1,720,970 research outputs found
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
On the Effect of an Aggressive Inlet Swirl Profile on the Aero-thermal Performance of a Cooled Vane
Full text linkAbstractA high-pressure vane equipped with a realistic film-cooling configuration has been studied. The vane is characterized by the presence of multiple rows of fan-shaped holes along pressure and suction side while the leading edge is protected by a showerhead system. Steady three-dimensional Reynolds-Averaged Navier-Stokes (RANS) simulations have been performed. A preliminary grid sensitivity analysis has been performed (with uniform inlet flow) to quantify the effect of the spatial resolution. Turbulence model has been assessed in comparison with available experiment data. The effects of a realistic inlet swirl on the aero-thermal performance of the cooling system are then investigated by means of comparison between two different kinds of simulations. The first one using a uniform inlet flow while the second one with aggressive swirl derived from the EU-funded project TATEF2. Clocking effects are also accounted for. The effect of the swirling flow in determining the coolant transport are investigated, evidencing the key role that these phenomena have in determining the effectiveness of the cooling
Uncertainty Quantification in Hydrodynamic Bearings
Full text linkAlthough it is possible to imagine a strong sensitivity of hydrodynamic bearings performance to geometrical and fluid dynamic uncertainties, a small amount of scientific contributions have been found in literature about the use of uncertainty quantification techniques for the numerical modeling of bearings. In the present paper we aim at quantifying the effects of the aleatory uncertainty of some relevant input values on key parameters related to rotordynamic effects in turbomachinery, and in particular on the rotor thermal instability problem (e.g. the equilibrium position and the dynamic coefficients). A methodology is initially developed in order to study the propagation of the uncertainties in the numerical analysis of Tilting Pad Journal Bearings (TPJB). Due to the characteristics of the in-house finite element code TILTPAD considered for the UQ analysis, the Monte Carlo method has been selected among the possible approaches. The analysis here presented considers the effects of both manufacturing tolerances on the assembled bearing clearance and of the tolerances adopted for the characterization of the viscosity grade of the oil. The test case adopted for the analysis is the Kingsbury D-140 TPJB. Considering the individual variation of the selected parameters, it is possible to observe that the standard deviation (STD) of the the non-dimensional dynamic coefficients is up to 2.1% in case of viscosity variation and up to 9.1% in case of clearance variation. The STD of the frictional power losses is about 2.2% and 1.4% respectively. Considering the simultaneous variation of the selected parameters, it is possible to observe a STD of the non-dimensional dynamic coefficients comprised between 6.4% and 9.4%, while the STD of the frictional power losses is about 2.7%
Effects of realistic inflow conditions on the aero-thermal performance of a film-cooled vane
Full text linkA high-pressure vane equipped with a realistic film-cooling configuration has been studied. The vane is characterized by the presence of multiple rows of fan-shaped holes along pressure and suction side while the leading edge is protected by a showerhead system of cylindrical holes. Steady three-dimensional Reynolds-Averaged Navier-Stokes simulations have been performed. A preliminary grid sensitivity analysis has been performed with uniform inlet flow to quantify the effect of the spatial resolution. Turbulence model has been assessed in comparison with available experimental data. The effects of a realistic inflow condition on the thermal behaviour of the cooled vane are then investigated by means of comparison between two conjugate heat transfer simulations. The first one is characterized by a uniform inlet flow while the second one presents a temperature distortion and a superimposed aggressive swirl derived from the EU- funded TATEF2 project. The effect of the swirling flow in determining the metal temperature distribution is investigated with particular attention to the consequences on the operation of the film cooling system
On the Effect of Inlet Swirl and Temperature Profiles on the Aero-Thermal Performance of a Heavily Film-Cooled Vane
No full textA linear cascade of high-pressure vanes equipped with a realistic film-cooling configuration has been studied. The aim is to provide an accurate analysis of a heavily cooled high-pressure vane subjected to aggressive inlet swirl. The analyzed vane is characterized by the presence of multiple rows of fan-shaped holes along pressure and suction side while the leading edge is protected by a showerhead system. Numerical simulations have been performed on hybrid unstructured grids using a steady approach with the commercial code ANSYS Fluent®. The transitional kT-kL-ω model by Walters and Cokljat has been selected as turbulence closure. A realistic computational domain that mimic a combustor/vane count of 1:2 has been used. The classical analysis approach with uniform inlet flow has been compared with an approach that takes into account inlet swirl motion considering two clocking positions of such velocity distortion. This latter have been obtained through a non-reacting swirl generator experimented during the EU-funded TATEF2 Project and representative of modern aeroengines. Results highlight the importance of considering realistic boundary conditions for cooling system analysis and quantify the effects of swirl in affecting external heat transfer
Clocking effects of inlet nonuniformities in a fully cooled high-pressure vane: A conjugate heat transfer analysis
No full textA high-pressure vane (HPV) equipped with a realistic film-cooling configuration has been studied. The vane is characterized by the presence of multiple rows of fan-shaped holes along pressure and suction side, while the leading edge (LE) is protected by a showerhead system of cylindrical holes. Steady three-dimensional Reynolds-averaged Navier–Stokes simulations have been performed. A preliminary grid sensitivity analysis with uniform inlet flow has been used to quantify the effect of spatial discretization. Turbulence model has been assessed in comparison with available experimental data. The effects of the relative alignment between combustion chamber and HPVs are then investigated, considering realistic inflow conditions in terms of hot spot and swirl. The inlet profiles used are derived from the EU-funded project TATEF2. Two different clocking positions are considered: the first in which hot spot and swirl core are aligned with passage; and the second in which they are aligned with the LE. Comparisons between metal temperature distributions obtained from conjugate heat transfer (CHT) simulations are performed, evidencing the role of swirl in determining both the hot streak trajectory within the passage and the coolant redistribution. The LE aligned configuration is determined to be the most problematic in terms of thermal load, leading to increased average and local vane temperature peaks on both suction side and pressure side with respect to the passage-aligned case. A strong sensitivity to both injected coolant mass flow and heat removed by heat sink effect has also been highlighted for the showerhead cooling system
An Efficient Iterative Coupled Model for the Study of the Insurgence of the Morton Effect in Tilting Pad Journal Bearings
No full textThe introduction of the tilting pad journal bearing (TPJB) technology has allowed the achievement of important goals regarding turbomachinery efficiency in terms of high peripheral speed, enhanced power density, higher efficiency, and tolerated loads. That kind of technology overcomes the typical dynamic instability problem that affects fixed geometry bearings but, under certain working conditions, can be subjected to thermal instability phenomena, which are particularly significant at high peripheral speeds. In this work, the authors propose an innovative iterative procedure to forecast the thermal instability onset by using two coupled models, a thermo-structural one and a fluid dynamic one. The first one calculates the vibrations and the deformations due both to the external forces and to the temperature distribution applied on the rotor. The fluid dynamic model calculates the temperature profile by using as inputs the characteristics of the rotor, of the bearing and of the orbits, obtained by the thermos-structural code. After a general description of the iterative procedure is given, details of each tool are provided. Code validation is presented by means of comparison with available experimental and numerical data. Finally, the results of the iterative procedure are shown to prove its potential in forecasting instability thresholds. The model has shown a good trade-off between accuracy and efficiency, which is very critical when dealing with the extended time windows characterizing thermal instabilities. This research activity is in cooperation with the industrial partner Baker Hughes, a GE company, which provided the experimental data obtained thorough a dedicated experimental campaign
Development of a One-Dimensional Model for the Prediction of Leakage Flows in Rotating Cavities Under Non-Uniform Tangential Pressure Distribution
Full text linkRegenerative pumps are characterized by a low specific speed that place them between rotary positive displacement pumps and purely radial centrifugal pumps. They are interesting for many industrial applications since, for a given flow rate and a specified head, they allow for a reduced size and can operate at a lower rotational speed with respect to purely radial pumps. The complexity of the flow within regenerative machines makes the theoretical performance estimation a challenging task. The prediction of the leakage flow rate between the rotating and the static disks has the greatest impact on the prediction of global performance. All the classical approaches to the disk clearance problem assume that there is no relevant circumferential pressure gradient. In the present case, the flow develops along the tangential direction and the pressure gradient is intrinsically non-zero. The aim of the present work is to develop a reliable approach for the prediction of leakage flows in regenerative pumps. A preliminary numerical simulation on a virtual model of a regenerative pump where the disk clearance is part of the control volume has been performed for three different clearance aspect ratios. The outcome of that campaign allowed the authors to determine the behavior of the flow in the cavity and choose correctly the baseline hypotheses for a mathematical-physical method for the prediction of leakage flows. The method assumes that the flow inside of the disk clearance is two-dimensional and can be decomposed into several stream-tubes. Energy balance is performed for each tube, thus generating a system that can be solved numerically. The new methodology was tuned using data obtained from the numerical simulation. After that, the methodology was integrated into an existing one-dimensional code called DART (developed at the University of Florence in cooperation with Pierburg Pump Technology Italy S.p.A.) and the new algorithm was verified using available numerical and experimental data. It is here demonstrated that an appropriate calibration of the leakage flow model allows for an improved reliability of the one-dimensional code
On the CFD Analysis of a Stratified Taylor-Couette System Dedicated to the Fabrication of Nanosensors
Full text linkSince the pioneering work of Taylor, the analysis of flow regimes of incompressible, viscous fluids contained in circular Couette systems with independently rotating cylinders have charmed many researchers. The characteristics of such kind of flows have been considered for some industrial applications. Recently, Taylor-Couette flows found an innovative application in the production of optical fiber nanotips, to be used in molecular biology and medical diagnostic fields. Starting from the activity of Barucci et al., the present work concerns the numerical analysis of a Taylor-Couette system composed by two coaxial counter-rotating cylinders with low aspect ratio and radius ratio, filled with three stratified fluids. An accurate analysis of the flow regimes is performed, considering both the variation of inner and outer rotational speed and the reduction of fiber radius due to etching process. The large variety of individuated flow configurations provides useful information about the possible use of the Taylor-Couette system in a wide range of engineering applications. For the present case, the final objective is to provide accurate information to manufacturers of fiber nanotips about the expected flow regimes, thus helping them in the setup of the control process that will be used to generate high-quality products
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