bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of Foil Geometry on the Static Performance of Thrust Foil Bearings
Authors: Fu Gen , Untaroiu Alexandrina , Swanson Erik 
Published: August 2017

Gas foil bearings use ambient air as working fluid, which forms the hydrodynamic wedge between the rotor surface and the compliant foil structure, therefore they are oil-free and environmental friendly. Compared to traditional bearings, gas foil bearing can operate in extreme conditions such as high temperature and high rotating speed. They also provide better damping and stability characteristics and have larger tolerance to debris and rotor misalignment. Gas foil bearings have been successfully applied to micro and small sized turbomachinery, such as micro gas turbine and cryogenic turbo expander. In the previous decades, a lot of theoretical and experimental work have been conducted to investigate the properties of gas foil bearings. However, very little work has been done to study the influence of the foil bearing pad geometry configuration using computational fluid dynamics (CFD). This study proposes a robust approach to analyze the effect of the foil geometry on the performance of a six pads thrust foil bearing.
In this study, a 3D CFD model for a parallel six pads thrust foil bearing is built using ANSYS-CFX software. The full Navier-Stokes equations are solved in the fluid domain. Taking into account the rotational periodicity, the computational model is simplified and only one pad of the thrust foil bearing is analyzed. In order to predict the thermal property, the total energy with viscous dissipation is used. Practical boundary conditions are used in the baseline CFD model. The results of the baseline modes is further compared with a modified Reynolds equation solution. Based on this model, the geometry of the thrust foil bearing is parameterized and analyzed using the design of experiments (DOE) methodology. In this paper, the selected geometry parameters of the foil structure include: minimum film thickness, inlet film thickness, the ramp extent on the inner circle, the ramp extent on the outer circle, the arc extent of the pad, and the orientation of the leading edge. A factorial design technique is employed to sample the design space in DOE. The objectives in the sensitivity study are selected as load force and friction torque. An optimal foil geometry is derived based on the results of the DOE process by using a goal driven optimization technique to maximize the load force and minimize the friction torque. The results show that the geometry of foil structure is a key factor for foil bearing performance. The numerical approach proposed in this study is expected to be useful from the thrust foil bearing design perspective.
Printed Circuit Heat Exchanger Flow Distribution Measurements
Authors: Lance Blake W. , Carlson Matthew D. 
Published: August 2017

Printed circuit heat exchangers (PCHEs) have an important role in supercritical CO2 (sCO2) Brayton cycles because of their small footprint and the high level of recuperation required for this power cycle. Compact heat exchangers like PCHEs are a rapidly evolving technology, with many companies developing various designs. One technical unknown that is common to all compact heat exchangers is the flow distribution inside the headers that affects channel flow uniformity. For compact heat exchangers, the core frontal area is often large compared with the inlet pipe area, increasing the possibility of flow maldistribution. With the large area difference, there is potential for higher flow near the center and lower flow around the edges of the core. Flow maldistribution increases pressure drop and decreases effectiveness. In some header geometries, flow separation inside the header adds to the pressure drop without increasing heat transfer.
This is the first known experiment to test for flow maldistribution by direct velocity measurements in the headers. A PCHE visualization prototype was constructed out of transparent acrylic for optical flow measurements with Particle Image Velocimetry (PIV). The channels were machined out of sheets to form many semi-circular cross sections typical of chemically-etched plates used in PCHE fabrication. These plates were stacked and bolted together to resemble the core geometry. Two header geometries were tested, round and square, both with a normally-oriented jet.
PIV allows for velocities to be measured in an entire plane instantly without disturbing the flow. Small particles of approximately 10 micrometers in diameter were added to unheated water. The particles were illuminated by two laser flashes that were carefully timed, and two images were acquired with a specialized digital camera. The movement of particle groups was detected by a cross-correlation algorithm with a result of about 50k velocity measurements in a plane. The velocity distribution inside the header volume was mapped using this method over many planes by traversing the PCHE relative to the optical equipment. The level of flow maldistribution was measured by the spatially-changing velocity coming out of the channels. This effect was quantified by the coefficient of variation proposed by Baek et al. The relative levels of flow maldistribution in the different header geometries in this study were assessed. With highly-resolved velocity measurements, improvements to header geometry to reduce flow maldistribution can be developed.
Planar Velocity Measurements at 100 kHz in Gas Turbine Combustors With a Continuous Laser Source
Authors: Mazur Marek , Scouflaire Philippe , Richecoeur Franck , Cunha Caldeira Mesquita Léo , Vie Aymeric , Ducruix Sébastien 
Published: August 2017

This work aims at presenting a novel approach to measure planar velocity in gas turbine combustors at very high sampling frequencies. For this purpose, a continuous wave laser is used in order to illuminate particles that are seeded into the flow. The Mie scattering images are acquired with a high-speed camera at 100 kHz with a constant time between each frame. The velocity fields are then obtained by applying classical PIV algorithms on successive particle scattering images. While this approach has been recently used in other research fields, such as aerodynamics or hydrodynamics, it is relatively new in combustion studies, where pulsed laser systems with higher power levels are usually preferred. The proposed technique is an economical and ergonomic solution to determine velocity fields at very high sampling frequencies. It is highly portable and safe and convenient to use and align. The main drawback is the long image exposure duration due to the low laser energy. This leads to a smearing effect of the captured particles and acts as a low-pass filter. It has the consequence that the PIV algorithm does not determine the displacement of “dots”, but of “traces”.
The measurement technique is tested experimentally on a model gas turbine combustor at a laboratory scale. The test is performed in three steps: (1) The instantaneous velocity fields are analysed in order to verify, whether the flame topology is represented correctly. (2) The mean and RMS velocity fields that are obtained with the present technique are compared with those obtained by classic low speed PIV. (3) Instantaneous synthetic Mie scattering fields are generated from a large eddy simulation (LES) on a similar combustor to test the algorithms. The planar velocity fields are calculated from these images and compared for the two techniques. Finally, possible error sources of the new technique are discussed.
Effect of Inlet Distortion Features on Transonic Fan Rotor Stall
Authors: Page James H. , Hield Paul , Tucker Paul G. 
Published: August 2017

The effect of inlet distortion from curved intake ducts on jet engine fan stability is an important consideration for next generation passenger aircraft such as the boundary layer ingestion (BLI) Silent Aircraft. Highly complex inlet flows which occur can significantly affect fan stability. Future aircraft designs are likely to feature more severe inlet distortion, pressing the need to understand the important factors influencing design.
This paper presents the findings from a large CFD investigation into which aspects of inlet distortion cause the most significant reductions in stall margin and, therefore, which flow patterns should be targeted by mitigating technology. The study considers the following aspects of distortion commonly observed in intakes: steady vortical distortion due to secondary flow, unsteady vortical distortion due to vortex shedding and mixing, static pressure distortion due to curved streamlines, and low momentum endwall flow due to thickened boundary layers or separation. Unsteady CFD was used to determine the stall points of a multipassage transonic rotor geometry with each of the inlet distortion patterns applied. Interesting new evidence is provided which suggests that low momentum flow in the tip region, rather than distortion in the main body of the flow, leads to damaging instability.
Fault Detection Through Model Based Diagnostics of AE-T100 Micro Gas Turbine
Authors: Mahmood Mariam , Martini Alessio , Traverso Alberto 
Published: August 2017

Micro gas turbines (mGT) are emerging power sources for distributed generation facilities considering their environment friendliness, high fuel flexibility and efficient cogeneration of heat and power (CHP). Numerical simulation and diagnostic tools are essential for cycle optimization of mGT and prediction of performance degradation.
This work is focused on fault diagnostics of T100 mGT through the application of AE-T100 model, a simulation tool, which has been developed within a collaboration between the University of Genoa (Unige) and Ansaldo Energia. This model has been developed for steady-state simulation of mGT in off-design conditions. Leveraging on previous efforts for model development, tuning, first phase of validation through Ansaldo Enegia test rig (AE-T100) and diagnostic application of the model, the present work deals with further utilization of the diagnostic capability of the model for mGT cycles. For this purpose, it was used in real operating conditions through the AE-T100 test rig for the diagnostic aim of some unexpected machine behavior. The model results indicated high deviation from the actual field data in terms of fuel flow and efficiency, and so justified the diagnostic capability of the AE-T100 tool.
Afterwards, based on the experimental observation of some bypass leakage from the burner in the same test rig, AE-T100 model was applied to model such leakage through the variation of AS ratio, leakage from the recuperator outlet to the ambient, combustor pressure drop and turbine section modification, and carried out the sensitivity analysis of these parameters. Sensitivity analysis has verified that the accurate impact of this leakage on overall mGT performance cannot be modeled with the help of AE-T100 tool in its current capacity. Therefore, some other investigations like analysis of compressor maps must be carried out to explain such a performance deviation in case of leakage. Afterwards, the analysis of compressor maps resulted from the operating conditions of the test conducted on the same machine after leakage repair, has highlighted the change in compressor operating point and thus, more efficient compressor functioning, which results in higher net power. Hence, this analysis has provided a more comprehensible explanation of this leakage impact on the mGT performance.
Egress Interaction Through Turbine Rim Seals
Authors: Scobie James A. , Hualca Fabian P. , Sangan Carl M. , Lock Gary D. 
Published: August 2017

Engine designers require accurate predictions of ingestion (or ingress) principally caused by circumferential pressure asymmetry in the mainstream annulus. Cooling air systems provide purge flow designed to limit metal temperatures and protect vulnerable components from the hot gases which would otherwise be entrained into disc cavities through clearances between rotating and static discs. Rim seals are fitted at the periphery of these discs to minimise purge. The mixing between the efflux of purge (or egress) and the mainstream gases near the hub end-wall results in a deterioration of aerodynamic performance.
This paper presents experimental results using a turbine test rig with wheel-spaces upstream and downstream of a rotor disc. Ingress and egress was quantified using a CO2 concentration probe, with seeding injected into the upstream and downstream sealing flows. The probe measurements have identified an outer region in the wheel-space and confirmed the expected flow structure. For the first time, asymmetric variations of concentration have been shown to penetrate through the seal clearance and the outer portion of the wheel-space between the discs. For a given flow coefficient in the annulus, the concentration profiles were invariant with rotational Reynolds number. The measurements also reveal that the egress provides a film-cooling benefit on the vane and rotor platforms. Further, these measurements provide unprecedented insight into the flow interaction, and provide quantitative data for CFD validation, which should help reduce the use of purge and improve engine efficiency.
Numerical and Experimental Comparison of Forced Response of Free-Standing and Single-Connected Last Stage Blades
Authors: Piraccini Francesco , Rice Tim , Auciello Jury , Evtushenko Alexey , Mossom Michael 
Published: August 2017

In this paper a comparison is made between numerical forced response simulations and experimental measurements for two popular Steam Turbine Last Stage Blade (LSB) architectures: a free standing LSB and a single-connected LSB with a mid-span wing (also known as a ‘snubber’). These variants share the same aerodynamic design in operation, i.e. they have the same ‘operating’ geometry. The focus of this study is the level of vibration response induced on the blades by resonance with a non-synchronous excitation. This can reduce the maximum condenser pressure safely attainable by a Steam Turbine during low volume flow (LVF) operation.
The study develops a common method to effectively represent the LVF excitation in FEM harmonic analysis for both free standing and single-connected LSB’s. This is valuable to LSB designers during the initial phase of a new development (when an assessment of the LVF capability is required before experimental measurements have been taken). In addition, the method can be used to evaluate the suitability of an existing LSB design to a revised environment, as it is often the case in retrofit applications.
Optimizing a Helical Groove Seal With Grooves on Both the Rotor and Stator Surfaces
Authors: Watson Cori , Wood Houston G. 
Published: August 2017

Helical groove seals are non-contacting annular seals commonly used in pumps within the impeller stages to sustain a pressure differential for a given leakage. Helical groove seals have continuously cut grooves, like the threads of a screw, on the surface of the rotor, the surface of the stator, or both. The two main components of the flow within helical groove seals are axial flow and groove flow. The axial flow serves to reduce the leakage by dissipating kinetic energy as the fluid expands in the grooves and then is forced to contract within the jet stream region. The groove flow serves to reduce the leakage by acting as screw pump. The fluid within the grooves is displaced towards the high pressure region as it spins with the rotor.
Previous work has shown that seals with grooves on both the surface of the rotor and the surface of the stator can sustain higher pressure differentials for a given leakage than seals with grooves on only one surface. The goal of this study is to optimize the leakage performance of a double surface helical groove seal for a given set of operating conditions. To accomplish this goal, simulations are run in ANSYS CFX. A sufficient mesh with appropriate boundary layers is determined from the mesh independence study. The turbulence model is k-ε turbulence for water at 25°C. This is the first paper to present numerical results for the performance of helical groove seals with grooves on both the rotor and the stator.
The design parameters used in the optimization are inner (rotor) groove size, inner helix angle, outer (stator) groove size, and outer helix angle. A Kennard-Stone algorithm, which optimally spaces the simulations within the design space, is used to select the designs to be simulated. A multifactor quadratic regression is derived. Backward regression is used to reduce the performance function to only statistically significant terms. Finally, the optimal seal design is derived from the performance function and is simulated to demonstrate the predictive power of the performance function. Interaction terms for the rotor and stator design parameters will be used to explore the mechanism whereby helical groove seals with grooves on both the rotor and the stator surfaces are able to have lower leakage than helical groove seals with grooves on just one surface. The end result of this study is a seal design which minimizes leakage and therefore improve machine efficiency.
Inverted Brayton Cycle With Exhaust Gas Condensation
Authors: Kennedy Ian , Chen Zhihang , Ceen Bob , Jones Simon , Copeland Colin D. 
Published: August 2017

Approximately 30% of the energy from an internal combustion engine is rejected as heat in the exhaust gases. An inverted Brayton cycle (IBC) is one potential means of recovering some of this energy, in order to improve the overall system efficiency. When a fuel is burnt, water and CO2 are produced and expelled as part of the exhaust gases. In an IBC, in order to reduce compression work, the exhaust gases are cooled before compression up to ambient pressure. If coolant with a low enough temperature is available, it is possible to condense some of the water out of the exhaust gases, further reducing compressor work.
In this study the condensation of exhaust gas water is studied. The results show that the IBC can produce an improvement of approximately 5% in BSFC at the baseline conditions chosen and for a compressor inlet temperature of 310 K. The main factors that influence the power output are heat exchanger pressure drop, turbine expansion ratio, coolant temperature and turbine inlet temperature. A lower coolant temperature significantly increases power output, particularly when condensation occurs. Larger turbine expansion ratios produce more power and slightly lower the temperature at which condensation onset occurs. The system is very sensitive to heat exchanger pressure drop, as larger pressure drops increase the compressor pressure ratio whilst leaving the turbine expansion ratio unchanged. Higher turbine inlet pressures can also increase net power, but the higher exhaust backpressures may increase engine pumping losses.
Finally, for conditions when condensation is possible, the water content of the exhaust gas has a significant influence on power output. The hydrogen to carbon ratio of the fuel has the most potential to vary the water content and hence the power generated by the system. If there is no condensation, water content has a small impact on performance. The effect on power in the condensing region is predominantly due to reduced mass flow in the compressor.
Effect of RANS Method on Stall Inception Eigenvalue Approach
Authors: Xie Zhe , Liu Yangwei , Liu Xiaohua , Lu Lipeng , Sun Xiaofeng 
Published: August 2017

The effect of mesh, turbulence model and discretization scheme in Reynolds-averaged Navier-Stokes method (RANS) on a stall inception eigenvalue approach is investigated in a transonic compressor rotor. The most influencing flow structures on the result of eigenvalue approach are also identified. The compressor stall point is calculated by a recently developed eigenvalue model. Based on the 3D Navier-Stokes equations, the body-force term and small disturbance were used to transform the original equations into the eigenvalue approach. Because the eigenvalue mainly relies on the results from RANS, the sensitivity of the eigenvalue to the mesh density, turbulence model, and numerical scheme needs to be clearly identified before it is applied to engineering.
The effect of mesh density is firstly specified. Several grids with different densities and distributions are employed in RANS. The eigenvalue results indicate that the solution converges at the same grid density as RANS does. Besides, the eigenvalue approach has the ability to predict a more accurate stall point compare to RANS with a coarse computational grid. The investigation of the detailed flow field indicates that the flow structures in the vicinity of blade tip region change significantly with three different grid densities, the eigenvalue is also influenced. Two important flow mechanisms are found to be the decisive factors for the eigenvalue, namely the blockage generated by the shock-vortex interaction, the separated flow and the wake near the trailing edge. These flow patterns are consistent with the flow mechanisms of the compressor stall inception. Further investigations are conducted with four different turbulence models combined with three different spatial discretization schemes. Calculated eigenvalue proves that the turbulence model changes the eigenvalue with an over-prediction of stall point at about 1%. The spatial discretization scheme has small effect on stall point prediction using k-ε and SA models, whereas it has large effect when using SST model. The scheme shows great influence in the simulations with specific turbulence model by changing the predicted stall point at least 1.7%. The existence of blockage, the separation and the wake flow are identified as the major and secondary factor which contributes to an unstable prediction of eigenvalue approach, respectively.
Comparative Analysis of Different Numerical Techniques to Analyze the Wake of a Wind Turbine
Authors: Bianchini Alessandro , Balduzzi Francesco , Gentiluomo Domenico , Ferrara Giovanni , Ferrari Lorenzo 
Published: August 2017

The analysis of wind turbine wakes’ structure and interaction with other machines installed in the same array or park has become a key element in the current wind energy research due to the notable impact that wakes can have on the actual energy production of the turbines themselves.
The present frontier of the research in this field is leading to the massive use of large-eddy simulations to completely solve the flow field surrounding the rotors. By doing so, however, enormous calculation resources are needed, which are often not available in an industrial context and also generally not compatible with extended optimization analyses (e.g. for a park layout definition). In this latter case, several cases need to be solved in a reasonable amount of time and therefore more computationally efficient methods are still needed.
Within this context, the present study reports a comparative analysis between three different techniques to analyse the wind turbine wakes’ structure and their mutual interaction. In particular, a state-of-the-art 3D RANS calculation of the famous NREL Phase VI rotor was used as a benchmark for comparison with two other methods. The first one is based on the Virtual Blade Model (VBM) of the commercial solver ANSYS® FLUENT®, in which a 3D RANS calculation of the flow field is carried out for the outer domain, while the effect of the rotating blades on the fluid is simulated through a body force, which acts inside a disk of fluid with an area equal to the swept area of the turbine. The value of the body force is time-averaged over a cycle from the forces calculated by a simplified Blade Element Method. In the present study, a stall delay model was also implemented within the VBM module. The second one is instead based on the even more simple approach with an Actuator Disk Model (ADM), in which the turbine presence is actually modelled as a sink of momentum in the main flow. Cross-comparisons between the techniques are shown, both in terms of single wake description and of wake-turbine interaction, leading to the conclusion that the VBM model may represent a valuable and computationally affordable tool in many wind energy applications.