Integrated design and hydraulic model optimization of model horizontal main pump based on CFD fluid dynamics analysis

There are few studies and reports related to horizontal main pumps for nuclear power plants. The installation of vertical main pumps for small reactors requires the design of special elbows on the pressure vessel. Horizontal main pump, as a new main pump structure, can reduce the diameter of the main pressure vessel and simplify the structure of the pressure vessel. Compared with the vertical main pump, the horizontal main pump needs to control its axial dimension to improve its stiffness, which makes it impossible to set a long straight pipe section at the inlet, which has an impact on the pump performance. On the other hand, the compression of the axial dimension of the pump casing makes the overflow area of the press-out chamber small, resulting in large hydraulic losses. In this paper, in order to improve the efficiency of the pump and reduce the influence of non-uniform incoming flow, the optimization design is carried out with the SST k-ω model, and it is shown that: 1. the suction port of the curved impeller is easier for the fluid to enter into the impeller in the axial direction; and 2. the axially symmetric worm gear produces more vortex flow losses.


Introduction
The International Atomic Energy Agency (IAEA) defines a nuclear reactor power plant with a generating power of less than 300 MW as a "small reactor" [1].In recent years, many nuclear power technology developed countries are committed to research and development of good safety, economic competitiveness of small and medium-sized multi-purpose reactors.At present, the single development of coastal large nuclear power units can not fully adapt to the needs of nuclear energy widely used, the future demand for nuclear power will be transferred to the mobile nuclear power plant and water and other constraints of the inland areas, and the design of small reactors is also focused on the design of the target to eliminate accidents from the root cause.At present, small reactor nuclear main pump technology is imperfectly developed domestically, so transverse main pump technology for small nuclear reactors is even more lacking.
The function of the horizontal main pump is to drive the coolant to carry out closed circulation in the main system, transfer the heat energy generated by the reactor to the steam generator, so that the medium in the second circuit is heated into steam, and drive the turbine to do work.At present, the main pump of nuclear power plant at home and abroad generally adopts vertical structure, there are structures with motor on top and pump on the bottom, and there are also structures with motor on the bottom and pump on the top, but there is almost no relevant literature and report on the structure of horizontal shielded main pump seen.Cai Long [2] designed a structure of horizontal nuclear main pump; Qiang Zhuang [3] and other orthogonal tests on the guide vane and impeller of small reactor nuclear main pump to optimize the design, and get the optimized hydraulic model; Yang Minjuan [4] and other research on the position of the guide vane inlet side and the number of guide vanes on the hydraulic performance of the hybrid nuclear main pump model; Zhang Qinzhao [5] and other Fortran language programming to achieve the space of the high-ratio rotational mixed flow pump The hydraulic design of guide vane; Wang Chunlin [6] et al. studied the effect of the relative position of the guide vane inlet side on the performance of the pump; Poullikkas [7] et al. investigated the effect of liquid-gas two-phase flow on the performance of nuclear reactor cooling pumps; Li Tianbin [8] et al. investigated the optimal design of the hydraulic of the main pump of the nuclear small-scale reactor and orthogonal test, verified the feasibility of the optimal design of orthogonal test and obtained the performance of the outer characteristics of the main pump of the small-scale reactor.test important raw data; LU [9] and others proposed a joint optimal design technique integrating constrained shape design, CFD analysis, LHS method, prediction model and MOPSO algorithm to optimise the impeller blades and guide vane blades, and to improve the main performance factors such as nuclear main pump external characteristics and pressure pulsation.
In summary, a number of scholars have studied the internal flow of the nuclear main pump to some extent, but there are relatively few studies on the hydraulic optimisation of the transverse nuclear main pump.Taking the transverse nuclear main pump as the research object, the design input parameters are flow Q=2500m 3 /h, rotational speed n=2920r/min, head H=30 m.The hydraulic component design of the transverse nuclear main pump is carried out by using the velocity coefficient method, and the hydraulic performance optimisation is carried out by using numerical simulation method, which provides the theoretical basis for the subsequent optimisation of the hydraulic characteristics of the transverse nuclear main pump.

Computational model and meshing
The basic geometrical parameters of the key hydraulic components are determined based on the design input parameters of the transverse nuclear main pump, the impeller and outlet guide vane of the transverse nuclear main pump are modelled with multi-parameters using the commercial CFturbo software, and the inlet domain, the worm casing and the outlet section are modelled as a water body using NX.The hydraulic performance calculation of the transverse nuclear main pump generally divides the watershed into four sections, the inlet duct, the impeller, the pressurised water chamber and the outlet duct.Outlet guide vane meshing.The mesh is generated using TurboGrid, which is a professional turbine impeller channel meshing software that can produce high-quality structured meshes for complex shaped blades and impeller channels in a short period of time.For the open impeller model, the Shroud Tip is set to 0.5 mm, and the core hydraulic components are meshed as shown in Figures 1-4.In order to ensure the computational accuracy and save computational resources, the above components are discretized with hexahedral meshes, and the total number of meshes is about 2 million.

Boundary condition setting
The computational software of choice for this project is Ansys CFX, a general-purpose fluid dynamics software that has been used to solve a wide range of flow problems over the past 20 years.For the design of the main pump in the calculations using constant calculations, the calculation process using more suitable for rotating fluid machinery, the near-wall vortex and boundary layer capture more accurate SST k-turbulence model, using SIMPLE algorithm for pressure and velocity coupling.For the open impeller, the wheel cover (Shroud) surface is set as Counter-rotating Wall, the interface between the rotating and stationary domains of the impeller is adopted as Frozen Rotor interface, the No Slip Wall condition is adopted for the normal wall surface, and the Reference Pressure is set as 1 atm.mass flow inlet and pressure outlet, the outlet pressure is set to 1atm, and the convergence accuracy of the average residual RMS is set to 10 -5 .

Hydraulic performance analysis of pipeline inlet and outlet models
After numerical simulation of the above hydraulic model using the CFD model described above, the external characteristic values of the main pump at rated flow conditions are shown in Table 1, and the internal flow state of the main pump at the rated point is shown in Figure 5, and the combination of the two shows that the worm casing loss accounts for the largest percentage of the loss, which is about 15.04%, and this is mainly attributed to the use of a worm casing to convert the axial velocity of the axial pump into the radial velocity of the impact loss and vortex losses caused by the This is mainly attributed to the impact loss and eddy current loss caused by using a worm gear to convert the axial velocity into radial velocity.
Figure 6 Pressure distribution on the flow surface of S1 at different spreading positions of the impeller.At the same time, the key hydraulic components of the impeller and the export guide vane internal flow situation is analysed, from Figure 6 and Figure 7 impeller different spreading position S1 flow pressure distribution and streamline distribution trend can be seen, the impeller from the hub to the rim of the regional inlet of the stationary point are basically the trailing edge of the wing-type head position, indicating that the impeller inlet angle is set up reasonably, there is no obvious head impact loss.Figure 8 and Figure 9 gives the exit guide vane different spread position S1 flow surface pressure distribution and streamline distribution, guide vane head stationing point distribution is more reasonable, near the hub of the 0.1 spread position in some of the leaf grille channel there is a local phenomenon, which is mainly due to the centrifugal effect of the high-speed impeller resulting in the hub region of the flow velocity is low, but from the overall flow pattern and guide vane loss ratio are in a reasonable range.
Table 1.CFD prediction of hydraulic performance at rated operating point.   .Watershed-wide model.Since the project is for engineering, and through the preliminary calculations, it is found that the vertical baffle and the form of the inlet of the worm and blade have a greater impact on the overall flow field, and in order to minimize the modification of the model, so the least modification margin is selected.In summary, the vertical baffle and the form of worm and blade inlet are selected to optimize the model.

Model meshing
Fluent meshing software is used to mesh the fluid domain model.Mainly unstructured meshes are used, and the number and quality of meshes are regulated by controlling the mesh size in the mesh construction, and ensuring that the mesh quality of important flow channel models is above 0.3.The final overall mesh number is about 7.2 million.

Calculation process settings
The mesh file is imported into the CFX solver, the constant flow calculation is selected, the turbulence model is selected as SST k-ω turbulence model, the monitoring points of the total pump head, impeller head, guide vane loss, and volute loss are set, and the convergence accuracy of the numerical simulation calculation is set to 10-4.
In the flow model, the impeller part is rotating area, and other parts are static area, which are connected with each other through the interface surface, and the dynamic and static interfaces are set by the Frozen rotor method.The inlet adopts the mass inflow port, the outlet adopts the pressure outlet, the solid boundary adopts the no-slip wall condition, and the near-wall region adopts the standard wall function.The rotational speed in the impeller rotation region is set to 2920 rpm.

Calculation of rated working conditions
The performance of the three models at rated operating conditions was calculated and the data are shown in Table 2: From the data in the table, it can be seen that the total head of the pumps under the rated working conditions are all around 30 m. From the data in the table, it can be seen that, compared with the initial model and the improved model 1, the impeller head of the improved model 1 is larger than that of the initial model by 0.24 m, and the losses of the guide vane and the worm gear are less by 1.87 m, which can be seen that the curved inlet can reduce the losses; compared with the improved model 1 and the improved model 2, the impeller head of the improved model 2 is smaller than that of the Compared with improved model 1 and improved model 2, the impeller head of improved model 2 is smaller than that of the initial model by 0.66 m, and the losses of guide vane and volute are more than 1.1 m, which can be seen that the model of the volute becomes smaller and increases the losses; the impeller efficiencies of the improved model 1 and improved model 2 are both about 95% and larger than that of the initial model, which indicates that the hydraulic performance of the impeller is basically guaranteed by the optimization of the inlet profiles in line with the calculations of piping system in the previous chapter.In summary, under the condition of guaranteeing the installation space, the improved model 2 can meet the use requirements.
Table 2. Basin-wide rated condition performance data.

Flow field analysis
In order to better compare the three fluid domain models, the internal flow states of their key flow components and improvement locations are analysed:  critical flow components: Combining Figure 7 and Figure 9, it can be seen from Figure 12 to Figure 17      From the comparison of Figure .18 and Figure .19, it can be seen that the curved inlet is easier than the straight inlet for the fluid to enter the impeller in the axial direction; from the comparison of Figure 19 and Figure 20, it can be seen that the axially symmetric worm gear produces more vortex flow loss, which is mainly due to the residual ring volume of the fluid at the outlet of the guide vane is in the same direction of rotation of the eccentric worm gear, and the helix produced by the eccentric worm gear helps in the control of the outlet flow field.In summary, the improved model 2 is the finalised hydraulic component structure of the transverse main pump.
During the experimental test, in order to obtain the pressure at the guide vane outlet position, three columnar fascia plates are added to arrange the pressure measurement holes through the inside of the pump body on the basis of the improved model 2, as shown in Figure 21.The same setup method was used to simulate the reinforced model, and the rated point performance data is shown in Table 3, which shows that the reinforced pick-up holes do not have a significant effect on the performance of the main pump.

Conclusion
 Under the condition of pipeline type inlet and outlet, the impeller head is 38.745m, the impeller efficiency is 95.4%, and the total head is 31.381m, of which the volute loss of 5.826m (15.04%) is the main flow loss of the overflow component. In the model under the test bench considering the inlet and outlet of the tank, under the condition of satisfying the Φ720mm mounting holes, the easier-to-machine structure of the non-eccentric volute is adopted, and the impeller head of 37.5m is calculated, the impeller efficiency of 95.1%, and the total head of 30.1m.  During the experimental test, the increase of ribbed plate pressure-taking holes in the guide vane outlet has little effect on the performance of the main pump, which can be ignored. A curved impeller suction port is more likely to allow fluid to enter the impeller in the axial direction than a straight one. An axially symmetrical volute produces more swirl flow losses.

Figure 3 .
Figure 3. Impeller meshing.Figure4.Outlet guide vane meshing.The mesh is generated using TurboGrid, which is a professional turbine impeller channel meshing software that can produce high-quality structured meshes for complex shaped blades and impeller channels in a short period of time.For the open impeller model, the Shroud Tip is set to 0.5 mm, and the core hydraulic components are meshed as shown in Figures1-4.In order to ensure the computational accuracy and save computational resources, the above components are discretized with hexahedral meshes, and the total number of meshes is about 2 million.

Figure 4 .
Figure 3. Impeller meshing.Figure4.Outlet guide vane meshing.The mesh is generated using TurboGrid, which is a professional turbine impeller channel meshing software that can produce high-quality structured meshes for complex shaped blades and impeller channels in a short period of time.For the open impeller model, the Shroud Tip is set to 0.5 mm, and the core hydraulic components are meshed as shown in Figures1-4.In order to ensure the computational accuracy and save computational resources, the above components are discretized with hexahedral meshes, and the total number of meshes is about 2 million.

Figure 12 .
Figure 12.Streamline distribution of S1 flow surface at different spreading positions of the initial model impeller.

Figure 13 .Figure 14 .Figure 15 .Figure 16 .Figure 17 .
Figure 13.Streamline distribution of S1 flow surface at different spreading positions of guide vanes in the initial model.

Figure 19 .
Figure 19.Cross-sectional streamline distribution in the improved position of improved model 1.

Figure 20 .
Figure 20.Cross-sectional streamline distribution in the improved position of improved model 2.From the comparison of Figure.18 and Figure.19, it can be seen that the curved inlet is easier than the straight inlet for the fluid to enter the impeller in the axial direction; from the comparison of Figure19and Figure20, it can be seen that the axially symmetric worm gear produces more vortex flow loss, which is mainly due to the residual ring volume of the fluid at the outlet of the guide vane is in the same direction of rotation of the eccentric worm gear, and the helix produced by the eccentric worm gear helps in the control of the outlet flow field.In summary, the improved model 2 is the finalised hydraulic component structure of the transverse main pump.During the experimental test, in order to obtain the pressure at the guide vane outlet position, three columnar fascia plates are added to arrange the pressure measurement holes through the inside of the pump body on the basis of the improved model 2, as shown in Figure21.

Figure 21 .
Figure 21.Improved model 2 with reinforcement.The same setup method was used to simulate the reinforced model, and the rated point performance data is shown in Table3, which shows that the reinforced pick-up holes do not have a significant effect on the performance of the main pump.Table3.Rated operating condition performance data.