Moving blade and shell have radial clearance, the pressure of liquid on the surface of the blade through the radial clearance flow to non working surface area of low pressure. In the blade inlet and outlet pressure difference between the two sides, through the clearance of the liquid with a high flow rate, thereby forming a partial pressure and cavitation. Cavitation moves along the surface of the shell with the rotation of the blade. This type of cavitation is called the clearance cavitation. Clearance cavitation has little effect on the performance of the pump, but the cavitation increases and the leakage rate increases. Experience shows that, if the blade pressure side of the edge of the edge of the circle, or the blade forward along the bend, can reduce the clearance cavitation.
The airfoil head of the axial flow pump blade is thicker, and the non working surface of the blade is formed when the liquid flow around the airfoil is low, in addition, the low pressure may also be caused by a larger impact angle between the airfoil and the flow. Airfoil head blade pressure caused by the cavitation, known as the airfoil cavitation.
Axial flow pump of the basic theory of cavitation and centrifugal pump the same, in order to prevent the axial flow pump cavitation is the key to control the nD value. Foreign drainage pumping station nD generally take 350 ~ 400, the large axial flow pump nD are made relatively low.
The anti cavitation performance of the axial flow pump impeller is related to the lift coefficient of the airfoil, the greater the lift coefficient, the greater the pressure difference between the working face and the non working surface, the worse the anti cavitation performance of the airfoil. Therefore, the lift coefficient of the airfoil and the axial flow pump anti cavitation performance of the two are contradictory.
There is no direct relation between the average pressure difference and the minimum pressure of the airfoil. From the analysis of the pressure distribution on the working face and the non working surface of the airfoil, the minimum pressure is the airfoil's head near the leading edge. But the low pressure area is very small, the liquid flow is still too late to vaporization on the very soon. Therefore, the low pressure zone has little effect on the pump cavitation performance. At the back of the second low pressure areas, although the absolute pressure is higher than the first low pressure area, but because of its wide area, the impact of large cavitation.
In order to reduce the load on the blade, increase the number of blades or increase the consistency of the cascade, the anti cavitation performance of axial flow pump can be improved. The increase of the thickness of the cascade means that the leaf area is increased, the unit load on the blade is reduced, and the resistance to cavitation is favorable.
