Cavitation is a common phenomenon that appears during the operation of the hydraulic turbomachines reducing performance and life of Centrifugal pumps. The main goal of this work is primarily a CFD-simulation of the whole Centrifugal Pump-Turbine including the suction cone, impeller, diffuser blades and volute, in order to characterize and evaluate its performance under cavitation conditions. The CFD simulations results were compared with experimental data under cavitation and non-cavitation conditions. A good agreement has been obtained under non-cavitation conditions for global performance parameters. After the implementation of the Rayleigh Plesset cavitation model, the required Net Positive Suction Head (NPSHr) has been predicted from CFD simulations. Finally, a full cavitation test can be reproduced for a Hydraulic Turbomachine to avoid this dangerous phenomenon.
Cross Flow Turbines (CFT), also known as Banki or Ossberger turbines are broadly used in small scale hydropower generation. Easy construction and operation, low CAPEX and OPEX and fairly independent efficiency from flow rate are the main characteristics of the CFT. However, they also tend to have a modest efficiency (80%), hence they are not considered for large scale power plants. Previous work have focused on use of Internal Deflectors (ID) for CFT efficiency improvement. However experimental flow observation and characterization inside CFT is hard to achieve. This work proposes use of Computational Fluid Dynamic (CFD) tools as an aid in ID design. A transient regime, two-dimensional, numerical model of a CFT without any internal deflectors was carried out. Deviation from experimental results at BEP was close to 5%. CFT w/o ID results were used as ID design starting point. Parameters: Upper Blade Position and ID Length were defined and varied obtaining six different ID versions. Numerical models were carried out for evaluation of ID effect on CFT. CFT hydraulic efficiency improvement was achieved for all ID versions studied (range 0.5%–3%, average:1.9%). Output power was also augmented (range 0.3%–4%, average:2.5%)for all cases.
Air cooling via evaporation of water droplets injected at the compressor intake duct is the process known as Fogging System, which is among the most used technologies for increasing output power of gas turbines nowadays. The optimal design of this system must consider numerous variables, such as: air temperature (Ta), air relative humidity (RH), duct geometry, amount of water injected (mw), droplets size (Dd), and nozzles location. Since there are so many variables the flow under study is very complicated. In consequence the analytical determination of an optimal Fogging System design is not feasible. In this paper, a numerical model was developed in order to characterize the injection of water at the air intake duct of a Gas Turbine. First, the expressions characterizing the model were included in the CFD software ANSYS CFX v-11 and simulated in a simple geometry (rectangular duct). Validation of CFD results was carried out by comparison with experimental data. Good agreement between numerical results of a control case and experimental data was achieved (deviation < 2%). Then, the influence of key parameters such as: Ta, RH, Dd, mw over the performance of the air cooling system was investigated. Finally, the model was used to design a Fogging System for an existing 120 MW Gas Turbine. For this gas turbine operating under real conditions, the model predicts a net power increment of 2% [7].
Cross-Flow Turbines (CFT) also known as Banki Turbines, are often considered for small scale hydroelectric generation. They are known for their simple construction, maintenance and operation, which means they incur in lower CAPEX and OPEX when compared to other types of turbines. However, they also tend to have a modest efficiency (82% [1-3]), hence they are not considered for big scale operations. Little is known about the flow characteristics inside the runner of the CFT. The objective of this investigation is to better understand the flow inside CFTs using Computational Fluid Dynamics (CFD) tools. Steady and Transient State simulations were performed for a CFT at an specific speed N S = 45. SST and κ − ε turbulence models were compared in terms of simulation requirements and obtained results. A proposed runner-nozzle interface, considering real CFT existent gap between these two components (free space) was evaluated as well. Results were compared to available experimental data. Maximum, numerically calculated efficiency deviation from reported experimental global efficiency was 15%. Pressure and velocity profiles along nozzle outlet, energy transfer stages location and CFT reaction ratio values were addressed. Results were compared in terms of runner-nozzle interface (gap vs no-gap), turbulence model (SST vs κ − ε) and calculation regime (steady vs transient regime). Only calculation state (steady vs transient) was found to have major influence over results. Transient state calculations better representing complex flow inside the CFT. Obtained degrees of reaction (no runnernozzle gap, SST, transient state) were 0.12 and 0.08, for 1st and 2nd stages respectively. Hence the CFT is defined, according to this numerical models, as an impulse turbine.
A useful methodology in the design of a Turgo Type Turbine (TTT) has been accomplished through the study of a particular three dimensional potential flow, known as Rankine Ovoids. The obtained streamlines solution for this flow was modified implementing several algorithms in order to select a suitable flow profile that could be adapted as a prediction of the flow passing through the buckets of a Turgo runner. Afterwards, the selected profile was incorporated with other geometric parameters, which were based on the hydrodynamic and geometric conditions presented in a TTT, in the design methodology proposed for this investigation. In addition, the equations to calculate power and efficiency of this kind of turbines are included. The global efficiency in the 3DT methodology was reported to be 80.8% for the designed TTT, which it is an expected value for this kind of turbines.
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