Impeller Shroud Research Articles

Inception of Inlet Reverse Flow in Centrifugal Pump Impeller.

In the previous reports, effects of an impeller blade tip clearance and the blade number on the inlet reverse flow starting point were clarified for three and five blades open-type and closed-type impellers with good suction performences. Then, in this paper, for the close-type and open-type impellers with the blade inlet angle decided by conventional design method, the same experiment was carried out to know effects of the blade inlet angle on the inlet flow condition at low flow rates and the inlet reverse flow starting point, and the results were compared with the previous ones. Furthermore, the inlet reverse flow occurrence mechanism of the centrifugal pump impeller was examined based on the experimental results. It was particularly shown that the inlet reverse flow inception point can be predicted by investigating the relative flow along the impeller shroud surface.

Fluid-Structure Interaction Forces at Pump-Impeller-Shroud Surfaces for Axial Vibration Analysis

Solutions are presented for the dynamic axial forces developed by pump-impeller-shroud surfaces. A bulk-flow model of the leakage path between the impeller and the housing is used for the analysis consisting of the path-momentum, circumferential-momentum, and continuity equations. Shear stresses at the impeller and housing surfaces are modeled according to Hirs’ turbulent lubrication model. The governing equations were developed earlier to examine lateral rotordynamic forces developed by impellers. A perturbation expansion of the governing equations in the eccentricity ratio yields a set of zeroth and first-order governing equations. The zeroth-order equations define the leakage rate, velocity distributions, and the pressure distribution for a centered impeller position. The first-order equations define the perturbations in the velocity and pressure distributions due to axial motion of the impeller. Integration of the perturbed pressure and shear-stress distribution acting on the rotor yields the reaction forces acting on the impeller face. Calculated results yield predictions of resonance peaks of the fluid within the annulus formed by the impeller shroud and housing. Centrifugal acceleration terms in the path-momentum equation are the physical origin of these unexpected predictions. For normalized tangential velocities at the inlet to the annulus, uθo(0) = Uθo(0)/Riω of 0.5, the phenomenon is relatively minor. As uθo(0) is increased to 0.7, sharper peaks are predicted. The fluid modes are well damped in all cases. Numerical results are presented for a double-suction single-stage pump which indicate that the direct stiffness of the perturbed impeller shroud forces are negligible. Small but appreciable added-mass and damping terms are developed which have a modest influence on damping and peak-amplitude excitation frequency. The forces only became important for pumps with very low axial natural frequencies in comparison to the running speed, viz., ten percent of the running speed or lower.

Haemodynamic approach to reducing thrombosis and haemolysis in an impeller pump

In the experimental and clinical support of the failing heart, the impeller-type centrifugal pumps continue to be of interest because of their inherent advantages; however, the blood compatibility of these pumps still remains to be improved. From the viewpoint of haemodynamics, thrombosis and haemolysis could be reduced by eliminating the stagnation and turbulence of blood flow within the pump, which frequently takes place near the blood contracting surfaces of the pump, when the impeller contours do not coincide with the stream surfaces of the blood. It is suggested that it could be advantageous to design impeller contours according to the stream surfaces, by solving the partial differential equations of continuity, motion and energy. An impeller shroud and vane based on this approach would be fully rinsed by non-turbulent flow and there would then be neither stagnation nor turbulence within the pump, with the result that thrombosis and haemolysis could be reduced. A new impeller pump, developed according to this method, was evaluated as a left ventricular device in four dogs. The bypass flow was controlled at 40–50% of the total flow, each test lasting 6 h. All of the haematological parameters, measured every 2 h, remained within normal range. There was no thrombosis, and coagulation in the pump was avoided by a small dose of heparin to maintain the activated coagulation time (ACT) under 200″ in the experiments.

A Parametric Study of Radial Turbomachinery Blade Design in Three-Dimensional Subsonic Flow

An aerodynamic design method is described and used to implement a parametric study of radial turbomachinery blade design in three-dimensional subsonic flow. Given the impeller hub and shroud, the number of blades and their stacking position, the design method gives the detailed blade shape, flow, and pressure fields that would produce a prescribed tangentially averaged swirl schedule. The results from that study show that decreasing the number of blades increases the blade wrap, and that the blade loading is strongly affected by the rate of change of mean swirl along the mean streamlines. The results also show that the blade shape and the pressure field are rather sensitive to the prescribed mean swirl schedule, which suggests that, by carefully tailoring the swirl schedule, one might be able to control the blade shape and the pressure field and hence secondary flow.

A New Total Heart Design Via Implantable Impeller Pumps

Hemolysis and thrombosis have been considered as the main limitations of the impeller pump and other centrifugal pumps to total heart applications. One of the solutions is to choose the impeller shroud and vane according to the stream surfaces of blood flow, so as to vanish the turbulence and stagnation which cause the hemolysis and thrombosis, respectively, in the pump. This paper describes the method of deducing the stream surfaces together with the velocity distributions in the impeller from the fundamental dynamical equations, and presents a prototype design of the impeller total heart which promises to be an ideal alternative to the problematic diaphragm total heart.

Low haemolysis pulsatile impeller pump: design concepts and experimental results

A pulsatile fully implantable impeller pump with low haemolysis has been produced by developing a pulsatile impeller for a nonpulsatile pump also developed in this laboratory. The impeller was designed according to the 3-dimensional theory of fluid dynamics. The impeller shroud retains the same parabolic form and the vane has a form compacted by a radial logarithmic spiral and an axial helical spiral so that the absolute vibration velocity of the blood in a peripheral direction is a minimum as the impellar changes its speed periodically to generate a physiological pulsatile blood flow. Thus the Reynolds shear and the Newton shear are a minimum for the required pulse pressure. The mean volume and mean pressure are controlled by adjusting the voltage. The shape of the pressure pulse is determined by a square was of vollage and the systole/diastole ratio. In order to abolish regurgitation of the pump, a 40 per cent systole period and a 5 V voltage pulse are desirable for 40 mmHg pulse pressure (80–120 mmHg mean pressure). The pulse frequency has almost no effect on pump output. The pump can delivery 4 l/min mean volume and 100 mmHg mean pressure (40 mmHg pulse pressure), and these conditions result in an index of heamolysis (IH) for porcine blood of 0.020—only slighly more than the nonpulsatile pump (0.016). When the pulsatile impeller was used under nonpulsatile conditions its IH was almost doubled, but when the nonpulsatile impeller was used under pulsatile conditions the IH reached 0.13. The power consumption is approximately equal to that for the nonpulsatile pump: 3 W for 4 l/min and 100 mmHg output. The pump weights 240 g (including 190 g for the motor) and, because a commercial motor has been used, costs only a few hundred US dollars.

Fluid-Structure Interaction Forces at Pump-Impeller-Shroud Surfaces for Rotordynamic Calculations

Governing equations of motion are derived for a bulk-flow model of the leakage path between an impeller shroud and a pump housing. The governing equations consist of a path-momentum, a circumferential-momentum, and a continuity equation. The fluid annulus between the impeller shroud and pump housing is assumed to be circumferentially symmetric when the impeller is centered; i.e., the clearance can vary along the pump axis but does not vary in the circumferential direction. A perturbation expansion of the governing equations in the eccentricity ratio yields a set of zeroth and first-order governing equations. The zeroth-order equations define the leakage rate and the circumferential and path velocity distributions and pressure distributions for a centered impeller position. The first-order equations define the perturbations in the velocity and pressure distributions due to either a radial-displacement perturbation or a tilt perturbation of the impeller. Integration of the perturbed pressure and shear-stress distribution acting on the rotor yields the reaction forces and moments acting on the impeller face. Calculated results yield predictions of possible resonance peaks of the fluid within the annulus formed by the impeller shroud and housing. Centrifugal acceleration terms in the path-momentum equation are the physical origin of these unexpected predictions. For normalized tangential velocities at the inlet to the annulus, uθ0(0) = Uθ0(0)/Riω of 0.5, the phenomenon is relatively minor. As uθ0(0) is increased to 0.7, sharp peaks are predicted. Comparisons for rotordynamic coefficient predictions with test results of Bolleter et al. show reasonable agreement for cross-coupled stiffness and direct damping terms. Calculated results are provided which make comparisons between seal forces and shroud forces for a typical impeller/wear-ring-seal combination.

The Influence of Geometric Parameters on F. C. Centrifugal Fan Noise

In this report, through experiments the influence of five major geometric parameters on noise and aerodynamic performance of forward curved (F.C.) centrifugal fans was studied. The parameters considered are: (1) width-inner diameter ratio of impeller, (2) axial clearance between the fan inlet nozzle and the impeller shroud plate, (3) blade-setting angle, (4) blade pitch-chord ratio, and (5) spiral extension index of the scroll. Noise characteristics were evaluated by means of the specific noise level (A-weighted) at every operating point except the surging region. The optimal values for the parameters to realize low noise are discussed. Design diagrams for low noise F.C. centrifugal fans are shown.

Casing Headloss in Centrifugal Slurry Pumps

The headlosses in slurry pump casings are caused by the energy dissipation in the flowing liquid-solid mixture, and the particle-casing wall interaction. This paper presents a quasi-three-dimensional flow analysis, in two steps: (1) the casing width-averaged parameters are obtained from the two-dimensional flow simulation in a normal plane to the pump axis, (2) the results are then used for flow computation in the casing radial cross-sections. The governing equations are averaged over the typical time interval for the periodic flow in the casing caused by the finite number of impeller blades. Finite element techniques with linear three-node and quadratic nine-node elements are used in the numerical simulation. The effect of the casing shape on the headlosses is illustrated by comparing the performances of three centrifugal slurry pumps with quasi-spiral, semi-annular and annular casings at various flowrates. The flow recirculated between the casing tongue and impeller shrouds is shown to play a major role in the overall headloss. The flow results may also be used for determining the wear in the casing and thrust on the impeller, as well as for optimizing the casing dimensions.

Prediction of Stiffness and Damping Coefficients for Centrifugal Compressor Labyrinth Seals

A theory is presented for the calculation of stiffness and damping coefficients of the fluid-rotor interaction in centrifugal compressor labyrinth seals based on turbulent flow calculations. The theory has been confirmed by measurements on labyrinth test stands and on a centrifugal compressor impeller shroud seal at pressures up to 140 bar. Predicted rotor stability limits based on the theory are in agreement with those observed in real compressors.

A New Device to Control Axial Thrust of Radial Flow Turbomachinery

In high-head and large-sized radial flow turbomachinery some devices, such as balance holes or balance disks, are usually used to balance the axial thrust. But these devices sometimes result in an unbalanced axial force owing to wear or aging of wearing rings. This report puts forward a new device which is quite effective to control and balance the axial thrust with ease and convenience. Even if the above mentioned unbalanced force is caused, it is possible to balance it by this new device even during the operation of a machine. In the former report, it was suggested that it was possible to balance the axial force by controlling the angular momentum of the flow between an impeller shroud and a casing wall. this device uses the phenomenon of angular-momentum defect caused by the wake flow behind several short cylindrical pegs attached onto the casing wall in the rotating flow. The effects of cylindrical pegs on the thrust control are studied theoretically and experimentally.

Effect of Dimensional Parameters of Impellers on Performance Characteristics of a Radial-Inflow Turbine

An experimental radial-inflow turbine having a pressure ratio of 2:1 was constructed, and the effect of dimensional parameters of an impeller was examined by performance tests of the turbine using compressed air. A series of four impellers having different passage areas and another series of three impellers having different mean diameters of exducer exits but with the same exit annular areas were employed to examine the effect. In addition, the effects of side clearance between impeller shroud and casing as well as radial clearance between nozzle exit and impeller inlet were examined.

Dredge-Pump Pressures and Thrust Loads

Abstract The Port of Portland engineering staff, desiring to determine the actual thrust loads and pressures acting on the impeller of a dredge pump, carried on some tests which should be of interest to members of this society. Pressure gages were installed at controlling points in both the front and back heads of the 2700-b.hp. 30-in. pump of the dredge Clackamas and a series of tests run to determine the effect of varying lengths of pipe line, impeller clearances, and impellers with and without relief holes in the back shroud. The results of the tests showed clearly: 1 That relief holes in the back of the impeller materially reduced the thrust load and did not reduce the efficiency of the pump to any noticeable degree. 2 That renewable clearance rings and small vanes on the outside of the impeller shrouds are necessary to maintain close clearances and control the thrust loads. 3 While large clearances produce low thrusts, close clearances do not create a prohibitive thrust and are necessary to maintain a high pump efficiency. 4 That the pressures on the front and back shrouds are not the same and are considerably less than the pump discharge pressure. 5 That the whirlpool pressures are not as high as the discharge pressure of the pump, and that the pressures in the lower half of the pump casing are somewhat greater than those in the upper half. 6 That it is not necessary to carry the same pressure on the water service to the pump-shaft seal as is the discharge pressure of the dredge pump. 7 That as might be expected the thrust load increased with the total head. 8 That the thrust load is greater for any given condition when pumping material rather than water. 9 That the thrust loads are less for any given heads with the smaller high-speed pump impeller as compared with larger slow-speed impellers.