Spatial-temporal Characteristics And Excitation Mechanism Of Bubble Flow Pattern In Self-priming Process Of Centrifugal Pump

Self-priming pumps are widely applied to agriculture irrigation,municipal water treatment,petrochemical transportation and ocean areas,et al.With the increasing technical demands for the self-priming pump,high self-priming performance and low vibration level have to be considered as key improved performance.It is noticed that the self-priming process is a very complicated gas-liquid two-phase flow process,and the internal flow characteristics and unsteady excitation characteristics have not been fully revealed yet.Therefore,in this study,the bubbly flow patterns during the self-priming process were captured by high-speed photography technique.Meanwhile,the vibration levels of pump during self-priming process were recorded by acceleration sensor.The relationship between the bubbly flow patterns and the vibration features was investigated,and some experimental data and technical guidelines were provided for the optimal design of self-priming pumps.The main contents and results are as followed:(1)A transparent self-priming pump was designed and manufactured by acrylic for fulfilling the experimental requirements.A test facility for the gas-liquid two-phase flow characteristics and vibration tests during self-priming process of self-priming pump was constructed.Meanwhile,the hydraulic performance and vibration level of pump in a normal operation mode under different flow rate conditions were measured in a closed loop test system.A high-speed camera was used to capture the bubbly flow patterns during the self-priming process.The vibration acceleration sensors were arranged in the sensitive areas of the centrifugal pump to measure the vibration characteristics of different regions on the centrifugal pump.The spatial and temporal characteristics of the bubbly flow during self-priming process of centrifugal pump and its excitation mechanism were revealed in use of the image processing technology and signal frequency spectrum analysis.(2)The change of liquid level of the inlet and outlet pipes during the self-priming process was observed and analyzed.The self-priming process was divided into three periods,including initial period,middle period and end period according to the change of liquid level in the inlet and outlet pipes.In the initial period,the flow rate and pressure at the outlet of volute gradually increase due to the rotation speed of impeller being fast.The gas enters the pump through the inlet,and the gas-liquid mixture is formed under the interactions between the gas pocket and the blades.The bubble recirculates in the chamber and forms gas pocket at the reflux hole.The gas pocket appears at the front cavity and grows up.In the middle period,a large amount bubbles is generated inside the pump.The gas is inclined to gather at the bottom of the gas-liquid separation chamber and enters impeller through reflux hole.More gas refluxes longer self-priming time is.In the end period,the volume fraction of liquid in pump increases and the gas pocket at the front cavity disappears.Meanwhile,the rate of gas reflux decreases and the rate liquid reflux increases.(3)The variation of vibration level and its excitation mechanism at different measuring points in the self-priming process of pump were analyzed,and the influence of different positions of reflux hole on self-priming performance and its excitation mechanism were explored.The change of vibration level at five measuring points keeps same tendency during the self-priming process.The vibration level at the x-axis direction of the bearing body is the strongest,and the vibration level of the base is the weakest.The vibration level of the whole pump is mainly affected by the motor in the initial period.The vibration level of the y-axis direction of the pump body and the bearing body fluctuates obviously in the middle period due to the shearing effect on the impeller by recirculating fluid from reflux hole and volute.The volume fraction of gas and liquid keeps stable in the end period.The decrease of gas volume fraction makes lower vibration level in the case of the less bubbles break.The impact strength of the jet flow at the outlet of volute is also decreased.It further reduces the vibration level in the y-axis direction of the pump.Furthermore,in the comparison of self-priming time under different positions of reflux hole,it is found that self-priming time is mainly influenced by the position of reflux hole,and the self-priming time of No.4 reflux hole is shortest.In addition,no any impact on the vibration level from position of reflux hole.(4)The vibration characteristics under different flow rate conditions of the centrifugal pump under normal operation were measured.By analyzing the vibration spectrum characteristics of different measuring points,the internal relationship between the vibration characteristics and the flow condition of the pump under normal working condition was established.In comparison with the vibration level during self-priming process,it is found that the vibration level of each measuring point decreased under normal operation condition.Flow rate has the greatest influence on the y-axis vibration of the bearing body,but has little influence on the x-axis vibration of the bearing body.The vibration level of the bearing body and the pump body in the x-axis direction is the largest,which fluctuates and rises with the increase of flow rate.It may be due to the increase of the unstable electromagnetic force between the stator and rotor of the motor under large flow rate conditions.The maximum vibration level of the five measuring points occurs in the frequency range of 0-200 Hz in different flow rate,and the overall vibration level of the pump base is the lowest,which is less affected by the variation of flow rates.The vibration level of the x-axis direction is larger than that of the y-axis,and the vibration level of the pump body is larger than that of the bearing body.Mechanical induced vibration is the main part,and it is mainly caused by misalignment between motor and bearing body.In addition,the vibration level on the pump body is partly caused by the dynamic and static interference between impeller and volute.