| The multi-blade fan has been widely used in heating, ventilating and air-conditioning engineering because of its smaller size and lower noise. This kind of fan has two types of blade profile: aerofoil-shaped profile and equal-thickness circular arc profile. The aerofoil-shaped profile blade has better flow characteristics and hence higher efficiency. But it also has some disadvantages: higher weight, higher cost and more difficult to manufacturing. So, the equal-thickness circular arc profile blade has been widely used in small and medium sized fan for its impeller is easier to manufacturing and the its cost is lower, though its efficiency is slightly lower due to the serious flow separation in its blade channel.In order to optimize the performance of the equal-thickness circular arc profile blade fan and improve its efficiency, one of effective way is to use appropriate-shaped equal-thickness blade to substitute the mono-arc blade that is widely used at present. One type of this kind of blade is dual-arc blade, which is composed by two circular-arc that have different curvature and central angle. By setting different inlet mount angle, arc curvature, central angle and their combinations, we can change the shape of the blade to simulate the aerofoil-shaped blade, so that the flow characteristics in the blade channel could approximately close to that in the aerofoil-shaped blade channel. However, very few works is done on this kind of blade.In this paper, the effect of dual-arc blade on performance of fan was studied. The evaluation criterion used was whether the fan could output more flow rate and higher pressure. During the research, several blades were designed by changing inlet mount angle, arc curvature, central angle and their combinations. After that, whole flow field's 2-D simulation was conducted using Fluent program. The effect of the blade profile parameters on the performance of the fan was studied, and several better-performance blades were selected. 3-D simulation was conducted on the whole flow field of the fan using selected blade whereafter. The 2-D simulation results indicated that static pressure decreases, dynamic pressure and total pressure increase gradually with the advance of the fluid flowing to the outlet in the spiral,. The minimum value of static pressure and the maximum value of dynamic pressure and total pressure are presented at the place near to the outlet of the spiral. Analysis on different blades showed that the central angle and curvature of the arc are interactional. If the central angle of the first arc was small, such as less than 60 degree, the advantages of the dual-arc blade can not be get the best out of, sometimes is even worse than the mono-arc blade. Curvature ratio (the ratio of curvature of the first arc to the second arc) should be moderate. If the angle of the first arc was small, it should be decreased or vice versa. General speaking, central angle of the first arc should be 75°-90°, curvature ratio should be 3-4. Research on the inlet mount angle indicated that the fluid direction at the inlet of the blade channel is not along the tangent direction of the arc, so it has impact to the pressive side of the blade. The larger the inlet angle is, the more intensive the impact on the blade is and the more the energy loss is; While the smaller inlet angle could make the blade guide the stream and decrease the impact to the pressive side of the blade. From the research of the paper, the inlet mount angle should be 60° or so.The 3-D simulation results indicated that the stream velocity is mainly axial direction near the front plate in the inlet. And then gradually turns to radial direction at the middle part of the impeller. The outflow from the middle impeller caused an intensive volution, and most of them flow back to the impeller near the front plate. The stream near the back plate in the inlet is already turned to radial direction, most of them flow out of the fan. Thus we can conclude that the flow field near the back plate is the main factor to affect the performance of the fan. Because of the axial volution between the front plate and the back plate and its block to the outflow, the velocity in the outlet from 3-D simulation is smaller than that form 2-D simulation at the same outlet static pressure. However, due to the flow field near back plate has the decisive effect on the performance of the fan, which also similar the flow field gained from 2-D simulation, the conclusion from 3-D simulation was same to that from 2-D simulation.
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