bluuebunny/crossref_metadata_2025_split huggingface dataset:
Computational Cost Reduction of MIMO Controllers for Active Magnetic Bearing Systems
Authors: Sahinkaya Alican , Hawkins Larry , Sawicki Jerzy T. 
Published: January 2021

Abstract
Generalized MIMO controllers such as H∞ and μ have not been widely adopted in the magnetic bearing industry, partially due to high computational cost relative to simpler SISO schemes. Computational cost is important to industrial magnetic bearing vendors as their controller hardware is based on embedded processors that have limited bandwidth. Studies to mitigate the problem of high order controllers show the limit of the existing methods in order reduction while still maintaining satisfying robust performance. A novel method is proposed to reduce the computational cost of robust controllers by identifying the bounds in their dynamic response, such that an implementation of a controller within those bounds results in the robust performance. The bounds are used to develop two computational cost reduction schemes for controller implementation, i.e., 1) identifying a dual-rate implementation of a single-rate controller which uniformly reduces the computational cost via interlacing technique, and 2) redesign of a controller by identifying its negligible dynamics based on the identified bounds in the controllers’ dynamic response. The results of both approaches are demonstrated on two active magnetic bearing (AMB) systems, a model of a 300 kW turbine generator with permanent magnet biased AMBs and an experimental high-speed AMB machining spindle. μ-synthesis controllers are designed for both systems and the proposed method and schemes are applied accordingly. The comparison of standard implementation of the synthesized controllers and the proposed new implementations is presented. The results demonstrate considerable reduction in the computational cost in terms of required number of multiply-accumulate (MAC) operations.
Comparison of Endwall Loss Reduction Techniques in a High-Lift Turbine Passage
Authors: Marks Christopher R. , Fletcher Nathan , Sondergaard Rolf 
Published: January 2021

Abstract
The development of techniques that reduce the losses in the endwall region is an important area of research as it supports an increase in the turbine design space through the use of higher lift blade designs while maintaining high efficiency. Several active and passive shape contouring methods that reduce losses generated by the secondary flow have been developed and investigated on a high-lift front-loaded LP turbine research profile in a low-speed linear cascade configuration. This paper summarizes and compares alterations to the three-dimensional secondary flow field by the application of three different techniques: blade profile contouring, optimized endwall shape contouring, and localized low mass coefficient jets. Each method was applied to identical research blade profiles and compared in the same linear cascade wind tunnel resulting in a unique perspective into the loss reduction mechanisms associated with each technique. The design strategy will be discussed along with a detailed description of changes to the secondary flow field using in- and out-of-passage total pressure loss measurements and high-speed stereoscopic particle image velocimetry. The key findings include the loss reduction mechanisms associated with each approach and the manipulation of key endwall flow structures such as the passage vortex and a strong suction surface corner separation.
Generalized Model for the Approximation of Coupled Acousto-Mechanical Natural Frequencies in High-Pressure Centrifugal Compressors
Authors: Heinrich Christoph Rocky , Kühhorn Arnold , Steff Klaus , Petry Nico 
Published: January 2021

Abstract
The oil and gas, chemical, and process industries employ centrifugal compressors for a wide range of applications. Due to this, the conditions under which centrifugal compressors have to operate, vary significantly from case to case. Gas pipeline compressors, for example, may feature discharge pressures well over 100 bar. In other fields of application, like gas injection, which is used to enhance oil recovery, this quantity can reach considerably higher values. Here, discharge pressures over 600 bar and gas densities over 300 kg/m3 are not uncommon. During the last several decades, comprehensive research was conducted on the impact of high pressure operating conditions on the vibrational behavior of centrifugal compressor wheels. Nowadays, it is well-known that an increase in gas pressure levels leads to a more pronounced interaction between the side cavities and the impeller, which results in a frequency shift of the acoustic and structural modes.
For the safe operation of compressors, it is necessary to predict these coupled natural frequencies accurately. The state-of-the-art approach to achieve this objective is the finite element method. While this technique provides high-quality results, the simulation of acousto-mechanical systems is still a time-consuming process that incurs high computational costs. Therefore, finite element models are, in this case, not suitable for probabilistic studies, sensitivity analyses, and comprehensive simulations of the full operating range of the compressor. In 2013, Magara proposed a simplified model based on an annular plate between two cylindrical cavities to solve this problem. While this method reduces the required computational effort significantly, its use is limited to platelike impellers.
The authors of the current paper propose a more generalized method to overcome the challenges mentioned above. It uses the uncoupled structural and acoustic modes of the actual impeller and side cavities in a modal superposition to approximate the natural frequencies of the coupled acousto-mechanical system. In this way, the intended design geometries of the impeller and side cavities are considered while maintaining the advantages of Magara’s model regarding the computational effort. In a numerical study, Magara’s method and the generalized model are applied to different systems of increasing complexity. The investigation starts with a simple annular plate in a cylindrical cavity and ends with an actual compressor impeller. At every complexity level, the results of both approaches are compared to a finite element analysis. Moreover, measurement data of a simplified rotor in a cylindrical cavity is used to validate the numerical models. Finally, the paper concludes with a discussion of the limitations and benefits of all employed numerical methods.
Effects of Circumferential Nonuniform Tip Clearance on Flow Field and Performance of a Transonic Turbine
Authors: Zheng Yun , Jin Xiubo , Yang Hui , Gao Qingzhe , Xu Kang 
Published: January 2021

Abstract
The numerical study is performed by means of an in-house CFD code to investigate the effect of circumferential nonuniform tip clearance due to the casing ovalization on flow field and performance of a turbine stage. A method called fast-moving mesh is used to synchronize the non-circular computational domain with the rotation of the rotor row. Four different layouts of the circumferential nonuniform clearance are calculated and evaluated in this paper. The results show that, the circumferential nonuniform clearance could reduce the aerodynamic performance of the turbine. When the circumferential nonuniformity δ reaches 0.4, the aerodynamic efficiency decreases by 0.58 percentage points. Through the analysis of the flow field, it is found that the casing ovalization leads to the difference of the size of the tip clearance in the circumferential direction, and the aerodynamic loss of the position of large tip clearance is greater than that of small tip clearance, which is related to the scale of leakage vortex. In addition, the flow field will become nonuniform in the circumferential direction, especially at the rotor exit, which will adversely affect the downstream flow field.
Impact of Secondary Air Flow on Combustor Emissions
Authors: Mohammad Bassam S. , Volk Brian , McManus Keith 
Published: January 2021

Abstract
It is a common practice to relate emissions performance of Dry Low Emissions (DLE) combustion systems to the flame temperature that is estimated from the mass flows of air and fuel flowing through the premixer. In many combustion systems, the exit temperature (or turbine nozzle inlet temperature) is quite low and is not a good parameter for estimating combustion emissions. The difference between the combustion flame temperature and exit temperature is mainly due to secondary air dilution. To our knowledge there are no detailed published data that quantify the impact of this temperature difference on combustion emissions. The target of this study is to quantify the impact of secondary air variation on emissions, both globally and locally. High pressure experiments are conducted at H class gas turbine operating conditions using a DLE combustion system. In the context of this DLE system, secondary air refers to cooling and leakage flows because direct air dilution of the combustion gasses is not necessary. This is because the flame stabilized downstream of the premixer is well mixed and fuel-lean. With NOx requirements moving toward single digit (ppm) levels, it becomes essential to accurately quantify the impact of reducing the secondary air percentage on emissions performance. In addition to the need to carefully study the impact of local interaction of the secondary air with the flame. The combustion system is configured with two independently controlled mixers along with a variable secondary air circuit that can change the secondary air fraction from 14 to 8%. Multiple emissions rakes are used at the combustor exit to delineate the interaction and relate it to the flame structure. The system is configured to enable sampling from individual rakes to study local emissions and the rakes can be ganged together to measure the bulk-averaged combustion emissions. This research provides a quantification of the improvement of the NOx margin with a decrease in the secondary air percentage. The study shows that the increase in margin is not a simple re-estimate of the combustor emissions using the NOx design curve due to flame quenching effects. The results also show that the secondary air can be used to improve the NOx emissions via controlling the interaction with the primary flame. The impact is quantified in terms of emissions, acoustics and metal temperatures.
Generation and Development of Klebanoff Streaks in Low-Pressure Turbine Cascade Under Upstream Wakes
Authors: Sun Shuang , Wu Xingshuang , Tan Tianrong , Zuo Canlin , Pan Sirui , Liu Fulin 
Published: January 2021

Abstract
At low Reynolds numbers operating condition, the boundary layer of the high-lift low-pressure turbine (LPT) of aero-engines is prone to separate on the suction surface of the airfoil. The profile losses of the airfoil are largely governed by the size of the separation bubble and the transition process in the boundary layer. However, the wake-induced transition, the natural transition and the instability induced by the Klebanoff streaks complicate the transition process.
The boundary layer on the suction surface of a high-lift LPT was investigated at Re = 50,000 with upstream wakes. The numerical simulation was performed with the CFX software using large eddy simulations (LES), and the experiment was performed on a linear cascade. In this study, the wake is divided into the wake center and the wake tail, the unsteady formation process of the streaks and the wall shear stress caused by the wake are analyzed. A new mechanism of generation and development of Klebanoff Streaks was presented to better understand the effect of the wake on the boundary layer.
Moreover, it was found that after entering the blade passage, the wake center does not contact the blade but causes the wall shear stress of the front part on the suction surface to increase. However, it is not possible to form strong Klebanoff streaks at the leading edge of the blade by shear sheltering effect. Only the wake tail can form Klebanoff streaks when it contacts the blade.
Dynamic Modelling of Small Scale and High Temperature ORC System Using Simulink and CoolProp
Authors: Dhanasegaran Radheesh , Uusitalo Antti , Turunen-Saaresti Teemu 
Published: January 2021

Abstract
In the present work, a dynamic model has been developed for the small-scale high-temperature ORC experimental test rig at the LUT University that utilizes waste heat from a heavy-duty diesel engine exhaust. The experimental facility consists of a high-speed Turbogenerator, heat exchanger components such as recuperator, condenser, and evaporator with a pre-feed pump to boost the working fluid pressure after the condensation process constituting a cycle. The turbogenerator consists of a supersonic radial-inflow turbine, a barske type main-feed pump, and a permanent magnet type generator components connected on a single shaft. Octamethyltrisiloxane (MDM) is the chosen organic working fluid in this cycle. Matlab-Simulink environment along with the open-source thermodynamic and transport database Cool-Prop has been chosen for calculating the thermodynamic properties of the dynamic model. A functional parameter approach has been followed for modeling each block component by predefined input and output parameters, aimed at modeling the performance characteristics with a limited number of inputs for both design and off-design operations of the cycle. The dynamic model is validated with the experimental data in addition to the investigation of exhaust gas mass flow regulation that establishes a control strategy for the dynamic model.
Aerodynamic Retrofit Design for a High Pressure Compressor Using a High Hub/Tip Ratio Mixed-Flow Compressor
Authors: Xiang Hang , Chen Jiang , Cheng Jinxin , Niu Han , Liu Yi , Song Xiancheng 
Published: January 2021

Abstract
A high-load mixed-flow compressor with an extremely high inlet hub/tip ratio (0.889) is designed and analyzed for replacing the rear stages of a multistage high pressure axial compressor. The effects of blade number, splitter blades and dimensionless geometric parameters on the impeller performance are investigated by an improved loss model. A full-surface parameterization control method is adopted for blade optimizations of the mixed-flow impeller and the tandem stator. As a retrofit design of the multistage axial compressor, an unconventional type of axial-co-mixed-flow combined compressor scheme is proposed and discussed. Further, in order to minimize the axial dimension and maximize the load, this paper also proposed preliminary designs of the twin-stage mixed-flow compressor and the twin-stage counter-rotating mixed-flow compressor respectively equipped with the high hub/tip ratio mixed-flow compressor. The results indicate that the mixed-flow impeller configuration with 42 principal blades and splitter blades with a fifth of principal blade length has the maximum efficiency at design flow rate. Blade height/pitch ratio is a considerable parameter which demonstrates the interaction among hub/tip ratio, aspect ratio and solidity especially for high hub/tip ratio cascade designs. The mixed-flow compressor can greatly improve the load capacity of the high pressure compressor with slight impact on efficiency and surge margin. At low rotate speed, the mixed-flow impeller can maintain relatively high efficiency level and even carry a higher proportion of the load, while the tandem stator limits the overall efficiency improvement. Besides, structures with no return channel of the three unconventional combined compressors are beneficial to the reduction of dimension and cost, which shows the potential application prospects of high hub/tip ratio mixed-flow compressors.
Influence of Blade Pitch and Number of Blades of a Pump Inducer on Single and Two-Phase Flow Performance
Authors: Mansour Michael , Parikh Trupen , Thévenin Dominque 
Published: January 2021

Abstract
This study investigates the influence of various inducer configurations upstream of a pump impeller on the single and two-phase flow performance. Three pitch values (P = 0.151, 0.251, and 0.351 m), as well as three different numbers of blades (N = 2, 3, and 4 blades), were studied, leading to a total of 9 different inducer geometries. The main objective of the present study is to analyze and compare the corresponding performances and the two-phase mixing behavior, which is necessary for improving the two-phase pumping ability. 3D steady-state simulations using the Moving Reference Frame (MRF) approach were applied for single-phase flow, while a transient setup using a moving-mesh approach was employed for two-phase simulations. Turbulence was modeled by the Reynolds Stress Model (RSM), whereas the Volume of Fluid (VOF) method was applied to model air-water interactions. The results show that the increase in the number of blades leads to a high performance drop at overload (high-flow) conditions, but only to a slight performance enhancement at part-load (low-flow) conditions. Additionally, the effective flow range of the inducer corresponding to high efficiency becomes narrower for a higher number of blades. Concerning the inducer pitch, at part-load conditions, a lower pitch is slightly beneficial to smoothly suck the flow and damp the low-flow vortices; employing a high pitch at these conditions results in intensified flow vortices, reducing slightly the performance. On the other hand, the blade pitch is very influential for the performance at optimal and overload conditions, where a lower pitch causes flow blockage, leading to significant performance deterioration and a very limited range of applications. Generally, it was found that a modification of the inducer configuration affects the performance much more at overload compared to part-load conditions. Concerning two-phase mixing performance, the highest pitch provides the best mixing since the inducer is able to effectively churn the two phases. Similarly, an increase in the number of blades amplifies the turbulence between the two phases, thus improving mixing. Overall, a higher inducer pitch and a low to moderate number of inducer blades best ensure high performance, wide working range, and efficient two-phase mixing.
Loss and Thrust Modeling of Rocket Engine High Density Turbopumps
Authors: Gloger Jonathan , Lettieri Claudio , Souverein Louis 
Published: January 2021

Abstract
Turbopumps constitute an essential component of high thrust liquid rocket engines. They are characterized by a compact design, providing a large shaft power at high rotation rates. This is necessary to deliver the propellants at high pressure into the combustion chamber to generate the required engine thrust. Recirculating fluid around the pump has a major influence on axial loads and fluid dynamical losses, impacting turbopump performance and life. Therefore, simplified modelling approaches are required early on for the preliminary design of the pump impeller, and its side cavities and seals. Indeed, past experience within ArianeGroup for pumps and secondary circuits indicates that the coupling between the main flow and the leakage has to be considered at an early stage of the design. The empirical correlations of the flow in the cavities shall be carefully selected, accounting for the particularities of each new configuration. Furthermore, it is also recommended for the impeller design (e.g. for blade leading edge and pressure relieve orifices positioning), that the effects of leakage reinjection into the main flow shall be taken into account.
In order to obtain first estimates for early design optimization without the cost of full scale 360° high fidelity computational dynamic simulations (CFD), a reduced model is developed to predict losses and axial thrust on the rotor, including effects of fluid recirculation and reinjection. A two-step approach is followed: Firstly, an empirical model developed by Gülich et al. [1] is applied to characterize leakage loss analytically. Secondly, a reduced numerical model is implemented which features a single passage impeller geometry including seals and side wall gaps. The accuracy of both the analytical model and the simplified numerical model are verified in comparison to high fidelity CFD calculations, evaluating the loss contributions in the leakage path and axial thrust for a range of operating points.
In line with expectation, the highest impact on the pump performance are the volumetric losses due to the recirculation of pressurized fluid, with and efficiency decrease of up to 20 % in the investigated cases. The implemented analytical model captures the overall loss mechanisms with a 20 % uncertainty in the design point, disk friction is underpredicted and axial thrust is mostly over-predicted. Due to the simplified numerical model with the single passage impeller geometry including side cavities, the uncertainty can be decreased to about 5 %. At part load operation, the accuracy of both models reduces. It is noted, that thrust prediction is subject to the highest uncertainties.
The current work has provided a simplified numerical model that offers the higher flexibility required for the early design phase as compared to a full annulus CFD simulation of the pump, with an increased accuracy as compared to the analytical models.
Evaluation of the Capacity of RANS/URANS/LES in Predicting the Performance of a High-Pressure Turbine: Effect of Load and Off Design Condition
Authors: Dombard Jérôme , Duchaine Florent , Gicquel Laurent , Odier Nicolas , Leroy Kevin , Buffaz Nicolas , Le-Guyader Sébastien , Démolis Jacques , Richard Stéphane , Grosnickel Thomas 
Published: January 2021

Abstract
This paper aims at addressing design issues of turbomachinery configurations by use of Large-Eddy Simulation (LES). To do so, a research state-of-the-art high-pressure turbine stage, without technological details and for which experimental data are available, is computed with the three methods: i.e. RANS, URANS and LES. Starting from the nominal operating design, a database is acquired varying the design space (three Zweifel numbers), load (three pressure rates) and rotation speed (three reduced speeds). The analysis of the database is carried out incrementally from a design perspective. Numerical results are systematically compared to experimental ones.
Main conclusions are threefold: 1/ Calibrated RANS provides excellent results at the nominal operating point but lacks of accuracy at off design conditions. Only unsteady methods (both URANS and LES) allow a good agreement with experiment along the whole database. 2/ Although very good on the overall performances, LES provides radial profiles and 2D maps leaving room for improvement in comparison with the URANS predictions. 3/ LES and standard law-of-the-wall is validated against experiments in a high-pressure turbine without technological details but still representative of a realistic and recent industrial design. From an aero design point, this paper shows the interest in using URANS for off design conditions. It also represents a milestone for LES that had to be passed before addressing more complex issues which URANS hardly addresses.