How to Control Pump Instability Characteristics and Experimental Research
This paper mainly from the design point of view, to clarify the formation mechanism of these small flow instability and analyze its influencing factors to guide the design of low speed high speed induced wheel centrifugal pump, the high speed centrifugal pump head flow characteristics of the line H ~ Q There is a positive slope rise, that is, high-speed centrifugal pumps have good low-flow working stability.
Instability phenomenon The mechanism of small flow instability is mainly caused by the swirling flow generated at the outer diameter of the leading edge of the inducer, the return flow at the inlet of the centrifugal wheel, the secondary flow in the impeller flow path, the wake in the impeller flow path - the jet flow Structure and flow separation, as well as impeller and volute work together when the impeller exit secondary flow and so on. The existence of these factors, on the one hand affect the high-speed centrifugal pump flow field distribution, on the other hand consume a lot of energy, resulting in low-flow area of ​​the head and efficiency decline, it is easy to make high-speed centrifugal pump characteristic line is positive Slope rise, so that high-speed centrifugal pump in the small flow conditions instability. The following describes the mechanism of these unstable factors.
1. The Mechanism of Imported Backflow The mechanism of imported backflow of impellers has been studied by many scholars at home and abroad. Stepanoff was one of the earliest scholars to study centrifugal pump impeller inlet backwash mechanisms, arguing that liquid flow is maintained by energy slopes and that when the flow rate drops to near zero, the impeller is likely to be imported due to the inertial force of the liquid As the peripheral velocity increases, the energy in the vicinity of the tube wall increases, which renders the slope of the energy necessary to maintain the liquid flowing along the streamline out of the way, so that the liquid flow near the impeller inlet is reversed. Fraser assumes that the centrifugal lift is constant for a given impeller diameter and flow, and the dynamic head is a function of flow rate. At some point on the head flow curve, once the dynamic head exceeds the centrifugal head, the pressure gradient at these points To, led to the opposite direction of flow, that is, the phenomenon of reflux. Literature 3 analyzes the mechanism of the inlet backflow at the impeller of low specific speed centrifugal pump from both theoretical and experimental aspects. It is considered that the rotational speed component is the main reason of the impeller inlet backflow. It is pointed out that the backflow is the main reason causing the small flow instability.
Because designers often design positive-angle method when designing low-speed high-speed induction wheel centrifugal pump, in order to ensure that the lift head produced by the induction wheel can meet the energy requirement of the centrifugal inlet, the inlet angle of the induced wheel is larger than the flow angle, In order to obtain better cavitation performance of the centrifugal wheel, the blade inlet angle is also taken as greater than the flow angle. In addition, in order to obtain higher efficiency, the design of ultra-low speed high-speed induction wheel centrifugal pump is generally adopted to increase the flow design, This makes the actual operating angle of the flow angle is less than the design conditions of the flow angle, so that the inducer and the leading edge of the centrifugal wheel have a non-uniform circumferential velocity component, resulting in flow around the whirlpool. Therefore, the inducing wheel and the centrifugal wheel inlet reflux is actually due to the rotating blade edge of the liquid at the circumference of the non-uniform velocity caused by the vortex is perpendicular to the axial plane and flow around the whirlpool swirling flow.
2. Centrifugal impeller runner flow and stratification effect of the current flow field analysis and flow test studies have shown that the flow in the centrifugal impeller flow path is basically a relatively small wake area and the approximate non-viscous jets Zone, the wakespace closely to the front cover of the impeller and the non-work surface, the wider the wake area, the thinner the shear layer between the jet and the wake, the greater the speed gradient between the two, meaning The jet - wake structure stronger, the greater the loss within the impeller. The formation and development of wakes is formed by the interaction between the development of the boundary layer, the development of the secondary flow, the separation of flows and the stratification effect.
On the secondary flow formation and its impact on the wake, many scholars at home and abroad made a study, the qualitative terms can be used to analyze the impeller rotation flow channel secondary flow:
EMBEDEquation.2 (2-1)
In the above equation, EMBEDEquation.2 is the rotation stagnation pressure, EMBEDEquation.2 is the relative streamline rotation component, and EMBEDEquation.2 is the partial derivative of I to the normal direction and the rotation axis respectively. The above equation shows that the vortex in the direction of the opposite streamline is generated by two factors: one is the streamline curvature with radius Rn, and the other is the angular velocity ω.
Swirling stagnation pressure I is the sum of the dynamic pressure EMBEDEquation.2 and the translational static pressure EMBEDEquation.2, and the viscosity effect causes I to decrease. Since there is a large relative velocity gradient in the rotating boundary layer of the impeller runner, the minimum value of I in the boundary layer with a uniform hydrostatic pressure appears on the wall surface and its value is equal to p *.
Considering the BB flow in the impeller runner, assuming that the velocity profile shown in the figure has been generated due to the friction on the wall of the inlet tube, considering the flow surface ABCD of the flow path BB and the point A near the outer diameter of the flow path of the impeller, The blade curvature is generated. The rotational pressure gradient in the second normal direction is caused by the loss of the boundary layer of the front cover. The first component produces a positive rotational component of the streamline EMBEDEquation.2. While near the inner diameter of the B point, causing a negative EMBEDEquation.2, the result is the formation of the front cover and rear cover surface boundary layer of the secondary flow so that the front and rear cover surface boundary layer low I The micelles flow to the non-working surface and, from the continuity, drive the low-micelles on the working surface to the non-working surface, thus thickening the non-working boundary layer. Since the I gradient is almost perpendicular to ω, the secondary flow caused by the second term of equation (2-1) is small. Because at the exit of the impeller C and D are located at the runoff part of the runner, the positive and negative EMBEDEquation.2 and the secondary flow of the direction as shown in the figure are mainly caused by the second one. In this way, the front and back covers The low-energy micelles in the slab boundary layer are driven to the non-working face and the boundary layer on the non-working face is increased.
The same analytical method is applied to the meridional plane. When the streamline curves from the axial direction to the radial direction, secondary flow vortices are formed on the working face and the non-working face boundary layer. They combine the working face and the non-working face upper boundary layer Within the low I micro-drive to the front cover, thickened the boundary layer of the front cover surface.
The above analysis shows that there are three sources of secondary flow vortices in the streamline direction:
1) curved blade; it makes the flow direction from the entrance to the axis of rotation angle, the front and rear cover surface layer of low I fluidic microfacies driven to the non-working surface, the boundary layer due to the low I fluid micelles are unstable and therefore also driven to non-work surfaces.
2) turning radially to the radial direction; transferring the low-I fluid micelles in the working and non-working faces and the boundary layer on the back cover surface to the front cover surface due to the curvature of the mezzanine upper and lower cover profile lines.
3) rotation; with the flow from the axial to the radial, the contribution of rotation to the secondary flow vortex increases, Coriolis secondary flow generated low I fluid from the front and rear cover surface and the unstable surface of the work surface Of the low I fluid transferred to the non-working surface due to the stratification effect, the high-energy fluid micelles accumulate on the working face and the rear cover side to facilitate the flow rate to accelerate and the boundary layer grow slowly, reducing the tendency of separation. In the non-working face and the front cover side, there are low-energy fluid micro-accumulation, thus reducing the flow rate, exacerbating the boundary layer growth, contributing to the separation tendency of the boundary layer.
3. Wake - jet structure and flow separation It has been mentioned above that the flow in a centrifugal impeller channel is basically composed of a relatively small wake area and an approximately non-stick jet zone, taking into account the viscous effect of the real fluid, BB channel surface and non-working surface are formed boundary layer, the blade curvature and rotation, the non-working surface boundary layer due to the influence of the secondary flow more and more thick, easy at a small flow Stalls occur, causing the boundary layer to separate.
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