This study compared mixing efficiencies of the symmetric and asymmetric airfoil blades in a continuous stirred tank reactor (CSTR) at various mixing velocities and angles of attack. The symmetric airfoil blades (NACA0009 and NACA0015) and asymmetric airfoil blades (NACA2414 and NACA4412) were set at different angles of attack and mixing velocities. The tank reactor was equipped with two sets of three airfoil blades at the upper and the lower parts of the stirring shafts at the angles of attack 0 deg, 10 deg, 16 deg, and 20 deg, and the mixing velocities of 80, 110, 140, and 190 rpm. The mixing efficiencies were evaluated from the homogenous appearance of plastic particles (5 mm in diameter) dispersed in water by an image processing technique. The results indicated that the mixing efficiencies of both the symmetric and asymmetric airfoil blades increased with increasing mixing velocities and at the angles of attack 0 deg and 10 deg, and slightly decreased with increasing mixing velocities at the angles of attack 16 deg and 20 deg due to the blade stall and mixing saturation as well as short-circuiting flow from the high flowrate. There was no significant mixing velocity effect on mixing efficiencies at the angles of attack 10 deg, 16 deg, and 20 deg except 0 deg of the symmetric and asymmetric airfoil blade systems. The two asymmetric airfoil blade types gave higher mixing efficiencies than the two symmetric airfoil blade types. The results from this study can be applied for a novel blade design for an efficient mixing flow, which will be beneficial for industrial biogas production.
The objective of this study is to analyze angular velocities, surrounding air pressure and velocities of various micro vertical axis Savonius wind turbine prototypes by the Computational Fluid Dynamics (CFD) method. The angular velocities of turbines are the basic parameters for determining the major parameters of the wind turbines, such as efficiency, torque, power and electricity etc. Eight models of the micro vertical axis Savonius wind turbine made of acrylic were designed by varying three parameters including the turbine distance between the pivot point and the tip, the distance between the apex curvature and the center plane curvature and the cross section types (circular, square, triangular and trapezoid). The eight models were divided into three groups including two models (B1, B2) with two different cross section types (triangle and rectangular) in the first group, three models (C1, C2, C3) with three different distances from the curvature tip to the centerline of 110, 85 and 40 millimeters in the second group and three models (D1, D2, D3) with different distances between the pivot point and the tip in the third group. Then, the angular velocities, the surrounding air pressure and velocities of the micro vertical axis Savonius wind turbine at the five wind speeds at 5.59, 7.67, 9.76, 10.45 and 11.84 m/s were measured and evaluated by the ANSYS program. The simulation by the CFD method of the angular velocities of the designed wind turbines was compared with the experimental results. Some discrepancies due to the absence of friction in the CFD results were observed. However, discrepancies between the simulation and the experimental results were decreased when the wind speed was increased due to the increase of torque and force in the experimental results which has overcome the turbine core friction. It has indicated that at the highest wind speed (11.84 m/s), the designed micro vertical axis Savonius circular cross section wind turbine with the curvature of the distance from the apex curvature to the center plane at 110 millimeters gave the highest turbine angular velocity of 441 rpm. At this highest turbine velocity by the CFD simulation, the high surrounding air pressure was not the smallest and the low turbine surrounding air pressure area was not the largest. This might be due to the effect of the suitable apex curvature distance to the center plane causing the efficient air flow to overcome the effect of the surrounding air pressure. The wind turbine of the second group gave the highest angular velocities followed by the first and the third group. These designed micro vertical axis Savonius wind turbines can be used as a preliminary model for the design and construction of the micro wind turbine to generate electricity at low wind speed region, such as Thailand.
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