Numerical Study of Wake Characteristics in a Horizontal-Axis Hydrokinetic Turbine

ABSTRACT Over the years most studies on wake characteristics have been devoted to wind turbines, while few works are related to hydrokinetic turbines. Among studies applied to rivers, depth and width are important parameters for a suitable design. In this work, a numerical study of the wake in a horizontal-axis hydrokinetic turbine is performed, where the main objective is an investigation on the wake structure, which can be a constraining factor in rivers. The present paper uses the Reynolds Averaged Navier Stokes (RANS) flow simulation technique, in which the Shear-Stress Transport (SST) turbulent model is considered, in order to simulate a free hydrokinetic runner in a typical river flow. The NREL-PHASE VI wind turbine was used to validate the numerical approach. Simulations for a 3-bladed axial hydrokinetic turbine with 10 m diameter were carried out, depicting the expanded helical behavior of the wake. The axial velocity, in this case, is fully recovered at 12 diameters downstream in the wake. The results are compared with others available in the literature and also a study of the turbulence kinetic energy and mean axial velocity is presented so as to assess the influence of proximity of river surface from rotor in the wake geometry. Hence, even for a single turbine facility it is still necessary to consider the propagation of the wake over the spatial domain

Assessment of turbulence modelling for CFD simulations into hydroturbines: spiral casings

Spiral casings are applied to distribute the water, as evenly as possible, to the stay vanes and wicket gates and then to the turbine runner. In a well-designed spiral casing, the pressure head of the fluid should be made available to the runner with minimum loss, hence the analysis of the flow through a spiral casing is important for the design of efficient hydraulic turbines. The goal of this work is based on turbo machines spiral casings three-dimensional flow modeling, simulation and characterization. Governing equations related to their study are used in the mathematical modeling part. Therefore, several turbulence models available will be tested with a finite volume method based commercial software CFX 5.5.1 which generates default tetrahedral meshing. Qualitative and quantitative results validation are proved by workshop benchmark experiments already done, in order these results can obtain a turbulence model that represents better the flow complexity inside spiral casings

A tropical rainforest clearing experiment by biomass burning in the state of Para, Brazil

Results are described of a forest clearing experiment conducted in Tome Acu, located approximately 250 km south of Belem, the capital of the Brazilian northern state of Para. An area of 3 ha of virgin forest was cut in July 1994 and left to dry until October of the same year, when fire was set. Post burning was also performed 30 days after the main fire. The test location biomass content per hectare was measured by indirect methods using formulas with parameters of forest inventories. The carbon content of the several biomass compartments was determined in a CHN analyzer. The combustion completeness was estimated by selecting ten 2 x 2 m2 areas and 24 large trunks and examining their consumption rates by fire. The 2 x 2 m2 areas were used to determine the completeness of small parts of biomass (those whose characteristic diameters were lower than 10 cm) and the trunks to determine the efficiency of the larger parts (characteristic diameters larger than 10 cm). The overall process combustion completeness was estimated to be 20.1%. Considering that the combustion gases of carbon in open fires contain approximately 90% of CO2 and 10% of CO in volumetric basis, the emission rates of these gases by the burning process were estimated as 70.2 and 5.0 t ha-1, respectively.Results are described of a forest clearing experiment conducted in Tome Acu, located approximately 250 km south of Belem, the capital of the Brazilian northern state of Para. An area of 3 ha of virgin forest was cut in July 1994 and left to dry until October of the same year, when fire was set. Post burning was also performed 30 days after the main fire. The test location biomass content per hectare was measured by indirect methods using formulas with parameters of forest inventories. The carbon content of the several biomass compartments was determined in a CHN analyzer. The combustion completeness was estimated by selecting ten 2×2 m2 areas and 24 large trunks and examining their consumption rates by fire. The 2×2 m2 areas were used to determine the completeness of small parts of biomass (those whose characteristic diameters were lower than 10 cm) and the trunks to determine the efficiency of the larger parts (characteristic diameters larger than 10 cm). The overall process combustion completeness was estimated to be 20.1%. Considering that the combustion gases of carbon in open fires contain approximately 90% of CO2 and 10% of CO in volumetric basis, the emission rates of these gases by the burning process were estimated as 70.2 and 5.0 t ha-1, respectively