Compared with the periodic unsteady flow generated by the single-stage centrifugal pump, the internal flow state of the multistage centrifugal pump is more complex and the flow is more turbulent. In this paper, the startup process is divided into two stages: valve closing transition and valve opening transition, the transient working process is numerically simulated, and the pump performance and the evolution characteristics of the internal flow field are analyzed. Research has shown that during the startup process of the valve closing transition of the multistage centrifugal pump, when the impeller does the same work, compared with the steady state at the same speed, the mechanical energy in the internal flow field is converted into more kinetic energy and the pressure decreases. Compared with the steady state at the same flow rate, at the initial stage of the valve opening process, the stall group in the flow channel of the impeller during the transient process is larger and the number of stall groups is larger, which enhances the rotational stall flow inside the impeller and makes the flow more turbulent. As a result, the amplitude of the hump oscillations increases compared with the steady state of the same flow rate. The study of transient characteristics of low-frequency multistage centrifugal pumps under starting conditions is of great importance for the safety and reliability of nuclear power equipment and systems.
Multistage centrifugal pumps require frequent startup due to the change of operating conditions. During the startup process, the pump speed increases rapidly from zero to thousands of revolutions. The flow state inside the pump changes from laminar to strong turbulent. and the static interaction between the impeller and the pressure water chamber. Due to the periodic unsteady flow generated by the pump, the flow inside the pump exhibits abnormal transient characteristics. In this paper, the CAP1400 capacitive water pump (CVS), which is a 13-stage low-speed centrifugal pump, is selected as the research object. The capacitive water make-up pump is an extremely important class D pump in nuclear power plants and the key equipment to ensure the safe and reliable operation of nuclear power plants.
In the operation of multi-stage centrifugal pump, frequent starting is required due to the change of working conditions. The rapid increase of rotation speed and flow rate during the starting process will cause great changes in pressure, velocity, radial force and other parameters. flow field in a short period of time. At present, the research on the starting process of centrifugal pumps generally uses the quasi-steady-state assumption method instead of the transient starting process (Wu et al., 2009). However, due to the obvious transient effects in the starting process, the numerical simulation is inaccurate. With the expansion of the starting process of centrifugal pumps in various application fields and the increase of system complexity in recent years, it has attracted more and more attention from scholars.
Tsukamoto and Ohashi (1982) and Tsukamoto et al. (1995) considered that the pulsating pressure during the starting process of small centrifugal pumps with low specific speed and the flow hysteresis around the blades are the main reasons for the dimensionless pressure curve to be lower than the pressure curve in the quasi-steady state, and the impeller speed oscillation frequency is directly related to the transient effect of the pressure curve. Thanapandi and Prasad (1995) used the future analysis method to establish a numerical model of the screw pump with different orifices, analyzed the transient dynamic characteristics of the pump, and confirmed through experiments: they found that the dynamic characteristics of the pump deviate greatly from its steady-state characteristics. Xu Jianjun et al. (2010), Li Jianjun et al. (2010), and Li Jianjun (2009) established a complete system model including circulation pipes and pumps. In the numerical simulation of the fast starting process of a centrifugal pump using the sliding mesh method, the evolution of the unsteady flow inside the centrifugal pump under transient conditions was analyzed. Ping Zhiqiang et al. (2007) established the basic equation for describing the fast starting of centrifugal pumps. Taking a mixed-flow fast starting pump as an example, they carried out theoretical analysis and numerical calculations of the transient processes of the flow field under the fast transient state and found that the initial characteristics of the pump at startup are significantly different from those of the quasi-steady process. Wang Zhiqiang et al. (2017) studied the transient characteristics of an ultra-low specific speed centrifugal pump during the starting process under valve closing conditions by comparing with the quasi-steady process and found that the size of the starting acceleration has an important effect on the final transient shock pressure of valve closing. At the beginning of the starting process, the difference in static pressure distribution between the two is the largest. As the rotating speed increases, the difference between them gradually decreases.
At present, domestic and foreign scientists have a deep understanding of the transient characteristics of the starting process of centrifugal pumps, but most of the objects of research on the transient characteristics of the starting process are limited to single-stage centrifugal pumps. Compared with single-stage centrifugal pumps, the complex sequential structure of multi-stage centrifugal pumps makes the internal flow more complex (Zhang Jianjun et al., 2012; Liu Jianjun et al., 2014; Wang Jianjun et al., 2017). and due to the transfer of flow states between stages, the starting process. The mid-transient internal flow characteristics will be more turbulent.
CAP1400 is a new third-generation reactor, and it is necessary to study the performance of many kinds of equipment, especially pumping equipment, which is critical to safety. The research objective and innovation of this paper is to identify the transient response of new nuclear power pumps by existing methods, so as to provide a basis for the safety design of nuclear power plants. Therefore, based on the previous work, this paper further studies the transient response of multistage centrifugal pumps by numerical simulation methods. Conducting research on the transient response of low-frequency multistage centrifugal pumps under start-up conditions is of great significance to the safety and reliability of nuclear power equipment and systems.
Zhang Jianguo (2013) derived a generalized Euler equation suitable for the transient starting process of a centrifugal pump based on the angular momentum theorem and the energy conservation law, as shown in Equation (1).
In the formula, Vu1 and Vu2 are the circumferential components of the absolute velocity of the fluid particles at the leading and trailing edges of the impeller, respectively, u1 and u2 are the circumferential velocities of the fluid particles at the leading and trailing edges of the impeller, respectively, ω is the instantaneous angular velocity of the impeller, Q is the instantaneous volumetric flow rate, and D is the nominal diameter of the impeller. For a radial centrifugal impeller, D=D2. ΩJ and ΩM are two different blade influence coefficients, their dimensions are related to the geometric parameters of the impeller, such as the shape and thickness of the blades.
From the formula, it can be seen that the lift of the centrifugal pump during the transient process of starting is mainly composed of two parts. One part is the stable lift of the centrifugal pump at the corresponding speed during the starting process, as shown in the figure. the first term on the right side of the formula; the other part is the lifting force during the starting process and the additional pressure of the flow inertia, as shown in the second and third terms on the right side of the formula, the latter is the main cause of the transient effect in the starting process of the centrifugal pump.
The starting process of a centrifugal pump is a necessary process for the normal operation of a pumping system. This process generally refers to the stage where the flow rate gradually increases from zero to the rated flow rate (Li Jianjun, 2012). In order to prevent the starting power of a multi-stage centrifugal pump from being overloaded, the starting process generally includes two processes: valve closing starting and valve opening control. This paper adopts full voltage starting and the motor speed changes linearly. As shown in Figure 1, the valve closing transition stage is divided into two stages: the first stage is the impeller acceleration stage, and the second stage is the valve closing stage. Under the condition of the closed valve dead center flow, the impeller speed quickly increases from zero to the rated speed, and then the multi-stage centrifugal pump runs in the critical dead center state with a stable speed. In the valve closing starting process, under the conditions of meeting the minimum torque and maximum flow rate, full pressure starting is preferable. The third and fourth stages are the valve opening transition stages. In the third stage, after the impeller reaches the rated speed, the valve opens according to the set rule, and the flow rate gradually increases from zero to the rated flow rate. The fourth stage is the stable operation stage. In the transient stage of valve opening, since the change of boundary conditions during the starting process is unclear, this paper will use Flowmaster and CFX software for simulation respectively. First, the Flowmaster software is used to establish a working model of the multi-stage centrifugal pump and obtain the parameters of the corresponding boundary conditions. Then, the boundary conditions are input into the CFX software to carry out numerical simulation of the valve. opening stage.
The main design parameters of the multistage centrifugal pump selected in this article are: flow rate Qd=34.1 m3/h, pressure H=1800 m, rotation speed n=2985 rpm, ns=26.3. The main geometric parameters of the flow parts of the model pump are given in Table 1.
Using the commercial software Cero, a three-dimensional model of the flow part of the multi-stage centrifugal pump was created, including the suction chamber, impellers of all stages, radial vanes and the pump casing. Compared with the guide vanes of other stages, the guide vane of the last stage has no anti-guide vanes, and the outlet of the guide vane of the last stage is in direct contact with the flow field of the pump casing. The flow field model of the multi-stage centrifugal pump is shown in Figure 2. Considering that the internal fluid flow state changes greatly during the startup process, the impeller is divided into a structured hexagonal mesh, as shown in Figure 3.
Figure 2. Liquid region of a multistage centrifugal pump. (A) Suction chamber, (B) pump casing, (C) end guide vane, (D) impeller, (F) radial guide vane, (G) integral flow region of a multistage centrifugal pump.
The radial guide vanes, suction chamber and pump casing are divided into unstructured tetrahedral meshes, which are easily adaptable and allow the creation of meshes of complex structures, guaranteeing a mesh quality higher than 3.0. In the early stage of the study, the network independence was verified for a single-stage pump. As shown in Figure 3A, when the total number of grates was 1,584,810, the head change was <0.5%. Finally, it was found that the number of cells in the suction chamber, pump casing and impeller were 570,325, 107,906 and 326,751, respectively. The number of final radial guide vanes was 144,854, and the total number of grates was 13,763,729.
This paper completes the numerical calculation of the startup transient simulation based on ANSYS CFX 18.1. The turbulence model adopts the shear stress transport (SST) model, using the steady-state flow field results at zero velocity and zero flow rate as the input file. The interface between the impeller and the radial vane is set to the stator-rotor mode with frozen rotor, the wall roughness is set to 0.125 mm, and the wall boundary condition is set to no-slip wall. The inlet boundary condition is set to the total pressure inlet, and the reference pressure is set to 1 atm. The outlet boundary condition is set to the mass flow rate outlet. The turbulence intensity is medium (Intensity = 5%). When the closing valve is at the closing dead center at startup, the flow rate can be considered as 0. But in fact, the internal flow of the multistage centrifugal pump is still circulating at a small flow rate. The flow rate can be regarded as a constant value during the entire process of closing and starting the valve (Shao Jianjun, 2016). The rate is 1% to 5% of the design flow rate of the pump. In this paper, the mass flow rate is taken as 0.01 kg/s. In the transient stage of valve opening, the boundary condition of the flow rate is the flow parameter change value obtained by Flowmaster simulation.
In this paper, the total calculation time of the valve closing transition stage is set to 2.5 s, the total calculation time of the valve opening transition stage is set to 3.8 s, and the time step is 0.002 s. In order to ensure absolute convergence at each time step, the maximum number of iterations in a time step is set to 50, and the convergence residual value is set to 0.0001.
The rated speed is 2985 rpm, and the total calculation time is 2.5 s. During 0 ≤ t ≤ 2.1 s, the speed is uniformly accelerated to the rated speed of 2985 rpm. During 2.1 s ≤ t ≤ 2.5 s, the speed is stable at 2985 rpm.
The formula for calculating the velocity expression function at the transition stage of valve closing is shown in equation (3).
During the transition phase of valve opening, the rotation speed nt remains constant, that is, nt=-2.985[rpm-1].
In equation (3), Ttol=2.1[s], that is, the startup time is 2.1 s. The Step() function is a function of the CFX itself, and its expression value is shown in equation (4).
The calculation formula, written as a function of the efficiency expression η, is shown in equation (9).
In the process of starting the valve closing transition section, the rotation speed increases linearly with the starting time. In order to study the relationship between the pump characteristics and the rotation speed changes during the starting process of the multistage centrifugal pump, this paper uses the rotor rotation period of the user-defined function f to describe the valve closing transition. The change pattern of the pump characteristics during the starting process. The rotor rotation period f represents the time required for each revolution of the impeller. The calculation formula is shown in Equation (). 10).
Figure 4 shows the change in the characteristics of a multistage centrifugal pump depending on the start-up time during the valve closing transient process. In order to make the nature of the change in the function f more obvious, the range of values of the ordinates of the function tf in Figure 4 uses natural logarithmic interpolation.
It can be seen from Figure 4 that at the initial stage of valve closing of stage I 0≤t≤0.25s, the rotor rotation period quickly decreases from 25.2 to 0.169s, and the pressure head and power remain almost unchanged. In this time range, the multistage centrifugal pump has just started up, and the liquid is in a steady state before starting up. Although the impeller starts to rotate, doing work on the flow, the liquid still maintains its original state characteristics under the action of liquid inertia force. At level II, when the starting process develops to 0.25≤t≤2.1s at the end of starting up, the rotor rotation period slowly decreases to a linear decreasing trend, and the pressure head and power slowly increase in the early stage of valve closing, and as the starting time increases, the rotor rotation period gradually increases until it begins to have a linear increasing trend (Zhang et al., 2019a). It is obvious that the change of pressure and power is closely related to the change of the rotor rotation period. In this time range, with the development of the valve closing transition starting process, the rotation speed increases, the working power of the liquid increases, the centrifugal force of liquid rotation increases, the ability to overcome the inertial force of the liquid increases, and the pressure and power begin to increase linearly. At the end of the valve closing transition process 2.1≤t≤2.5s, the rotor rotation period remains unchanged and is 0.0201s. At the end of the starting process, the liquid still maintains the state of rotation acceleration, and the pressure and power will continue to increase for a short period of time. As time increases, the pressure and power gradually show periodic fluctuations. During the entire closing valve transition starting process, the liquid has obvious transient flow characteristics.
In order to study the transient effect of the multistage centrifugal pump during the startup process of the valve closing transition section, the flow field in the steady state with the same speed at the corresponding time was simulated and compared with the calculation results of the transient processes of the startup process in the valve closing transition section. In this paper, 5 rotation speeds are adopted: 600, 1200, 1800, 2400 and 2985 rpm, and the corresponding startup processes of the valve closing transition section are 0.42, 0.84, 1.27, 1.69 and 2.1 s, respectively. Figure 5 is a comparison chart of the pump characteristics in the startup process and the steady state at the same speed as the valve closing transition section.
Figure 5. Comparison of the valve closing transition start process and the steady state pump characteristics at the same speed. The S index represents the steady state, and the T index represents the transient state.
Figure 5 shows that under the same speed, the change trends of the steady-state pressure head and power curves and the working pressure head and power curves of the valve closing transition section are basically the same, which indicates that the numerical simulation results in this chapter for the valve closing transition section startup process are correct. During the startup transition time 0≤t≤2.1s, the corresponding values of the power and lift in the steady state are higher than those in the transient state. At the end of the startup process, the power and lift states are basically the same, indicating that the valve is closed. During the process, there is a certain hysteresis in the fluid state compared with the fluid state at steady speed. In the startup process of the valve closing transition section, on the one hand, the flow state is unstable due to the rotation of the fluid and the acceleration of the water flow, on the other hand, due to the inertia of the fluid flow during. In the startup process of the transition section of the valve, the flow field is extremely uneven, which aggravates the hydraulic loss in the channel. Therefore, the pressure and power during the transient process of starting a closed valve are lower than in the steady state.
In this section, the flow with the same speed and the steady-state flow at the corresponding time are combined, the difference in the distribution of the velocity field and the pressure field in the valve closing transition startup process is compared, the effect of transient processes on the internal flow distribution is analyzed, and the effect of the transient state during the valve closing process startup on the internal flow distribution is discussed (Zhang et al., 2019b). Among them, Figures 6 to 8 respectively show the valve closing transition startup process and the speed distribution diagram of the first stage impeller in the steady-state constant speed state at n = 600, 1800, and 2985 rpm. Figures 9 to 11 respectively show the valve closing transition startup process and the static pressure distribution diagram of the first stage impeller in the steady-state constant speed state at n = 600, 1800, and 2985 rpm.
Figure 6. First stage impeller speed distribution at n = 600 rpm. (A) Transient valve closure process at start-up and (B) steady state at the same speed.
Figure 7. First stage impeller speed distribution, n = 1800 rpm. (A) Transient process when the valve closes, (B) Steady state at the same speed.
Figure 8. First stage impeller speed distribution, n = 2985 rpm. (A) Valve closing transient at start-up and (B) steady state at the same speed.
Fig. 9. Static pressure distribution of the first stage impeller at n = 600 rpm. (A) Transient valve closing process at start-up and (B) steady state at the same speed.
Fig. 10. Static pressure distribution of the first stage impeller at n = 1800 rpm. (A) Transient process of valve closure at start-up and (B) steady state at the same speed.
Fig. 11. First stage impeller static pressure distribution, n = 2985 rpm. (A) Transient valve closure process at start-up and (B) steady state at the same speed.
Comparing the static pressure distribution and the velocity distribution in the two states, it can be seen that as the impeller speed increases, the velocity at the inlet of the first stage impeller gradually increases, and the static pressure in the flow channel also gradually increases. This is because, with the increase of time, the impeller speed continues to increase, the force acting on the liquid also increases, and the total mechanical energy converted into the liquid also increases, that is, both the dynamic head and the static head increase. However, compared with the steady-state speed starting process, at the corresponding time, the internal speed of the impeller is relatively large during the variable speed starting process in the transient state, and the static pressure of the static head in the flow channel is relatively small. This shows that when the impeller does the same work, the mechanical energy is converted into kinetic energy during the variable speed starting transient process, and the pressure energy is smaller. This is a typical manifestation of the transient processes of the valve closing transient state and the starting transient state. This is also consistent with the phenomenon that the pressure curve of the transition flow at the corresponding moment is lower than the pressure curve of the steady flow at the same velocity.
In the transient start-up process of the closed valve, there are a large number of zero-velocity sections from the impeller inlet to the middle channel zone. The velocity sections of different sizes are mainly concentrated near the rear outlet region of the blade, while there. are less distributed in the outlet zone of the blade working surface, and the maximum velocity zone is mainly in the middle channel near the outlet of the impeller. Under the action of the centrifugal force of the impeller, a small amount of liquid is distributed at the rear. blade outlet. During the transient start-up process of the closed valve, the zero-velocity zone is a dead water zone, that is, no liquid passes through this zone. If this state persists for a long time, the impeller will be in a large volume. consumed to perform its function, and the pump casing and liquid will generate heat during the start-up process (Li et al., 2019). Compared with the transient start-up process of the variable speed, in the valve closing transition, the velocity gradient in the impeller channel during the start-up process with a steady-state speed at the corresponding time is more uniform. There is no zero velocity zone at the impeller outlet, but there are scattered zero velocity zones at the impeller outlet during the variable speed startup process in the transition state. In the static pressure cloud diagram of the startup process in the valve closing transition section, the static pressure distribution at different times is the same. All four main low-pressure vortex regions are located at the impeller outlet and are distributed symmetrically in the center. The steady startup process at the corresponding time, the low-pressure vortex regions. The area of the zone is larger, the central pressure is lower, the pressure gradient is larger, and there are more numbers, indicating that the valve closing startup process transition section is more turbulent and complex than the steady state of the internal flow at the corresponding time.
The studied transient start-up process of valve opening is the next stage of the transient start-up process of the closed valve. During the transient start-up process of the valve opening, the impeller maintains the rated speed, and the flow rate increases rapidly from zero to the specified. rated flow rate. Since it is difficult to simulate the transient process of the transient process of the valve opening of the multistage centrifugal pump in the laboratory, and it is also impossible to obtain the boundary conditions for the numerical simulation of the calculation of the beginning of the valve opening transition, Therefore, Flowmaster and CFX software are used to simulate the valve opening of the multistage centrifugal pump. The start-up transition process is simulated numerically. Firstly, Flowmaster software is used to establish a working model of the multistage centrifugal pump to obtain the outlet flow rate and inlet pressure. Secondly, the boundary conditions are entered through the UDF function in CFX to simulate the internal flow of the multistage centrifugal pump. stage centrifugal pump during the transient process of valve opening. The steps of the modeling process are: (1) collect and systematize the component parameters; (2) import parameters and check the working status of components; (3) use Flowmaster for modeling and simulation; (4) import boundary condition parameters into CFD simulation; numerical calculation software.
This paper uses Flowmaster software to create a centrifugal pump operation model. The centrifugal pump operation model is shown in Figure 12. The operation model mainly consists of circulation pipes, elbows, centrifugal pumps, torque regulators, ball valves and water tanks. In order to make the simulation process closer to the real situation, the setting parameters of the centrifugal pump are determined based on the actual test measurement of the centrifugal pump. According to the start-up test report of the multi-stage centrifugal pump, this chapter determines that the time required for the flow rate to change from zero to rated flow rate in the valve start-up stage is 3 seconds, and the total calculation time for the numerical simulation of the entire start-up process in the valve opening transition section is 3.8 seconds.
Fig. 12. Working model of multistage centrifugal pump. 8: Water tank; 10: Centrifugal pump; 11: Ball valve; 15, 17, 9, 14, 18: Pipes; 12, 13, 16: Elbow; Other: Torque regulator;
During the transient start-up process of a multi-stage centrifugal pump with a fully open valve, the flow rate changes with the start-up time. The change trend of the pump characteristics with time in the transient start-up state is shown in Figure 13. Figure 14 shows the changes in the pressure head of different stages during the transient start-up process of a fully open valve. The single-stage lift is obtained by calculating the pressure difference corresponding to the impeller inlet and the guide vane outlet. Since there are too many levels, only typical levels 1, 2, 3, 7, 8, and 13 are selected for analysis.
It can be seen from Figure 13 that in the process of starting the valve opening transition section, the flow rate and efficiency change depending on the starting time are the same in the early starting period of 0 ≤ t ≤ 0.25 s and the late starting period of 2.75 s. ≤ t ≤ 3.04 s, the flow rate and efficiency increased steadily. At the starting stage, both the flow rate and efficiency increased linearly. At the beginning of the starting, the pressure decreased with the increase of the starting time. At t=1.01 s, the pressure began to increase, and at t= the first local peak appeared. 1.22 s. A large value, then the pressure decreases again, and a local minimum appears at t=1.61 s. Then the pressure begins to rise again, and the second local maximum appears near t=1.89 s, and the two maximum values are almost equal. Then, as the starting time increases, the head continues to fall.
As can be seen from Figure 14, the pressure head of the first stage is very different from that of other stages, especially the sharp fluctuations in two periods of t≤1s and 1.75s≤t≤2.66s. The other stages are different. the degree of fluctuations in the early stages of startup. In the transient numerical simulation of the startup process of the transition section of the valve opening, the boundary condition at the inlet is the total pressure, which does not completely correspond to the actual situation of the startup process of the transition section of the valve opening, which leads to sharp changes in the static pressure at the inlet, and the rise of the first stage is different from the rises of other stages. The difference is obvious. Except for the first stage, the change trend of the pressure head in the other single stages is the same as that of the total pressure, and double humps are present to varying degrees. The first hump of the flow pressure curve at each level appears at about t=1.21s, but the second hump appears at different times. The second ridge of the 8th level hydraulic pressure curve appears last, and the single ridge of the 8th level appears last. The water pressure is also the highest.
In order to further analyze the influence of transient processes on the pump performance in the transient state of valve opening, especially the influence on the flow hump, the pump performance in the steady state and transient starting states at the same flow rate were compared. The plugs here are 6.84, 8.10, 9.75, 11.39, 14.44, 17.48, 18.91, 20.33, 27.20, 34.20. m3/h, the corresponding starting transition times are 0.93, 1.01, 1.11, 1.22, 1.41, 1.61, 1.71, 1.89, 2.35, 3.10 s, respectively. Figure 15 shows the comparison of the pump performance between the transient state of valve opening and the steady state starting process at the same flow rate. The index S in the figure represents the steady state, and the index T represents the transient state.
Fig. 15. Comparison of transient and steady-state characteristics during startup with constant flow rate at the valve opening stage.
As can be seen from Figure 15, in the same steady state of flow and the beginning of the valve opening transition process, when the valve opening transition starts at a speed of 27.20 m3/h ≤ Q ≤ 34.20 m3/h in the middle and late stages of development, the flow efficiency curve basically follows the flow pressure curve; when Q ≤ 20.33 m3/h, three protrusions appear on the pressure curve under the same steady state of flow, and the amplitude of the hump oscillations is smaller than the amplitude of the hump oscillations of the flow. The pressure curve during the valve opening transition starting process is shown in Fig. 15.
In order to study the effect of the transient process on the protrusion during the startup process in the transition section of the valve opening, four flow points are selected in this paper: the maximum and minimum positions of the protrusion, Q=8.10 m3/h, Q=11.39. m3/h, Q=17.48 m3/h, Q=20.33 m3/h, a comparative analysis of the distribution of streamlines between the blades of the second stage impeller at different flow points under the same steady-state flow regime and the transient effect of the startup process in the transition section of the valve opening. Figure 15 shows the distribution of streamlines between different flow blades during the startup process of the transition section of the valve opening. Figure 16 shows the distribution of streamlines between different flow blades in the same steady-state flow state.
Fig. 16. Distribution of streamlines between blades with different flow velocities during the transient start of valve opening. (A) Q = 8.01 m3/h, (B) Q = 11.39 m3/h, (C) Q = 17.48 m3/h, (D) Q = 20.33 m3/h.
Comparing Figures 16 and 17, it can be seen that no matter the startup process occurs in the valve opening transition section or in the steady state with the same flow condition, the flow disorder in the impeller passage and different degrees of stall groups are the main causes of the hump on the flow pressure curve. However, during the startup process of the valve opening transition section, the stall group appears in a larger range and the number of stall groups is larger, for example, when the flow rate Q = 17.48 m3 / h and Q = 20.33 m3 / h, there are stall groups not only at the inlet and outlet of the impeller but also in the middle flow channel. During the startup process of the valve opening transition section, the inertia of the fluid flow will cause the flow rate change to be out of sync with the pressure change, and the increase in flow rate will aggravate the stall. Therefore, during the startup process of the valve opening transition section, there will be a hump oscillation exceeding the steady state with the same flow rate.
Figure 17. Distribution of streamlines between blades with different flow rates in steady-state constant flow mode. (A) Q = 8.01 m3/h, (B) Q = 11.39 m3/h, (C) Q = 17.48 m3/h, (D) Q = 20.33 m3/h.
This paper uses a two-stage (second) impeller to analyze the internal flow field changes during the valve opening transition startup process. The static pressure distribution of the second stage impeller during the valve opening transition startup process is shown in Figure 18.
Post time: Jan-10-2025