1,720,977 research outputs found
Hydraulics/turbo machinery research. 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
On the choice of the vane curvature of centrifugal impellers
No full textA computer programme has been develop to investigate the influence that curvature of the blade exercises on the fluid in radial and mixed flow impellers. Point by point method is used for shaping the blade surface. The results of numerical analysis show that a linear variation of the blade angle with the ordinate in the conformal plane is a satisfactory compromise between flow behaviour and geometrical requirements. Blades having NACA profile sections are compared with other configurations. It was observed that the former produce a remarkable fluctuation of the angle of the absolute velocity at the discharge section
Optimal assets management of a water distribution network for leakage minimization based on an innovative index
No full textLeakage reduction in water distribution networks is an absolute priority and several pressure management
strategies have been proposed in the literature to tackle this issue. However, the definition of an effective relationship
between leakage and relevant and measurable parameters still represents a challenge.
This paper presented a novel performance parameter, the Leakage Performance Index (LPI), to minimize
leakages starting from pressure and flow rate measurements. This parameter creates a ranking among the different
nodes in the network, by properly weighting the pressure of each node with the output flow from the node
in order to focus the pressure management strategy on those nodes whose impact, in terms of leakage, is expected
to be greater.
To verify the effectiveness of the proposed LPI, a model of an existing water distribution district in Italy was
developed in EPAnet and validated by comparison with experimental results. The valve settings of the model
were then used as variables of time-dependent optimization procedures aimed at minimizing different objective
functions. Different scenarios were considered by varying the minimum guaranteed pressure at the customer
points.
The LPI minimization strategy was efficient insofar as it indirectly minimized the leakages, achieving the
same results of the leakage minimization strategy
Numerical simulation of a pump-turbine transient load following process in pump mode
Full text linkThis paper presents the simulation of the dynamic behaviour of variable speed pump-turbine. A power reduction scenario at constant wicket gate opening was numerically analysed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (pressure, blade torque, flow rate in blade passages). The unsteady pattern in return channel strengthened emphasizing its characteristic frequency with the rotational velocity decreasing reaching a maximum and then disappearing. At lower rotational speed, the flow field into the wickets gates channel start to manifest a full three-dimensional flow structure. This disturbance was related to the boundary layer separation and stall and it was noticed by a specific frequency
Analysis of the Inner Fluid-Dynamics of Scroll Compressors and Comparison between CFD Numerical and Modelling Approaches
Full text linkScroll compressors are widely adopted machines in both refrigeration systems and heat
pumps. However, their efficiency is basically poor and constitutes the main bottleneck for improving
the overall system performance. In fact, due to the complex machine fluid dynamics, scroll design
is mainly based on theoretical and/or semi-empirical approaches. Designs strategies that do not
guarantee an in-depth analysis of the machine behavior can be supplemented with a Computation
Fluid Dynamics (CFD) approach. To this purpose, in the present work, the scroll compressor inner
fluid dynamics is numerically analyzed in detail using two CFD software and two different modelling
strategies for the axial gap. The analysis of the fluid evolution within the scroll wraps reveals unsteady
phenomena developing during the suction and discharge phases, amplified by the axial clearance
with negative impact on the main fluid flow (e.g., −13% of average mass flow rate for an axial gap
of 30 μ) and on the scroll performance (e.g., +26% of average absorbed power for an axial gap of
30 μ). In terms of accuracy, the k-" offers good performance on the estimation of average quantities
but proves to be inadequate for capturing the complexity of the unsteady phenomena caused by the
axial gap (e.g., −19% of the absorbed power in case of perfect tip seal). The need for considering
specific geometric details in design procedures is highlighted, and guidelines on the choice of the
most suitable numerical model are provided depending on the analysis need
Numerical analyses of a cavitating pelton turbine
No full textErosive wear of hydro turbine runners is a complex phenomenon, which depends upon many parameters and which leads to a decrease of the performance in time and/or in extreme cases to the rotor mechanical failure. Consistently, the study of this wearing process is an important step to improve the impeller design, to avoid or minimize the rise of extraordinary maintenance. In the present paper the cavitation mechanics of a Pelton turbine was investigated using CFD analyses. A Pelton affected by pitting cavitation was taken as test case. The Pelton geometry was modelled and analyzed using unsteady Reynolds averaged Navier-Stokes (RANS) multiphase analyses. The homogeneous approach was used to describe the multiphase flow composed by water, water vapour and air. Numerical results discriminated the vapour production processes during the cut in of the bucket on the water jet. The design and the part load flow rates were analyzed and the cavitation process compared. A simple procedure to identify the locations of higher damage risk was presented and verified on the test case runner
Characterization of the hydrodynamic instabilities in a pump-turbine operating at part load in turbine mode
No full textPump-turbines (RPT) nowadays represents the most common mechanical equipment adopted in the new generation of storage hydro plant. In order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources, RPT are forced to rapidly switch between the pumping and generating mode also extending their operation under off-design conditions in unstable operating areas. Because of the design criterion adopted for the development of a RPT, an unstable behavior represented by a typical S-shaped profile with a positive slope in the machine's characteristic can occur near to the runaway condition.
With the purpose of evaluating the evolution of the fluid field near to the no-load condition, an in-depth CFD analysis of the RPT model test of the Norwegian Hydropower Center is performed by retracing the machine's characteristic curve running through the flow-speed characteristic curve up to the turbine brake region for fixed guide vanes opening. To validate the numerical results, a comparison with the experimental results in terms of characteristic curves and pressure signals is performed.
The results allow to capture the 3D characteristics of the unsteady phenomena, progressively evolving in an organized rotating stall, highlighting also the influence of the flow rate change from partial loads to the turbine brake operation on their development. In order to characterize the pulsating nature of the instability phenomena developing in the runner and in the rotor-stator interaction, a time-frequency analysis is performed on the numeric pressure and torque signals. The combination between fluid-dynamic and time-frequency analysis makes it possible to identify and characterize three evolution phases: inception, growth and consolidation
Influence of the fluid-dynamic properties of organic fluids on pump performance
No full textIn the industrial sector, medium, low and ultra-low temperature waste heat represents a significant source of energy loss as well as constitutes a harmful environmental effect which must be avoided. In this context, the Organic Rankine Cycle (ORC) technology is a proven solution because, being the working fluid an organic substance with low boiling temperature, it is better than water when low grade heat needs to be recovered. However, the recovery process presents several challenges when dealing with low and ultra-low temperature (<150°C) heat sources and numerous studies have been already carried out with the aim of, from the one hand, improving the ORC performance from a global point of view and, on the other hand, focusing on the performance of key components like heat exchangers and turbines. Only few works focused on ORC pumps performance because it is considered an established component. The result is that ORC models available in literature completely disregarded the influence of different properties (i.e. density, viscosity, ...) of the considered working fluid on the pump performance by assuming a constant value in the range from 65% to 85%. This simplified approach to the pump models generally brings, on one side, to an overestimation of the achievable ORC efficiency and, on the other side, to an unfair comparison between different working fluids behavior. This paper presents an in-depth analysis of the influence of the organic substances fluid-dynamic properties on the pump performance. The performance of a multi-stage centrifugal pump, designed to serve with water but suitable for ORC applications, is experimentally and numerically investigated by means of the commercial CFD code, Ansys CFX. Flow fields and performance of the pump operating with eight organic fluids typically used in ORC applications (R134a, R141b, R245fa, R152a, R142b, Acetone, Benzene and Toluene) are investigated by properly combining the CFD code with CoolProp. All the fluids are assumed to be sucked by the pump at a condensing temperature of 30°C in pure liquid condition. A negligible heat transfer from the machine to the environment is considered during the numerical simulation. Besides the expected density factor, which modifies the best efficiency point in terms of design mass flow rate and head, the comparison of the pump dimensionless performance, within the frame work of the similarity laws, highlighted a clear influence of the Reynolds number that is greatly affected by the different fluid properties (density and viscosity). Differences in efficiency of approximately 2%, being the flow coefficient equal, are detected due to different viscous losses. This finding influences the ORC system efficiency, stuck around 10-12% in case of low-grade waste heat recovery applications. Possible developments of prediction methods as well as of new design strategies, based on the different working fluid properties and, hence, on the expected flow regime, will be foreseen
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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