Semi-open Impeller Research Articles

Calculation of axial gasodynamic forces, disk friction losses and overflow in semiopen impellers of centrifugal compressors

The methods for calculating axial forces, disk friction losses and overflow are developed on the basis of experimental data on the radial pressure distribution at both sides of the semi-open impeller, disk friction and overflow through clearances between the blades and the case.

20316 ステップ動圧軸受の最適化による動圧浮上遠心ポンプの血液適合性改善(人工臓器と医用工学(4),OS.8 人工臓器と医用工学)

We have developed a hydrodynamic levitation centrifugal blood pump with a semi-open impeller for a mechanically circulatory assist. The impeller is levitated with original hydrodynamic bearings without any complicated control and sensors. In this paper, in order to improve the hemocompatibility of the developed blood pump, we studied the hydrodynamic step bearing using the numerical analysis based on Reynolds equation and the measurement test of the hydrodynamic force. As a result, we found that the step bearing having the step length of 66% of the vane generated the maximum hydrodynamic force, and was able to improve the hemocompatibility.

Cavitation in semi-open centrifugal impellers for a miniature pump

Cavitation in miniature pumps was investigated experimentally for two semi-open centrifugal impellers. Although both impellers had the same blade cross-section, one impeller had a two-dimensional blade, while the other had a leaned blade. The flows were also analyzed using a numerical model of the three-dimensional turbulent flow in the pumps near the peak efficiency point using the k-ɛ turbulence model and the VOF cavitation model. The average cavitation performance of each impeller was satisfactorily predicted by the numerical simulations. The results show that the miniature pumps have similar cavitation performances as an ordinary-size pump, with the cavitation performance of the semi-open impeller reduced by increased axial tip clearances. Also, both the hydraulic and cavitation performance of the semi-open impeller were improved by the leaned blade. The results also show that uniform flow upstream of the impeller inlet will improve the cavitation performance of a miniature pump.

PTV 계측법에 의한 극저비속도 원심펌프의 내부유동특성에 관한 연구

In the range of very low specific speed (nSUBs/SUB＜0.25, non-dimensional), the perfomance of a centrifugal pump is much different from that of a centrifugal pump of normal nSUBs/SUB and the efficiency of the pump drops rapidly with the decrease of nSUBs/SUB. In order to examine the reason of unstable performance characteristics of the very low nSUBs/SUB centrifugal pump, the internal flow of the pump with a semi-open impeller is measured by a PTV(Particle Tracking Velocimetry) system. The purpose of this study is to make clear the internal flow characteristics and to obtain basic knowledge of the pump perfornance. The results show that the leakage flow through tip clearance give a strong effect on the flow pattern of impeller passage. A large vortex in the impeller passage and a strong reverse flow at impeller outlet are formed in the range of small flow rates, and the vortex and the reverse flow together reduce the absolute tengential velocity at the impeller outlet and cause the performance instability.

Effects of tip clearance and impeller geometry on the performance of semi-open ceramic centrifugal fan impellers at elevated temperatures

Centrifugal fans are being widely used in many industrial applications. However, when handling gases with temperatures exceeding 800 °C, the use of centrifugal fan impellers are of particular interest since the conventional steel impellers would not be operated at such elevated temperatures. In the present experimental study, three semi-open centrifugal fan impellers have been designed and fabricated using ceramic materials to provide high resistance to temperature. Experiments have been conducted to investigate the performance characteristics of these impellers and the deteriorations in their performance due to varying tip clearance. Factors have been determined to estimate the tip clearance losses. Results showed that the simple impeller geometries of ceramic materials were less sensitive to the varying tip clearance. In addition, the gas temperature has been found to have almost no influence on the performance degradation due to the tip leakage flow.

Performance and Internal Flow Characteristics of a Very Low Specific Speed Centrifugal Pump

In very low specific speed range (ns<0.25), the efficiency of the centrifugal pump designed by the conventional method becomes remarkably low. Therefore, positive-displacement pumps have been widely used for long. However, the positive-displacement pumps remain associated with problems such as noise and vibration and they require high manufacturing precision. Since the recently used centrifugal pumps are becoming higher in rotational speed and smaller in size, there appear to be many expectations to develop a new centrifugal pump with high performance in the very low specific speed range. The purpose of this study is to investigate the internal flow characteristics and its influence on the performance of a very low specific speed centrifugal pump. The results show that large reverse flow at the semi-open impeller outlet decreases absolute tangential velocity considerably which in turn decreases the pumping head.

극저비속도 원심펌프의 성능과 내부유동특성에 관한 연구

In the very low specific speed range (N_s < 0.25, non-dimensional), the efficiency of centrifugal pump designed by a conventional method is very low in common. Therefore, positive-displacement pumps have long been used widely. Recently, since the centrifugal pumps are becoming higher in rotational speed and smaller in size, there expects to develop a new centrifugal pump with a higher performance to replace the positive-displacement pumps. The purpose of this study is to investigate the internal flow characteristics of a very low specific speed centrifugal pump and to examine the effect of internal flow pattern on pump performance. The results show that the theoretical head definition of semi-open impeller should be revised by the consideration of high slip factor in the semi-open impeller, and the leakage flow through the tip clearance results in a large effect on the impeller internal flow. Strong reverse flow at the outlet of semi-open impeller reduces the absolute tangential velocity considerably, and the decreased absolute tangential velocity increasese the slip factor with the reduction of theoretical head.

PIV를 이용한 극저비속도 세미오픈임펠러의 내부유동 계측

Internal flow measurement of very low specific-speed semi-open impellers has been carried out by PIV in order to understand better the internal flow patterns that are responsible for the unique performance of these centrifugal pumps operating in the range of very low specific speed. Two types of impellers, one equipped with six radial blades(Impeller A) and the other with four conventional backward-swept blades(Impeller B), are tested in a centrifugal pump operating at a non-dimensional specific-speed of n_s=O.24. Complex flow patterns captured by PIV are discussed in conjunction with the overall pump performance measured separately. It is revealed that Impeller A achieves higher effective head than Impeller B even though the flow patterns in Impeller A are more complex, exhibiting secondary flows and reverse flows in the impeller passage. It is shown that both the localized strong outward flow at the pressure side of each blade outlet and the strong outward through-flow along the suction side of each blade are responsible for the better head perfornance of Impeller A.

Numerical simulation of flow of pulp fiber suspensions in stock pump with semi-open impeller

The flow of pulp fiber suspensions with a consistency below 5% through a pump falls into the turbulent region. The 2-phase 3D turbulent flow of the mixture is simulated in the hydraulic components composed by suction chamber, front board, semi-impeller and volute casing for the first time.The pseudo-fluid model and the k-e turbulent model are modified. Phase coupled SIMPLE algorithm is applied to make pressure-velocity correction equation. Based on the results of the numerical simulation, the distribution of velocity, pressure and density of liquid and particle phase are compared and analyzed. The flow analysis in the tip between front board and impeller is emphasized, which reflects the advantage of semi-open impeller when it transports.2-phase medium. This calculation result will be helpful for the design of pump impeller and the establishment of design method of centrifugal stock pump.

PIV measurement of internal flow characteristics of very low specific speed semi-open impeller

Detailed particle-image velocimetry (PIV) measurements of flow fields inside semi-open impellers have been performed to understand better the internal flow patterns that are responsible for the unique performance of these centrifugal pumps operated in the range of very low specific speed. Two impellers, one equipped with six radial blades (impeller A) and the other with four conventional backward-swept blades (impeller B), are tested in a centrifugal pump designed to be operated at a non-dimensional specific speed of ns=0.24. Complex flow patterns captured by PIV are discussed in conjunction with the overall pump performance measured separately. It is revealed that impeller A achieves higher effective head than impeller B even though the flow patterns in impeller A are more complex, exhibiting secondary flows and reverse flows in the impeller passage. It is shown that both the localized strong outward flow at the pressure side of each blade outlet and the strong outward through-flow along the suction side of each blade are responsible for the better head performance of impeller A.

ANALYSIS OF THE FLUID DYNAMIC FORCES ON THE CENTRIFUGAL PUMP IMPELLER

The radial thrust acting on the impeller of a centrifugal blood pumps is to be minimized in order to enhance the durability of the suspending mechanism, including pivots and magnetic bearings. However, it is a challenging task because 1) the pump flow rates fluctuate instantaneously by beatings (if the natural heart and gradually by variations of the systemic- resistance, and because 2) the surrounding space inside the casing ate usually too small to have uniform velocity distribution. This study deals with computational analysis about the radial fluid force acting on the impeller. We developed the computational model of the pump with the semi-open impeller that the authors designed and conducted flow visualization studies. The pump was originally fabricated as the double scale model. The numerical simulation involved the whole pump area me incorporated a turbulent model (SST k-omega model). The computations of unsteady, three-dimensional flows were conducted using the commercial software. Computed flows were reasonable in comparison with the results of the performance testing and the fluid visualization. The computed radial thrust at 500 pm and 3.4L/mm ranged from 0.1 53N to 0.194N, fluctuated in synchronization with the blade passing frequency. The peak radial thrust was observed when the position of the blade tip coincided with the casing tongue point. In conclusion, the radial thrust acting on the impeller surface was quantitatively estimated by numerical computation, indicating thrust fluctuation occurs at the blade passing frequency.

1301 小型低比速度ポンプの高効率・高信頼性に関する研究

It is well known that centrifugal pump efficiency becomes worse with the decrease of pump size and specific speed. The small-sized pump with low specific speed, in which the impeller outlet velocity was larger, was designed by an optimum design method combined with blade surface singularity method The results show as follows : The semi-open impeller designed by the optimum design method, yielded the highest efficiency of 61% among test impellers in narrow tip clearance c/b_2. In larger c/b_2,however, the leakage loss becomes dominant and the efficiency of impeller with larger blade outlet angle is higher. This fact implies that the c/b_2 should be considered as an additional factor for optimum design.

Improvement of washout flow in a centrifugal blood pump by a semi-open impeller.

To reduce the possible thrombogenicity of the pump studied, pump characteristics and washout conditions were compared between a pump with a semi-open and a pump with a full-open impeller. A difference in hydrodynamic performance was observed between the semi-open impeller and the full-open impeller; the pressure in the former was less by approximately 10%, and the maximum attainable efficiency decreased from 0.41 to 0.34. The flow pattern, as visualized by the oil film method, showed that the washout condition was enhanced by addition of the shroud, especially at the bottom region of the pump where the blood flow tended to be stagnant. The stagnant area was observed in the suction side of the impeller in both models, where the vortices shed from the impeller tip contributed to the washout. It was also shown that the flow entering the bottom region was circumferentially uniform in the full-open impeller, whereas in the semi-open impeller the flow was not uniform and entered primarily from the vicinity of the outlet port. The semi-open impeller, thus, was demonstrated to have better washout conditions than the full-open impeller regardless of a slight decrease in hydrodynamic efficiency.

Internal Flow in a Centrifugal Pump of a Low Specific Speed with Semi-Open Impeller.

The internal flow in a centrifugal pump with semi-open impeller is visualized and measured by PTV method. In this pump, the design type number is 0.244 of low specific speed. The experimental results of two impellers whose outlet angles are 30 and 90 degree are compared. At the outlet of impeller, most of the quantity of fluid flows out along the pressure side of the vane, but some climbs over the top of the vane and flows into the suction side through the tip clearance. When the total quantity is changed, the meridian velocity distribution changes only near the tongue area, and in other area the meridian velocity is almost same. When the quantity becomes small, large vortices are produced between vanes, and this vortex reduces the tangential velocity at the outlet of impeller. This causes the reduce of theoretical head and instability in the performance curve.

411 油膜法によるセミオープンミニ羽根車内二次流れの観察

411 油膜法によるセミオープンミニ羽根車内二次流れの観察

Impeller Geometry Suitable for Mini Turbo-Pump

The objective of the present study is to investigate the suitable impeller geometry for a mini turbo-pump, which is defined as the size having impeller diameter between around 5 mm and 50 mm. This is treated because those pumps having the above size are regarded as low efficiency machines if efficiency is less than 40 percent or 50 percent. Considering that not only low Reynolds number and tip clearance but the design method are the major causes for low performance, we carried out the performance test experimentally using two semi-open centrifugal impellers: one is named as Impeller B of 36 mm diameter and the other is Impeller C of 34 mm diameter. And the former is designed by a conventional method and the latter is based on the proposed method. In the performance tests, rotational speed was varied between 3000 rpm and 10,000 rpm and the axial clearance at the blade tip of impeller exit was between 0.2 mm and 0.8 mm. It is clearly seen from the results that Impeller C gives the better hydraulic performance. It is also clarified that the effect of tip clearance on Impeller C performance is much smaller than that of the Impeller B. Further, we conducted numerical calculation of impeller performance, where the commercial CFD code named TASCflow was used with k-ω turbulence model. From the results, the turbulent flow analysis is reasonably usable to study the flow in the above mini impellers.

Passive Control of Rotating Stall in a Parallel-Wall Vaned Diffuser by J-Grooves

In order to control and suppress instabilities caused by swirl flow, the authors have proposed a very simple passive method utilizing shallow grooves mounted on a casing wall or diffuser wall(s) parallel to the pressure gradient. The groove is termed a “J-groove.” The method is theoretically analyzed and experimentally proved capable of suppressing rotating stall in a vaneless diffuser. The performance curve instability is characterized by the positive slope of the head-capacity curve of a mixed flow pump for the entire flow range. In continuation, this work is aimed at realizing experimentally the effect of J-grooves on suppressing rotating stall in the vaned diffuser of a centrifugal turbomachine. Thirteen double curvature vanes with various radial positions, various setting angles, and J-grooves of different dimensions are tested in a parallel wall vaned diffuser with a semi-open radial impeller with and without J-grooves. The results show that J-grooves can also suppress rotating stall in the vaned diffuser for the entire flow range.

1114 極低比速度ポンプの性能向上に関する研究 : クローズド羽根車とセミオープン羽根車の比較

1114 極低比速度ポンプの性能向上に関する研究 : クローズド羽根車とセミオープン羽根車の比較

Fluid Dynamic Characteristics of Monopivot Magnetic Suspension Blood Pumps

A monopivot magnetic suspension blood pump is a centrifugal pump under development with a magnetic suspension and a ceramic pivot to support the impeller with minimum contact. The pump size has been reduced by implementing a direct impeller drive mechanism in place of a magnetic coupling and motor. Flow visualization studies revealed that high shear, which seems to be closely related to hemolysis, concentrates in boundary layers near the walls. This implies that fluid dynamic shear can be reduced not by widening the gap, but by reducing the impeller velocity. Therefore, compared with the results of the previous semi-open curved vane impeller model, impeller velocity was reduced by 30% with a closed impeller having radial straight vanes, and smaller impeller/housing gaps. The volute shape around the impeller tip was also changed such that the outflow from the impeller enters along the center plane of the volute. To examine the effect of the improvements, hemolysis testing was conducted and found that the newly developed closed impeller model generated a lower level of hemolysis than the previous semi-open impeller model.

Design of a centrifugal blood pump with magnetic suspension.

A new concept blood pump, whose impeller is suspended by permanent magnets and a mechanical pivot without seals or ball bearings, is presented in this paper. The primary aim of the blood pump is an application to implantable artificial hearts. The prototype model is of a centrifugal type with a four-vaned semiopen impeller 50 mm in diameter. Since this mechanism has no seals or ball bearings, flow stagnation or heat generation that might cause blood cell denaturation is expected to be small. The results of performance testing for the prototype model 2 were satisfactory regarding pump head and efficiency. The radial-suspension magnets and the magnetic coupling were stable. As a result, the present mechanism has been verified to be a candidate applicable to implantable artificial hearts.