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Research Papers

J. Eng. Gas Turbines Power. 2018;141(4):041001-041001-10. doi:10.1115/1.4041628.

In a cover-plate system, rotating receiver hole is an important component, because its structure and characteristics directly influence the aerodynamic loss and cooling performance in the preswirl system. A new type of vane shaped (VS) receiver hole was designed and presented in this paper. Numerical simulations were carried out to compare the performances among high-radius direct transfer system (model-A), low-radius cover-plate system with simple drilled (SD) receiver holes (model-B), and low-radius cover-plate system with VS receiver holes (model-C). Results indicate that for the operating conditions simulated here, temperature drop effectiveness of the high-radius preswirl system is much better compared to the low-radius system with SD receiver hole. With VS receiver hole, the aerodynamic loss in model-C is the lowest. The nondimensional static pressure at preswirl nozzle exit is only 0.93, around 10% lower than model-B. Moreover, it has a more remarkable cooling performance. The temperature drop effectiveness of model-C can be as high as 0.52, around 67.7% higher compared to model-A. The system with VS receiver hole could not only realize the advantage of low leakage flow as a low-radius system, but also could achieve higher temperature drop compared to high-radius system.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041002-041002-9. doi:10.1115/1.4041648.

The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I: Design Concept and Typical Performance of a Swirl Breaker.” When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and the rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In Part II, five cases of geometry with different axial distances between the swirl breaker and the rotor shroud, which covered a range for the stage axial distance of actual high and intermediate pressure (HIP) steam turbines, were investigated using a single-rotor computational fluid dynamics (CFD) analysis and verification tests in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and the rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and the rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the measured maximum pressure fluctuation in the swirl breaker cavity was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041003-041003-9. doi:10.1115/1.4041650.

In high and intermediate pressure (HIP) steam turbines with shrouded blades, it is well known that shroud leakage losses contribute significantly to overall losses. Shroud leakage flow with a large tangential velocity creates a significant aerodynamic loss due to mixing with the mainstream flow. In order to reduce this mixing loss, two distinct ideas for rotor shroud exit cavity geometries were investigated using computational fluid dynamics (CFD) analyses and experimental tests. One idea was an axial fin placed from the shroud downstream casing to reduce the axial cavity gap, and the other was a swirl breaker placed in the rotor shroud exit cavity to reduce the tangential velocity of the leakage flow. In addition to the conventional cavity geometry, three types of shroud exit cavity geometries were designed, manufactured, and tested using a 1.5-stage air model turbine with medium aspect ratio blading. Test results showed that the axial fin and the swirl breaker raised turbine stage efficiency by 0.2% and 0.7%, respectively. The proposed swirl breaker was judged to be an effective way to achieve highly efficient steam turbines because it not only reduces the mixing losses but also improves the incidence angle distribution onto the downstream blade row. This study is presented in two papers. The basic design concept and typical performance of the proposed swirl breaker are presented in this part I, and the effect of axial distance between a swirl breaker and rotor shroud on efficiency improvement is discussed in part II [8].

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041004-041004-7. doi:10.1115/1.4041657.

The response of a silicon carbide (SiC) fibrous ceramic composite to foreign object damage (FOD) was determined at ambient temperature and velocities ranging from 40 to 150 m/s. Target specimens were impacted, at a normal incidence angle and in a partially supported configuration, using 1.59 mm diameter hardened steel ball projectiles. Qualitative analysis of the damage morphologies of targets and projectiles was made via scanning electron microscopy (SEM). In addition, the extent of impact damage was characterized by determining the post-impact strength of each target specimen as a function of impact velocity. Relative to the as-received (As-R) strength, the fibrous composite showed limited strength degradation due to impact with the maximum reduction of 17% occurring at 150 m/s. A quasi-static analysis of the impact force prediction was also made based on the principle of energy conservation and the results were verified via experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041005-041005-9. doi:10.1115/1.4041658.

The flow in intake manifold of a heavily downsized internal combustion engine has increased levels of unsteadiness due to the reduction of cylinder number and manifold arrangement. The turbocharger compressor is thus exposed to significant pulsating backpressure. This paper studies the response of a centrifugal compressor to this unsteadiness using an experimentally validated numerical method. A computational fluid dynamic (CFD) model with the volute and impeller is established and validated by experimental measurements. Following this, an unsteady three-dimensional (3D) simulation is conducted on a single passage imposed by the pulsating backpressure conditions, which are obtained by one-dimensional (1D) unsteady simulation. The performance of the rotor passage deviates from the steady performance and a hysteresis loop, which encapsulates the steady condition, is formed. Moreover, the unsteadiness of the impeller performance is enhanced as the mass flow rate reduces. The pulsating performance and flow structures near stall are more favorable than those seen at constant backpressure. The flow behavior at points with the same instantaneous mass flow rate is substantially different at different time locations on the pulse. The flow in the impeller is determined by not only the instantaneous boundary condition but also by the evolution history of flow field. This study provides insights in the influence of pulsating backpressure on compressor performance in actual engine situations, from which better turbo-engine matching might be benefited.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041006-041006-12. doi:10.1115/1.4041676.

Lean burn combustion is increasing its popularity in the aeronautical framework due to its potential in reducing drastically pollutant emissions (NOx and soot in particular). Its implementation, however, involves significant issues related to the increased amount of air dedicated to the combustion process, demanding the redesign of injection and cooling systems. Also, the conditions at the combustor exit are a concern, as high turbulence, residual swirl, and the impossibility to adjust the temperature profile with dilution holes determine a harsher environment for nozzle guide vanes. This work describes the final stages of the design of an aeronautical effusion-cooled lean burn combustor. Full annular tests were carried out to measure temperature profiles and emissions (CO and NOx) at the combustor exit. Different operating conditions of the ICAO cycle were tested, considering Idle, Cruise, Approach, and Take-off. Scale-adaptive simulations with the flamelet generated manifold (FGM) combustion model were performed to extend the validation of the employed computational fluid dynamics (CFD) methodology and to reproduce the experimental data in terms of radial temperature distribution factor (RTDF)/overall temperature distribution factor (OTDF) profiles as well as emission indexes (EIs). The satisfactory agreement paved the way to an exploitation of the methodology to provide a deeper understanding of the flow physics within the combustion chamber, highlighting the impact of the different operating conditions on flame, spray evolution, and pollutant formation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(4):041007-041007-8. doi:10.1115/1.4041659.

One method frequently used to reduce NOx emissions is exhaust gas recirculation, where a portion of the exhaust gases, including NOx, is reintroduced into the combustion chamber. While a significant amount of research has been performed to understand the important fuel/NOx chemistry, more work is still necessary to improve the current understanding on this chemistry and to refine detailed kinetics models. To validate models beyond global kinetics data, such as ignition delay time or flame speed, the formation of H2O was recorded using a laser absorption diagnostic during the oxidation of a mixture representing a simplistic natural gas (90% CH4/10% C2H6 (mol)). This mixture was studied at a fuel lean condition (equivalence ratio = 0.5) and at atmospheric pressure. Unlike in conventional fuel-air experiments, NO2 was used as the oxidant to better elucidate the important, fundamental chemical kinetics by exaggerating the interaction between NOx and hydrocarbon-based species. Results showed a peculiar water formation profile, compared to a former study performed in similar conditions with O2 as oxidant. In the presence of NO2, the formation of water occurs almost immediately before it reaches more or less rapidly (depending on the temperature) a plateau. Modern, detailed kinetics models predict the data with fair to good accuracy overall, while the GRI 3.0 mechanism is proven inadequate for reproducing CH4/C2H6 and NO2 interactions.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;141(4):042501-042501-12. doi:10.1115/1.4041123.

Labyrinth gas seals (LSs) commonly used in turbomachines reduce secondary flow leakage. Conventional see-through labyrinth seal designs include either all teeth-on-stator (TOS) or all teeth-on-rotor (TOR). Experience shows that an interlocking labyrinth seal (ILS), with teeth on both stator and rotor, reduces gas leakage by up to 30% compared to the conventional see-through designs. However, field data for ILS rotordynamic characteristics are still vague and scarce in the literature. This work presents flow predictions for an ILS and a TOS LS, both seals share identical design features, namely radial clearance Cr = 0.2 mm, rotor diameter D = 150 mm, tooth pitch Li = 3.75 mm, and tooth height B = 3 mm. Air enters the seal at supply pressure Pin = 3.8, 6.9 bar (absolute) and temperature of 25 °C. The ratio of gas exit pressure to supply pressure ranges from 0.5 to 0.8, and the rotor speed is fixed at 10 krpm (surface speed of 79 m/s). The analysis implements a computational fluid dynamics (CFD) method with a multi-frequency-orbit rotor whirl model. The CFD predicted mass flow rate for the ILS is ∼ 21% lower than that of the TOS LS, thus making the ILS a more efficient choice. Integration of the dynamic pressure fields in the seal cavities, obtained for excitation frequency (ω) ranging from 12% to 168% of rotor speed (sub and super synchronous whirl), allows an accurate estimation of the seal dynamic force coefficients. For all the considered operating conditions, at low frequency range, the TOS LS shows a negative direct stiffness (K < 0), frequency independent; whereas the ILS has K > 0 that increases with both frequency and supply pressure. For both seals, the magnitude of K decreases when the exit pressure/inlet pressure ratio increases. On the other hand, the cross-coupled stiffness (k) from both seals is frequency dependent, its magnitude increases with gas supply pressure, and k for the ILS is more sensitive to a change in the exit/inlet pressure ratio. Notably, k turns negative for subsynchronous frequencies below rotor speed (Ω) for both the TOS LS and the ILS. The direct damping (C) for the TOS LS remains constant for ω > ½ Ω and has a larger magnitude than the damping for the ILS over the frequency range up to 1.5 Ω. An increase in exit/inlet pressure ratio decreases the direct damping for both seals. The effective damping coefficient, Ceff = (C-k/ω), whenever positive aids to damp vibrations, whereas Ceff < 0 is a potential source for an instability. For frequencies ω/Ω < 1.3, Ceff for the TOS LS is higher in magnitude than that for the ILS. From a rotordynamics point of view, the ILS is not a sound selection albeit it reduces leakage. Comparison of the CFD predicted force coefficients against those from a bulk flow model demonstrates that the later simple model delivers poor results, often contradictory and largely indifferent to the type of seal, ILS or TOS LS. In addition, CFD model predictions are benchmarked against experimental dynamic force coefficients for two TOS LSs published by Ertas et al. (2012, “Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio,” ASME J. Eng. Gas Turbines Power, 134(4), pp. 04250301–04250312) and Vannini et al. (2014, “Labyrinth Seal and Pocket Damper Seal High Pressure Rotordynamic Test Data,” ASME J. Eng. Gas Turbines Power, 136(2), pp. 022501–022509.)

Commentary by Dr. Valentin Fuster

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