The dirt suction self-cleaning filter is a new type of filtration equipment, and the dirt suction device is its important component. This article uses computational fluid dynamics (CFD) research methods to conduct fluid mechanics analysis of the sewage suction device. Study its internal flow field pressure distribution, velocity distribution, and other fluid mechanics characteristics. Three structural improvement measures are proposed to achieve the purpose of uniform suction of sewage by each sewage suction pipe.

Today, with the increasing scarcity of water resources and extremely serious water pollution, comprehensive wastewater treatment is becoming more and more important. The self-cleaning filter is a new type of filter developed in the late 1970s. It has many advantages, the most important of which is the use of water pressure for self-operation, self-cleaning, and small pressure loss. As a new type of filtration equipment, the suction-type self-cleaning filter can provide an uninterrupted water supply during cleaning. It is widely used in filtration and separation in industry, agriculture, municipal administration, seawater desalination process, etc.

The traditional self-cleaning filter has a simple structure. However, the amount of sewage inhaled by each sewage suction pipe during self-cleaning is different. As a result, the filtering effect is not optimal. This article focuses on the traditional sewage suction device structure, combines filtration theory, and uses modern computational fluid dynamics (CFD) research methods to improve its design.

## The structure and function of the sewage suction device

The schematic diagram of the dirt-suction self-cleaning filter is shown in Figure 1. Including cylinder, self-cleaning screen, sewage suction device, drain valve, driving mechanism, and other components. The dirt suction filter is an important part of the filter. It is a hollow structure. Several sewage suction pipes are evenly distributed at a certain distance on its axis, and the sewage suction pipes are connected to a sewage discharge valve.

When the drain valve is opened, the pressure difference formed between the water pressure inside the filter and the outside atmosphere causes the sewage suction pipe to generate strong suction. The high-speed flow of water washes down the impurities carries them into the sewage suction device, and then discharges them through the sewage valve.

The dirt suction unit is driven by a bidirectional motor connected to a lead screw assembly. It makes a spiral motion at a fixed speed so that several suction nozzles can suck all over the inner surface of the filter.

The structure of using a sewage suction pipe to absorb sewage has the following characteristics:

- High efficiency and compact size.
- The mechanical structure is reliable.
- The minimum flushing flow rate is required to ensure the cleaning effect even when the filter pressure is low.
- Flushing requires a small amount of water and does not cause interruption of downstream water supply.

## Flow field analysis of sewage suction device

Establish a finite element model through ANSYS and import it into CFX software for computational fluid dynamics (CFD) analysis. Set boundary conditions in CFX pre-processing, then solve, post-process and analyze.

### Finite element model and boundary conditions

The structure and related dimensions of the sewage suction device analyzed in this article are shown in Figure 2, to establish a finite element model.

Based on the actual conditions of the filter, this analysis sets the outlet flow rates of the sewage suction pipe to 2.45, 4.53, and 6.61 m/s respectively, and the inlet pressures of sewage suction pipe 1 to sewage suction pipe 4 are set to 1MPa. The other faces are set as fixed boundaries.

### Calculation results and analysis

The convergence accuracy is controlled as follows: the maximum residual MAX is less than 10-4; the average residual RMS is less than 10-5.

Applying different flow rates to the sewage suction device is to change the boundary conditions of the outlet pressure. When the sewage pipes are of equal diameter, the flow velocity of the sewage suction pipe under different flow rates and the relative variance and range analysis are shown in Table 1. The relative variance refers to the mean deviation of the flow velocity of each sewage suction pipe divided by the average value, and the relative range refers to the range of the flow velocity of each sewage suction pipe divided by the average value.

The analysis shows that when the sewage pipes are of equal diameter, the relative variance of the sewage suction pipe flow rate is about 3.29%. The relative range is about 8.51%, and the flow rates of each sewage suction pipe vary greatly. For a sewage suction device with a certain shape, the relative variance and relative range of the flow rate of each sewage suction pipe change little at different flow rates. The ratio of the outlet flow rate of the sewage pipe to the flow rate of each sewage suction pipe remains unchanged.

To improve the suction and filtration effect, it is necessary to optimize the design of the flow field by changing the structural dimensions of the suction device components. Keep the flow rate of each sewage suction pipe consistent. In this way, when the sewage suction pipe sucks sewage, some sewage suction pipes will not have a large flow rate, causing a waste of cleaning water. Some sewage suction pipes have a small flow rate and incomplete sewage suction.

Item | 1 | 2 | 3 |

Export speed | 2.45 | 4.53 | 6.61 |

Suction pipe 1 | 6.81 | 12.59 | 18.31 |

Suction pipe 2 | 6.90 | 12.75 | 18.61 |

Suction pipe 3 | 7.13 | 13.18 | 19.23 |

Suction pipe 4 | 7.41 | 13.61 | 20.07 |

Relative variance | 3.28 | 3.05 | 3.53 |

Relative range | 8.47 | 7.83 | 9.24 |

## Improved design of flow field of sewage suction device

### Sewage pipe reducing diameter design

When the sewage pipe is of equal diameter, the flow velocity of straws 1 to 4 shows a decreasing trend and the flow resistance increases. Increasing the diameter of the sewage pipe is beneficial for the uniform distribution of flow resistance, thereby ensuring that the flow rate of each suction pipe is evenly distributed. Therefore, first, only one section of the sewage pipe is designed with variable diameter, that is, increasing the diameter at D2 to find consistency in the flow rate of each suction pipe.

- Finite element model and boundary conditions. Establish a finite element analysis model according to the structure of the suction device shown in Figure 3. Take D1-D2 as the following sets of values: 52-62; 52-72; 52-82; 52 88; 52-92. 52-92 refers to the left end D1=52mm, the right end D2=92mm, and so on. The outlet flow rate of the sewage pipe of the suction device is set to 6.61 m/s. The inlet pressure of the straw is set to 1MPa.
- Calculation results and analysis. The flow field of the sewage pipe with an equal diameter of 52-52 is shown in Figure 4, and the flow field of the sewage pipe with a variable diameter of 52-92 is shown in Figure 5. Comparing Figure 5 and Figure 4, it is found that after the diameter of the sewage pipe is changed, the flow velocity of each suction pipe tends to be consistent. The specific relative variance and range of flow velocity and flow rate are shown in Table 2. As shown in Figure 3 and Table 2, D2 is between 52 and 88mm. As D2 increases, the relative variance decreases and the flow velocity in each pipe tends to be uniform. But at D2=92mm, the relative variance increases again, indicating that the degree of change in the diameter of the sewage pipe is not necessarily better.

D1-D2 | 52-52 | 52-62 | 52-72 | 52-82 | 52-88 | 52-92 |

Suction pipe 1 | 18.31 | 18.74 | 19.05 | 19.32 | 19.21 | 19.37 |

Suction pipe 2 | 18.61 | 18.73 | 18.79 | 18.91 | 18.92 | 18.75 |

Suction pipe 3 | 19.23 | 19.23 | 19.12 | 18.96 | 19.23 | 19.05 |

Suction pipe 4 | 20.07 | 19.5 | 19.49 | 19.23 | 19.06 | 19.26 |

Relative variance | 3.53% | 1.73% | 1.30% | 0.90% | 0.656% | 1.21% |

Relative range | 9.24% | 4.04% | 3.66% | 2.15% | 1.62% | 3.246% |

### Design of variable diameter sewage suction pipe

If only the cross-sectional size of each sewage suction pipe is changed (the size of the sewage discharge pipe is not changed), when the diameter of the sewage suction pipe becomes smaller, its flow rate will become smaller. On the contrary, the flow rate becomes larger. Theoretically, adjusting the cross-sectional size of the sewage suction pipe can make the flow rate of each sewage suction pipe equal. Adjust the size of sewage suction pipes 1-4 and establish several sets of finite element models for analysis. The results are shown in Table 3.

Calculate the flow rate based on the flow rate and cross-sectional area of the sewage suction pipe, and then adjust the diameter of the sewage suction pipe based on the difference between the flow rate and the average flow rate. Finally, it is concluded that when the sewage suction pipe size in analysis 3 in Table 3 is used, the relative variance of the sewage suction pipe flow rate is reduced from 3.53% when the sewage suction pipe is of equal diameter to 0.73%. The relative range is reduced from 9.24% to 1.98%, achieving the purpose of uniform dirt absorption.

Item | 1 | 2 | 3 | |||

OD | Flow | OD | Flow | OD | Flow | |

Suction pipe 1 | 16.3 | 17.39 | 16.3 | 17.08 | 16 | 17.49 |

Suction pipe 2 | 16.3 | 17.33 | 16.3 | 17.04 | 16 | 17.42 |

Suction pipe 3 | 15.3 | 17.99 | 15.8 | 17.69 | 15.8 | 18.02 |

Suction pipe 4 | 15.3 | 18.81 | 15.3 | 18.64 | 15.3 | 18.84 |

Relative variance | 3.75% | 1.66% | 0.73% | |||

Relative range | 9.17% | 3.91% | 1.98% |

Item | 1 | 2 | 3 | |||

OD | Flow | OD | Flow | OD | Flow | |

Suction pipe 1 | 15 | 19.73 | 14.8 | 19.95 | 14.4 | 20.68 |

Suction pipe 2 | 14.6 | 20.07 | 14.6 | 19.97 | 14 | 21.01 |

Suction pipe 3 | 13.8 | 22.45 | 13.8 | 22.2 | 13.6 | 22.7 |

Suction pipe 4 | 13 | 23.76 | 13.4 | 23.18 | 13.4 | 23.38 |

Relative variance | 3.75% | 1.76% | 1.43% | |||

Relative range | 9.97% | 4.88% | 4.05% |

### Design of variable diameter nozzle

Similarly, the inner diameters of sewage suction pipes 1-4 are all 15.3mm, and the suction nozzle height is 3mm. The analysis is conducted with different suction nozzle inner diameters. The results are shown in Table 4.

Calculate the flow rate based on the flow rate and cross-sectional area of the sewage suction pipe, and then adjust the suction nozzle diameter based on the difference between the flow rate and the average flow rate. Finally, it is concluded that when the nozzle size in analysis 3 is used, the relative variance decreases from 3.53% when the nozzle is of equal diameter to 1.43%. The relative range was reduced from 9.24% to 4.05%, achieving the optimization purpose.

## In conclusion

This article applies computational fluid dynamics to analyze the flow field of the self-cleaning filter suction device. And change the structure of the dirt suction device to achieve the purpose of uniform dirt suction. The conclusions are as follows:

- When the sewage pipes have equal diameters, the relative variance of the sewage suction pipe flow rate is about 3.29%. The relative range is about 8.51%, and the flow rates of each sewage suction pipe vary greatly.
- Analysis of the flow field of the sewage suction device under different variable-diameter sewage pipes shows that increasing the diameter of the end of the sewage pipe within a certain range is beneficial to the balance of the pressure drop in the flow field of each sewage suction pipe, thereby improving the flow distribution.
- Reasonable adjustment of the inner diameter of each sewage suction pipe or suction nozzle can ensure that the flow rate of each sewage suction pipe is equal and the sewage suction capacity is similar.