| Axial thrust is one of the most important factors affecting the service life and stable operation of the centrifugal pump.Due to the incomplete research on the fluid flow mechanism in the cavity of th.e impeller cover,the axial thrust of the centrifugal pump cannot be accurately calculated and balanced.It has become an important research topic in the pump industry.In the method of balancing axial thrust of a centrifugal pump,the number of balan.ce holes on the rear cover is closely related to axial thrust.Therefore,I selected a closed impeller centrifugal pump with a double sealing ring and a semi-open impeller centrifugal pump with a high-speed inducer as the research objects,The influence of flow conditions and balance holes on the flow mechanism and axial thrust characteristics was studied on the back cavity of the centri.fugal pump.In the centrifugal pump with the balance hole and the sealing ring,I analyzed the back cavity flow mechanism and the axial thrust characteristics on the flow conditions from 0.6Qsp to 1.2Qsp and the balance holes diameter from 0 to 12 mm of the design point The main causes of flow distribution were pointed out in the cavity.The ratio of the cavity liquid rotational angular velocity to th.e impeller angular velocity was found.The disc friction loss and the leakage flow loss were calculated.The distribution of the pressure and the axial thrust characteristics,were revealed in the cavity’ The.main phenomenon is as follows.1.At the same flow point,the flow is mainly restricted by the section area and the main flow of the volute in the back shroud cavity of the centrifugal pump.From the volute to the sealing ring,the radial leakage flow exists in the 0° and 90°of the back shroud cavity.The radial leakage flow of rear sealing ring gap and the axial leakage flow of balance hol,es are the main reasons for the change of flow characteristics in the hub cavity.As the flow rate increases,the values of disc friction loss tend to be equal,the radial leakage rate reduces,and the radial pressure difference becomes smaller between volute and sealing ring in the back shroud cavity.Meanwhile,the rotational angular velocity rises,and the disc friction loss,the axial thrust of balance cavity accounted for the proportion of the total axial thrust,the axial thrust of the closed impeller in the balance cavity all turns to smaller.2.The larger the diameter of the balance hole is,the larger the rotational angular velocity of the liquid in the back shroud cavity and the hub cavity will be,and the radial leakage velocity of the turbulent boundary layer nearby the pump cover increases,while the axial leakage velocity of the balance hole drops.When there is no balance hole,the rotational angular velocity of the hub cavity is about 0.4 times in the impeller.When there are some balance holes,the rotational angular velocity of the impeller is larger than 0.4 times in the hub cavity.When the balance holes diameter is larger than the design value,the disc friction loss of the cavity almost remains the same,the leakage is about 2%of the design flow,the plate force of the hub cavity reaches the minimum,and the axial thrust of the closed impeller remains stable.Compared with the no balance holes,the total axial thrust can be offset by 70%.On the flow conditions from 0.8Qt to 12Qt and the number of balance holes from 0 to 8 at design flow rate,the research is carried out in the centrifugal pump with a high-speed inducer The circumferential and radial velocity components of the flow rule is analyzed in the cavity of a semi-open impeller.The influence of the flow condition and the balance holes on the friction loss,the leakage flow loss,the pressure distribution,and the axial thrust characteristics are discussed in the cavity.The results show that:3.The flow of the cavity is mainly restricted by the main flow of the volute in the centrifugal pump.The eddy is mainly concentrated in the 900 near the impeller cover and the pump cover in the pump cavity.When the flow rate is larger,the eddy area is larger near the impeller cover,while the eddy area is smaller near the pump cover,and the radial movement from the hub to the volute.As flow rate increases,the flow direction of the same angle mainstream flow remains the same in the pump cavity,the range of the circumferential and the radial velocities are increased,rotation angular velocity is increased,and the disc friction loss and the pressure is decreased in the pump cavity.4.When the number of balance holes increases,the mainstream flow direction is the same in the pump cavity of centrifugal pump,the velocity showed a rising trend and the maximum can reach near the 900 of hub,the pressure is reduced,the leakage eddy structure and position always remain the same near the 900 of volute in the pump cavity.On the same number of balance holes,the radial leakage flow is mainly at 0° and 270° from the volute to the hub in the pump cavity.The pressure of 900 gradually increases from the hub to the volute,just like a parabolic distribution.The pressure has the greatest value near 0°of the volute.There are the right amounts of balance holes on the semi-open impeller that can increase liquid rotational angular velocity,reduce the disc friction loss and the pressure,and decrease the pressure difference between pump cavity and impeller passage.It functions as balancing the axial thrust in the pump cavity.Compared with the no balance holes,the total axial thrust can be offset by 26%.Combining the flow mechanism and the axial thrust characteristics of two centrifugal pump impellers,the distribution rule of flow was found in the cavity.The main influence factors are obtained in the axial thrust of centrifugal pump.A simple test method is presented to measure the axial thrust of the cavity and centrifugal pump.The main results are as follows:5.The flow characteristics are the two-dimensional viscous laminar flow,which is composed of the circumferential shear flow and the radial pressure difference flow in the back cavity of the centrifugal pump.The rotational angular velocity of the liquid in the cavity is not equal to 0.5 times as the rotational angular velocity of the impeller When the balance holes exist,the back cavity of the impeller is affected by the radial leakage flow of the turbulent boundary layer and the axial leakage flow of the balance holes.The flow velocity is not axisymmetric along the circumferential or the radial in the cavity.The direction of the leakage flow in the balance holes is always the axial flow from the cavity to the impeller passage.When the flow rate is larger,the disc friction loss near the hub cavity and the axial thrust of the centrifugal pump become smaller When the balance hole area increases,the disc friction loss,the leakage flow loss and the axial thrust will be reduced first and then remain unchanged in the centrifugal pump.At the same radial position,the balance holes area is larger than 2.4%of the area on the impeller wall of the cavity,the two impellers can achieve the optimal balance effect of the axial thrust.The double sealing ring structure of closed impeller is more effective than that of the semi-open impeller without sealing ring.The main influence factor of the axial thrust is the impeller cover force near the hub cavity.In the measurement experiment of the cavity pressure and the axial thrust only measuring the radial center pressure along the cavity of 1800 is needed,and the axial thrust can be approximately obtained in the centrifugal pump.The assumptions are inconsistent with facts on the theoretical formulas of the axial thrust and the disc friction loss in the centrifugal pump.The theoretical formulas are failed to consider the influence of the flow rate,the diameter and the number of the balance holes.These formulas need to be further developed. |
