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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):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

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

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

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

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