Effect of reduced diameter pipeline and impeller position on the performance of reactor coolant pump

In the reactor coolant pump experiments, there are cases where different impellers are replaced on the same volute for experiments, but due to the different diameter of the impeller inlet, there will be a sudden expansion between the inlet pipeline and the impeller inlet, resulting in a decrease in the efficiency of the pump. In order to solve this problem, a scheme of using an inlet pipe with a reduced diameter structure and changing the position of the impeller was proposed, and based on the three-dimensional incompressible Reynolds N-S equation, the reactor coolant pump device was numerically simulated by CFD software. The data simulation results of the reduced diameter structure, straight pipe flow field and changing the position of the impeller were compared with the model experiments, and the reliability of the calculation results was verified. The results show that the inlet pipe with reduced diameter structure and the method of changing the position of the impeller can effectively solve the problem of efficiency decline caused by the sudden expansion of the flow channel.


Introduction.
Nuclear power is an important component of the power industry, and vigorously developing nuclear power construction is an important policy for China's medium to long-term energy construction.The primary coolant circulation pump, also known as the reactor coolant pump, is one of the key equipment in pressurized water reactor nuclear power plants.It drives the circulation of coolant within the RCP (reactor coolant system) system and is located at the core of the nuclear power plant.Its operational performance and safety are key research issues [1][2][3][4][5][6].The inlet passage of the reactor coolant pump plays an important role in the liquid circulation process of the pump, which can reduce the liquid turbulence, improve the suction efficiency and avoid negative effects such as cavitation.Its design needs to comprehensively consider the liquid flow state, flow, pressure and other parameters to ensure the performance and safety of the reactor coolant pump.Meanwhile, different impeller positions will also change the flow channel inside the volute, change the shape of the flow channel, and affect the efficiency of the impeller.Therefore, the size design of the inlet pipeline and the impeller position have a crucial impact on the performance and service life of the reactor coolant pump.
When conducting localization research on reactor coolant pumps, it is sometimes necessary to replace impellers of different sizes but share the same pump body.In this case, the inlet of the same pipe may suddenly shrink due to size mismatch when facing impellers of different sizes, leading to a decrease in pump efficiency.In view of this problem, the conjecture of adding a reduced diameter tube and changing the position of the impeller to improve the efficiency of the impeller is proposed.
At present, domestic and foreign scholars have conducted relevant research on the impact of pipe diameter and contraction angle on pump efficiency.Wang Yan et al [7] studied the unsteady pressure pulsation characteristics inside the pump and pipeline, and found that reducing the cold section pipe diameter would increase the efficiency of the reactor coolant pump, but the overall system efficiency decreased by 1.3%;The cold section with a larger pipe diameter can significantly reduce the pressure fluctuation amplitude of the transition section; Zhu Rongsheng et al [8] studied the effect of contraction angle on the performance of reactor coolant pumps, and the results showed that the contraction angle has a significant impact on the front and rear areas at the junction of the pressurized water chamber and the outlet.When the contraction angle is within the range of 12 ° to 16 °, the reactor coolant pump efficiency is above 70%, and the aging rate reaches the maximum value of 74.2% when the contraction angle reaches 15 °.Similarly, many scholars have studied the influence of different impeller positions on the impeller.Zhang Yangliu [9] studied the effect of impeller installation at different positions on the full flow field of centrifugal blower.The study shows that the higher the position of the impeller, the wider the distribution of high pressure area, the greater the pressure of the whole machine, the higher the speed, but the little influence on the speed change range and the small impact on the speed distribution.Zhan Tingjun [10] studied the influence of the relative position of the impeller and the volute on the performance of the centrifugal fan.
Based on previous research results and the three-dimensional incompressible Reynolds N-S equation, the article uses CFD software to perform numerical calculations on the reactor coolant pump and obtains the effect of the reducing tube and changing the impeller position on pump efficiency.At the same time, efficiency experiments were conducted on the hydraulic model prototype of the reactor coolant pump on the pump test bench, and numerical calculations and experimental results were compared and analyzed, providing reference for optimizing the efficiency of the reactor coolant pump.

Establishment of parameters and model for reactor coolant pump.
The research object of this article is the model reactor coolant pump, which delivers clean water at room temperature.The main parameters of the model pump are as follows: design flow rate 1470 m 3 /h, design head 21 m, design speed 1490 r/min, 4 impeller blades, and 12 guide vanes.
The three-dimensional water body model of reactor coolant pump was established using CFturbo and UG software for the inlet section, volute, impeller, guide vane, outlet section, and other flow areas, as shown in Figure 1.

Mesh.
The water body of the entire reactor coolant pump model is sequentially divided into the inlet section, impeller, guide vane, volute, and outlet section.Using ANASYS ICEM, the water body in the calculation domain of the model reactor coolant pump was structured into unstructured grids, and local grid refinement was performed on the impeller blades and guide vanes.
The calculation domain grid of the main components of the reactor coolant pump is shown in Figure 2.

Mesh independence verification.
To avoid the impact of the number of grids on computational accuracy, it is necessary to perform grid independence verification on the computational domain grids.Using CFX software to calculate the head at the rated operating point, the results show in the picture 3 that when the grid number of the reactor coolant pump reaches 6×10 6 , the simulated head stabilized with an error of within 0.5%.Considering the calculation capacity and progress, the total number of cells for fluid domain grid division is finally determined to be about 6 × 10 6 .After grid quality inspection, it was found that the overall quality is relatively high.

Boundary condition settings.
The fluid motion control equation used in the calculation is the three-dimensional incompressible Reynolds time averaged N-S equation, and the computational domain k-Epsilon turbulence model (which is currently the most widely used engineering turbulence model).The inlet of the reactor coolant pump adopts inlet velocity conditions, and the outlet is given pressure conditions.At the same time, it is assumed that the pressure on the inlet section is uniformly distributed.Select the standard wall function near the wall, set it as the adiabatic non slip wall boundary condition, and use the standard wall function near the wall area, with a residual convergence accuracy of 10 -4 ; In terms of setting the time step for steady calculation, the time step should be small enough.After analysis, it has been determined that the time step for steady calculation is 0.00067114s.The parameter terms in the momentum and turbulent flow transport equations adopt a firstorder upwind difference scheme [11].
Taking the head and hydraulic efficiency of the reactor coolant pump as the objective functions, the calculation formula is as follows: In the formula: Pout-total pump outlet pressure, Pa; Pin-total inlet pressure of the pump, Pa; -Clear water density, kg/m3; g-Gravitational acceleration, m/s 2 ; Q-design flow rate, m 3 /h; M-total rotational torque, N • m;-Pump shaft rotation angle speed, rad/s.

Experimentation.
The above reactor coolant pump was experimentally verified.Drawing its external characteristic curve, and comparing the external characteristic results obtained from numerical simulation with the experimental external characteristic results.Near the rated operating point (0.9Qn-1.1Qn), the variation trend of the external characteristic curve of the test data and simulation data is basically consistent, and the overall error between the numerical simulation and test results does not exceed 5%, which is in line with engineering practice.This result indicates that the results obtained from numerical simulation are reliable.

Variant parameters.
Based on relevant literature and previous research experience, the inlet pipe slope and impeller position were selected as variables for optimization design.The difference between the straight pipe inlet section and the reduced diameter pipe inlet section is shown in Figure 7. Using the slope of the inlet pipe as a variable, a model is designed every two degrees for calculation to obtain the impact of changes in the slope of the inlet pipe on the head and efficiency of the reactor coolant pump.

Analysis of Simulated Result.
The numerical simulation results of different slopes of the reducing tube are shown in the figure 9.
Figure 9.The variation of head and efficiency with slope.From the head curve in Figure 9, it can be seen that the head of the reactor coolant pump generally increases with the increase of the slope of the inlet section reducing pipe.The head starts to rise at 0°, fluctuates after reaching 6° which arrive to 20.489m, and then continues to rise steadily; From the efficiency curve, it can be seen that the efficiency first increases to the highest efficiency point with the increase of the slope of the reducing pipe in the inlet section, and then gradually decreases and tends to be flat, but the overall efficiency is slightly higher than that of the straight pipe inlet.When the slope of the reducing tube is around 4 °, the reactor coolant pump reaches the highest efficiency point, with head of 20.4878m and efficiency of 84.54%.At the slope of 4°, the highest efficiency is 84.54%, which is an increase of 1.1% compared to 83.49% of straight pipes.From this, it can be seen that the efficiency of the inlet section reducing pipe has slightly improved compared to the straight pipe for the reactor coolant pump.
The numerical simulation results of different impeller positions are shown in the figure 10.From the head curve in the figure, it can be seen that under the same conditions, the head of the reactor coolant pump with the impeller position on the side is 0.2m higher than the head of pump with the impeller position in the middle; From the efficiency curve, the efficiency of the reactor coolant pump with the side impeller is 83.49%, while the efficiency of the reactor coolant pump with the side impeller centered decreases to 78.02%.It can be seen that the efficiency of the reactor coolant pump with the side impeller has a significant advantage compared to the reactor coolant pump with the centered impeller.

Internal Flow Field Analysis.
The nephogram of the impeller inlet pressure of the reactor coolant pump under the same impeller position and different slope reducing pipes is shown in the following figure 11.The dark color at the inlet of the impeller in the figure represents the low-pressure area, while the light color represents the high-pressure area.As shown in the figure, as the slope of the reducing tube gradually increases, the low-pressure area at the impeller inlet gradually increases and merges towards the center, resulting in a gradual decrease in pressure at the impeller inlet; However, there is no significant change in the outlet pressure of the pump.According to the head calculation formula, the head will increase with the increase of the slope of the reducing pipe.As the slope gradually increases, the loss of the inlet section of the reduced diameter pipe also gradually increases.The increase in head and the increase in losses in the inlet section will result in a maximum point in efficiency, which at a slope of 4° The pressure nephogram of the reactor coolant pump under the same straight pipe and different impeller positions is shown in the following figure 12.It can be seen from the pressure cloud diagram of different impeller positions that the internal pressure and pressure difference of the reactor coolant pump with side impeller are generally larger than the internal pressure and pressure difference of the reactor coolant pump located at the centered impeller position.The simulation results show that the efficiency of the reactor coolant pump centered on the impeller is 78%, which is much smaller than the 84.26% of the reactor coolant pump at the impeller position.

Conclusion.
(1) Compared to the straight pipe inlet, the head and efficiency of the reduced diameter pipe inlet have a slight improvement.The head continuously increases with a slope of 2 °~12 °.
(2) When the slope of the reduced diameter pipe is 4 °, the efficiency of the reactor coolant pump reaches its highest point, which is about 1.1% higher than the straight pipe inlet, and the head increases by 0.2m.
(3) The reactor coolant pump with the impeller located on the edge has a higher head and efficiency compared to the reactor coolant pump with the impeller located in the center, with a head difference of 0.2m and an efficiency difference of 5.47%.

Figure 2 .
Figure 2. Calculation Domain Grid for Impeller and Guide Vane of reactor coolant pump.

Figure 5 .
Figure 5. Actual drawing of the reduced diameter pipe.Near the rated operating point (0.9Qn-1.1Qn), the variation trend of the external characteristic curve of the test data and simulation data is basically consistent, and the overall error between the numerical simulation and test results does not exceed 5%, which is in line with engineering practice.This result indicates that the results obtained from numerical simulation are reliable.

Figure 6 .
Figure 6.Head and efficiency of experiment and simulation.

Figure 7 .Figure 8 .
Figure 7.Comparison between straight and reduced diameter pipes.The changes in the positions of the two types of impellers are shown in Figure 8.Using the impeller position as a variable, two model pumps are designed: the impeller position center and the impeller position edge, and ensure that the cross-sectional area of the flow channel inside the pump body is the same.Perform data simulation on two models to obtain the impact of impeller position changes on the head and efficiency of the reactor coolant pump.

Figure 10 .
Figure 10.The numerical simulation results of different impeller positions.From the head curve in the figure, it can be seen that under the same conditions, the head of the reactor coolant pump with the impeller position on the side is 0.2m higher than the head of pump with the impeller position in the middle; From the efficiency curve, the efficiency of the reactor coolant pump with the side impeller is 83.49%, while the efficiency of the reactor coolant pump with the side impeller centered decreases to 78.02%.It can be seen that the efficiency of the reactor coolant pump with the side impeller has a significant advantage compared to the reactor coolant pump with the centered impeller.

Figure 11 .
Figure 11.Impeller inlet pressure nephogram.The dark color at the inlet of the impeller in the figure represents the low-pressure area, while the light color represents the high-pressure area.As shown in the figure, as the slope of the reducing tube gradually increases, the low-pressure area at the impeller inlet gradually increases and merges towards the center, resulting in a gradual decrease in pressure at the impeller inlet; However, there is no significant change in the outlet pressure of the pump.According to the head calculation formula, the head will increase with the increase of the slope of the reducing pipe.As the slope gradually increases, the loss of the inlet section of the reduced diameter pipe also gradually increases.The increase in head and the increase in losses in the inlet section will result in a maximum point in efficiency, which at a slope of 4°The pressure nephogram of the reactor coolant pump under the same straight pipe and different impeller positions is shown in the following figure 12.