Aerodynamic characteristics of blade installation angle of adjustable blade axial fan

In actual operation, the adjustable axial fan of the moving blade has poor running performance and frequent failure due to the complex structure of the rotor, the precision and the many rotating parts. At present, the research on axial flow fan failure mainly focuses on stall and surge, blade breakage, bearing temperature rise and fan vibration, and proposes corresponding preventive measures, which have achieved obvious results. The operation practice shows that the phenomenon that the fan efficiency decreases and the noise increases due to the deviation of the installation angle of the blade from the design value also occurs from time to time, and the faults and phenomena caused by it are gradually paid more attention.

Due to the complex internal structure of the fan, the current one. Among them, the near hub surface (foot = 480), S is the intermediate flow surface (foot = 600), and C is the near top surface (foot = 720). Limited by the length of the space, only the feature flow surface is analyzed.

The characteristic flow surface diagram is the total pressure distribution map under the design flow rate. It can be seen from (a) that when the installation angle is normal, the total pressure distribution in each flow channel is the same. The total pressure on the pressure surface of the blade increases from the leading edge to the trailing edge. On the suction side of the blade, the pressure difference between the leading edge and the trailing edge is larger, and the trailing edge pressure is higher than the leading edge pressure, forming a large reverse pressure. The gradient makes it more likely that the airflow will separate at the trailing edge of the blade. At the trailing edge of the airfoil, the total pressure is significantly reduced due to the wake flow at the trailing edge of the airfoil, resulting in energy loss and a reduction in local total pressure. When the installation angle deviates from the design value, the total pressure distribution on the abnormal blade (the intermediate position blade in the figure) and its adjacent blades changes significantly. When A6y=6 ((b)), the total pressure on the pressure side of the abnormal blade increases, and the pressure gradient in the flow direction is smaller than the pressure gradient on the pressure surface of the adjacent normal blade. The pressure gradient at the front and rear edges of the abnormal blade suction is larger than that of the normal blade. This is because the abnormal blade installation angle increases, resulting in an increase in the negative angle of attack between the incoming flow and the inlet side of the blade, and a large pressure gradient on the suction surface. The trailing edge loss at the trailing edge of the anomalous airfoil increases. With the further increase of A ((c) and 3(d)), the pressure gradient on the pressure side of the abnormal blade gradually decreases, and the pressure gradient at the trailing edge of the suction surface is significantly larger than the pressure gradient at the suction surface of the adjacent blade, indicating The disturbance caused by the airflow on the suction side is enhanced as the degree of deviation of the mounting angle increases. The reason may be that as the Ay increases, the angle of attack of the guide vane increases when the airflow enters, and a vortex is generated at the suction surface of the trailing vane, which hinders the flow of the airflow and generates a large disturbance on the suction surface of the abnormal blade.

A streamline diagram of the feature flow surface. It can be seen from (a) that when the blade mounting angle is normal, the airflow smoothly passes through the flow path, and the speed changes uniformly. When A=6 ((b)), the disturbance is generated at the trailing edge of the suction surface of the abnormal blade, but the overall change of the flow field is not large; when A/y=12 ((c)), the airflow generates a rotation with the impeller at the trailing edge. The flow of the total pressure distribution of the opposite flow surface, the flow loss increases, and the flow condition deteriorates; when a=18. ((d)), a large scale vortex is generated at the trailing edge of the suction side of the blade, and the generation of the vortex not only changes. The flow of fluid through the direction of the flow path, and the flow loss is further increased, which will result in a reduction in the total pressure of the fan and a decrease in efficiency.

The radial distribution of the total pressure at the impeller outlet for the design flow.

As can be seen from the figure, the total pressure gradually increases in the radial direction, and the total pressure at the tip of the blade is significantly reduced. This is because the tip leakage flow interferes with the main flow, causing local loss, resulting in a decrease in total pressure. In addition, as the degree of deviation of the installation angle increases, the total pressure shows a tendency to increase first and then decrease, but at different radial positions, the total pressure changes are different. In the leaf root to 25% leaf height, ie 0.45 (a), the total pressure distribution on the outlet flow surface presents a mainstream zone and a non-mainstream zone, and the total pressure value in the main zone between adjacent blades is higher, and is located at The total pressure of the non-mainstream zone at the same height of the tip and root of the blade is lower due to the influence of the thicker boundary layer and the larger wake loss at the end wall. When A/y=6 ((b)), the total pressure of the adjacent flow channel with the abnormal blade increases significantly, while the low pressure region after the blade wake flow decreases and moves to the middle and the middle of the leaf (region); Continue to increase ((c) and 8(d)), the total pressure of adjacent flow channels will be further increased, while the average total pressure of the outlet flow surface will decrease, the wake loss of abnormal blades will increase, and the lowest total pressure zone The range is enlarged and the total pressure is reduced (regions S and C), indicating that the flow loss caused by the abnormal blade is mainly concentrated in the middle of the leaf height as A increases, which is consistent with the obtained results.

2.4 The change of performance curve is to investigate the influence of A on the overall running performance of the fan. The full pressure and internal efficiency characteristic curves of the abnormal blade installation angle in four cases are given. The design flow rate is 37.12m3/s. The figure shows that with A When /y is increased, the full pressure characteristic and the internal efficiency characteristic exhibit different variations. Under the design flow rate, the full pressure shows a trend of increasing first and then decreasing. This is because with the increase, the total pressure increase through the abnormal blade airflow is greater than the flow loss caused by the abnormal blade; when a certain critical value is exceeded, the full pressure The added value is less than its flow loss. In addition, when A is small, the total pressure change is not obvious. When A=18, the total pressure drop is very significant. Under the design condition, the total pressure drops by 4.8%. With the increase of A/, the internal efficiency under various working conditions of the fan Both showed a downward trend and the efficiency decreased most at the design operating point. When A/ was 6°, 12 and 18, the internal efficiency decreased by 2.9%, 4.8% and 9.1% respectively. Under the same A/y value, when the flow exceeded When designing values, the internal efficiency decline is reduced. It is worth noting that when A/ changes in 012, although the fan output is small, that is, the total pressure does not change much, the efficiency is obviously reduced.

3 Conclusion When the installation angle of a single blade deviates from the design value, not only the inflow characteristics of the abnormal blade and the adjacent blade are changed, but also the full pressure and efficiency characteristic curves of the fan are affected to varying degrees. Specifically, the total pressure of the pressure surface increases from the leading edge to the trailing edge, and there is a back pressure gradient at the suction surface; with A/increased, the trend of total pressure loss at the trailing edge of the blade gradually increases, and the abnormal blade pressure surface from the leading edge The pressure gradient to the trailing edge decreases, and the pressure gradient of the suction surface increases, and a vortex is generated at the trailing edge of the suction surface.

The total pressure of the impeller outlet gradually increases along the radial direction, and the total pressure at the tip of the blade decreases significantly. The flow loss caused by A/increased is mainly concentrated in the middle of the blade height; there is a low pressure zone at the impeller exit, which increases with A/, adjacent flow The total pressure of the channel gradually increases, the average total pressure of the outlet flow surface gradually decreases, and the range of the low pressure zone expands continuously and the total pressure decreases.

With the increase of A/, the whole pressure shows a trend of increasing first and then decreasing, and the efficiency of the fan is decreasing, and the efficiency decreases most under the design flow.

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