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How to control the unstable characteristics of water pumps and experimental research

19 Nov 2020Email

How to control the unstable characteristics of water pumps and experimental research

 


This article mainly starts from the design point of view, clarifies the formation mechanism of these small flow instability and analyzes its influencing factors, so as to guide the design of low specific speed high speed inducer centrifugal pump, so that the head flow characteristic line of high speed centrifugal pump H~Q is different There is a positive slope rising section, that is, high-speed centrifugal pumps have good low-flow working stability.  

 

The mechanism of instability   
The reasons for the instability of the small flow are mainly the swirling flow generated at the outer diameter of the leading edge of the inducer inlet, the backflow of the centrifugal wheel inlet, the secondary flow in the impeller channel, the wake-jet structure and flow separation in the impeller channel, and The secondary flow at the exit of the impeller when the impeller and volute work together. The existence of these factors affects the flow field distribution of the high-speed centrifugal pump on the one hand, and consumes a lot of energy on the other hand, resulting in a decrease in the head and efficiency of the small flow area. Therefore, it is easy to make the characteristic line of the high-speed centrifugal pump appear positive. The slope rises section, which makes the high-speed centrifugal pump unstable under small flow conditions. The following describes the generation mechanism of these unstable factors. 

 

 
1. The mechanism of inlet reflux   
Many scholars at home and abroad have done research on the mechanism of impeller inlet reflux. Stepanoff is one of the earliest scholars who studied the mechanism of centrifugal pump impeller inlet reflux. He believes that the liquid flow is maintained by the energy gradient. When the flow rate drops to close to zero, the impeller may make the inlet flow due to the inertial force of the liquid. The surrounding circumferential speed increases, so the energy near the tube wall increases, which makes the energy gradient necessary to maintain the liquid flow along the streamline no longer present, so the liquid flow near the impeller inlet reverses. Fraser believes that the centrifugal head is constant for a given impeller diameter and flow, and the dynamic head is a function of the flow. At certain points on the head-flow curve, once the dynamic head exceeds the centrifugal head, the pressure gradient at these points is reversed. Direction, leading to the opposite flow direction, that is, backflow phenomenon. Literature 3 analyzes the mechanism of low specific speed centrifugal pump impeller inlet backflow from both theoretical and experimental aspects. It is believed that the rotation speed component is the main reason for the impeller inlet backflow, and points out that the backflow is the main reason for the small flow instability.  

Because designers often use the positive angle of attack method when designing low specific speed and high speed inducer centrifugal pumps, that is, in order to ensure that the head generated by the inducer can meet the energy requirements of the centrifugal wheel inlet, the inlet angle of the inducer blade is greater than the liquid flow angle, and at the same time In order to obtain better cavitation performance of the centrifugal wheel, the blade inlet angle is also selected to be larger than the liquid flow angle; in addition, in order to obtain higher efficiency, the design of increased flow is generally adopted when designing ultra-low specific speed high-speed inducer centrifugal pumps. This makes the actual liquid flow angle under operating conditions smaller than the liquid flow angle under design conditions, so that both the leading edge of the inducer and the inlet of the centrifugal wheel have uneven circumferential velocity components, resulting in a vortex around the streamline. Therefore, the inlet reflux of the inducer and the centrifugal wheel is actually caused by the uneven circumferential partial velocity of the liquid flow at the edge of the rotating blade, and it contains the vortex perpendicular to the axial plane and the swirling flow around the streamline vortex.

 

 

2. The secondary flow and stratification effect in the flow channel of the centrifugal impeller   
Current flow field analysis and flow test research have shown that the flow in the centrifugal impeller channel is basically composed of a wake zone with a relatively low relative velocity and a jet zone that is almost inviscid. The wake zone is close to the front cover of the impeller. On the plate and non-working surface, the wider the wake area, the thinner the shear layer between the jet and the wake, and the greater the velocity gradient between the two, which means that the jet-wake structure is stronger, and the loss in the impeller is also It gets bigger. The formation and development of the wake are formed by the mutual influence of the development of the boundary layer, the development of the secondary flow, the flow separation and the stratification effect.  
Regarding the formation of the secondary flow and its influence on the wake, many scholars at home and abroad have done research. Qualitatively speaking, the following formula can be used to analyze the secondary flow in the impeller rotating channel:   


In the above formula, EMBEDEquation.2 is the rotation stagnation pressure, EMBEDEquation.2 is the rotation component of the relative streamline, and EMBEDEquation.2 is the partial derivative of I to the subnormal direction and the direction of the rotation axis respectively. The above formula shows that the vortex relative to the streamline direction is produced by two factors: one is the curvature of the streamline with the radius Rn, and the other is caused by the rotational angular velocity ω.  
Rotation stagnation pressure I is the sum of dynamic pressure EMBEDEquation.2 and converted static pressure EMBEDEquation.2, the effect of viscosity makes I drop. Because there is a large relative velocity gradient in the rotating boundary layer of the impeller channel, the minimum value of I in the boundary layer with uniformly converted static pressure appears on the wall, and its value is equal to p*.  

Consider the BB flow of the impeller channel. Assuming that the velocity profile as shown in the figure has been generated due to the friction of the inlet pipe wall, consider a flow surface ABCD of the BB channel, close to point A of the outer diameter of the impeller channel, and the streamline curvature is determined by The curvature of the blade is generated, the rotational pressure gradient in the subnormal direction is caused by the loss of the boundary layer of the front cover, and the first term produces a positive streamline rotational component EMBEDEquation.2. At point B near the inner diameter, negative EMBEDEquation.2 is caused, which results in the formation of secondary flow on the boundary layer of the front cover and the rear cover, so that the low I in the boundary layer of the front and rear cover The micro-clusters flow to the non-working surface, and from the continuity, the low-I micro-clusters on the working surface are also driven to the non-working surface, thus thickening the boundary layer on the non-working surface. Since the I gradient is almost perpendicular to ω, the secondary flow caused by the second term of equation (2-1) is small. Since the two points C and D at the exit of the impeller are located in the runoff part of the flow channel, the second term mainly causes the positive and negative EMBEDEquation.2 and secondary flow in the direction shown in the figure, so that the front and rear covers are The low-energy clusters in the boundary layer of the plate are driven to the non-working surface, increasing the boundary layer on the non-working surface.  


Applying the same analysis method to the meridian plane, when the streamline turns from the axial to the radial direction, secondary flow vortices are formed on the boundary layer of the working surface and the non-working surface. The low-I microclusters inside drive to the front cover, thickening the boundary layer on the front cover.  

From the above analysis, it can be concluded that there are three sources of secondary flow vortex in the streamline direction:   
1) Curved blades; it turns the flow from the direction of the entrance angle of attack to the axis direction, and drives the low I fluid clusters in the boundary layer on the front and rear cover plates to the non-working surface, due to the low pressure in the boundary layer of the working surface I fluid micelles are unstable and are therefore driven to the non-working surface.  
2) Axial to radial turning; due to the curvature of the front and rear cover profile on the meridian surface, the low I fluid clusters in the boundary layer on the working surface, non-working surface and the back cover surface are transferred to the front cover surface.  
3) Rotation; as the flow goes from axial to radial, the contribution of rotation to the secondary flow vortex is increasing. The secondary flow generated by Coriolis force causes the low I fluid to flow from the front and back cover surfaces and unstable working surface surfaces. Of the low I fluid transferred to the non-working surface
Due to the stratification effect, high-energy fluid clusters accumulate on the side of the working face and the rear cover, which promotes the acceleration of the incoming flow, and the slow growth of the boundary layer, which reduces the separation tendency. On the non-working surface and the front cover side, low-energy fluid micro-clusters accumulate, which reduces the incoming flow velocity, aggravates the growth of the boundary layer, and promotes the tendency of boundary layer separation. 

 

 
3. Wake-jet structure and flow separation   
As mentioned above, the flow in the centrifugal impeller channel is basically composed of a relatively small wake area and a nearly non-viscous jet area. Taking into account the viscous effect of the real fluid, the working surface and non-working surface of the BB channel are A boundary layer is formed. Under the action of blade curvature and rotation, the boundary layer on the non-working surface becomes thicker and thicker due to the influence of the secondary flow. It is easy to stall at a small flow rate, which leads to the separation of the boundary layer.

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