bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental and Modeling Study on Tension Characteristics of a 2.5D Woven Composites
Authors: Teng Xuefeng , Shi Duoqi , Yang Xiaoguang
Published: November 2019

2.5D woven composites have recently attracted much attention in the fields of aerospace and automobile industry due to their excellent properties such as low density, high thermal shock resistance, high specific strength and enhanced mechanical properties. Before the 2.5D woven composites are applied as load-bearing structure, it is necessary to have an in-depth understanding of their mechanical behavior and load transfer mechanism under external loads. In this paper, in-plane tensile tests including longitude direction and transverse direction were conducted for a 2.5D woven SiO2f/SiO2 composites at room temperature. With the full-field displacements and strains retrieved by digital image correlation (DIC) method, the mechanical properties and deformation features of the 2.5D woven composites were obtained and observed. The results show that the composite has a lower elastic modulus and fracture strength in the weft direction, and the warp yarn has weaker mechanical properties than weft yarn in the loading direction due to its crimp feature. Remarkable deformation features of the full-field displacement and strain distributions were observed, indicating that the woven structure has a great influence on the deformation evolution and load-bearing mechanism of the composite. Also, based on the actual geometrical architecture of the composite, a mesoscale finite element (FE) model was established and the deformation characteristics of the material were analyzed. The crimp of the warp yarns causes the local fiber axis to rotate with respect to the global coordinate system, which causes the effective modulus in the global direction to vary. Local coordinate systems were assigned to each node such that the local 1 direction is always parallel to the local fiber axis along the length of the warp yarns, which improves the accuracy of the simulation results. Deformation features of the 2.5D woven SiO2f/SiO2 composites were obtained in FEM simulation and discussed comparing with experimental results.
Impact of Foreign Object Damage to Leading Edge on Fatigue Capability of CMC Turbine Vanes
Authors: Obuchi Kenro , Watanabe Fumiaki , Kuroki Hiroshi , Arai Kazuyoshi
Published: November 2019

Ceramic matrix composites (CMCs) are expected to make turbine components lighter because they have lower density than nickel-based super alloys which have been used for turbine components. Also they are expected to improve the efficiency of gas turbine engines by realizing higher turbine inlet temperature (TIT), because they have higher temperature capability.

One of the technical issues of CMCs is that they have relatively low impact resistance comparing with nickel-based alloy. In the previous work, it was estimated that the foreign object damage (FOD) on CMC turbine vanes under the actual engine condition is less than 0.36-mm-depth. The concern of such damaged CMC turbine vanes is a decrease in a fatigue capability due to the dent particular to the leading edge (L/E). The fatigue strength reduction can be caused by the stress concentration due to the dented shape and the oxidation due to the coating spallation. There are various works about the impact resistance or the fatigue capability of CMCs, but there are little works which assess the fatigue capability of CMCs which was damaged by foreign object.

The objective of this work was to clarify the impact of FOD to L/E on the fatigue capability of SiC/SiC CMC turbine vane.

The tension-tension low cycle fatigue (LCF) tests and high cycle fatigue (HCF) tests were conducted using four types of test pieces at elevated temperature in the steam environment. The first one was smooth test piece with anti-oxidation coating, the second one was the test piece which had notch with coating, the third one was the test piece which had notch without coating at notched area and the fourth one was the test piece which was damaged by dropped weight test simulating FOD.

The result showed that impact of stress concentration due to notch shape on fatigue capability is small. It was also cleared that uncoated condition did not have impact on the LCF capability but had impact on the HCF capability. The HCF strength reduction of TP without coating to the TP with coating was about 35% at 107 cycles. More fiber breakage was observed at the fracture surface of the TP without coating tested in HCF condition, on the other hand more fiber pullout was observed at that of other TPs. The results suggest that the HCF strength reduction was caused by oxidation when the bare CMC was exposed by FOD.

From this work, it was concluded that the loss of coating due to FOD impacted on HCF strength of CMC turbine vane, but it can be accepted because turbine vane is normally designed so that it can endure the stress concentration at L/E due to FOD (typically kt = 3).
Investigation of Turbulence Characteristics in a Tip Leakage Flow Model Using Large-Eddy Simulation
Authors: Gao Yanfei , Liu Yangwei , Lu Lipeng
Published: November 2019

A simple tip leakage flow (TLF) model which consists of a square duct with a longitudinal slit on the top of a side wall is proposed to reproduce the jet flow/main flow shear mechanism of the tip leakage vortex (TLV) rolling-up in turbomachinery. Large-eddy simulation (LES) is employed to investigate the turbulence characteristics of the flow model under low Reynolds number condition. The geometry and boundary conditions of the flow model are simplified from a compressor rotor and modified to apply to low-Re condition for LES. The vortex structures and turbulence characteristics of the LES results are compared with the measurements of the rotor. It is found that the flow model could reproduce similar flow field and turbulence structures compared with the TLF in the real rotor, thus it can be used to investigate the turbulence in practical flows. Reynolds-Averaged Navier-Stokes (RANS) calculations are also carried out. The mean flow and turbulence behaviors of different cases are analyzed. The budgets of turbulent kinetic energy (k) are analyzed to investigate the turbulence transport nature in the TLF model, indicating that the non-equilibrium transport process of k is significant, especially the pressure and turbulent transport, which is not predicted by RANS.
Design of 20 kW Turbomachinery for Closed Loop Supercritical Carbon Dioxide Brayton Test Loop Facility
Authors: Seshadri Lakshminarayanan , Sathish Sharath , Kumar Pramod , Giri Gaurav , Nassar Abdul , Moroz Leonid , Setty Raghavendra , Gopi Pramod Chandra , Namburi Adi Narayana
Published: November 2019

Indian Institute of Science, Bangalore in collaboration with Sandia National Labs has developed a 140kW (thermal) simple recuperated supercritical CO2 (s-CO2) test facility to enable power generation of up to net 20 kWe output using turbomachinery components. The primary intent of the test loop is to understand the design and operational aspects of an s-CO2 Brayton cycle for distributed power generation. This paper describes the development of suitable turbomachinery to be deployed in the test loop. Turbomachinery design study is primarily performed using a commercial design tool AxStream® for both design and off-design operating conditions with a maximum cycle temperature limit of 525°C and a pressure of 145 bar. Present design considers a decoupled turbine and compressor driven independently by an electrical motor and a generator pair. This arrangement provides flexibility to independently assess compressor and turbine prototypes and also helps establish stable operation of the s-CO2 Brayton test loop. A range of single stage compressor and turbine geometries are independently evaluated considering un-coupled shafts and appropriate loss models using the above boundary conditions. Specific geometries are filtered based on total-to-total efficiency for a given shaft speed. The speed of the turbo-machinery is restricted to 40,000 rpm to enable independent testing and characterization using direct drive high-speed Switched Reluctance (SRM) motor-generator pair that is being developed in-house for this purpose. The investigation reveals the absence of a suitable compressor and turbine geometry at desired operating speed, hence, to circumvent the problem of low blade heights in the preliminary impeller design at 40,000 rpm, the turbomachinery is designed for 65,000 rpm and the off-design condition is taken for study.
Application of the Nozzle Analogy to Easily Predict Steam Turbine Performance With Minimal Geometric Input
Authors: Fuls W. F. , Clark J. S. , Marx A. C.
Published: November 2019

A simple method for modelling steam turbines using a nozzle analogy was recently published. The method describes both stator and rotor components of a turbine with an appropriate nozzle definition, and makes use of the traditional velocity triangles to determine work output. What makes the method convenient is that it only requires a reasonable estimate of the effective nozzle throat area per blade row, as well as properly chosen loss coefficients. A complete turbine train with multiple stages can easily be solved using a simple 1-D pressure correction method. The method was demonstrated to match detailed inter-stage turbine data quite accurately.

This paper elaborates on the application of the method. It presents a more thorough procedure for establishing the nozzle throat areas using a turbine lay-out drawing, as well as consideration of possible reaction ratio variation between successive stages. Furthermore, the loss coefficients are determined using the methods described by Traupel and Zehner. The paper presents these in a compressed and digitized format for easy implementation in a software code.

A case study is presented to show how the model can quantify improvements to blade profiles. Another case study models a complete boiler feed pump turbine connected to the feedwater pumps and includes the partial arc governing stage and cold re-heat supply. This specific model demonstrated the method’s ability to accurately model a turbine at low load and high back pressure conditions.

Turbine system modelers can apply the method presented here to their own equipment and obtain trustworthy results without the need for detailed profile geometry or complex CFD analyses. The method is however limited to subsonic flow between blade rows, hence some modern transonic last stage blade designs might not be adequately represented.
Igniter Testing for sCO2 Oxy-Combustion
Authors: Katcher Kelsi M. , Allison Timothy C. , Coogan Shane , Jin Yuin , Lim Chansun , Chang Sungho
Published: November 2019

Utilizing direct-fired sCO2 oxy-combustion is attractive for power generation applications because of the cycle’s inherent carbon capture, high efficiency, and small machinery footprint. However, there is a large amount of uncertainty regarding the combustion process of natural gas in carbon dioxide diluent at supercritical pressures. One such area of uncertainty is in regards to the ignition system. The performance of most common ignition systems is not proven at the elevated pressures and densities typical of these cycles.

This paper presents an evaluation and down-selection of potential ignition systems considered for a sCO2 oxy-combustor igniter. The ignition systems considered include spark ignition, laser ignition, heating element auto-ignition, external preheat auto-ignition, ignition using solid or liquid fuels, and external torch ignition. After a preliminary review, spark ignition and laser ignition were chosen for system reliability and repeatability. To further quantify the practicality of each system, a spark igniter and laser igniter were lab-scale tested to determine breakdown energies associated with these igniters. The spark igniter was tested using gaseous CO2 and SF6 (to attain higher fluid densities). The laser igniter was tested using supercritical CO2 and gaseous CO2. An additional round of testing was conducted using the laser igniter in a constant volume combustion chamber (CVCC). Natural gas was combusted with oxygen in varying levels of CO2 dilution to determine the required laser power for stable, reliable ignition and to quantify the high dilution flammability limit. Based on these test results, a laser igniter was selected as the most practical option for high-pressure sCO2 combustor ignition.
Aspects of Creep Fatigue Lifetime Assessment for High Temperature Components With Accumulative Model
Authors: Linn Stefan , Kontermann Christian , Oechsner Matthias
Published: November 2019

Alternating temperatures induce thermomechanical stresses in thick-walled components such as turbine rotors or housings, which can lead to fatigue and superimposed creep. Subsequently, damage can occur at their heated surfaces. Under the nowadays prevailing operating conditions of power plants with multiple cold, warm and hot starts as reaction to the high volatility of electric demand from fossil fired power plants for ensuring grid stability, methods for lifetime assessment are coming more into the focus of investigations and research. Engineers are trying to estimate the residual lifetimes of in-service components and operators of power plants ask for strategies to minimize the calculative material damage while simultaneously providing a maximum flexibility with shortest response times on altered demands. Among constitutive models, which are not subject of this paper, accumulative models for lifetime assessment were introduced several decades ago and are partially considered in applicable standards. Such models based on a damage accumulation are easy to apply but they are considered to be either very imprecise or very conservative, while the conservatism reflects the necessity of large safety margins.

This paper summarize a few measures, which are suitable to improve the predictive quality of models based on a simple time-fraction rule. The proposed model is based on a synthesis of hysteresis loops for isothermal and non-isothermal conditions, concepts for consideration of cyclic softening or hardening during lifetime, concepts for dealing with internal back stresses, mean strains or stresses, and for accounting of creep-fatigue interaction. The latter is based on a so-called transition time concept, where the creep damage during dwell times partially attributes to the portion of fatigue damage, which in turn is determined from fatigue life curves for dwell time experiments. In addition, the model comprises a concept for the post-processing of transient FEM calculations and dealing with multiaxial loading conditions.

Since the essentials of the proposed method with the transition time concept were published more than 10 years ago, the listed modifications improve the benefit for daily engineering usage. Validation experiments provide evidence of the models predicting capabilities with acceptable uncertainty.
Geometric Mistuning Reduced Order Model Development Utilizing Bayesian Surrogate Models for Component Mode Calculations
Authors: Beck Joseph A. , Brown Jeffrey M. , Kaszynski Alex A. , Carper Emily B. , Gillaugh Daniel L.
Published: November 2019

By design, Integrally Bladed Rotors (IBRs) are meant to be tuned, rotationally periodic structures. However, unique variations in geometries and material properties from sector-to-sector, referred to as mistuning, destroy the structural periodicity. This results in mode localization that can induce forced response levels greater than what is predicted with a tuned-structure analysis. Furthermore, mistuning and mode localization are random processes that require stochastic treatments when analyzing the distribution of fleet responses. Generating this distribution can be computationally intensive when using the full finite element model. To overcome this expense, Reduced Order Models (ROMs) have been developed to accommodate fast calculations of mistuned forced response levels for a fleet of random IBRs. Usually, ROMs can be classified by two main families: frequency-based and geometry-based methods. Frequency-based ROMs assume mode shapes do not change due to mistuning. However, this assumption has been shown to cause errors that propagate to the fleet distribution. To circumvent these errors, geometry-based ROMs have been developed to provide accurate predictions. However, these methods require recalculating modal data during ROM formulations. This increases the computational expense in computing fleet distributions. A new geometry-based ROM is presented to reduce this cost. The developed ROM utilizes a Bayesian surrogate model in place of sector modal calculations required in ROM formulations. This method, referred to as the Surrogate Modal Analysis for Geometry Mistuning Assessments (SMAGMA), will propagate the uncertainties of the surrogate prediction to the forced response. Assessments of the ROM accuracy are made by comparing results to the true forced response levels and results computed by a frequency-based ROM.
Flow in a Scaled Turbine Coolant Channel With Roughness due to Additive Manufacturing
Authors: Hanson David R. , McClain Stephen T. , Snyder Jacob C. , Kunz Robert F. , Thole Karen A.
Published: November 2019

Additive manufacturing (AM) approaches such as Laser Powder Bed Fusion (L-PBF) are being explored to reduce manufacturing costs of gas turbine components. Surfaces of additively manufactured components exhibit distinctive roughness characteristics that significantly affect the pressure losses and heat transfer. For this study, a coupon with a vertical internal flow passage was created using L-PBF and characterized using x-ray tomography. The roughness pattern was extracted using the spanwise-planar extraction approach. Also, two surfaces consisting of distributions of ellipsoids were created to capture the important statistical characteristics of the original rough surface. The resulting roughness geometries were scaled by 102x and applied to the internal wall of the Roughness Internal Flow Tunnel (RIFT). Measurements of friction losses and velocity profiles were obtained. Detailed Reynolds-Averaged Navier Stokes (RANS) simulations using grid-resolved roughness of flow in the RIFT were also performed using an in-house Computational Fluid Dynamics (CFD) code. The present approach combining experimental measurements and CFD simulation of an up-scaled roughness coupon provides a more detailed picture of the flow-field within the roughened channel than was previously available. The measured friction factor of the baseline up-scaled rough channel is within 15% of the original engine-scale channel within the fully turbulent regime. The ellipsoid surfaces are seen to have a friction factor 30% lower than the up-scaled rough channel; the authors hypothesize that this is due to bulk span-wise ridges being present in the up-scaled rough surface but not in the ellipsoid surfaces.
Investigating Instabilities in a Rotating Detonation Combustor Operating With Natural Gas-Hydrogen Fuel Blend: Effect of Air Preheat and Annulus Width
Authors: Roy Arnab , Bedick Clinton , Ferguson Donald , Sidwell Todd , Strakey Peter
Published: November 2019

Propagation characteristics of a detonation wave in an air-breathing Rotating Detonation Combustor (RDC) using natural gas-hydrogen fuel blends is presented in this paper. Short duration (∼up to 6s) experiments were performed on a 152.4mm OD uncooled RDC with two different annulus gap widths (5.08mm and 7.62mm) over a range of equivalence ratios (0.6–1.0) at varying inlet air temperatures (∼65°C-204°C) and natural gas content (up to 15%) with pre-combustion operating pressure slightly above ambient.

It was observed that the RDC, with an annulus gap width of 5.08mm, was inherently unstable when natural gas (NG) was added to the hydrogen fuel while operating at pre-combustion pressures near ambient and at an inlet air temperature of 65°C. Increasing the annulus gap width to 7.62mm improved the stability of the detonation wave at similar temperatures and pressure permitting operation with as much as 5% NG by volume. While observed speeds of the detonation waves were still below theoretical values, an increase in inlet air temperature reduced the variability in wave speed. The frequency analysis thus explored in this study is an effort to quantify detonation instability in an RDC under varying operational envelope. The data presented is relevant towards developing strategies to sustain a stable detonation wave in an RDC using natural gas for land based power generation.
Validation Studies of Linear Oscillating Compressor Cascade and Use of Influence Coefficient Method
Authors: Phan H. M. , He L.
Published: November 2019

Advanced predictions of blade flutter have been continually pursued. It is noted however that validation cases of unsteady CFD methods against experimental cases with detailed 3D unsteady pressures are still rather lacking. The main objectives of the present work are two-folds. Firstly, validate and understand the characteristics of blade tip clearance, as well as a bubble-type flow separation for an unsteady CFD solver against a 3D oscillating cascade experiment. And secondly, examine the applicability of the Influence Coefficient Method (ICM) as widely used in an oscillating linear cascade setup.

In the first part, the capability of a widely used commercial solver (CFX) for unsteady flows induced by a 3D oscillating compressor cascade is examined. The present computations have shown consistently a destabilizing effect of increasing blade tip clearance, in agreement with the experiment. More remarkably, the computational analyses reveal a distinctive interplay between the inlet endwall boundary layer and the tip clearance in relation to the aerodynamic damping. Different inlet endwall boundary layer thicknesses are shown to lead to qualitatively different aeroelastic stability characteristics in relation to tip clearance. The aero-damping variation with the tip-clearance under the influence of the inlet endwall boundary layer seems to correlate closely to a balancing act between the passage vortex and the tip-leakage vortex. The tip clearance aeroelastic behaviour seems also in line with a simple quasi-steady analysis. On the other hand, the mid-chord laminar bubble separation on suction surface, though with a clear signature in the local aero-damping, has negligible effects on the overall stability.

The second part aims to examine computationally the applicability of the influence coefficient method in a linear cascade setup. The comparison between the cascade based ICM data and a baseline ‘tuned cascade’ shows that the differences in the sensitivity to the far-field treatment can be significant, depending on interblade phase angles. On the other hand, non-linearity effects closely relevant to the basic linear assumption of the ICM are shown to only have a small influence. The present results suggest that extra caution should be exercised when comparing a CFD-based tuned cascade model with a finite cascade-based ICM model, at conditions close to acoustic resonance. The resultant discrepancies may well arise from the inherently different far-field sensitivities between the two models, rather than those typical numerical and physical modelling aspects of interest (e.g. meshing, spatial and temporal discretization errors as well as turbulence modelling).
Assessing the Sensitivity of Stall-Regulated Wind Turbine Power to Blade Design Using High-Fidelity CFD
Authors: Sanvito A. G. , Persico G. , Campobasso M. S.
Published: November 2019

This study provides a novel contribution towards the establishment of a new high–fidelity simulation–based design methodology for stall–regulated horizontal axis wind turbines. The aerodynamic design of these machines is complex, due to the difficulty of reliably predicting stall onset and post–stall characteristics. Low–fidelity design methods, widely used in industry, are computationally efficient, but are often affected by significant uncertainty. Conversely, Navier–Stokes CFD can reduce such uncertainty, resulting in lower development costs by reducing the need of field testing of designs not fit for purpose. Here, the compressible CFD research code COSA is used to assess the performance of two alternative designs of a 13–meter stall–regulated rotor over a wide range of operating conditions. Validation of the numerical methodology is based on thorough comparisons of novel simulations and measured data of the NREL Phase VI turbine rotor, and one of the two industrial rotor designs. An excellent agreement is found in all cases. All simulations of the two industrial rotors are time–dependent, to capture the unsteadiness associated with stall which occurs at most wind speeds. The two designs are cross-compared, with emphasis on the different stall patterns resulting from particular design choices. The key novelty of this work is the CFD–based assessment of the correlation among turbine power, blade aerodynamics, and blade design variables (airfoil geometry, blade planform and twist) over most operational wind speeds.
Flow Instabilities in Gas Turbine Chute Seals
Authors: Horwood Joshua T. M. , Hualca Fabian P. , Wilson Mike , Scobie James A. , Sangan Carl M. , Lock Gary D. , Dahlqvist Johan , Fridh Jens
Published: November 2019

The ingress of hot annulus gas into stator-rotor cavities is an important topic to engine designers. Rim-seals reduce the pressurised purge required to protect highly-stressed components. This paper describes an experimental and computational study of flow through a turbine chute seal. The computations — which include a 360° domain — were undertaken using DLR TRACE’s time-marching solver. The experiments used a low Reynolds number turbine rig operating with an engine-representative flow structure. The simulations provide an excellent prediction of cavity pressure and swirl, and good overall agreement of sealing effectiveness when compared to experiment.

Computation of flow within the chute seal showed strong shear gradients which influence the pressure distribution and secondary-flow field near the blade leading edge. High levels of shear across the rim-seal promote the formation of large-scale structures at the wheel-space periphery; the number and speed of which were measured experimentally and captured, qualitatively and quantitatively, by computations.

A comparison of computational domains ranging from 30° to 360° indicate that steady features of the flow are largely unaffected by sector size. However, differences in large-scale flow structures were pronounced with a 60° sector and suggest that modelling an even number of blades in small sector simulations should be avoided.
Use of Vector Autoregressive Model for Anomaly Detection in Utility Gas Turbines
Authors: Goyal Vipul , Xu Mengyu , Kapat Jayanta
Published: November 2019

This study is based on time-series data from the combined cycle utility gas turbines consisting of three-gas turbine units and one steam turbine unit. We construct a multi-stage vector autoregressive model for the nominal operation of powerplant assuming sparsity in the association among variables and use this as a basis for anomaly detection and prediction. This prediction is compared with the time-series data of the plant-operation containing anomalies. Granger causality networks, which are based on the associations between the time series streams, are learned as an important implication from the vector autoregressive modelling. Anomaly is detected by comparing the observed measurements against their predicted value.
A Computational Technique to Evaluate the Relative Influence of Internal and External Cooling on Overall Effectiveness
Authors: Bryant Carol E. , Rutledge James L.
Published: November 2019

Gas turbine hot gas path components are protected via coolant that travels through internal passageways before being ejected as external film cooling. Modern computational approaches facilitate simulation of the conjugate heat transfer that takes place within turbine components, allowing prediction of the actual metal temperature, usually nondimensionalized in the form of the overall effectiveness. Efforts aimed at improving cooling are often focused on either only the internal cooling or the film cooling; however, the common coolant flow means the internal and external cooling schemes are inextricably linked and the coolant holes themselves provide another convective path for heat transfer to the coolant. The relative influence of internal cooling, external cooling, and convection through the film cooling holes on overall effectiveness is not well understood.

Computational fluid dynamics (CFD) simulations were performed in order to isolate each cooling mechanism, and thereby determine their relative contributions to overall effectiveness. The conjugate CFD model was a flat plate with five staggered rows of shaped film cooling holes. Unique boundary conditions were used to isolate the cooling mechanisms. The internal cooling was modeled with and without heat transfer on the internal surface in order to isolate the effects of plenum cooling. Convection through the coolant holes was isolated by making the inside of the film cooling holes adiabatic. This was done in order to evaluate the influence of the internal cooling provided by the cooling holes themselves. The effect of film cooling was removed through the novel use of an outlet boundary condition at the exit of each hole that allowed unaltered internal coolant flow without external coolant ejection.
Application of Multi-Scale Methodology for Transonic Turbine Blade Tip Cooling Design
Authors: Duan P. H. , He L.
Published: November 2019

In recent research the cooling flow at transonic turbine tip is found to have several unique flow features such as strong interaction with the base flow, acceleration at divergent duct, strong shock-boundary layer interaction and weak correlation between the aerodynamic loss and the heat transfer. These flow features require the cooling design to be considered together with the tip geometry shaping. However due to the large disparity of the length scales between the cooling holes and the turbine blade, the combination of the cooling design and tip geometry shaping tends to be too computationally expensive to be employed. In this study the multi-scale method is implemented in a commercial solver to provide a fast and accurate solution for the turbine tip cooling design. The method uses source terms added on the coarse mesh to generate the solution that is rather close to the solution obtained with a well resolved mesh. The source terms are found to present not only the influence of mesh resolution but also the flow injection. The computed results have been validated against the experimental data and the fine mesh results. The good agreement shows a great potential for this multi-scale method to be used in cases with large length scale disparity.
Development of High Speed and High Temperature Atomic Layer Thermopiles
Authors: Bhatnagar Lakshya , Paniagua Guillermo
Published: November 2019

This work aims to provide a technique with which high frequency heat flux measurement data can be acquired in systems with high operational temperatures and high-speed flows with quantifiable and accurate uncertainty estimates. This manuscript presents the detailed calibration and application of an atomic layer thermopile, for heat fluxes with a frequency bandwidth of 0 to 1MHz. Two calibration procedures with a detailed uncertainty analysis. The first procedure consists using a laser to deliver radiation heat flux, while the second consists of a convective heat blowdown experiment. The use of this probe is demonstrated in a high-speed environment at Mach 2. The sensor effectively captures the passage of the normal shock wave and the values are compared with those computed using surface temperature measurement. Finally, a numerical study is carried out to design a cooling system that will allow the sensor to survive in high temperature conditions of 1273K while the sensor film is maintained at 323K. A two-dimensional axisymmetric conjugate heat transfer analysis is carried out to obtain the desired geometry.
Airfoil Optimization for a Wind Turbine Operating in a Particle-Laden Environment
Authors: Diab Aya , Hossam El-Din Ahmed
Published: November 2019

Dust may be challenging to the blades of wind turbines deployed in the harsh environment of the Sahara. In this paper, the airfoil sections of a wind turbine have been customized for low sensitivity to surface roughness at the wind conditions prevailing in Hurghada—Egypt to avoid serious power degradation. To this end, a two-dimensional a computational model is developed using ANSYS-FLUENT 15.0 to understand the distinguishing features that govern the specific behavior of NACA-63-215 (root section) and NACA-63-415 airfoils (midspan and tip sections) with respect to dust deposition and sand erosion. Subsequently, a two-objective genetic algorithm is developed in MATLAB 16.0 and used to customize the airfoil geometry, enhancing the lift-to-drag ratio while simultaneously minimizing the deposition and erosion rates. The whole optimization process is realized through coupling MATLAB 16.0 with ANSYS-FLUENT 15.0 via the ICEM meshing tool to predict the optimum blade shape based on its aerodynamic performance in a dust-loaded environment. The optimization process enhanced the aerodynamic performance for the aforementioned airfoils under particle laden conditions with up to 38.34% higher lift-to-drag coefficients ratio in addition to 70 % and 99.267 % drop in dust deposition and sand erosion, repectively.
Preliminary Design and Performance Assessment of an Underwater CAES System (UW-CAES) for Wind Power Balancing
Authors: Astolfi Marco , Guandalini Giulio , Belloli Marco , Hirn Adriano , Silva Paolo , Campanari Stefano
Published: November 2019

A key approach to large renewable power management is based on implementing storage technologies, including batteries, power-to-gas and compressed air energy storage (CAES). This work presents the preliminary design and performance assessment of an innovative type of CAES, based on underwater storage volumes (UW-CAES) and intended for installation in the proximity of deep water seas or lakes. The UW-CAES works with constant hydrostatic pressure storage and variable volumes. The proposed system is adiabatic, not using any fuel to increase the air temperature before expansion; a sufficient TIT is instead obtained through a thermal energy storage system which recovers the compression heat.

The system includes (i) a set of turbomachines (modular multi-stage compressor, with partial intercooling; expansion turbine); (ii) a thermal energy storage (TES) system with different temperature levels designed to recover a large fraction of the compression heat, allowing the subsequent heating of air prior to the expansion phase; (iii) an underwater modular compressed air storage, conceived as a network of rigid but open tanks lying on the seabed and allowing a variable-volume and constant pressure operation. The compressor operates at variable loads, following an oscillating renewable power input, according to strategies oriented to improve the overall system dispatchability; the expander can be designed to work either at full load, thanks to the stability of the air flow rate and of the TIT guaranteed by the thermal storage, or at variable load.

The paper first discusses in detail the sizing and off-design characterization of the overall system; it is then simulated a case study where the UW-CAES is coupled to a wind farm for peak shaving and dispatchability enhancement, evaluating the impact of a realistic power input on performances and plant flexibility.

Although the assessment shall be considered preliminary, it is shown that round trip efficiency in the range of 75%–80% can be obtained depending on the compressor section configuration; making the UW-CAES a promising technology compared to electrochemical and pumped-hydro storage systems. The technology is also applied to perform peak-shaving of the electricity production from a wind park; annual simulations considering part load operation result in global round trip efficiency around 75% with a 10 to 15% reduction in the average unplanned energy injection in the electric grid. The investigated case study provides an example of the potential of this system in providing power output peak shaving when coupled with an intermittent and non-predictable energy source.
Design Procedure and Performance Analysis of a Microturbine Combustor Working on Biogas for Power Generation
Authors: Bazooyar Bahamin , Darabkhani Hamidreza Gohari
Published: November 2019

Design of the combustor is of high priority in microturbine generators (MTG) due to the small and compact configuration of these type of generators and high range of the shaft revolution (normally over 100k rpm). Design process of the MTG components including the micro combustor and turbomachinery also require accurate description of the combustion phenomena, heat transfer, emission level and performance analysis of the system. Design of combustors for renewable fuels such as biogas has several complications including overcoming the lower heating value of the biogas (normally 1/3 of the natural gas), combustion instabilities and corrosion effects of burning these types of fuels. The main benefit of burning a carbon neutral fuel (e.g., biogas), however will be in reducing the carbon emission by avoiding fossil fuels and achieving the environmental targets (e.g., Paris Agreement). The tubular combustors are in the centre of attention in design and operations of the microturbines due to their low cost and the level of emission. This research work presents the design procedure and CFD modelling of a tubular combustor for a biogas burnt microturbine engine assembly. The biogas is generated from anaerobic digestions of agriculture waste and include a 57% and 43% mixture of methane and CO2 respectively. All the combustor parts are designed with empirical and practical equations and dimensions are optimised by CFD simulations. Operation of the combustor is then analysed in terms of its gaseous emissions. Finally, the operation of the new combustor in a closed heat and power cycle was verified and compared with conventional combustor of the microturbine burning diesel fuel, and as a result all the benefits and considerations for the application of biogas in microturbine assembly are carefully remarked and discussed.
The Effects of Exit Boundary Condition on Precessing Vortex Core Dynamics
Authors: Mason Danielle , Clees Sean , Frederick Mark , O’Connor Jacqueline
Published: November 2019

Many industrial combustion systems, especially power generation gas turbines, use fuel-lean combustion to reduce NOx emissions. However, these systems are highly susceptible to combustion instability, the coupling between combustor acoustics and heat release rate oscillations of the flame. It has been shown in previous work by the authors that a precessing vortex core (PVC) can suppress shear layer receptivity to external perturbations, reducing the potential for thermoacoustic coupling. The goal of this study is to understand the effect of combustor exit boundary condition on the flow structure of a swirling jet to increase fundamental understanding of how combustor design impacts PVC dynamics. The swirling jet is generated with a radial-entry, variable-angle swirler, and a quartz cylinder is fixed on the dump plane for confinement. Combustor exit constriction plates of different diameters are used to determine the impact of exit boundary condition on the flow field. Particle image velocimetry (PIV) is used to capture the velocity field inside the combustor. Spectral proper orthogonal decomposition, a frequency-resolved eigenvalue decomposition that can identify energetic structures in the flow, is implemented to identify the PVC at each condition in both energy and frequency space. We find that exit boundary diameter affects both the structure of the flow and the dynamics of the PVC. Higher levels of constriction (smaller diameters) force the downstream stagnation point of the vortex breakdown bubble upstream, resulting in greater divergence of the swirling jet. Further, as the exit diameter decreases, the PVC becomes less energetic and less spatially defined. Despite these changes in the base flow and PVC coherence, the PVC frequency is not altered by the exit boundary constriction. These trends will help inform our understanding of the impact of boundary conditions on both static and dynamic flame stability.
Large Eddy Simulation of Bypass Transition in Vane Passage With Freestream Turbulence
Authors: Kanani Yousef , Acharya Sumanta , Ames Forrest
Published: November 2019

High Reynolds flow over a nozzle guide-vane with elevated inflow turbulence was simulated using wall-resolved large eddy simulation (LES). The simulations were undertaken at an exit Reynolds number of 0.5×106 and inflow turbulence levels of 0.7% and 7.9% and for uniform heat-flux boundary conditions corresponding to the measurements of (Varty, J. W., and Ames, F. E., 2016, ASME Paper No. IMECE2016-67029). The predicted heat transfer distribution over the vane is in excellent agreement with measurements. At higher freestream turbulence, the simulations accurately capture the laminar heat transfer augmentation on the pressure surface and the transition to turbulence on the suction surface. The bypass transition on the suction surface is preceded by boundary layer streaks formed under the external forcing of freestream disturbances which breakdown to turbulence through inner mode secondary instabilities. Underneath the locally formed turbulent spot, heat transfer coefficient spikes and generally follows the same pattern as the turbulent spot. The details of the flow and temperature fields on the suction side are characterized and first and second order statistics are documented. The turbulent Prandtl number in the boundary layer is generally in the range of 0.7–1, but decays rapidly near the wall.
Experimental Demonstration of Inverse Heat Transfer Methodologies for Turbine Applications
Authors: Cuadrado David G. , Lozano Francisco , Paniagua Guillermo
Published: November 2019

Gas turbines operate at extreme temperatures and pressures, constraining the use of both optical measurement techniques as well as probes. A strategy to overcome this challenge consists of instrumenting the external part of the engine, with sensors located in a gentler environment, and use numerical inverse methodologies to retrieve the relevant quantities in the flowpath. An inverse heat transfer approach is a procedure used to retrieve the temperature, pressure or mass flow through the engine based on the external casing temperature data.

This manuscript proposes an improved Digital Filter Inverse Heat Transfer Method, that consists of a linearization of the heat conduction equation using sensitivity coefficients. The sensitivity coefficient characterizes the change of temperature due to a change in the heat flux. The heat conduction equation contains a non-linearity due to the temperature-dependent thermal properties of the materials. In previous literature, this problem is solved via iterative procedures that however increase the computational effort. The novelty of the proposed strategy consists of the inclusion of a non-iterative procedure to solve the non-linearity features. This procedure consists of the computation of the sensitivity coefficients in function of temperature, together with an interpolation where the measured temperature is used to retrieve the sensitivity coefficients in each timestep. These temperature-dependent sensitivity coefficients, are then used to compute the heat flux by solving the linear system of equations of the Digital Filter Method.

This methodology was validated in the Purdue Experimental Turbine Aerothermal Lab (PETAL) annular wind tunnel, a two minutes transient experiment with flow temperatures up to 450K. Infrared thermography is used to measure the temperature in the outer surface of the inlet casing of a high pressure turbine. Surface thermocouples measure the endwall metal temperature. The metal temperature maps from the IR thermography were used to retrieve the heat flux with the inverse method. The inverse heat transfer method results were validated against a direct computation of the heat flux obtained from temperature readings of surface thermocouples. The experimental validation was complemented with an uncertainty analysis of the inverse methodology: the Karhunen-Loeve Expansion. This technique allows the propagation of uncertainty through stochastic systems of differential equations. In this case, the uncertainty of the inner casing heat flux has been evaluated through the simulation of different samples of the uncertain temperature field of the outer casing.
Transfer Function Modeling and Experimental Variation of Rotor System Considering Morton Effect Caused in Journal Bearing
Authors: Yabui Shota , Chiba Jotaro , Suzuki Takafumi , Tomimatsu Shigeyuki , Inoue Tsuyoshi
Published: November 2019

In recent years, a rotating machinery are required to operate at high rotational speed for high efficiency. However, the rotating machinery may become unstable due to the increase of rotational speed. One cause of unstable vibration is the Morton Effect generated in a journal bearing. To avoid unstable vibration due to the Morton Effect, construction of a mathematical model for predicting it becomes an important subject. Many researches on the Morton Effect have been conducted previously. Conventional researches are mostly divided into two types. The first one is a study based on detailed numerical simulation using computational fluid dynamics (CFD), thermoshydrodynamics (THD) and so on. It tries to find solution of a differential equation which indicates the Morton Effect induced vibration for a specific machine or a test rig. Therefore, this approach has led not comprehensive model. The other one is a study expressed by a simple mathematical formula. However, modeling in the time domain has been mainly focused and modeling in the frequency domain has not been investigated in detail. In this research, a model based on the frequency response that can quantitatively evaluate the Morton Effect induced vibration in the rotating machinery supported by the journal bearing is developed. First, experimental data was collected for modeling by using an experimental rig. Using these experimental data of journal position in the journal bearing and temperature of journal, a model of the Morton Effect was constructed based on frequency responses. In the proposed method, the characteristic of the journal bearing was considered as a proportional differential controller from control engineering point of view. In addition, the proposed model considers the Morton Effect induced vibration as a new bending mode of a rotating shaft caused by thermal difference. Then, the developed model of the Morton Effect was evaluated in the frequency domain. The characteristics of vibration calculated by the proposed model indicated good correlation with that of the experimental data. Finally, the behavior of the rotating shaft at another rotational speed was predicted by using the proposed model. It was confirmed that the experimental data well agreed with the predicted results. These results show the usefulness of the proposed method of this research for predicting the Morton Effect.