bluuebunny/crossref_metadata_2025 huggingface dataset:
Fabrication of Double-Layered Alginate Capsules Using Coaxial Nozzle
Authors: Jin Yifei , Zhao Danyang , Huang Yong
Published: August 2017

Multilayered encapsulation has been of great interest for various pharmaceutical, chemical, and food industries. Fabrication of well-defined capsules with more than one shell layer still poses a significant fabrication challenge. This study aims to investigate the feasibility of using a coaxial nozzle to fabricate double-layered (core–shell–shell) capsules during vibration-assisted dripping. A three-layered coaxial nozzle has been designed, manufactured, and tested for double-layered capsule fabrication when using sodium alginate solutions as the model liquid material for inner and outer shell layers and calcium chloride solution as the core fluid. To facilitate the droplet formation process, a vibrator has been integrated into the fabrication system to provide necessary perturbation for effective breakup of the fluid flow. It is demonstrated that double-layered alginate capsules can be successfully fabricated using the proposed three-layered coaxial nozzle fabrication system. During fabrication, increasing the core flow rate leads to an increase in capsule and core diameters while the inner and outer shell layer thicknesses decrease. Increasing annular flow rate results in an increase in capsule diameter and inner shell layer thickness while the outer shell layer thickness decreases. An increase in the sheath flow rate leads to an increase in capsule diameter and outer shell layer thickness but has no significant effect on the core diameter and inner shell layer thickness.
Methodology to Develop Geometric Modeling of Accurate Drilled Cooling Holes on Turbine Blades
Authors: Dong Yiwei , Zhao Qi , Li Xiaolin , Li Xiaoji , Yang Jun
Published: August 2017

Film cooling is one of most developed technologies which enhance the gas turbine blades operation in today’s high-thrust-to-weight-ratio gas engines. The determination of the accurate cooling holes of turbine blades is of vital importance to improving the cooling performance. However, in all current methods for manufacturing drilled cooling holes on turbine blades, the complexity of the production processes will ineluctably cause unexpected consequences such as the deformation of the turbomachinery parts, the locating error caused by fixture layouts. These issues will cause deviations in the geometry and position of the drilled holes. In this paper, a methodology is proposed to analyze the deviation and to establish an improved geometric model of drilled cooling holes with accurate positional and geometrical parameters on turbine blades. The discontinuous deformation, including the non-uniform wall thickness and shrinkage distribution, was established by deformation characteristics decoupling analysis; the surface error generated by locating error was obtained using locating error analysis. An accurate model for film cooling holes can be established by considering the process-induced deviations in geometry and positioning. The relevance of the proposed method is verified using numerical and experimental results.
Numerical Efforts of Aerodynamic Re-Design in a Single-Stage Transonic Axial Compressor: Part 1 — Stator Design
Authors: Oh Justin (Jongsik)
Published: August 2017

In many aerodynamic design parameters for the axial-flow compressor, three variables of tailored blading, blade lean and sweep were considered in the re-design efforts of a transonic single stage which had been designed in 1960’s NASA public domains. As Part 1, the re-design was limited to the stator vane only. For the original MCA (Multiple Circular Arc) blading, which had been applied at all radii, the CDA (Controlled Diffusion Airfoil) blading was introduced at midspan as the first variant, and the endwalls of hub and casing (or tip) were replaced with the DCA (Double Circular Arc) blading for the second variant. Aerodynamic performance was predicted through a series of CFD analysis at design speed, and the best aerodynamic improvement, in terms of pressure ratio/efficiency and operability, was found in the first variant of tailored blading. It was selected as a baseline for the next design efforts with blade lean, sweep and both combined. Among 12 variants, a case of positively and mildly leaned blades was found the most attractive one, relative to the original design, providing benefits of an 1.0% increase of pressure ratio at design flow, an 1.7% increase of efficiency at design flow, a 10.5% increase of the surge margin and a 32.3% increase of the choke margin.
High-Pressure Gas Turbine Vane Turbulent Flows and Heat Transfer Predicted by RANS/LES/DES
Authors: Dong Ping , Amano R. S.
Published: August 2017

The lifetime of the modern gas turbines greatly depends on the durability of hot section components operating at high temperatures. Film cooling is key to air cooling technologies in modern gas turbine and widely used in high-temperature and high-pressure blades as an active cooling scheme.

The requirements of accurate prediction of aerodynamic flow and heat transfer in gas turbine blades lay the essential foundation of cooling effectiveness improvement and working life estimation. In recent days, Large Eddy Simulations (LES) is considered as a useful tool to predict turbulent flows and heat transfer around gas turbine blades, but, comparing to the Reynolds-Averaged Navier–Stokes (RANS) methods, the LES method usually needs more computing resource and depends on computational power and mesh quality.

In this paper, LES/DES (Detached Eddy Simulation) predictions were compared to RANS prediction with interest in the accuracy and improvement of turbulent flow and heat transfer phenomena around NASA’s C3X high-pressure gas turbine vane with leading edge cooling film. RANS/LES/DES were detailed and further investigated to assess their ability to predict flow and heat transfer in boundary layer around C3X vane.

The current predictions showed that the mix between film cooling injections and free stream resulted in complex flow and heat transfer in the boundary layer on the external vane surface. The predictions of the aerodynamic load along the C3X vane with RANS/LES/DES were almost identical and agreed well with the experimental results. However, the heat transfer predictions with RANS/LES/DES were different. The transition prediction showed the best agreement with the experiment data in the most region. The LES prediction only partially agreed with the experimental data before separation point on the suction side and mild pressure gradient region on the pressure side. The DES and RANS predictions agreed with the experiment data after separation point on the suction side and most region on the pressure side.
Short Helical Combustor: Flow Control in a Combustion System With Angular Air Supply
Authors: Ariatabar Behdad , Koch Rainer , Bauer Hans-Jörg
Published: August 2017

The concept of the novel Short Helical Combustor (SHC) was investigated in our previous work [1, 2].

Based on the insight gained from these previous investigations, we propose a generic design improvement to address the tremendous loss of initial angular momentum as well as inhomogeneous flow and temperature field at the outlet of the SHC.

In the present paper, the main features of this design are introduced. It is shown that a three-dimensional shaping of the sidewalls, the dome, and the liners can effectively counteract the suboptimal interaction of the swirl flames with these surrounding walls.

As a result, the flow at the outlet of the combustor features a high angular momentum and exhibits a uniform flow angle and temperature field.

The insight gained from these generic investigations, and the resulting design optimization provides a useful framework for further industrial optimization of the SHC.
Response of a Compact Recuperator to Thermal Transients in a Supercritical Carbon Dioxide Brayton Cycle
Authors: Clementoni Eric M. , Cox Timothy L. , King Martha A.
Published: August 2017

Supercritical carbon dioxide (sCO2) power cycle designs are typically highly recuperated, transferring heat from the high temperature turbine exhaust stream to the compressor discharge stream thereby increasing overall cycle efficiency. Compact heat exchangers are preferred for this application due to their high surface area-to-volume ratio enabling much smaller heat exchangers as compared to conventional designs. However, compact heat exchangers have a higher metal density than conventional heat exchangers which could result in thermal lag during rapid temperature transients.

The Naval Nuclear Laboratory has been operating the Integrated System Test (IST) with the objective of demonstrating the ability to operate and control an sCO2 Brayton power cycle over a wide range of conditions. Rapid turbomachinery startups and power transients result in thermal transients on the recuperator. This paper presents thermal transients observed in the IST recuperator during loop startup and power transients and illustrates the time to achieve thermal equilibrium following the transients.
Assessment of Real Turbine Blade Roughness Parameters for the Design of a Film Cooling Test Rig
Authors: Glasenapp Tobias , Puetz Franz , Schulz Achmed , Bauer Hans-Jörg
Published: August 2017

The average roughness height Ra was used for characterizing surface roughness in many past investigations. In order to fully describe heat transfer and fluid flow over a rough surface, statistical parameters, however, are not sufficient. There is a lack of data in the literature in terms of three-dimensional real turbine roughness and the distribution of roughness parameters on the turbine airfoils. This study closes this gap and provides detailed surface roughness data of high-pressure turbine vanes that have been in service in land-based gas turbines and aero engines. Furthermore, samples of thermal barrier coatings from two different application techniques are investigated. High-resolution 3D profilometer and optical measurements are performed at various locations on both the pressure and suction side. Key parameters such as roughness shape and density are quantified. Equivalent sand grain roughness height (ks) values are obtained and related to the fluid flow by calculating the corresponding dimensionless roughness heights.

The results show that roughness parameters are spread over a wide range. No correlation with position on the vane surface is found. The distribution of the roughness heights deviates from a Gaussian distribution. Roughness on the suction side is generally smaller than on the pressure side. Dimensionless roughness heights are found to be mainly in the transitional regime and not in the fully rough regime as expected. The data presented in this paper is of great importance to the design of test surfaces for laboratory experiments and as input for numerical models.
The Impeller Exit Flow Coefficient As a Performance Map Variable for Predicting Centrifugal Compressor Off-Design Operation Applied to a Supercritical CO2 Working Fluid
Authors: Liese Eric , Zitney Stephen E.
Published: August 2017

A multi-stage centrifugal compressor model is presented with emphasis on analyzing use of an exit flow coefficient vs. an inlet flow coefficient performance parameter to predict off-design conditions in the critical region of a supercritical carbon dioxide (CO2) power cycle. A description of the performance parameters is given along with their implementation in a design model (number of stages, basic sizing, etc.) and a dynamic model (for use in transient studies). A design case is shown for two compressors, a bypass compressor and a main compressor, as defined in a process simulation of a 10 megawatt (MW) supercritical CO2 recompression Brayton cycle. Simulation results are presented for a simple open cycle and closed cycle process with changes to the inlet temperature of the main compressor which operates near the CO2 critical point. Results showed some difference in results using the exit vs. inlet flow coefficient correction, however, it was not significant for the range of conditions examined. This paper also serves as a reference for future works, including a full process simulation of the 10 MW recompression Brayton cycle.
Investigation of the American Petroleum Institute’s Support Stiffness Ratio Specification
Authors: Griffin David J. , Fittro Roger L. , Rockwell Robert D. , Goyne Christopher P.
Published: August 2017

In the design phase of developing a rotating machine, it is important to consider the effect that the supporting structure has on the rotordynamic behavior of the system. The American Petroleum Institute’s (API) Standard Paragraphs state that if the support stiffness is greater than 3.5 times the bearing oil film stiffness, the designer may omit the supports from the rotordynamic analysis. As discussed in this paper, there is concern that machines operating near the second critical speed may not be adequately modeled using the rigid support assumption allowed by API. Due to the suspected influence of support dynamics on problem machines in industry, this paper investigates the effectiveness of the support to bearing stiffness ratio threshold of 3.5. A Jeffcott rotor model is used to capture the effect of the support on the separation margin and amplification factors of the first and second critical speeds. The trends are then validated using numerical models of two case studies. Results show that the API support stiffness ratio of 3.5 has limitations as a rigid support threshold and should be used with caution when the second critical speed is near a separation margin boundary. The paper proposes an addendum to the current API specification intended to reduce the risk of separation margin encroachment due to a rigid support assumption. Unlike other papers discussing the influence of flexible supports on rotating machinery, this investigation studies the effects at API’s support stiffness ratio of 3.5 and compares them to a rigid support system.
Numerical Approach for Real Gas Simulations: Part I — Tabular Fluid Properties for Real Gas Analysis
Authors: Moraga Francisco , Hofer Doug , Saxena Swati , Mallina Ramakrishna
Published: August 2017

Recently there has been increased interest in the use of carbon dioxide (CO2) in closed loop power cycles. As these power cycles capitalize on the non-ideal gas behavior of CO2, their analysis both at the system level and at the detailed component level requires an advanced equation of state. Commonly used analytical equations of state as BWRS (BenedictWebbRubin equation of State) or Peng-Robinson are known to have high errors near the critical point and are thus unsuitable for the analysis of cycles or components where the flow conditions approach the critical point. An accurate equation of state is required at all phases of the development process from high level cycle calculations to the detailed component CFD. The NIST RefProp software package provides accurate CO2 fluid properties across the thermodynamic space but suffers from high computational over-head.

This study is presented in two parts. Part I (this part) of this paper describes an approach to creating a tabular representation of the equation of state that is applicable to any fluid. This approach is applied to generating an accurate, fast and robust tabular representation of the RefProp CO2 properties and an error analysis is performed to meet the accuracy requirements. The paper also discusses two approaches used to define speed of sound in the two-phase region and their sensitivity analysis on the 3D compressor flow. Part II of the paper details the numerical simulations of a supercritical CO2 centrifugal compressor using the tabular approach.

This paper shows that table resolution can be tailored to match the accuracy requirements while minimizing the time used to evaluate the tabulated thermo-physical functions. Error analysis are shown to demonstrate the level of accuracy possible with this approach.
Numerical Study on Effects of Density Ratio on Film Cooling Flow Structure and Film Cooling Effectiveness
Authors: Sakai Eiji , Takahashi Toshihiko
Published: August 2017

To understand film cooling flow fields on a gas turbine blade, this paper reports a series of large-eddy simulations of an inclined round jet issuing into a crossflow. Simulations were performed at constant momentum ratio conditions, IR = 0.25, 0.5, 1.0 and Reynolds number, Re = 15,300, based on the crossflow velocity and the film cooling hole diameter. Density ratio, DR, is changed from 1.0 to 2.0, and effects of the density ratio on vortical structures around the film cooling hole exit and film cooling effectiveness are investigated. The results showed that the vortical structure of the ejected jet drastically changes with varying density ratio. When the density ratio is comparatively small, hairpin vortices are formed downstream of the hole exit. On the contrary, when the density ratio is comparatively high, the formation of the hairpin vortices is suppressed and jet shear layer vortices are formed on side edges of the cooling jet. The jet shear layer vortices conveys the coolant air to the wall surface. As a result, higher film cooling effectiveness is obtained at comparatively high density ratio conditions compared to comparatively low density ratio conditions. Additional simulations were performed to discuss a possibility of an improvement in the film cooling effectiveness by controlling the formation of the jet shear layer vortices.
A Semantic Differential for Evaluating the Sound Quality of Fan Systems
Authors: Feldmann Carolin , Carolus Thomas , Schneider Marc
Published: August 2017

Fans are main components e.g. in heating, ventilating and air conditioning systems for vehicles or buildings, cooling units of engines and electronic circuits, and household appliances such as kitchen exhaust hoods or vacuum cleaners. End-users increasingly demand a high sound quality of their system or device. The overall objective of a recent research project at the University of Siegen is a multidimensional assessment of fan sound quality.

In a first step an advanced novel semantic differential for the assessment of fan-related sounds is established with the aid of carefully designed jury tests. Eventually, this semantic differential is employed for sound quality jury tests of fans in kitchen exhaust hoods, heat pumps and air purifiers as a first case. Finally, a prediction model is suggested, which relates the outcome from the jury tests to objective metrics.

A principal component analysis is carried out and yields five main assessment criteria with 23 relevant adjective scales. The results show that the perceived sound quality of fan systems is mainly determined by the loudness and tonality of the sound. The spectral content (represented by the sharpness) as well as the time structure (represented by the roughness) have no significant impact on perceived sound quality of the fan systems investigated.
The Effect of the Pocket on the Heat Transfer of Endwall With Bluff Body in the Rear Part of Gas Turbine
Authors: Liu Jian , Hussain Safeer , Wang Lei , Xie Gongnan , Sundén Bengt , Abrahamsson Hans , Arroyo Carlos
Published: August 2017

A pocket cavity is generated at the connection of two parts, such as the transition part between the low pressure turbine (LPT) and outlet guide vane (OGV) in a gas turbine engine. A bluff body, working as a heat transfer enhancement part or supporting strength part, has tremendous engineering applications in turbomachinery. In the present work, the effect of the pocket on the heat transfer of endwall with a bluff body in the rear part of gas turbine is investigated. A simplified triangular pocket cavity is built in a rectangular channel and two bluff bodies, a cylinder or a cuboid is attached downstream on the endwall. The heat transfer and fluid flow on the endwall are investigated experimentally and numerically. Liquid Crystal Thermography (LCT) is employed to measure the heat transfer over the pocket surface with Reynolds number ranging from 87,597, to 218,994. The turbulent flow details are provided by numerically calculations based on the commercial software Fluent 17.0. Based on the results, high heat transfer areas are usually found at the boundary of the pocket cavity and vortex street shedding regions around the bluff body. When a pocket cavity is placed in the upstream of a bluff body, the endwall heat transfer around the bluff body is obviously decreased due to the disturbance by the pocket. There are no recirculating flows in front of the tested cylinder while this is not applicable for the cuboid case. The recirculating flow behind the bluff bodies forms a three-dimensional flow structure rotating in two directions.
Influence of Nozzle Design Upon the Primary Jet Breakup of High-Viscosity Fuels for Entrained Flow Gasification
Authors: Müller Thomas , Dullenkopf Alexa , Habisreuther Peter , Zarzalis Nikolaos , Sänger Alexander , Jakobs Tobias , Kolb Thomas
Published: August 2017

The research work of the present study is focused on the influence of design parameters of twin-fluid nozzles used for the atomization of high-viscosity fuels with respect to the primary breakup of the liquid jet. Two external mixing twin-fluid nozzles, which have already been investigated in previous studies [1, 2], were chosen as basic design. Based on the previous findings the web thickness between fuel and oxidizer supply was varied. In addition both designs were extended by a channel for internal mixing of gas and liquid with a length to diameter ratio of one. Moreover one of the basic nozzles was scaled by decrease of the effective areas in a way that momentum flux ratio as well as gas to liquid mass flow ratio was kept constant.

The newly designed atomizers were subsequently investigated with regard to the influence of the changes upon the primary jet breakup using CFD simulations. The numerical simulations were conducted by means of the open source package OpenFOAM. The Volume of Fluid method was used for the determination of the gas-liquid interface. These simulations were then compared with experimentally validated simulations of the basic nozzle designs with regard to the breakup morphology of the jet and the mode of the primary surface instability. In addition, the liquid structure was examined by comparison of breakup length and frequency.

The results of these simulations showed that small changes in the atomizer design heavily influence the primary breakup, which in turn influences the overall performance of the atomizer (e.g. SMD).

Moreover, these findings will contribute to a better understanding of the physics of the breakup of high-viscosity liquid jets and as well to create an experimentally validated CFD based tool for future burner development and optimization.
Facing the Challenges in CFD Modelling of Multistage Axial Compressors
Authors: Cozzi Lorenzo , Rubechini Filippo , Marconcini Michele , Arnone Andrea , Astrua Pio , Schneider Andrea , Silingardi Andrea
Published: August 2017

Multistage axial compressors have always been a great challenge for designers since the flow within these kind of machines, subjected to severe diffusion, is usually characterized by complex and widely developed 3D structures, especially next to the endwalls. The development of reliable numerical tools capable of providing an accurate prediction of the overall machine performance is one of the main research focus areas in the multistage axial compressor field.

This paper is intended to present the strategy used to run numerical simulations on compressors achieved by the collaboration between the University of Florence and Ansaldo Energia. All peculiar aspects of the numerical setup are introduced, such as rotor/stator tip clearance modelling, simplified shroud leakage model, gas and turbulence models. Special attention is payed to the mixing planes adopted for steady-state computations because this is a crucial aspect of modern heavy-duty transonic multistage axial compressors. In fact, these machines are characterized by small inter-row axial gaps and transonic flow in front stages, which both may affect non-reflectiveness and fluxes conservation across mixing planes. Moreover, the high stage count may lead to conservation issues of the main flow properties form inlet to outlet boundaries. Finally, the likely occurrence of partspan flow reversal in the endwall regions affects the robustness of non-reflecting mixing plane models.

The numerical setup has been validated on an existing machine produced and experimentally tested by Ansaldo Energia. In order to evaluate the impact on performance prediction of the mixing planes introduced in the steady-state computation, un-steady simulations of the whole compressor have been performed at different operating conditions. These calculations have been carried out both at the compressor design point and close to the surge-line to evaluate the effect of rotor/stator interaction along the compressor working line.
Investigation Into Flow Mechanism Leading to the Step Change in Aerodynamic Modes of Rotating Instabilities in a Subsonic Axial Compressor Rotor
Authors: Chen Zhiyang , Wu Yanhui , Yang Guowei , An Guangyao , Wang Bo
Published: August 2017

Rotating instability (RI) as a prestall wave is closely related to tip clearance noise, blade vibration and rotating stall/surge of axial compressors, so lots of researchers have focused on it. In this paper, the experimental and numerical investigations for a subsonic compressor are carried out to explore the flow mechanism leading to the step change in aerodynamic modes of rotating instabilities at different rotor speeds. The measurement results show that RIs only appear at operating points in a narrow mass flow range near the stability limit of the test rotor both at a medium and at a high rotor speeds. The dominant mode order frequency of RI decreases from increased rotor speed. The simulations show that circumferential traveling waves also appear near the stability limit of the test rotor at different rotor speeds. The dominant frequency/mode of the circumferential traveling waves decrease with the raise of rotor speed. Details of the simulated flow fields reveal that the periodical behavior of TLF will form the flow unsteadiness in one passage. There is a time shift of pressure oscillations in between rotor passages. RI originates when the flow unsteadiness propagates circumferentially no matter at MRS or HRS. In the rotating frame, the time shift of pressure oscillations reduces and the period of the static pressure oscillation increases from the increased rotor speed. The variation of the time shift of pressure oscillations and the period of the static pressure oscillation lead to the step change in the dominant frequency/mode of RI.
Robust Design Optimization of a Low Pressure Turbine Rotor Discs Secondary Air System
Authors: Antinori Giulia , Arsenyev Ilya , Fischersworring-Bunk Andreas
Published: August 2017

Low pressure turbine (LPT) rotor discs undergo high thermal and mechanical loads during normal aircraft missions. Therefore, to meet the minimum requirement for life, temperatures and stresses in the disk need to be maintained within certain limits. This is achieved by carefully designing the disk shape and the cooling system. The complexity of this multi-physics problem together with a large number of design parameters require the use of numerical optimization methods for the Secondary Air System (SAS) design. Moreover, possible variations in the boundary conditions due to ambient parameters (e.g. temperatures, pressures) and manufacturing tolerances of the SAS components should be taken into account within the system design and optimization phase. In this paper an application of robust optimization methods for the design of a LPT secondary air system is proposed. The objective is to increase the engine efficiency by minimizing the amount of cooling flow, which is needed to guarantee a minimum required number of life cycles and to keep maximal temperatures within the limits. In order to predict the disks life accurately, transient thermal-structural analysis is used, which is computationally demanding. For this reason, optimization should be performed with a very limited amount of system evaluations. The dimension of the parameter space is reduced through the application of global sensitivity analysis methods by selecting the parameters that most affect the results. Optimization methods are sped up by the use of surrogate models, created over the reduced parameter space, which approximate the objective function and the constraints.
On Boundary Layer Relaminarization in an Highly Accelerated High Pressure Turbine Stator Flow
Authors: Bader Pascal , Sanz Wolfgang
Published: August 2017

The boundary layer represents the thin layer between the wall and the free stream. Although this zone is small in respect to the free stream its state — whether it is laminar or turbulent — has a high impact on wall parameters like skin friction but also on the free stream.

The state of the boundary layer is influenced by free stream parameters like free stream turbulence intensity or acceleration. Transition can occur from laminar to turbulent, but also from turbulent to laminar. The latter process is called relaminarization.

Transition and relaminarization has already been detected in laboratory conditions but also occur in real-life flow around aeroplane wings, but also in turbomachines. In this work a high pressure stator of a two-spool, two-stage turbine is analyzed numerically regarding transition and relaminarization to show how the boundary layer flow along casing and blade is influenced by these phenomena.

It is found out in this work, that the boundary layer along the blade, hub and shroud undergoes transition and also relaminarization despite the high free stream velocities and turbulence levels inside the machine. The differences between a transitional and fully turbulent simulation regarding loss generation by wall friction are given.
Quantifying Blowing Ratio for Shaped Cooling Holes
Authors: Cerantola D. J. , Birk A. M.
Published: August 2017

Effusion cooling was a popular technology integrated into the design of gas turbine combustor liners. A staggering amount of research was completed that quantified performance with respect to operating conditions and cooling hole geometric properties; however, most of these investigations did not address the influence of the manufacturing process on the hole shape. This study completed an adiabatic wall numerical analysis using the realizable k-ε turbulence model of a laser-drilled hole that had a nozzled profile with an area ratio of 0.24 and five additional cylindrical, nozzled, diffusing, and filleted holes that yielded the same hole mass flow rate at representative engine conditions. The traditional methods for quantifying blowing ratio yielded the same value for all holes that was not useful considering the substantial differences in film cooling performance. It was proposed to define hole mass flux based on the outlet y-cross sectional area projected onto the inclination angle plane. This gave blowing ratios that correlated to better and worse cooling performance for the diffusing and nozzled holes respectively. The diffusing hole delivered the best film cooling due to having the lowest effluent velocity and greatest amount of in-hole turbulent production, which coincided with the worst discharge coefficient.
Fast Optimisation of a Three-Dimensional Bypass System Using a New Aerodynamic Design Method
Authors: Barbarossa F. , Rife M. E. , Carnevale M. , Parry A. B. , Green J. S. , di Mare L.
Published: August 2017

The propulsive efficiency of civil aviation power plants can be effectively improved by increasing the bypass ratio. Higher bypass ratios, however, exacerbate issues of performance, stability and integrity due to the interaction between the engine pylon, the outlet guide vanes (OGV) and the fan. These issues are due to the distortion of the static pressure field at fan exit due to the presence of the pylon and its transmission through the OGV bladerow and are more pronounced the closer the components of the low pressure compression (LPC) system are. These issues make a rational and effective design of the LPC system of paramount importance for the success of very high-bypass ratio engines.

At the preliminary design phase, methods that utilise computational fluid dynamics (CFD) are prohibitively expensive, particularly if they are used as part of optimisation processes involving highly three dimensional, non-axisymmetric OGV designs. An alternative method is being developed exploiting the simplicity and the accuracy of surface singularity element methods to investigate the sensitivity of the bypass system to changes in the design variables. Although the singularity method is based on simplified assumptions of inviscid, incompressible flow, it still performs remarkably well when combined with a tailored optimisation technique. This paper discusses the optimisation framework in detail, including the underlying mathematical models that describe the three-dimensional aerodynamic flowfield as well as the optimisation tools, variables and cost functions used within the optimisation process. The results show that the proposed approach can be used to explore quickly and efficiently a far wider design space than attempted so far in literature. Furthermore, the proposed method leads to non-axysymmetric cascade designs whereby every vane has the same load as the nominal vane whilst greatly reducing the static pressure distortion at fan exit.
The Design of a Large Diameter Axial Flow Fan for Air-Cooled Heat Exchanger Applications
Authors: Wilkinson Michael B. , van der Spuy Johan , von Backström Theodor W.
Published: August 2017

An axial flow fan design methodology is developed to design large diameter, low pressure rise, rotor-only fans for large air-cooled heat exchangers. The procedure aims to design highly efficient axial flow fans that perform well when subjected to off design conditions commonly encountered in air-cooled heat exchangers. The procedure makes use of several optimisation steps in order to achieve this. These steps include optimising the hub-tip ratio, vortex distribution, blading and aerofoil camber distributions in order to attain maximum total-to-static efficiency at the design point.

In order to validate the design procedure a 24 ft, 8 bladed axial flow fan is designed to the specifications required for an air-cooled heat exchanger for a concentrated solar power (CSP) plant. The designed fan is numerically evaluated using both a modified version of the actuator disk model and a three dimensional periodic fan blade model. The results of these CFD simulations are used to evaluate the design procedure by comparing the fan performance characteristic data to the design specification and values calculated by the design code. The flow field directly down stream of the fan is also analysed in order to evaluate how closely the numerically predicted flow field matches the designed flow field, as well as determine whether the assumptions made in the design procedure are reasonable.

The fan is found to meet the required pressure rise, however the fan total-to-static efficiency is found to be lower than estimated during the design process. The actuator disk model is found to under estimate the power consumption of the fan, however the actuator disk model does provide a reasonable estimate of the exit flow conditions as well as the total-to-static pressure characteristic of the fan.
Development of Reliable NARX Models of Gas Turbine Cold, Warm and Hot Start-Up
Authors: Bahlawan Hilal , Morini Mirko , Pinelli Michele , Spina Pier Ruggero , Venturini Mauro
Published: August 2017

This paper documents the set-up and validation of nonlinear autoregressive exogenous (NARX) models of a heavy-duty single-shaft gas turbine. The considered gas turbine is a General Electric PG 9351FA located in Italy.

The data used for model training are time series data sets of several different maneuvers taken experimentally during the start-up procedure and refer to cold, warm and hot start-up. The trained NARX models are used to predict other experimental data sets and comparisons are made among the outputs of the models and the corresponding measured data.

Therefore, this paper addresses the challenge of setting up robust and reliable NARX models, by means of a sound selection of training data sets and a sensitivity analysis on the number of neurons. Moreover, a new performance function for the training process is defined to weigh more the most rapid transients. The final aim of this paper is the set-up of a powerful, easy-to-build and very accurate simulation tool which can be used for both control logic tuning and gas turbine diagnostics, characterized by good generalization capability.
A Numerical Investigation of the Impact of Part-Span Connectors on the Flow Field in a Linear Cascade
Authors: Brüggemann C. , Schatz M. , Vogt D. M. , Popig F.
Published: August 2017

This paper presents a numerical investigation of the impact of different part-span connector (PSC) configurations on the flow field in a turbine passage. For this purpose a linear cascade based on a profile section of a typical reaction blade used in industrial steam turbines was modeled and 3D simulations with varying size, shape, axial position and yaw incidence angle of the PSC were performed. Air modeled as ideal gas was chosen as the working fluid.

Apart from a sensitivity study of the above mentioned parameters on the losses incurred by PSCs based on the numerical results, a detailed investigation of the flow field was carried out to highlight the interaction with the incoming flow. Moreover, the variation of the flow field behind the cascade was examined to assess the impact on the subsequent blade row. It is shown that depending on the geometry and the position of the PSC, different vortex structures are established in the wakes. These wakes interact with the main flow in the passage, thus influencing both dissipation and the downstream flow field. Major changes of the wake flow character and extent could be observed.

Comparisons of the CFD results against commonly used analytical loss correlations for PSC revealed large differences, especially as certain parameters such as the yaw incidence angle are generally not considered by the latter. As a consequence, the analytical models need to be improved and extended. The results of this study indicate that the possibility of reducing the losses incurred by PSC by careful selection of design parameters within the design space dictated by its mechanical constraints.
Wind Tunnel Study of a Generic Wind Turbine Nacelle Model
Authors: Manolesos Marinos , Chaviaropoulos Panagiotis
Published: August 2017

Three versions of a generic horizontal axis wind turbine nacelle model were tested in a wind tunnel. Force and wake velocity measurements were performed at a Reynolds number of 4.2e5 based on the complete length of the nacelle and the spinner. The effect of main body corner shape was examined as well as that of the spinner. It was found that rounded corners can lead to significant reduction in the aerodynamic forces experienced by the nacelle. At the same time, however, smooth curve surfaces cause instabilities in the separation location, which lead to sudden drops in the lift force. In terms of aeroelastic behavior this is an undesirable feature that leads to negative aerodynamic damping and structures prone to galloping.
Interaction of Concurrent Forced Response and Flutter Phenomena in a Compressor Stage
Authors: Mao Zhiping , Kielb Robert E.
Published: August 2017

This paper studies a subsonic compressor case with concurrent forced response and flutter by using the Harmonic Balance method, and was inspired by historical experimental data. Forced response was observed when the rotating speed was approaching a crossing on the Campbell diagram, where flutter appeared to be suppressed. CFD simulations are conducted by using a quasi-3D configuration at the mid-span of one stage of a 3.5-stage compressor. Due to the constraint of frequency domain methods, the research is conducted in the vicinity of the 1T-44EO crossing with a small frequency shift between flutter frequency and external excitation frequency. The influence from flutter to forced response is observed: a one-way crosstalk at forced response frequency is observed, presented as the anomaly of unsteady velocity and unsteady pressure near the rear section of rotor blades and in the rotor wake region. The anomaly is speculated as the presence of increasing intensity of shedding vortices induced by the vibration of the blade. To further prove the impact of this viscous effect, a numerical experiment was performed with inviscid rotor blades. In contrast to the crosstalk at forced response frequency, no obvious influence on the unsteady behavior is detected at the flutter frequency, and this observation is confirmed at multiple vibration amplitudes. Considering the relationship between unsteady pressure at flutter frequency and aerodynamic damping, we conclude the influence of forced response on the aerodynamic damping is negligible. In addition, a linearity of unsteady pressure at the flutter frequency vs. vibration amplitude is uncovered. The discoveries provide a proof to linearity assumption and single-frequency simplification widely adopted by industry in flutter simulations.
CFD-Based Prediction of Rotordynamic Performance of Smooth Stator-Grooved Rotor (SS-GR) Liquid Annular Seals
Authors: Mortazavi Farzam , Palazzolo Alan
Published: August 2017

Circumferentially grooved, annular liquid seals typically exhibit good whirl frequency ratios and leakage reduction, yet their low effective damping can lead to instability. The current study investigates the rotordynamic behavior of a 15 stage groove-on-rotor annular liquid seal by means of CFD, in contrast to previous studies which focused on a groove-on-stator geometry. The seal dimensions and working conditions have been selected based on experiments of Moreland and Childs. The precessional frequency ratios as high as 4 have been studied. The CFD model replicates the whirling motion imposed by the 2D shaker apparatus in Moreland and Childs experimental setup. Implementation of pressure-pressure inlet and outlet conditions obviates the need for loss coefficients at the entrance and exit of the seal. A computationally efficient quasi-steady approach is used to obtain impedance curves as functions of excitation frequency Ω. The effectiveness of steady-state CFD approach is validated by comparison with the experimental results of Moreland and Childs. Results show good agreement in terms of leakage, pre-swirl ratio and rotordynamic coefficients. Leakage is shown to decrease with spin rotational speed ω, whirl speed Ω and surface roughness ∈. The variation of pre-swirl ratio (PSR) and outlet-swirl ratio (OSR) with these parameters is presented. It was found that PSR will be about 0.3–0.4 at the entrance of seal in the case of radial injection and OSR always converges to values near 0.5 for current seal and operational conditions. The rotordynamic coefficients show negligible dependence on Ω in agreement with experiments. The small negative value of direct stiffness coefficients, large cross-coupled stiffness coefficients and small direct damping coefficients explain the destabilizing nature of these seals. Finally, influence of surface roughness on leakage, PSR, OSR and stiffness coefficients is discussed.
Static Load Performance of a Water Lubricated Hydrostatic Thrust Bearing
Authors: San Andrés Luis , Rohmer Michael , Wilkinson Scott
Published: August 2017

In rotating equipment, thrust bearings aid to balance axial loads and control shaft position. In turbomachinery, axial loads depend on shaft speed and pressure rise/drop on the impellers. This paper details a water lubricated test rig for measurement of the performance of hydrostatic thrust bearings (HTBs). The rig contains two water lubricated HTBs (105 mm outer diameter), one is the test bearing and the other a slave bearing. Both bearings face the outer side of thrust collars of a rotor. The paper shows measurements of HTB axial clearance, flow rate, and recess pressure for operation with increasing static load (max. 1.4 bar) and supply pressure (max. 4.14 bar) at a rotor speed of 3 krpm (12 m/s OD speed). Severe angular misalignment, static and dynamic, of the bearing surface against its collar persisted and affected all measurements. The HTB axial clearance increases as the supply pressure increases and decreases quickly as the applied load increases. The reduction in clearance increases the flow resistance across the film lands thus reducing the through flow rate with an increase in recess pressure. In addition, an estimated bearing axial stiffness increases as the operating clearance decreases and as the supply pressure increases. Predictions from a bulk flow model qualitatively agree with the measurements. Alas they are not accurate enough. The differences likely stem from the inordinate tilts (static and dynamic) as well as the flow condition. The test HTB operates in a flow regime that spans from laminar to incipient turbulent. Quantification of misalignment at all operating conditions is presently a routine practice during operation of the test rig.
Unsteady Behavior of Diffuser Stall in a Centrifugal Compressor With Vaned Diffuser
Authors: Fujisawa Nobumichi , Ema Daiki , Ohta Yutaka
Published: August 2017

In this study, the unsteady behavior of a diffuser rotating stall in a centrifugal compressor with a vaned diffuser was investigated through experiments and numerical analyses. From the casing static pressure measurements, it was determined that the diffuser stall propagated at 25% of impeller rotational speed in the vaneless space. The numerical results revealed the presence of a typical vortical structure on the diffuser’s leading edge. Under partial flow condition, a tornado-type vortex was generated on the diffuser’s leading edge. Furthermore, a longitudinal vortex at the shroud/suction surface corner (i.e., leading edge vortex (LEV)) was induced by the rolling-up flow on the diffuser suction surface. As the velocity was decreased, the development of the tornado-type vortex and LEV forms a substantial flow blockage within the diffuser passages. Furthermore, the diffuser stall cell was caused by the systematic vortical structure which consisted of the tornado-type vortex, LEV, and vortex in the throat area of diffuser passages. In addition to this, the developed LEV interacted with the next diffuser leading edge and formed the throat area blockage with the passage of time. Then, the tornado-type vortex and LEV developed by the throat area blockage and diffuser stall cell, which was caused by the systematic vortical structure, propagated to the succeeding diffuser vane. Therefore, the diffuser stall in the centrifugal compressor was caused by the evolution of the tornado-type vortex and LEV.