1,721,172 research outputs found
Technical Considerations on Cogeneration Systems with Gas Turbines
No full textAn analysis about the combined production of electrical and thermal energy with gas turbines is carried out to evaluate their consumption of primary energy and to determine their energy saving in comparison with the separate production of these two energy forms. Based upon these results a method is suggested to plan these congeneration systems
L'influenza delle perdite meccaniche sulle leggi di affinità nelle turbopompe.Effects of mechanical losses on the affinity characteristics of turbopumps.
No full textExperiments on three different designs of pumps, one fitted with a hydraulic thrust control bearing, employed various speeds and geometric similarity extending also to the wall surface roughness and the play between the rotor and the stator. The experiments show that the efficiency values can differ in homologous operating conditions, without any clearly defined law of affinity between the power and the shaft. Applying the similarity law to the overall power, disregarding the mechanical losses, however, permits accurate forecasting of the operating parameters thoughout. A recalculation formula considering separately the hydraulic and mechanical efficiencies, with correction base on a statistical analysis, is probably a more realistic solution to the problem
Scorrimento della Corrente Relativa nelle Giranti Operatrici
No full textSi analizza l’effetto della deviazione della corrente fluida in una girante centrifuga, dotata di pale a spirale logaritmica, e si calcola il valore adimensionale dell’altezza teorica Psit eseguendo operazioni sulla media ponderate dei valori della velocità, lungo i bordi di uscita e di ingresso della girante, ottenuti, al variare della portata e del numero delle pale, dalle diverse indagini numeriche su superfici intercalari. Si confrontano poi questi risultati con quelli che si ottengono dal calcolo del Psit con le formule teoriche monodimensionali, proposte da vari autori. Si considera infine l’effetto dell’inversione della corrente fluida all’ingresso della girante, per portate diverse da quelle nominali
Optimum Incidence Angle in Centrifugal Pumps and Radial Inflow Turbines
No full textA theoretical method is presented for establishing the optimum incidence angle in outward- and inward-flow runners. Relative through-flow and eddy vortex are considered separately in order to examine their influence on the vane inlet conditions. The action of the counter-rotating vortex and of the blade camber is analysed and an algorithm is given for the evaluation of the optimum incidence angle. The validity of the procedure is finally verified by comparing the theoretical results with experimental observations and with numerical computations
Analysis of Unsteady Impeller Diffuser Interaction in a Centrifugal Pump
No full textThe paper reports on acoustic and aerodynamic experiments conducted on a seven–blade centrifugal pump as part of an ongoing investigation into the flow field instabilities and the noise generated by these instabilities in centrifugal turbomachinery.
The test model is a low-pressure stage of a two-stage pump-tur¬bine used as a pump. It was fitted with two diffusers with twenty-two adjustable guide-diffuser vanes and a return channel with eleven continuous vanes. The diffuser was designed and built to permit pressure measurement both in the adjustable guide-diffuser vanes and in the continuous vanes surfaces.
In order to establish a link between the flow field structure at part load and acoustic radiation, mean flow velocity was meas¬ured in the vaneless part of the diffuser and piezoelectric trans-ducers, placed flush with the wall, were used to measure the pressure level inside the blade channels
Numerical investigation on transient flow of a high head pump-turbine in pump mode during rapid closure of wicket gates
No full textTransient flow in pump turbines appears because the hydropower storage plant needs to experience variable operation modes to make a balance between the electricity production and consumption, thus causing high-pressure fluctuations and shortening life expectancy. In this paper, three-dimensional numerical simulation, based on Detached Eddy Simulation (DES) turbulent model, was carried out to investigate the flow characteristics in a high head pump turbine in pump mode during rapid closure of wicket gates. The dynamic mesh technique was applied to simulate the rotation of the guide vane. In this work, the influence of water compressibility on pressure fluctuations was considered. The transient flow characteristics during load reduction were investigated by time-frequency analysis methods of the numerical data
Analysis of Unsteady Flow in a Vaned Diffuser Radial Flow Pump
No full textTo understand the phenomena that occur at off-design conditions, it is necessary to take into account unsteady ef¬fects that are mainly the consequences of rotor-stator inter¬actions. These interactions can now be analysed with the help of computer codes, but the numerical procedures still require experimental data for their validation.
A numerical analysis of the flow in a low-pressure stage of a two-stage pump with a seven-blade runner, is pre¬sented with experimental valida¬tion at various opera-tion conditions.
In order to focus upon the role of the stator-rotor inter-action on the unsteady effects and the recirculation on¬set, the single impeller was first analysed. Experimental and numerical analyses was carried out on the pump equipped with a van¬eless diffuser designed as the original multistage configuration of the vaned dif¬fuser passages. Successively, the annular cascade of stationary vanes was inserted and the influence of the stator/rotor interactions on the onset of unsteady reverse flow zones was analysed.
The velocity components were measured in the vane-less part of the diffuser, and pressure measurements were performed on diffuser vanes surfaces. Furthermore, a piezo¬electric transducer placed flush with the wall was used to measure the mean pres¬sure level inside the blade channels in order to find a link between the flow field at part load and acoustic radiation.
The experimental data were then compared with re-sults that were obtained with help of CFX-TASC Flow com¬puter code (AEA Technology), focusing on the chang-ing flow field at part load.
The paper summarizes the impeller flow structure, the development and decay of the blade, through-flow wake, and the main features of the diffuser flow with vane¬less and vaned combinations.
These evolutions was analysed considering both the lo¬cal impeller outlet flow conditions in the blade to blade section, the interactions with the diffuser blades and the influence of the leakage flow near the shroud. The leakage flow rate was experimentally evaluated
Theoretical evaluation of the effects of the Impeller Entrance Geometry and of the Incident Angle on Cavitation Inception in Centrifugal Pumps
No full textA method, based on quasi three-dimensional analysis, of describing pump cavitation behaviour is proposed. Cavitation performance is related to impeller entrance design and the influence of the angle of attack of the leading edges on the flow is studied. Coefficients are derived from the pressure drop due separately to the vanes and shroud. The influence of incident angle on cavitation is shown as a function of the blade geometry and discussed. By comparison with experimental data on centrifugal pumps, it is shown that the present model can simulate the characteristics of inception cavitation at design and off-design conditions
Comparison of different numerical approaches to the study of the H-Darrieus turbines start-up
No full textSelf-start capability is an important feature of wind turbines. It allows to obtain simpler and cheaper
turbines not actively controlled. Different approaches to describe the self-start of an H-blade Darrieus
rotor are presented and compared in the present work. The Blade Element Momentum (BEM) approach
was compared with two and three-dimensional CFD simulations. The tipespeed ratio versus power
coefficient curves and the evolution of the trust forces over a blade revolution highlighted the limits and
the strengths of each approach.
The BEM model showed remarkable limits to describe to describe the self-start behaviour of the tested
geometry. The principal limits of the BEM approach can be ascribed to the absence of well documented
aerofoil databases for low Reynolds number and the inadequate modelling of dynamics effects. The 2D
simulation allowed to highlight the unsteady features of the flow fields, and the presence of a complex
vortices pattern which interact with the blade. Furthermore the comparison between 2D and 3D data
demonstrated the importance of 3D effects such as secondary flows and tip effects. These effects were
proved to have a positive effect on start-up, increasing the torque characteristic for tipespeed ratio of 1.
The start-up capability of H-Darrieus appears to be influenced by many different factors, which include
secondary flows, three-dimensional aerodynamic effects and the finite aspect-ratio of the blades
Influence of the blade stacking on the flow through an axial flow runner and predictions of three-dimensional and quasi three-dimensional numerical codes
No full textThe influence of the stacking of the blade sections on the flow through an axial flow runner is investigated by means of quasi and three-dimensional finite element programs.
The results show up higher pressure gradients close to the leading edges, in the sections near the hub, when the stacking point is displaced towards the trailing edge. These gradients become noticeable near the trailing edge when the stacking point moves towards the inlet, even if in an attenuated way these trends reverse in sections close to the shroud.
The above-mentioned effects are highlighted only if three-dimensional codes or quasi three-dimensional codes with more than one hub-to-shroud surface are employed
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