602-112602-9. doi:10.1115/1.4038473.

The noise generated by the passage of acoustic and entropy perturbations through subsonic and choked nozzle flows is investigated numerically using an energetic approach. Low-order models are used to reproduce the experimental results of the hot acoustic test rig (HAT) of Deutsches Zentrum für Luft- und Raumfahrt (DLR), and energy budgets are performed to characterize the reflection, transmission, and dissipation of the fluctuations. Because acoustic and entropy perturbations are present in the flow in the general case, classical acoustic energy budgets cannot be used and the disturbances energy budgets proposed by Myers (1991, “Transport of Energy by Disturbances in Arbitrary Steady Flows,” J. Fluid Mech., 226, pp. 383–400.) are used instead. Numerical results are in very good agreement with the experiments in terms of acoustic transmission and reflection coefficients. The normal shock present in the diffuser for choked regimes is shown to attenuate the scattered acoustic fluctuations, either by pure dissipation effect or by converting a part of the acoustic energy into entropy fluctuations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112603-112603-13. doi:10.1115/1.4039818.

During the commissioning and stand-still cycles of wind turbines, the rotor is often stopped or even locked leaving the rotor blades at a standstill. When the blades are at a standstill, angles of attack on the blades can be very high, and it is therefore possible that they experience vortex-induced vibrations. This experiment and analysis helps to explain the different regimes of flow at very high angles of attack, particularly on moderately twisted and tapered blades. A single blade was tested at two different flow velocities at a range of angles of attack with flow tuft visualization and hotwire measurements of the wake. Hotwire wake measurements were able to show the gradual inception and ending of certain flow regimes. The power spectral densities of these measurements were normalized in terms of Strouhal number based on the projected chord to show that certain wake features have a relatively constant Strouhal number. The shedding frequency appears then to be relatively independent of chord taper and twist. Vortex generators (VGs) were tested but were found to have little influence in this case. Gurney flaps were found to modify the wake geometry, stall onset angles, and in some cases the shedding frequency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112604-112604-9. doi:10.1115/1.4040093.

Focus of this paper is aerodynamic investigation of tie-boss stabilization devices for extremely long rotor blades. This investigation covered measurements on multiple blade cascades and computational fluid dynamics (CFD) simulation of the flow past these cascades. Conclusions were drawn from results of the measurements and CFD and from the knowledge of prior investigation of the used blade cascade. Main focus of this paper is to describe influence of a tie-boss stabilization device on flow field in interblade channel. Tie-boss with more massive shape proved to cause lesser losses, while tie-boss with a tailored trailing edge showed lesser influence on flow turning.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112605-112605-11. doi:10.1115/1.4039837.

A fast preliminary design methodology for supersonic organic Rankine cycle (ORC) stator and rotor axial turbine blades with low degree of reaction is presented. First, the stator and rotor blade mean-line profiles are designed by using the two-dimensional (2D) method of characteristics (MOC), extended to gases governed by general equations of state (EOS). We focus more specifically on working fluids with medium to high molecular complexity, operating at thermodynamic conditions such that the fundamental derivative of gas dynamics Γ is lower than one in a significant portion of the flow field. For rotor blades, MOC is combined with a free-vortex method to achieve a smooth deflection of the supersonic incoming flow. A numerical approach is developed for solving the unique incidence problem in the case of gases governed by general EOS. Both stator and rotor blade geometries designed according to the inviscid MOC model are subsequently corrected to account for the development of viscous boundary layers by solving the compressible integral boundary layer equations extended to dense gases. The resulting blade designs are assessed by means of computational fluid dynamics (CFD) simulations based on a high-order finite volume solver equipped with advanced thermodynamic and transport-property models. Properly accounting for dense gas and viscous effects at an early design stage is found to improve the expected performance of ORC turbine rows significantly and delivers valuable baseline profiles for any further optimization.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(11):112801-112801-7. doi:10.1115/1.4039758.

The combustion of natural gas reduces fuel cost and generates less emissions of carbon dioxide and particulate matter (PM) than diesel and gasoline. Replacing diesel by natural gas in internal combustion engines is of great interest for transportation and stationary power generation. Dual fuel combustion is an efficient way to burn natural gas in internal combustion engines. In natural gas–diesel dual fuel engines, unburned hydrocarbon emissions increase with increasing natural gas fraction. Many studies have been conducted to improve the performance of natural gas–diesel dual fuel engines and reported the performance of combustion and emissions of regulated pollutants and total unburned hydrocarbon at various engine operating strategies. However, little has been reported on the emissions of different unburned hydrocarbon components. In this paper, an experimental investigation was conducted to investigate the combustion performance and emissions of various unburned hydrocarbon components, including methane, ethane, ethylene, acetylene, propylene, formaldehyde, acetaldehyde, and benzaldehyde, at a low engine load condition. The operating conditions, such as engine speed, load, intake temperature, and pressure, were well controlled during the experiment. The combustion and emissions performance of pure diesel and natural gas–diesel dual fuel combustion were compared. The effect of diesel injection timing was analyzed. The results show that appropriately advancing diesel injection timing to form a homogeneous charge compression ignition (HCCI)-like combustion is beneficial to natural gas–diesel dual fuel combustion at low load conditions. The emissions of different unburned hydrocarbon components changed in dual fuel combustion, with emissions of some unburned hydrocarbon components being primarily due to the combustion of natural gas, while those of others being more related to diesel combustion.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112802-112802-5. doi:10.1115/1.4040306.

In this work, engine-out particulate matter (PM) mass emissions from an off-highway diesel engine measured using a low-cost photometer, scanning mobility particle sizer (SMPS), elemental versus organic carbon (EC/OC) analysis, and a photo-acoustic analyzer are compared. Tested engine operating modes spanned the range of those known to result in high semivolatile particle concentration and those that emit primarily solid particles. Photometer measurements were taken following a primary dilution stage and a sample conditioner to control relative humidity prior to the instrument. Results of the study show that the photometer could qualitatively track total particle mass trends over the tested engine conditions though it was not accurate in measuring total carbon (TC) mass concentration. Further, the required photometric calibration factor (PCF) required to accurately measure total PM mass changes with the OC fraction of the particles. Variables that influence PCF include particle effective density, which changes both as a function of particle diameter and OC fraction. Differences in refractive index between semivolatile and solid particles are also significant and contribute to high error associated with measurement of total PM using the photometer. This work illustrates that it may be too difficult to accurately measure total engine PM mass with a photometer without knowing additional information about the sampled particles. However, removing semivolatile organic materials prior to the instrument may allow the accurate estimation of EC mass concentration alone.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112803-112803-18. doi:10.1115/1.4039466.

The rapid expansion of the market for remotely piloted aircraft (RPA) includes a particular interest in 10–25 kg vehicles for monitoring, surveillance, and reconnaissance. Power-plant options for these aircraft are often 10–100 cm3 internal combustion engines (ICEs). The present study builds on a previous study of loss pathways for small, two-stroke engines by quantifying the trade space among energy pathways, combustion stability, and engine controls. The same energy pathways are considered in both studies—brake power, heat transfer from the cylinder, short circuiting, sensible exhaust enthalpy, and incomplete combustion. The engine controls considered in the present study are speed, equivalence ratio, combustion phasing (ignition timing), cooling-air flow rate, and throttle. Several options are identified for improving commercial-off-the-shelf (COTS)-engine efficiency and performance for small, RPA. Shifting from typical operation at an equivalence ratio of 1.1–1.2 to lean operation at an equivalence ratio of 0.8–0.9 results in a 4% (absolute) increase in fuel-conversion efficiency at the expense of a 10% decrease in power. The stock, linear timing maps are excessively retarded below 3000 rpm, and replacing them with custom spark timing improves ease of engine start. Finally, in comparison with conventional-size engines, the fuel-conversion efficiency of the small, two-stroke ICEs improves at throttled conditions by as much as 4–6% (absolute) due primarily to decreased short-circuiting. When no additional short-circuiting mitigation techniques are employed, running a larger engine at partial throttle may lead to an overall weight savings on longer missions. A case study shows that at 6000 rpm, the 3W-55i engine at partial throttle will yield an overall weight saving compared to the 3W-28i engine at wide-open throttle (WOT) for missions exceeding 2.5 h (at a savings of ∼5 g/min).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112804-112804-12. doi:10.1115/1.4039810.

A triangular rotary engine includes several main components such as an eccentric shaft, a sun gear, a triangular rotor, a chamber, and apex seals. This study constructs the mathematical models for the chamber and triangular rotor profiles in a rotary engine, as well as for the kinematics and contact force of its apex seals by using the epitrochoid and envelope principles. The chamber profile is represented by design parameter, trochoid ratio, whose limitations are investigated together with the volume ratio. To simplify the calculation, the dynamics analysis model ignores the effects of combustion and thermal conditions in rotary engines. Gas force effect is taken into account by first constructing a fluid analysis model that measures the gas fluid moment on the triangular rotor. Then, based on the mathematical models of chamber and rotor, a systematic dynamics analysis model for a rotary engine is built. It allows analyzing the kinematics and the stress variations in all components of the engine. The dynamics model considers both output shaft torsion and fluid moment. The dynamics analysis then uses three cases of trochoid ratio to illustrate the effects of chamber profile design on the system dynamics properties of rotary engines. The results not only show the dynamic properties and differences in various mechanism designs but also indicate the stability and stress level in the components. In conclusion, the higher trochoid ratio with a larger variation in the chamber profile curvature reduces system stability and increases vibrations, stress fluctuations, and large stress peaks risk.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112805-112805-7. doi:10.1115/1.4040198.

Motivated by increasingly strict nitrogen oxides (NOx) limits, engine manufacturers have adopted selective catalytic reduction (SCR) technology to reduce engine-out NOx. In the SCR process, NOx react with ammonia (NH3) to form nitrogen and water vapor. The reaction is influenced by several variables, including stored ammonia on the catalyst, exhaust gas composition, and catalyst temperature. Currently, measurements from NOx and/or NH3 sensors upstream and downstream of the SCR are used with predictive models to estimate ammonia storage levels on the catalyst and control urea dosing. This study investigated a radio frequency (RF)-based method to directly monitor the ammonia storage state of the SCR. This approach utilizes the catalyst as a cavity resonator, in which an RF antenna excites electromagnetic waves within the cavity to monitor changes in the catalyst state. Ammonia storage causes changes in the dielectric properties of the catalyst, which directly impacts the RF signal. Changes in the RF signal relative to stored ammonia (NH3) were evaluated over a wide range of frequencies, temperatures, and exhaust conditions. The RF response to NH3 storage, desorption, and oxidation on the SCR was well correlated with changes in the catalyst state. Calibrated RF measurements demonstrate the ability to monitor the adsorption state of the SCR to within 10% of the sensor full scale. The results indicate direct measurement of SCR ammonia storage levels, and resulting catalyst feedback control, via RF sensing to have significant potential for optimizing the SCR system to improve NOx conversion and decrease urea consumption.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112806-112806-7. doi:10.1115/1.4040292.

The move to lead-free bearing materials is well known, and upcoming legislation, such as the restriction of hazardous substances, is increasing the drive to extend this trend toward heavy-duty diesel truck and off-highway applications. During the development of lead-free systems, new electroplated overlays and bronze-based substrates have been developed by various suppliers, but little attention has been given to the interlayer or diffusion barrier between the overlay and substrate materials. This interlayer is particularly necessary for tin-based solutions as it prevents the rapid diffusion of overlay species into the bronze substrate. The present development focuses on improving this often overlooked element in the system and provides a further robustness that could even be adapted to lead-based systems where increased performance is required. The incorporation of hexagonal boron nitride (hBN) as a solid lubricant in the nickel interlayer changes dramatically the interlayer properties and provides more typical bearing-like behavior for seizure resistance and scuff performance compared to nickel alone. The paper details the findings of respective rig tests as well as an actual engine test supporting the change in material characteristics and the associated improvement in seizure resistance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112807-112807-10. doi:10.1115/1.4040309.

Homogeneous charge compression ignition (HCCI) combustion has the potential for high efficiency with very low levels of NOx and soot emissions. However, HCCI has thus far only been achievable in a laboratory setting due the lack of control over the start and rate of combustion and its narrow operating range. In the present work, direct water injection (WI) was investigated to solve the aforementioned limitations of HCCI. This new advanced combustion mode is called thermally stratified compression ignition (TSCI). A three-dimensional computational fluid dynamics (3D CFD) model was developed using CONVERGE CFD coupled with detailed chemical kinetics to gain a better understanding of the underlying phenomena of the water injection event in a homogeneous, low temperature combustion (LTC) strategy. The CFD model was first validated against previously collected experimental data. The model was then used to simulate TSCI combustion and the results indicate that injecting water into the combustion chamber decreases the overall unburned gas temperature and increases the level of thermal stratification prior to ignition. The increased thermal stratification results in a decreased rate of combustion, thereby providing control over its rate. The results show that the peak pressure and gross heat release rate (HRR) decrease by 37.8% and 83.2%, respectively, when 6.7 mg of water were injected per cycle at a pressure of 160 bar. Finally, different spray patterns were simulated to observe their effect on the level of thermal stratification prior to ignition. The results show that the symmetric patterns with more nozzle holes were generally more effective at increasing thermal stratification.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112808-112808-11. doi:10.1115/1.4039348.

A detailed analysis of a common rail (CR) fuel injection system, equipped with solenoid injectors for Euro 6 diesel engine applications, has been performed in the frequency domain. A lumped parameter numerical model of the high-pressure hydraulic circuit, from the pump delivery to the injector nozzle, has been realized. The model outcomes have been validated through a comparison with frequency values that were obtained by applying the peak-picking technique to the experimental pressure time histories acquired from the pipe that connects the injector to the rail. The eigenvectors associated with the different eigenfrequencies have been calculated and physically interpreted, thus providing a methodology for the modal analysis of hydraulic systems. Three main modal motions have been identified in the considered fuel injection apparatus, and the possible resonances with the external forcing terms, i.e., pump delivered flow rate, injected flow rate, and injector dynamic fuel leakage through the pilot valve, have been discussed. The investigation has shown that the rail is mainly involved in the first two vibration modes. In the first mode, the rail performs a decoupling action between the high-pressure pump and the downstream hydraulic circuit. Consequently, the oscillations generated by the pump flow rates mainly remain confined to the pipe between the pump and the rail. The second mode is centered on the rail and involves a large part of the hydraulic circuit, both upstream and downstream of the rail. Finally, the third mode principally affects the injector and its internal hydraulic circuit. It has also been observed that some geometric features of the injection apparatus can have a significant effect on the system dynamics and can induce hydraulic resonance phenomena. Furthermore, the lumped parameter model has been used to determine a simplified transfer function between rail pressure and injected flow rate. The knowledge obtained from this study can help to guide designers draw up an improved design of this kind of apparatus, because the pressure waves, which are triggered by impulsive events and are typical of injector working, can affect the performance of modern injection systems, especially when digital rate shaping strategies or closely coupled multiple injections are implemented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(11):112809-112809-8. doi:10.1115/1.4039729.

Reliably starting the engine during extremely cold ambient temperatures is one of the largest calibration and emissions challenges in engine development. Although cold-start conditions comprise only a small portion of an engine's typical drive cycle, large amounts of hydrocarbon and particulate emissions are generated during this time, and the calibration of cold-start operation takes several months to complete. During the cold start period, results of previous cycle combustion event strongly influences the subsequent cycle due to variations in engine speed, residual fraction, residual wall film mass, in-cylinder charge and wall temperatures, and air flow distribution between cylinders. Including all these parameters in computational fluid dynamics (CFD) simulation is critical in understanding the cold start process in transient and cumulative manner. Measured cold start data of a production of four-cylinder spark-ignition (SI) direct-injection engine were collected for this study with an ambient temperature of −30 °C. Three-dimensional (3D) transient engine flow, spray, and combustion simulation over first three consecutive engine cycles is carried out to provide a better understanding of the cold-start process. Measured engine speed and one-dimensional (1D) conjugate heat transfer (CHT) model is used to capture realistic in-cylinder flow dynamics and transient wall temperatures for more accurate fuel–air mixing predictions. The CFD predicted cumulative heat release trend for the first three cycles matches the data from measured pressure analysis. The same observation can be made for the vaporized fuel mass as well. These observations are explained in the report.

Commentary by Dr. Valentin Fuster

Discussion

J. Eng. Gas Turbines Power. 2018;140(11):115502-115502-1. doi:10.1115/1.4040519.

The author is to be commended for his excellent paper showing potential efficiency and value advantage for alternative system architectures over today's highly evolved state of the art (SOA) gas turbine combined cycle (GTCC) offerings. Dr. Gülen has deftly outlined the second law constraints facing the engineers immersed in the difficult task of configuring next generation equipment and plant solutions. There are many technology options to be considered, each of which is uniquely positioned across the full complement of customer value parameters by thermodynamic entitlement, the maturity of core technologies, and the development and projected mature system costs. On top of these technical constraints are complex and rapidly changing market dynamics. Today's markets support a broad range of gas fired plants including simple cycle peakers (all sizes), small industrial size systems (often in CHP configurations), and large midrange or base-loaded plants of the H/J class. How will these markets change as low cost but intermittent renewables displace large base loaded fossil plants, including GTCC? What role(s) will gas fired plants play as renewables gain market share and storage goes mainstream? Which market niche will be large enough (and long enough lasting) to recoup the investments necessary to advance GTCC SOA? How scalable does the architecture need to be? How advantaged would a new architecture need to be for market acceptance? How much would it cost to validate it?

Commentary by Dr. Valentin Fuster

Closure

J. Eng. Gas Turbines Power. 2018;140(11):116001-116001-1. doi:10.1115/1.4039829.

I would like to thank to all discussers for their valuable comments. All discussers are unanimous in the main points that they make. Although its expression varies in language and extent among the discussers, the summary of their main arguments goes as follows [1,2]:

Commentary by Dr. Valentin Fuster

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