| Oil and gas mixed transportation technology has the advantages of high production and transportation efficiency,good flow safety and low investment cost.It is the preferred technology for deep-sea oil and gas production and transportation system.The core device of the oil-gas mixed transportation system is the gas-liquid two phase pump.The semi-open mixed-flow pump can operate safely under high gas volume fraction and large volume flow rate conditions,but there is pressure surging under the condition of gas-liquid two-phase.At this time,the pressurization capacity of the pump decreases significantly,accompanied by severe vibration,and the operation is extremely unstable.Aiming at the problem of operation instability of mixed flow pump,the two-phase operation characteristics of semi-open mixed-flow pump are numerically simulated and tested in this paper.The influence of operating parameters on its pressurization capacity was clarified,and the internal gas-liquid distribution was tested visually,revealing the gas-phase distribution in the impeller and its spatiotemporal evolution law under different working conditions.Finally,the energy loss and unsteady characteristics of pressure surging are analyzed in detail.The main research contents and conclusions are as follows:First,the external characteristic test of the semi-open mixed-flow pump is carried out under different Inlet gas volume fraction(IGVF)and liquid flow rate conditions,and the comprehensive characteristic curve of the pump is obtained.It is found that when the pump operates near the operating point of optimal liquid phase flow rate,its pressurization capacity decreases linearly with the increase of IGVF.Under the condition of small liquid flow rate,the pump pressure increment first decreases slowly with the increase of IGVF,and when the IGVF reaches a certain critical point,the pump performance drops suddenly,and the initial surging occurs.The pressure surging curve of the pump is obtained by the critical gas volume fraction when surging occurs under different liquid flow rates.The lower side of the surging curve is the unstable operation area,in which the pressure increment of the pump decreases with the decrease of liquid flow rate,and the pump stops working soon.At the same time,it is found that increasing the pump rotating speed can increase the gas phase transport capacity of the pump and expand its stable operation area.Second,the population balance model considering the mechanism of bubble breakup and coalensces is used in the numerical simulation of gas-liquid two-phase pump.Compared with the average bubble size model,the results of the external characteristics of the pump and the gas phase distribution predicted by this model are more consistent with the experimental results,and the performance prediction error is less than 5%.Through the analysis of numerical simulation results,it is found that the gas accumulation area in the flow field expands with the increase of IGVF,and the distribution of pressure and relative liquid flow angle in this area is uneven.The jet flow at the blade tip clearance of the semi-open impeller can effectively inhibit the gas phase aggregation in the flow field and slow down the decreasing rate of pressure increment of the pump.Third,The gas phase distribution,leakage vortex distribution and bubble evolution law in the pump under different working conditions,and different time and space are studied based on the visualization test.With the increase of IGVF and the decrease of liquid flow rate,the flow pattern in the flow field has experienced four stages:bubbly flow(BF),aggregated bubbly flow(ABF),gas pocket flow(GPF)and segregated flow(SF).The pressurization capacity of the pump has experienced three stages:slow decline,sudden decline and slow decline.When the flow pattern in the impeller changes from BF to GPF,the pump performance drops suddenly and the initial surging occurs.Under each working conditions,the bubbles in the bladeless area at the impeller inlet are evenly distributed,mainly in BF or ABF.Under the condition of small liquid flow rate,with the increase of IGVF,the GPF or SF is easy to form in the front half of the impeller channel,and the bubbles in the rear half of the impeller channel are more likely to break into smaller bubbles to form BF or ABF.Under the condition of small liquid flow rate and high IGVF,the gas column in the impeller flow channel is affected by the leakage flow at the tip clearance of the impeller,and shows an obvious dynamic cyclic evolution process of initiation,expansion,separation from blade surface,deformation and break up along the chord length of the blade..Finally,the input work,effective work and power loss in different areas of the impeller under pressure surging condition are analyzed based on the local-regional power loss analysis method.It is clarified that the reason for the large energy loss in the area of the blade pressure surface close to the hub is caused by gas phase accumulation.Through the unsteady numerical simulation of the pressure surging condition,the generation mechanism of the phenomena such as the increase of the radial force amplitude of the impeller,the eccentricity of the radial force trajectory and many low-frequency pulsation occurs in the flow field under the pressure surging condition is revealed. |
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