1,721,100 research outputs found
Uncertainty Quantification in CFD: The Matrix of Knowledge
No full textThe main difference between an experimental study and the corresponding numerical simulation is that the latter is usually considered a deterministic exercise, while the experiments are inherently affected by uncertainty. Despite this, the usage of numerical simulations is gaining more and more importance in aero-engine research thanks to their growing accuracy and accessibility. It must be underlined that even the most sophisticated numerical simulation cannot consider by default the impact of the uncertainties. Therefore, uncertainty quantification (UQ) techniques are increasingly coupled with deterministic calculations to include the most relevant variabilities. The overall goal of UQ is to investigate the impact of aleatory and epistemic uncertainties on a system response quantity of interest. The lesson learnt after applying UQ techniques to the numerical study of several aero-engine components is that to fully understand simulation results, it is imperative to incorporate uncertainty from the very beginning of the numerical procedure. To demonstrate that outcome, this chapter presents a discussion about the concepts of code verification and calculation validation, with a special interest in the analysis of the observed order of accuracy. A discussion about the definitions of aleatory and epistemic uncertainty follows, aiming at defining a common ground to start with the definition of what is called “uncertainty quantification” in engineering problems. A detailed list of limitations in deterministic computational fluid dynamics is also included in the chapter
Thermo-Hydrodynamic Analysis of Plain and Tilting Pad Bearings
Full text linkThe demand for higher efficiency and increased equipment compactness is pushing industrial compressors’ designers towards the choice of higher rotor peripheral speed. As a consequence, modern bearing-rotor systems are subject to complex thermal phenomena inducing a renewed interest on their real working conditions. This work is about the validation of the in-house numerical code TILTPAD developed at the Department of Industrial Engineering of the University of Florence for the thermo-hydrodynamic analysis of both plain and tilting pad journal bearings performance. TILTPAD is a steady-state code based on a 2D thin-film approach able to find either the resulting hydrodynamic load using the shaft equilibrium position and the rotational speed (i.e., direct problem) or the shaft equilibrium position once the load and the rotational speed are prescribed (i.e., inverse problem). In order to calculate pads’ pressure distribution a finite element approach is used to solve the Reynolds equation together with a mixed procedure to evaluate pads equilibrium positions. Two steady-state energy equations based on a Petroff-type simplification are implemented in the code. The first one is proposed in the work of Balbahadur and Kirk [1] while the second one is based on an improved mixing model and a temperature dependent viscosity. An iterative procedure is used between Reynolds and energy equations to account for the dependence of the dynamic viscosity on the temperature field. Super-laminar flow regimes are also modeled in the code by means of a simplified approach able to represents, with reasonable accuracy, the effects of Taylor-Couette vortex flows and of the transitional regimes up to the onset of a fully turbulent state. Under these hypotheses, the pressure field is slightly affected by the viscosity variation while dissipative effects are enhanced. The code has been validated by means of comparison with available experimental data. Particular attention is devoted to static working parameters (i.e., equilibrium position and frictional power loss), reproducing the global behavior of the bearing, although some local characteristic is also considered
Numerical Analysis of a Flow Control System for High-Pressure Turbine Vanes Subject to Highly Oscillating Inflow Conditions
Full text linkUnder the prism of introducing pioneering technologies in the propulsive field, the Rotating Detonation Engine (RDE) continuously attracts the Gas Turbine (GT) research community. However, how to effectively couple an RDE with High Pressure Turbine (HPT) stages is still debated. In fact, time dependent flow conditions from the RDE greatly affect turbine performance, thus reducing the positive impact of Pressure Gain Combustion (PGC) on the overall cycle efficiency. The present numerical work aims at analysing both the impact of a pulsating inflow on the performance of a newly designed high pressure turbine vane and the effectiveness of a flow control system in governing the oscillations within the vane passage. First, a baseline vane capable to ingest high enthalpy flow at an inlet Mach number of 0.6 is introduced. A total number of 297 samples are generated by varying the 18 geometrical parameters that characterize the vane endwalls and airfoil profile with the help of Latin Hypercube sampling method. Then, an optimization strategy is performed using steady inflow conditions allows for minimizing vane loss coefficient, thus providing the final geometry of the new vane. In the second part of the work, a flow control system is proposed by placing a series of holes in the endwalls of the vane. Air at constant stagnation conditions is injected upstream of vane leading edge. Unsteady calculations with and without flow control, including similar pulsating conditions from the RDE provide an insight to the generation and evolution of the secondary flow structures inside of the passage. The main outcome of this analysis is that the flow control system intensifies the passage vortices providing less oscillating flow at the vane exit section, which is beneficial for the aerodynamic performance of a subsequent blade row
Implementation of a high-order spatial discretization into a finite volume solver: Applications to turbomachinery test cases using an eddy-viscosity turbulence closure
Full text linkIn this study, the implementation of a high-order spatial discretization method into a Finite Volume solver is presented. Specific emphasis is put on the analysis of the performance over selected turbomachinery test cases. High-order numerical discretization is achieved by adopting the cell-centered Least-Square reconstruction, which is implemented in the in-house solver HybFlow. The validation of the adopted methodology is performed by assessing the solution of a turbulent flat plate with zero pressure gradient, using a eddy-viscosity transitional model. The test case also evidences the effect of the discretization of gradient-based source terms when a high-order reconstruction methodology is used. In the second part of the paper, the solver is used for the solution of relevant two-dimensional turbomachinery test cases, assessing the impact of 2nd and 3rd order reconstruction on the prediction of the aerodynamics and the heat transfer for respectively a low-pressure blade and a high-pressure turbine vane. It is shown how a high-order reconstruction allows for obtaining a better prediction of turbomachinery aerodynamics, with lower number of elements. The benefits over heat transfer predictions in high Reynolds number conditions are instead limited to the reduction of heat transfer coefficient spikes in under-resolved regions of the blade. Eventually, the methodology is validated for a three-dimensional low-pressure turbine cascade with realistic boundary layer inflow conditions
Numerical Analysis of a Flow Control System for High-Pressure Turbine Vanes Subject to Highly Oscillating Inflow Conditions
No full textUnder the prism of introducing pioneering technologies in the propulsive field, the rotating detonation engine (RDE) continuously attracts the gas turbine (GT) research community. However, how to effectively couple an RDE with high pressure turbine (HPT) stages is still debated. In fact, time dependent flow conditions from the RDE greatly affect turbine performance, thus reducing the positive impact of pressure gain combustion (PGC) on the overall cycle efficiency. The present numerical work aims at analyzing both the impact of a pulsating inflow on the performance of a newly designed high pressure turbine vane and the effectiveness of a flow control system in governing the oscillations within the vane passage. First, a baseline vane capable of ingesting high enthalpy flow at an inlet Mach number of 0.6 is introduced. A total number of 297 samples are generated by varying the 18 geometrical parameters that characterize the vane endwalls and airfoil profile with the help of a Latin hypercube sampling method. An optimization strategy is then performed under steady inflow conditions to minimize the vane loss coefficient, thereby determining the final geometry of the new vane. In the second part of the work, a flow control system is proposed by placing a series of holes in the endwalls of the vane. Air at constant stagnation conditions is injected upstream of the vane leading edge. Unsteady calculations with and without flow control, including similar pulsating conditions from the RDE, provide an insight into the generation and evolution of the secondary flow structures inside the passage. The main outcome of this analysis is that the flow control system intensifies the passage vortices providing less oscillating flow at the vane exit section, which is beneficial for the aerodynamic performance of a subsequent blade row
Methodology for the Residual Axial Thrust Evaluation in Multistage Centrifugal Pumps
No full textOne of the most challenging aspects in horizontal pumps design is the evaluation of the residual axial thrust acting on the rotating shaft. The thrust is affected by pump characteristics and working conditions. Solving this problem is easier for a single stage pump than for multistage pumps, even in partially self-balancing opposite impeller configuration. The challenge is then to individuate a procedure that will provide the residual thrust value with a moderate computational effort, dealing with the industrial requests of accuracy and reduced time consumption. A procedure is proposed, which consists in the numerical simulation of each pump component. For each component, the obtained mass-flow/thrust correlations are coupled by using a momentum balance equation used to calculate the axial thrust as a function of the working conditions. The main topic in multistage pump modeling is the leakage flows characterization by means of accurate numerical analysis. Therefore, the cavity flows behavior is investigated and the flow structures individuated. The numerical investigation of the pump’s components provides also a thorough knowledge of fluid dynamic fields. The proposed procedure is able to predict both the direction and the variation of the thrust in a selected range of flow rates, while the value of the thrust is affected by a non-negligible error generated by “real machine” effects
Unsteady Investigation of the Aero-Thermal Flow Field in a HP Gas Turbine
No full textL'articolo tratta dell'interazione non stazionaria in stadi di alta pressione di turbina
Editorial: Pressure Gain Combustion technologies for a Greener propulsion
Full text linkThis Research Topic was formulated to address some fundamental aspects of Rotating Detonation Combustor (RDC) performance and modelling strategies. Recently, great interest arose in developing novel cycles for gas turbines aimed at increasing overall efficiency and specific power. That effort is accompanied by the possibility of using hydrogen as fuel in Pressure Gain Combustion (PGC) cycles, thus decreasing the direct emission of CO2 (although NOx emissions are still an open topic) and exploiting its fast combustion properties. The actual performance of PGC-equipped gas turbines is highly debated, with the pressure rise in the combustor and the coupling with a turbine still under investigation by several research groups, including the ones to which the Editors of the Research Topic belong. The RDC exhibits complex aero-thermo-mechanical challenges that are difficult to evaluate experimentally due to the extremely high exhaust temperature, high Mach numbers and the high characteristic frequencies, preventing standard experimental approaches. A complete study of RDC performance is fundamental to appropriately design the turbine module, either subsonic or supersonic, aimed at fostering the manufacturing of enabling components for power generation or propulsion.
The present Research Topic addresses the impact of the injectors’ geometry on mixing efficiency and the definition of strategies for their simulation. Le Naour et al. provide insights into some fundamental numerical and experimental aspects of RDC, with particular interest in the injection configurations. Sato et al. studied the impact of two different boundary conditions on the flow field development inside the RDC chamber using high-fidelity Computational Fluid Dynamics (CFD) compared to experimental findings. Finally, Hellard et al. apply a modeling strategy for the simulation of transitory injection in RDCs, thus providing information about the possibility of studying reactants mixing at low computational cost
Numerical Study of Unsteady Rotating Structures in a Turbine Disk Cavity
No full textThe improvement of modern cooling systems in aero-engines has led to an increase in the turbine inlet temperature to improve cycle efficiency. In particular, the use of purge flow within the cavity between the stator rim and rotor platform has considerably reduced hot gas ingestion from the main flow and has ensured the maximum metal components operating life. The mechanisms governing axial turbine rim seal flow are complex and influenced by various factors, firstly by the geometry of the cavity. The resulting flow has a three-dimensional structure and a pattern that varies over time. For a wide axial cavity, hot gas ingestion zones can be mainly attributed to three phenomena. The first one is the disc pumping effect generated by the shaft rotation, which induces a radial outflow in the rotor disc boundary layer, creating a recirculation within the cavity that leads to ingestion in the stator disc area. The second one is the vane/blade relative position, which determines a circumferential pressure profile where the higher and lower pressure zones define the flow ejection or ingestion. The third phenomenon is the purge/mainstream flow interaction and its inherent instability that leads to large-scale unsteady flow features developing within the cavity. The formation of such rotating structures and their impact on the ingestion zones is a key research topic in this area. This study presents the results of unsteady numerical simulations (URANS) of an axial turbine's first-stage cavity with a focus on the analysis of 3D-unsteady flow structures. The simulations have been performed by imposing a circumferential pressure periodicity at the outlet, extracted from previous simulations of the stage. The static pressure values within the cavity have been validated by comparison with experimental data. Three purge flow rates have been tested, namely high purge, low purge, and ingestion flow. The paper provides a new perspective on the topic by performing a SPOD analysis on a characteristic plane to identify the main energy modes that dominate the flow and their influence on the ingestion phenomenon
Numerical Characterization of the Performance Curve of a Regenerative Pump-As-Turbine
Full text linkEnergy companies in the power generation field are continuously searching for green technologies to reduce pollutant emissions. In that context, small hydropower plants represent an attractive solution for distributed electricity generation. Reverse-running centrifugal pumps (also known as “pump-as-turbines”, PaT) are increasingly selected in that field. Amongst the existing type of pumps, drag-type regenerative pumps (RP) can perform similarly to radial centrifugal pumps in terms of head and efficiency for low specific speed values. For a fixed rotational speed, RPs with linear blades work as pump or turbine only depending on the flow rate. Such peculiarity makes it particularly intriguing to evaluate RPs working characteristic in the turbine operating mode. In the present paper, the performance of three Regenerative Pump-as-Turbine (RPaT) are analyzed using Computational Fluid Dynamics (CFD). The analysis is supported by an already validated in-house 1D code developed in cooperation with Pierburg Pump Technology Italy SPA. The obtained results are also discussed considering the theoretical behavior of the circulatory velocity in a regenerative machine as described by a widely used 1D model, which is extended in the present paper to the turbine working region. The numerical approach is validated using experimental data for both an RP (in the pump working region) and a regenerative turbine (RT) (in the turbine working region). Finally, the numerical simulation of a small-scale RP allows for the detailed characterization of both the pump and the turbine regions. The numerical analysis shows that for a RPaT it is possible to find a “switch region” where the machine turns from behaving as a pump to behaving as a turbine, the losses not being overcome by the turbine power output. The analysis of the RPaT also shows the inversion of the flow pattern and the positioning of the pivot around which the flow creates the typical helical structure that characterizes RPs
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