162,272 research outputs found
Flow and heat transfer mechanism for gas turbine internal cooling: DNS & LES study
No full textGas turbine are used for aircraft propulsion and in land-based power generation or industrial application. One of the key to increase turbine efficiency (or to reduce fuel consumption) is to increase turbine inlet temperature (TIT). It is crucial to design an efficient cooling system for turbine blade, to avoid penalties in the overall turbine efficiency. There is still an unsatisfactory comprehension of the mechanisms of turbulence and heat transfer in those configurations, considering also that, most of the times, they are designed using simple empirical correlations. The objective of this thesis is to analyse flow configurations representing the most complex parts of a turbine cooling system, using high-fidelity approaches (i.e. DNS and LES). These approaches are unique tools for a clear understanding of the physics behind the cooling process, but are unlikely to be appealing in the following years for industries, since they require large computational power and a lot of time to be performed (usually months). Increasing computing power does not necessarily make DNS or LES attractive, since some other issues may arise (e.g. the high storage required for the simulated data). Starting from the above considerations, URANS models, tailored to account for rotation and non linear turbulent flow features, were developed and validated in this research, to overcome all the issues related to DNS and LES
Computational fluid dynamics simulations of cavitating flows and numerical modelling of cavitation damage
No full textIn this thesis, Computational Fluid Dynamics (CFD) simulations are used to numerically model cavitation on different geometries: a NACA-66 (mod) hydrofoil profile, a horizontal axis tidal turbine (HATT) and an impinging nozzle test case. The Singhal (FCM) and the Schnerr and Sauer (SSCM) cavitation models have been selected from the literature, they are both based on the non-compressible bubble dynamics equation of Rayleigh-Plesset. Turbulence assessments involve the use of the Transitional Shear Stress Transport (TSST), the Scale Adaptive Simulation (SAS) and the Shear Stress Transport (SST) k-omega turbulence models to achieve the Reynolds Average Navier Stokes (RANS) equations closure. Based on the theory of Plesset, Chapmann and Lush a new cavitation damage index, capable to map the damage of solid boundaries due to vapour collapse has been proposed. Initially, in Chapter 1 a review of the major works contributing to the state of the art of cavitation and cavitation damage modelling is presented. Chapter 2 describes the physics of cavitation and cavitation induced damage, treating in depth phenomena ranging from bubble dynamics to water jet formations due to single bubble implosions. In Chapter 3, fundamental laws of turbulent and cavitating flows have been presented. The theoretical treatment involves derivation of the Navier-Stokes equations, presentation of the RANS approach used to model the cavitating flows and description of the turbulence models selected to close the RANS system. Then, the main equations ruling the mass transfer mechanisms are mentioned for both the FCM and SSCM, detailing peculiarities of each cavitation model. At the end of the Chapter, the formulation of a numerical framework aimed at defining a new cavitation damage index is explained. Its theoretical assumptions and the implementation are reported as well. In Chapter 4, the standard TSST and SAS turbulence models are used in conjunction with the FCM to detect pressure coefficients on the NACA-66 (mod) profile. Predicted results are compared to the experimental findings of Shen and Dimotakis. In Chapter 5 the Reboud density function is implemented to adjust the eddy viscosity computation with respect to the standard turbulence modelling. New results obtained on the same hydrofoil profile used in Chapter 4 are substantially improved in the cavitation region when the FCM is selected in combination with the SST k-omega turbulence model. The combination of the Reboud corrected turbulence model and the FCM is then exported to simulate cavitation occurrence for four different rotating conditions on a selected HATT. Cavitating simulations are performed at the end of a Mesh Sensitivity Analysis (MSA) aimed at identifying the optimal computational grid in terms of both turbine power and thrust. Final cavitation simulations reveal that the severest risk of cavitation inception occurs outside of the machine operating regime. In the end (Chapter 6), the work focuses on a campaign of simulations dedicated to validate the proposed cavitation damage model on the nozzle test case of Franc. The optimal computational grids are selected performing a MSA in terms of both static pressure and velocity magnitude. Results have been obtained by testing the corrected SST k-omega turbulence model and the cavitation damage model in conjunction with the FCM and the SSCM, with the latter returning the best performance in terms of damage peak location when the finest time step condition is implemented. Conclusions and future developments, exposed in Chapter 7, complete the thesis argumentation reporting the main outcomes of the work and its possible industrial applications
Direct numerical simulation of heat transfer of round subsonic impinging jets at high Reynolds number
No full textImpinging jets provide an effective cooling method for various applications such as the cooling of aircraft turbine blades. The latest generation of high performance computers allows us to investigate those at practically relevant Reynolds numbers Re by means of direct numerical simulations. In order to analyse the heat transfer of a confined round impinging jet, two direct numerical simulations are performed at Re=3300 and Re=8000 using a grid of 512 x 512 x 512 respectively 1024 x 1024 x 1024 points. Each configuration is fully turbulent. The first one features two annular regions with local maxima of heat transfer at the impinging plate. These effects are related to high wall-normal turbulent heat fluxes caused by vortical structures of the turbulent flow field. The second simultion is ongoing. Its results will also be presented on the conference
Interaction between the shear layer, shock-wave and vortex ring in a starting free jet injecting into a plenum
No full textWhile continuous free jets have been investigated and optimised during the last 60 years, the impulsively started jet is still relatively unexplored. We focus here upon the very first stage of a compressible free round jet, when the flow is only few diameters long and the vortex ring generated by the sudden expansion interacts with the shock-waves and the shear layer. Direct numerical simulations with more than 2*10^9 grid points are carried out, discretising the compressible Navier-Stokes equations to compute both the fluid flow and the noise radiated by the interaction of the shear layer, the shock-waves and the vortex ring in a compressible free round jet. As a result of the mentioned interaction, a sound level of 111[dB] at a distance of 100 diameters from the jet has been computed. An interaction between the shear layer, the shock-waves and the vortex ring has been investigated using numerical methods in an impulsively started supersonic free round jet and noise levels of order of the loudest acoustic phenomenon in the continuous jet have been identified and quantified
Vorgehenskonzept zur kriteriengestützten Selektion geeigneter Partner zum Benchmarking in der Beschaffungslogistik
No full textAm Beispiel der Beschaffungslogistik wird ein Vorgehenskonzept zur kriteriengestützten Selektion geeigneter Benchmarking-Partner entwickelt. Neben einer systematischen Vorgehensweise zur Partnerauswahl werden Kriterien und geeignete Bewertungsmethoden identifiziert, anhand derer potentielle Partner im Rahmen des Auswahlprozesses beschrieben und hinsichtlich ihrer Eignung als Benchmarking- Partner objektiv bewertet werden können. Um die Bewertung möglichst transparent zu gestalten und die Anwendung der Entscheidungsverfahren zu unterstützen, wird ein Software-Prototyp entwickelt, der die Datenerfassung, -aufbereitung und -bewertung in den einzelnen Vorgehensschritten unterstützt. (J. Sesterhenn
Effect of wall curvature of an asymmetric subsonic impinging jet: DNS study
No full textWe report on Direct Numerical Simulation (DNS) of a subsonic confined jet impinging on a curved surface. The configuration resembles a turbine leading edge cooling system. The bulk Reynolds number (based on bulk velocity Ub and jet diameter D) is 3300. The impinging wall was kept at a constant temperature higher than the jet bulk temperature. Comparisons with the jet impinging on a flat plate are carried out.
Our major findings are that the turbulent flow field is affected by the shape of the impinging wall, leading to an asymmetric development of turbulent structures. The frequencies which dominates the excess heat transfer mechanism change with respect to the reference case, changing the average heat transfer into the hot wall
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
DNS study of fusion reactor dust particle mobilization induced by a transonic jet incoming in a vacuum container
No full textThe flow induced motion of wall deposited particles
is highly linked with the instantaneous fluid structures.
Here we perform a two-phase flow DNS to analyze
the resuspension of solid particles from a surface
hit by a transonic jet into a low pressure container
in conditions similar to those which occur in a fusion
reactor vacuum vessel during a Loss of Vacuum Accident
(LOVA). The initial condition of pressure and
temperature were set to 49.5 mbar and 373 K, with
a Reynolds number of 3300 on a 512 512 512
grid properly refined in regions where high gradients
are present. The Thornton and Ning impact/adhesion
model is adopted, whereas an advanced resuspension
model, which also takes into account the dynamics
(rolling and sliding) of particles at the wall, is here
implemented. The initial deposited particles follow a
log-normal distribution with a count median diameter
of 2.21 μm, geometric standard deviation of 2.93 and
constant density of 8527 kg/m3. It has been found that
the resuspension phenomenon mostly affect particles
of the biggest diameters. Moreover, the jet-deposit interaction
is for the most part confined within a circumference
around the jet of radius equal to the jet diamete
- …
