bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance of FGM in Bluff-Body Stabilized H2-CO Diffusion Flame
Authors: Hofsteenge Jesse W. , Kok Jim B. W.
Published: October 2022

Blends of H2 with CH4 and/or CO have become of interest with hydrogen as sustainable fuel. In the past, numerical models were derived to simulate combustion of H2-CO (syngas) mixtures. This paper compares the performance of a newly implemented Flamelet Generated Manifold (FGM) combustion model to an earlier model by Correa and Pope and experimental data in the bluff-body stabilized syngas flame by Correa and Gulati. The FGM in the presented model consists of premixed flamelets stretching the flammable region, parametrized by mixture fraction and Computational Singular Perturbation-based reaction progress variable. RANS simulations are performed in Fluent with k-ε, k-ω SST and Reynolds Stress Model turbulence modelling. Radial and axial profile plots of temperature and major species concentrations showed generally improved results with the FGM model, although locally deviations occur. Causes for these deviations are scrutinized: the use of premixed opposed to diffusion flamelets and the unity Lewis number assumption.
Aerothermal Performance of Axially Varying Winglet-Squealer Blade Tips
Authors: Garg Anmol , Vadlamani Nagabhushana Rao , Srinivasan Balaji
Published: October 2022

High-pressure turbine blades are usually susceptible to secondary flow losses due to fluid flow between the casing and the blade tip. In this study, we have evaluated the performance of several blade tip designs for different combinations of winglets and squealer geometries towards mitigating tip leakage losses. The effects of considering Relative Casing Motion (RCM) on the aerothermal performance is also brought out. In particular, we have considered three different blade tip designs which include winglets, Top Winglet Bottom Squealer (TWBS) and Top Squealer Bottom Winglet (TSBW). Inspired by the partial winglet configurations studied in the literature, we have also examined designs with partial squealers and winglets. The performance of all the designs and the dynamics within the tip gap are discussed through the distributions of total pressure loss within the tip gap and Nusselt number over the blade tips. Of all the blade tip designs, aerothermal performance of a 100% TWBS design is demonstrated to be superior, both with and without relative casing motion. When compared to a flat tip design, a 100% TWBS design showed a 15% reduction in total pressure loss and a 22% reduction in the average Nusselt number over the blade tip. For this design, accounting for the relative casing motion showed a marked reduction in the total pressure loss and the heat transferred to the blade tip. In particular, RCM is shown to suppress the ‘hot-spot’ associated with a cavity vortex within the squealer cavity.
Comprehensive Analysis of a Cycloidal Rotor With Different Types of Blade Profiles
Authors: Staśko Tomasz , Majkut Mirosław , Dykas Sławomir , Smołka Krystian
Published: October 2022

From the historical point of view, the research on cyclorotors has been focused on aviation applications, such as propulsion systems for UAVs. There is a need for studies on the use of a cycloidal rotor fan (CRF) in internal-flow applications, such as power or HVAC applications. This paper presents considerations of the cycloidal rotor application for a fan. The possibility to change the flow angle and its direction makes the CRF a potential solution in HVAC systems. A very desirable feature is the lack of a diffuser, which simplifies construction and the ability to operate the CRF as a puller or a pusher fan in rectangular ventilation ducts. Two variants of the CRF were tested numerically and experimentally. The first variant was equipped with asymmetrical CLARK Y blades and the second one — with symmetrical NACA0012 blades. The blades were selected based on the universality of their use and their popularity in scientific research. For each rotor, experimental tests were carried out for the blade amplitude of 40 degrees, and rotational speed of 1000 rpm. The experimental results were compared with those obtained from numerical calculations, and the comparison results were satisfactory. Due to the small size of the fan, no significant differences were noticed in the speed profiles for the two rotor variants.
LES-CHT for a Rotating Cavity with Axial Throughflow
Authors: Hickling Tom , He Li
Published: October 2022

The flow and heat transfer within rotating cavities is often discussed as a conjugate problem: the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES).

The large timescale disparity between the fluid time steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid domain mesh can artificially amplify higher frequency temperature fluctuations in the fluid domain.

This paper addresses these challenges with a new method for LES-CHT where the low-frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud.

The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity — this interaction indicates that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures on the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modelling argument usually used to consider this region of the flow.
Market Opportunities for Power-to-X Solutions for Combined Cycles Flexibilization
Authors: Barberis Stefano , Cuneo Alessandra , Bellotti Daria , Sorce Alessandro , Vannoni Alberto
Published: October 2022

Power-to-X solutions are getting year by year more and more market interest in the present and future Renewable (RES) driven energy market scenario. Their potential coupling with Combined Cycles Gas Turbine (CCGT), which have to be present together with RES as back-bone to guarantee grid resilience and stability, is currently under investigation also in order to enhance CCGT flexibility as short-/middle-term storage solutions.

The H2020 EU Funded PUMP-HEAT and FLEXnCONFU projects1 study two different types of Power-to-X solutions: the former a power-to-heat concept by heat pump coupling and the latter a power-to-hydrogen/ammonia system via Electrolyser coupling), both targeting flexibility enhancement and emissions reduction. This paper presents and compares the market potential of Power-to-X solutions proposed in the two projects presenting guidelines for the upscale of the proposed Power-to-X solutions derived from a techno-economic assessment of the two Power-to-X solutions.

These guidelines will bring to a preliminary assessment of the flexibility and electric market potential of Power-to-X solutions coupled with CCGTs, presenting specific features per both of them such as the most techno-economic viable sizes or market characteristics to leverage Power-to-X potential. Furthermore, the market potential is presented by analysing bottlenecks and drivers (in current and future EU scenarios) that could facilitate or hamper the widespread of the proposed technologies.
System Parameter Identification of a Colored-Noise-Driven Rijke Tube Simulator
Authors: Vishnoi Neha , Gupta Vikrant , Saurabh Aditya , Kabiraj Lipika
Published: October 2022

In this paper, we report an experimental investigation on the influence of colored noise (generated by the Ornstein-Uhlenbeck process) on thermoacoustic coupling in an electroacoustic Rijke tube simulator. In the absence of noise, the simulator exhibits subcritical Hopf bifurcation. Although, noise in a practical system has a finite correlation time, yet the system identification methods are based on the assumption of white noise. In this study, we investigate the effects of correlation time and intensity of colored noise on the estimation of the growth rates of acoustic oscillations determined using Fokker Planck equation in stable, bistable and linearly unstable regions. Subsequently, we compare the findings against results obtained considering white noise approximations. We report the observed deviation of the estimated growth rates from the actual values as a function of noise intensity and correlation time. We find that with the colored noise model, the deviation in the estimated growth rates lies within the range of 0–10% compared to the deviation of 5–25% observed considering the white noise approximation. We also report that increasing noise amplitudes leads up to a deviation of approximately 30% in the estimated growth rates from the actual values.
The Effect of Rectangular Confinement Aspect Ratio on the Flame Transfer Function of a Turbulent Swirling Flame
Authors: Ånestad Aksel , Ahn Byeonguk , Nygård Håkon T. , Worth Nicholas A.
Published: October 2022

The Flame Transfer Function (FTF) has been experimentally investigated for a premixed, turbulent and highly swirled flame in rectangular enclosures with varying aspect ratio. A range of equivalence ratios and inlet bulk velocities are considered to vary the length of the flame. Increasing confinement is also shown to increase flame length. More confined flames become increasingly asymmetric, with significant changes to the mean flame shape. Characteristic peaks and dips are observed in the FTF gain for several configurations, caused by constructive and destructive interference between different sources of flow perturbations. Changes in the geometrical confinement distinctly affects these interactions, with FTFs in the most confined chamber containing more pronounced peaks and dips. Analysis of the phase-averaged dynamics has been conducted for two limiting operational conditions in the least and most confined enclosures: a short flame at a low bulk velocity and high equivalence ratio condition; and a long flame at a high velocity and low equivalence ratio condition. The analysis of the short flames shows similar behaviour in both enclosures, both in terms of the global response and the local structure of the heat release rate oscillations. Small differences in local response symmetry in the flame due to the close confinement do not affect the global flame response. By comparison, close confinement significantly affects the symmetry of the flame dynamics in the long flame. However, the changes in symmetry do not significantly modify the response, and more important is the change in flame length, which significantly alters the cut-off frequency, reducing the gain at high frequencies.
Novel Fuel Injector Geometry for Enhancing the Fuel Flexibility of a Dry Low NOx MicroMix Flame
Authors: Kroniger Daniel , Horikawa Atsushi , Okada Kunio , Ashida Yuji
Published: October 2022

A novel fuel injector is presented for enhancing the fuel flexibility of the dry micromix (MMX) combustion principle. Originally having been developed for pure hydrogen fueling, the micromix combustion is based on a non-premixed type jet-in-crossflow mixing for inherent safety against flashback, as well as flame miniaturization and multiplication for suppressing NOx emissions. This study investigates the potential of the novel geometry regarding the operation with higher natural gas content fuels. In experiments at atmospheric pressure conditions it could be shown that the new injector extends the fuel flexibility down to between 60 and 80 vol.% H2. The responsible flow phenomena are verified with numerical RANS simulations at engine pressure conditions using a detailed chemistry model. An experimental validation of numerical methods at atmospheric conditions based on OH chemiluminescence distributions showed that the unsteady LES model can predict the micromix flame more accurately regarding to its ignition point than the steady RANS model, although the reaction progress is underestimated by both models, which in comparison to the experiment results in a more stretched flame.
Large Eddy Simulation of Turbulent Flow Through a Compressor Cascade
Authors: Haider Rizvi Syed Anjum , Mathew Joseph
Published: October 2022

A study of large eddy simulations (LES) for compressor blade flows has been undertaken and assessed by comparisons with the measurements of Hobson et al. (AIAA J. Prop. Power, 17, 154 (2001)) on a linear cascade of controlled-diffusion airfoils at Reynolds numbers based on blade chord and inflow velocity of 210 000, 380 000 and 640 000. LES of the latter two conditions are reported here. LES is by an explicit filtering method. Compressible flow equations in curvilinear coordinates were solved with 8th-order, central, finite differences and 2nd order Runge-Kutta integration. At 380 000 there is close quantitative agreement of the blade surface pressures and blade normal profiles of velocity magnitudes and fluctuations. Features of the initial development of the suction surface boundary layer is typical of transition induced by freestream turbulence, but end-stage turbulent spots are missing because of premature completion of transition over a mid-chord separation bubble. At 640 000 this separation is absent and turbulent spots appear before transition is completed. Blade surface pressure distribution from the LES agrees closely with that from experiment and is consistent with absence of separation, as are velocity profiles. Velocity profiles from experiment however had shown substantial open separation from the suction surface. This discrepancy had been noted before and was supposed to be an endwall effect, but our LES, including one with endwalls, does not support that view.
Sensitivity Study on Species Diffusion Models in Turbulent Combustion of Hydrogen/air Jet in Crossflow Structure
Authors: Sun Xiaoxiao , Martin Harry , Gauthier Pierre , Sethi Bobby
Published: October 2022

CFD approach has been intensively used for novel hydrogen combustion technologies as well as emissions prediction. In order to minimise time and costs, simplified combustion CFD models have been developed to take into account of the effect of turbulence-chemistry interactions, coupled with both RANS and LES models. These models often employ assumptions that have been validated for conventional fuels such as Jet-A1 and natural gas. It is clear that the preferential diffusion and thermal diffusion play a vital role in hydrogen laminar premixed flames. Conventionally, it is believed that for high Re cases, turbulent diffusion is dominant, preferential diffusion and thermal diffusion only have a minor effect. For highly turbulent non-premixed flames, these effects have not been fully investigated. The appropriate modelling of these effects is essential for reaction rate, flame temperature and emissions prediction. There is a need to study the sensitivity of different species diffusion models for hydrogen-air flames.

Within the current study, a single micromix injector burning a hydrogen air mixture with an overall equivalence ratio of 0.4 has been simulated in a systematic RANS campaign, and compared with LES. The RANS campaign employs two combustion models: FGM with the kinetic rate closure and Complex Chemistry (CC) with both the laminar flame concept (LFC) and eddy dissipation concept (EDC) closures. The LES employs the same combustion models. The sensitivity of Lewis and Schmidt numbers have been investigated for FGM and EDC model. As expected, FGM is insensitive to the value of Le and Sc whereas for LFC, significant changes have been observed between Le = 0.25 and 0.5, Sc = 0.5 and 0.75. Several species diffusion coefficient models have been used with both LFC and EDC, including: mixture averaged, multi-component and multi-component with thermal diffusion coefficients. All three approaches of modelling molecular diffusivity yielded flame lengths and temperatures of a similar order. The micromix flame was found to be a mixture of both premixed and non-premixed, with jets of strongly premixed and non-premixed originating from the hydrogen injectors.

The sensitivity of turbulent Sc and Le numbers has also been studied with LFC models. It was clearly shown that these changes lead to more significant variation in flame length and temperature compared to those caused by different molecular diffusivity models. LES results with various multi-component diffusion models are compared, the results are in better agreement relative to RANS with different models. The similarity in flame length and insensitivity to CC closure type demonstrates the dominance of turbulence in determining flame structure, through turbulent mixing.
On the Impact of the Turbulence Model on the Secondary Flow Structure of a Highly-Loaded Compressor Stage
Authors: Toracchio Riccardo , Fontaneto Fabrizio , Hillewaert Koen
Published: October 2022

RANS turbulence models still represent a weak point of industrial simulations of turbomachinery flows. In presence of a large blade loading the turbulence model can impact severely on the prediction of the end-wall flow and secondary structures in the blade passage, with an important effect on the computed global performance and flow stability. This paper presents a turbulence model comparison for the characterization of a highly-loaded low-pressure compressor at multiple operating points using 3D RANS simulations. The turbulence models mostly employed for the design and analysis of axial compressors are considered, with the aim of providing a clear physical description of their impact on the critical flow features of the machine. It will be shown that no single turbulence model is generally better than others and the prediction depends on the operating point, with increasing discrepancies as the mass-flow is reduced.
Interaction Mechanism of Transonic Squealer Tip Cooling With the Effect of High-Speed Relative Casing Motion
Authors: Xie Wenbo , Jiang Hongmei , Lu Shaopeng , Peng Xu , Zhang Qiang
Published: October 2022

The relative casing motion can significantly influence the turbine blade tip aerothermal performance. In this study, experimental investigation was conducted in a newly developed high-speed disk rotor rig which can mimic engine realistic highspeed casing relative motion while enabling full optical access to a transonic turbine blade tip surface. Spatially-resolved tip heat transfer data, including heat transfer coefficient and film cooling effectiveness, were obtained for a cooled transonic squealer tip by infrared transient thermal measurement. Combined with closely coupled RANS CFD analysis, this paper reveals an interesting interaction mechanism between the cooling injections from the pressure side and the cavity floor with and without the effect of relative casing motion. Both experimental data and CFD results show a consistent trend in both heat transfer and cooling performance. With the cavity cooling only, the cooling performance reduces with the effect of relative casing motion. However, with additional cooling injection from the pressure side, a significant improvement in the combined cooling performance with the relative casing motion can be observed. Such opposite trend highlights the importance of relative casing motion when ranking different tip cooling designs. With the consideration of relative casing motion, extra tip cooling benefit can be obtained by combining cooling injections from two different locations.
Knowledge-Based Process Design Optimisation in Blisk Manufacturing
Authors: Landwehr Markus , Ganser Philipp , Vinogradov Georg , Bergs Thomas
Published: October 2022

The manufacturing process of blade-integrated disks (blisks) represents one of the most challenging tasks in turbomachinery manufacturing. The requirement is to machine complex, thin-walled blade geometries with high aspect ratios made of difficult-to-cut materials. In addition, extremely tight tolerances are required, since the smallest deviations can lead to a reduction in efficiency of the blisk in the later use.

Nowadays, the ramp-up phase for the manufacturing of a new blisk is time and cost intensive. To find a suitable manufacturing process that meets the required tolerances of the blisk, many experimental tests with different process parameters and strategies are necessary. The used approach is often trial and error which offers limited testing opportunities, is time consuming and wastes resources.

Therefore, the objective of this paper is to develop a knowledge-based process design optimization in blisk manufacturing. For this purpose, this paper picks up the results from our previous work. Based on these results, an experimental validation of the two process design tasks “number of blocks” and “block transition” is conducted. As part of the validation, the results of machining tests on a demonstrator blisk made of Inconel 718 are presented and discussed.
Effects of Part-to-Part Flow Variations on Overall Effectiveness and Life of Rotating Turbine Blades
Authors: Knisely Brian F. , Berdanier Reid A. , Wagner Joel H. , Thole Karen A. , Arisi Allan R. , Haldeman Charles W.
Published: October 2022

As firing temperatures in gas turbine engines continue to increase to achieve high efficiencies, components in the main gas path must be protected with cooling flows to ensure lifing targets are met. Manufacturing variations, however, influence the performance and life characteristics of components with the same nominal design. This study presents blade flow and overall cooling effectiveness measurements for nine true-scale, aero engine turbine blades with realistic manufacturing variations. Flow measurements were made through each blade at a fixed pressure ratio to determine flow variability between holes and between blades. Infrared thermography was used to capture spatially-resolved temperature measurements reported as overall effectiveness on the same nine blades under high-speed rotating conditions at the Steady Thermal Aero Research Turbine Laboratory. Thermal performance was correlated with blade flow performance indicating substantial blade-to-blade variations resulting from manufacturing differences. Measurements also indicated wide variations in cooling jet trajectories as well as overall cooling effectiveness. Finally, the observed blade-to-blade variations in effectiveness were scaled to engine conditions with lifing estimates showing some blades would be expected to last only half as long as others due to manufacturing variability.
A Numerical Investigation of Reheat Hydrogen Combustion in a Simplified Geometrical Configuration From Atmospheric Pressure to Full Load Conditions
Authors: Gruber Andrea , Heggset Tarjei , Duesing Michael , Ciani Andrea
Published: October 2022

An increasing amount of recent experimental evidence indicates that sequential combustion is particularly well-suited for burning highly-reactive fuels like hydrogen, while maintaining low emissions. A convenient feature of the sequential combustion system, resulting in a fundamental advantage compared to alternative approaches, is the possibility of controlling the second-stage flame position through its combustion characteristics, defined by a complex balance of propagation versus spontaneous ignition, based mainly on the reactants’ inlet temperature. At full-load conditions, requiring high pressure and high flame temperature, fuel mixtures with a hydrogen content approaching 100% still bring significant challenges, it is therefore of key importance for the further development of hydrogen-firing capabilities of the gas turbine to improve our present understanding of the interaction between flame propagation and spontaneous ignition and of its role in controlling flame stability and emissions. A series of DNS and LES calculations, featuring complex chemical kinetics and a fully-compressible representation of the reactive flow, are performed on simplified geometrical configurations, yet representative of a sequential combustion system. The present research effort provides novel insight about the combustion characteristics of hydrogen reheat flames at nominal part- and full-load conditions, defining their structure and stabilization mechanism for a range of reactants temperature, as well as modelling guidelines for a reliable numerical approach to reheat combustion.
Effect of Flare Geometry on the Flow Field of Radial-Radial Swirlers
Authors: Bay Ayşe , Kıyıcı Fırat , Perçin Mustafa
Published: October 2022

In this study, an experimental investigation is conducted to assess the impact of the flare geometry on the mean flow field generated by radial-radial swirlers. Two-dimensional two-component PIV measurements are performed on the mid-plane of a non-reacting planar combustor test section. Three-dimensional numerical simulations are conducted for selected cases to support experimental observations. In a previous study conducted in the same setup, counter-rotating radial-radial swirlers without flare extension were investigated. In this study, in addition to the previously studied baseline swirler geometry, four different swirlers are investigated with three different flare geometries (a rounded flare geometry with a radius of 4 mm and two chamfered flares at angles of 27.5° and 45°) with the rounded one having both co- and counter-rotating configurations. Analysis of the time-averaged flow fields reveals that there is an increase in radial velocity values and a decrease in axial velocity values as a result of the introduction of the flare geometry, which results in a sudden expansion of the swirling jet. When different flare geometries are compared, almost identical flow fields are observed and the formation of a CRZ is not observed for any geometry that employs a flare geometry. Although the maximum negative axial velocity values decrease for geometries with flare, due to the increase of the recirculation radius, the recirculating mass flow rate is higher than the baseline swirler. On the other hand, the recirculating mass flow rate is higher in the co-rotating swirler configuration due to stronger adverse pressure gradient along the central axis of the jet when compared to counter-rotating configuration. Coherent flow structures are identified by using the snapshot POD method and different mode shapes obtained for swirlers with and without flare geometry are reported. It is shown that the change of the sense of rotation and flare geometry does not bring about any differences in the POD modes and their energy contents for the given swirl number and confinement conditions.
Leakage Characteristics of a Stepped Balance Piston Labyrinth Seal in Supercritical CO2
Authors: Wilkes Jason , Smith Natalie R. , Wygant Karl , Patil Abhay , Kulhanek Chris
Published: October 2022

In an integrally geared compressor, there is a net thrust load on the pinion shaft from gear loads and aerodynamic forces. The net thrust on the pinion is resolved with either a thrust bearing or a thrust collar (rider ring). As the pressure increases, this increases the aerodynamic force proportionally. In high-pressure systems this can overwhelm the thrust bearing or thrust collar if thrust is mismanaged. In a recent test program for an sCO2 integrally geared compander (compressor/turbine), a balance piston was created on the opposite end of the turbine to provide an equalizing thrust; while this is not novel, the balance piston provided a test apparatus for measuring the leakage on a 20 tooth stepped labyrinth seal operating at pressures in excess of 250 bar (3600 psi).

The stepped balance piston seal is a 20 tooth tooth-on-rotor balance piston seal. This type of seal was selected since it was predicted to have substantially less leakage than a comparably sized straight through labyrinth seal. Comparisons between the two designs will be made, showing that the leakage of the stepped balance piston seal is considerably lower. The tooth clearance on the seal was selected to be 30 mils radial. While this seems large, a stack-up tolerance assessment showed that this magnitude of clearance was appropriate to prevent rotor stator contact.

During the testing campaign, data was recorded showing the leakage across this stepped balance piston seal at pressures in excess of 250 bar (3600 psi) with a pressure ratio of 2. This leakage data was measured, and will be compared to leakage data predicted with CFD. Two measurement methods are presented for the stepped balance piston seal: an orifice plate and valve.

The predicted seal leakage from a straight through seal was four times that of the predicted stepped labyrinth seal. The measured flow from the stepped seal was on the same order as the CFD prediction and offered adequate rotordynamic performance.
Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames
Authors: Wiseman Samuel , Gruber Andrea , Dawson James R.
Published: October 2022

Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NOx from fuel-bound nitrogen. Previous experimental and theoretical work has demonstrated that partially-dissociated ammonia (NH3/H2/N2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H2/N2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, NH3/H2/N2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H2/N2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cut-off frequency was found to be greater, due to a shorter flame length. The results suggest that NH3/H2/N2 blends may excite different thermoacoustic modes in gas turbines. In addition, the dependence of the flame response on forcing amplitude was measured for a forcing frequency of 650 Hz and the linearity of the NH3/H2/N2 flame response up to high forcing amplitudes suggests that particularly high-amplitude limit cycles may occur. For both CH4 flames and NH3/H2/N2 flames the confinement diameter was found to have a strong influence on peak gain values. The effect on the FTF phase was modest, except in the case of extreme confinement, where almost all of the flame is close to the wall, and in the case of a significant change in the flame stabilisation. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cut-off frequency and phase curves.
Investigation of Gas Turbine Internal Cooling Using Supercritical CO2 – Effect of Surface Roughness and Channel Aspect Ratio
Authors: Roy Arnab , Searle Matthew , Ramesh Sridharan , Straub Douglas
Published: October 2022

In this paper, an experimental and numerical investigation of internal cooling channels with rib turbulators is presented with sCO2 as the working fluid at process conditions (pressure-20.7 MPa and temperature up to 150 °C). The effect of channel aspect ratio up to 2:1 on thermal-hydraulic performance is explored in additively manufactured rectangular channels and square channels, both with and without 60° ribs on the top and bottom sides. The Wilson-plot method is employed to experimentally measure channel averaged Nusselt number over a Reynolds number range up to 370,000. The friction factor is calculated from pressure drop and mass flow rate and additionally, the overall thermal performance factor (TPF) is reported. A companion computational fluid dynamics (CFD) simulation is performed for the rib turbulated cooling configurations reported in the experiments using the Reynolds Average Navier-Stokes (RANS)-based turbulence model. The objective of the numerical study is to gain insight into the local heat transfer augmentation in the ribbed channels as a result of varying the aspect ratio, channel configuration (square vs rectangular), operating conditions (Reynolds number) and the surface roughness, an inherent outcome of the additive manufacturing process. Surface roughness is simulated using sand grain roughness height (KS) calculated from the experimental data, and a comparison is presented with the corresponding channel configuration with varying surface roughness heights starting from smooth surfaces (KS = 0). Experimental results indicate that the heat transfer augmentation is negligible in the rectangular channels with ribs on the long side while compared to the square channel. However, it is enhanced by 60% in comparison to placing ribs on the shorter side. The TPF remains constant around 1 for the entire range of Reynolds numbers consistent with prior work at NETL. The simulation results highlight that increased surface roughness can have a favorable considerable influence on Nusselt number and overall thermal performance enhancement.
Fused Deposition Modeling Fabrication Evaluation of a Ti-6Al-4V Centrifugal Compressor
Authors: Warner Justin , Celli Dino , Scott-Emuakpor Onome , George Tommy , Tomlin Trevor
Published: October 2022

Centrifugal compressors have a critical impact on the performance of a jet engine and can be made out of aluminum, steel, or titanium alloy. However, if made out of Ti-6Al-4V it would be more lightweight than if steel were used and stronger than if aluminum were used. Compressors manufactured using traditional techniques can be relatively expensive to manufacture, therefore, fused deposition modeling (FDM) or fused filament fabrication (FFF) can help reduce the cost while maintaining structural integrity. FDM/FFF usually prints polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS) layer-by-layer using an extruder with a spool of material feeding into the extruder. Recently, metal-polymer filaments have become commercially available using this same printing apparatus as common “hobby-class” FDM printers. Using FDM with a metal-polymer matrix allows for a lower cost of production because of the intricate designs it can accomplish with little to no machining involved. Additive printing with Ti-6Al-4V is challenging because of the health and safety concerns of the powder, but having the material in a spool eradicates these concerns of handling Ti-6Al-4V due to the Ti-6Al-4V powder being surrounded by a polymer. Even with the benefits of printing a spool of material using FDM, there has been very little research done of Ti-6Al-4V printed in a FDM format. Printing using the FDM process is known to create shrinkage of the part, it has been documented of different shrinkage percentages for steel using FDM but not Ti-6Al-4V. This document records the feasibility of using Ti-6Al-4V for a centrifugal compressor using FDM by incorporating a 3D structured blue light scanner before heat treatments while also analyzing manufacturing capabilities at the tips of blades as well as the base. This analysis is a first-stage to quantify FDM metal print characteristics and properties.
Experimental Determination of Blade Tip Rub Forces in a Multi-Blade System
Authors: Reifsnyder Robin , Keener Chris , Mathison Randall , D’Souza Kiran
Published: October 2022

As improved efficiency goals require ever smaller gaps between rotating turbomachinery blade tips and stationary outer casings, it becomes more important to understand the physics of what happens when a blade tip rubs against the casing. A key piece of information to be determined is the force exerted on the blade tip during the rub event. This paper presents a method of extracting the blade tip rub forces from experiments conducted at The Ohio State University Gas Turbine Laboratory. To identify the forces during the rub, three multi-axis load cells are mounted behind a segment of the casing to measure the forces exerted on the casing during the rub event. Recovering blade tip forcing from this load cell data requires creating an appropriate model that can relate the measured data at the load cells to the highspeed rotating blade loads. The continuous, repeated nature of the rubs in the experiment makes this a challenge. This paper discusses the creation and performance of a state-space realization model developed from stationary ping test data that is used to address these challenges. The model is augmented with information on the angular position of the blades from a rotary encoder and information on the angular span of the rub, which increases with rub depth. Challenges and solutions related to finding tangential frequency responses, simultaneous blade rubs, and irregularities in the experimental data are also discussed. This paper aims to summarize the process and effectiveness of this approach for finding blade tip forces, as well as lessons learned along the way.
Stall Margin Improvement in a Transonic Compressor With a Casing Treatment: Flow Mechanism
Authors: Hah Chunill
Published: October 2022

A compressor casing treatment with circumferential casing grooves (CCGs) is studied in detail. The primary objective of the current paper is to unearth the main driving fluid mechanism that changes the stall margin in a transonic compressor with CCGs.

Large Eddy Simulation (LES) is applied to calculate the transonic compressor flow fields with and without CCGs. Detailed comparisons between measured and computed flow fields show that changes in flow structures due to CCGs are very well represented with LES. The measured data show that flow blockage generation in the tip gap area is reduced significantly when CCGs are applied, which delays compressor stall onset. Calculated flow fields with and without CCGs are examined to understand how the blockage generation is reduced with CCGs.

Tip leakage flow typically travels upstream, opposite to the incoming main flow. When CCGs are applied on the compressor casing, part of the tip leakage flow goes into the grooves and is later reinjected into the main flow passage. Therefore, CCGs modify the collision between the incoming main passage flow and the tip leakage flow.

Two main aspects of the change of tip leakage flow due to CCGs are examined in the present study. The first aspect is whether the total mass flow through the tip gap changes substantially with CCGs. The second aspect is whether changes in the collision and mixing of the incoming main passage flow and the tip leakage flow due to CCGs substantially affect blockage generation.

The present investigation shows that CCGs reduce the mass flow through the tip gap by about 16 % near the stall-condition. Calculated flow fields show that most of this reduction of tip leakage flow occurs near the CCGs. Reinjected flow from the CCGs pushes the tip leakage flow radially inward below the casing and changes how the tip leakage flow collides with the incoming main passage flow. However, detailed examination of the calculated flow in the tip region shows that the reinjected flow does not contribute to the reduction of the overall blockage generation. The primary driver reducing blockage generation with CCGs is the reduction of overall mass flow rate through the tip gap.

It has been thought that circumferential grooves always incur small additional losses near the design condition due to mixing between the flow exiting the grooves and the main flow, in addition to an increased wetted area. In the present investigation, measurements show such a decrease in efficiency with CCGs, although the difference in efficiency is within the measurement uncertainty. Results from the LES simulation at the design condition with CCGs show that the tip leakage vortex (TLV) is pulled toward the blade suction side and double leakage flow is eliminated. The result is that the simulated efficiencies with and without CCGs are almost the same.
The Numerical and Experimental Evaluation of a Coupled Blade Dynamic Limit Response With Friction Contacts
Authors: Grant Kelly , Gastaldi Chiara , Berruti Teresa M. , Kellerer Rudolf
Published: October 2022

Increasing demand on turbine power and efficiency requires larger and higher loaded turbine blades, which in turn requires the consideration of aeromechanical interactions. Whilst CFD tools can reliably predict stability using aerodynamic damping as an indicator, the component of mechanical damping also needs consideration. An understanding of the mechanical damping in the system becomes key to a robust blade design. Mechanical damping for such a part comes predominantly from friction occurring at the coupling contact faces. It is well established and published that such contact forces are nonlinear in relation to the relative movement at the contact interface. Moreover, contact area, the rigidity in the contact, friction coefficient, and normal contact force must also be considered and included as parameters that influence the result. Consequently, the level of system damping is not a constant, and depends highly on the system response itself, as well as the other forementioned parameters. In the case of self-excited vibration such as flutter, the evaluation of the damped limit response is a part of the blade design process.

A tool has been developed to numerically simulate contact friction forces with the intention of parametrically evaluating the limit response and relating this to the mechanical integrity of the part. This paper presents the modelling of a coupled blade system with friction contact forces, results coming from this evaluation, and a comparison with test data.
Parametric Analysis for On-Board Thermal Regulation in a Hybrid-Electric Aircraft
Authors: Valsamis Mylonas Orestis S. , Gkoutzamanis Vasilis G. , Kalfas Anestis I.
Published: October 2022

The driving force behind the presented research is the need for more ecological applications in the aviation sector. Hybrid-electric propulsion systems are considered a promising alternative technology capable to provide fuel savings, reduced emissions and lower noise levels. Despite the added benefits, there are several pitfalls such as the commonly known issues of batteries’ low energy density — compared to that of jet-fuels —, as well as the need to efficiently regulate the various heat loads generated from the electrical propulsion system. The design of the Thermal Management System (TMS), for a hybrid-electric aircraft, alone constitutes of a ubiquitous task, due to its impact on the aircraft’s total weight, cooling drag and overall performance. This study focuses on investigating the possible operating media of a selected TMS, and their impact during the conceptual design phase of the system. On one hand, the air’s capability to transfer heat with increasing altitude is investigated. On the other hand, various liquid coolants are evaluated over their performance to remove heat loads but also, to not exceed their evaporation, boiling or autoignition points, while doing so. The simulations are conducted with the use of Modelon Impact, implementing the Modelica Liquid Cooling Library, and are transient to account for the dynamic effects of temperature change and thermal inertia of the various system elements. The results show that an atmospheric air mass flow of at least 0.848 kg/s is required to sufficiently dissipate the induced heat loads. Although, that is highly dependent on the flight altitude. If a liquid medium is to be selected, the best performing would be a propylene glycol water mixture with a mass flow of 0.204 kg/s, but the final selection is also subject to other factors that vary according to the priorities given.
Optimizing Internal Energy Streams in Micro Gas Turbines in Cogeneration Towards Flexible Heat-to-Power Ratio — Global Thermodynamic Performance Assessment and Specific Case Studies
Authors: De Paepe Ward , Clymans Tom
Published: October 2022

Although the simultaneous production of heat and power, so-called Combined Heat and Power (CHP), is from a thermo-dynamic point of view still the most efficient energy conversion method, cogeneration units have nowadays problems to position themselves in the current and future energy market. The increasing renewable energy penetration requires CHP units to become more flexible, especially on their currently fixed heat-to-power ratio. Within this framework, micro Gas Turbines (mGTs), as small-scale decentralized cogeneration units, offer opportunities. Since they use the recuperated Brayton cycle, they offer the theoretic option to adjust the internal heat streams to provide a flexible heat-to-power ratio as well as the unique feature of a tunable outlet temperature, making the unit feasible/interesting for a larger range of applications having a combined heat and power demand. Hence, in this paper, we assessed the impact of the use of a recuperator bypass for enhanced operational flexibility of mGTs. In a first step, the optimal pathway for the recuperator bypass, i.e., cold or hot side bypass, is selected for a typical mGT, the Turbec T100 (currently commercially available as the AE-T100), considering both thermodynamics as well as technological feasibility. Moreover, the potential performance impact on the electrical and total efficiency is calculated as well as on the total available thermal power. In a second step, the specific performance of the option of using a recuperator bypass is assessed for 2 specific cases: flexible heat-to-power ratio at low temperature and high temperature, i.e. steam generation, cogeneration. Thermodynamic simulations show that the impact on the electric efficiency remains rather limited (maximal 6% absolute efficiency reduction for a 40% bypass ratio), while the available thermal energy and exergy increase significantly: up to 60% increase for thermal power and even 115% increase in the exergy content of the flue gases. Moreover, there is no distinct difference between cold or hot bypass, leaving the selection of the optimal bypass route a pure technical choice. Finally, considering the specific cases studied, simulation results show that heat-to-power ratio could be increased by more than 50% for all power outputs for the low temperature CHP applications, even resulting in a global efficiency increase, while for the high temperature case, recuperator bypass allows for a significant increase in steam production, at total efficiencies comparable to the separate production (i.e. boiler and grid), clearly highlighting the benefits and potential of a recuperator bypass.