Background knowledge of twin-screw compressors and CFD applications

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Background knowledge on twin-screw compressors and the application of CFD. Overview of screw compressors: Twin-screw compressors generally consist of a pair of male and female rotors, as well as components such as machine bearings and shaft seals. Advantages: High reliability, easy operation and maintenance, good power balance, strong adaptability, and capability for multiphase fluid transport. Disadvantages: High cost; not suitable for high-pressure applications (above 4.5 MPa); cannot be designed in miniature form. Application areas: Widely used in industries such as mining, chemicals, power generation, metallurgy, construction, machinery, and refrigeration. Statistical data show that screw compressors account for over 80% of all sales of positive-displacement compressors. Classification of screw compressors: https://pic1.zhimg.com/v2-f6c6c5e10c994f2c0cb40d735b86a818_b.jpg The term “oil-free” refers to the situation where the gas does not come into contact with oil at any point during compression, meaning there is no oil lubrication within the compressor’s compression chamber or between the rotors. However, components such as bearings and gears in the compressor are still lubricated using conventional lubrication methods; it is only that effective shaft seals are used to isolate these lubrication areas from the compression chamber. In rotary vane compressors, a large amount of lubricating oil is sprayed into the gas being compressed, serving to lubricate, seal, cool, and reduce noise. There are no synchronous gears in the fuel injection machine; the pair of rotors functions like a pair of gears, with the male rotor directly driving the female rotor to rotate. Therefore, the structure of the fuel injection machine is simpler. Working principle of screw compressors 1. Intake process: The volume of the space between the teeth gradually increases as the rotor rotates, and this space becomes connected to the intake port. When the rotor rotates to a certain angle, the volume of this space exceeds that of the intake port, at which point it is disconnected from the intake port, and the intake process is complete. 2 Compression process: The meshing surface gradually moves toward the exhaust end, the gap between the meshing surface and the exhaust port gradually decreases, the gas within that gap is compressed, and the pressure rises. 3. Exhaust process: The meshing end face is connected to the exhaust port, allowing the compressed gas to be expelled until the exhaust port is closed. https://pic3.zhimg.com/v2-776e0873435658e2408dc14ab1c7b2c6_b.jpg Key aspects of screw compressor technology: 1. Research on rotor profiles and tooth shapes. 2. Impact of the injection process on compressor leakage, heat transfer, and friction. 3. Study of thermodynamic processes. 4. Investigation of cavitation performance. 5. Analysis of rotor vibration and noise. Key aspects of CFD technology: 1. The extremely small clearance between the male and female rotors. 2. The need for a high-quality initial mesh. 3. Since the rotor area experiences significant deformation, dynamic meshes are required. 4. It is necessary to take into account fluid dynamics, thermodynamics, and even cavitation. 5. Flow-induced vibration must also be considered. 6. The gas-liquid two-phase nature of the injection process needs to be taken into account. SCORG&PumpLinx solution: https://pic1.zhimg.com/v2-94c9266a48aeb641eabbea2aa60e5330_b.jpg Technical features of SCORG software: 1. Geometric model analysis: Profile editing, design of inlet/outlet channels, analysis of how the suction and discharge areas change with rotation angle, analysis of how the 2D area and 3D volume of the working chamber change with rotation angle. 2. Thermodynamic model calculations: Quasi-one-dimensional formulas, prediction of gas pressure and temperature, prediction of parameters such as leakage, efficiency, and power, setting of CFD boundary conditions. 3. 3D mesh generation for screw machinery: Mesh deformation in the rotor area, generation of meshes for inlet/outlet ports; seamless integration with PumpLinx. Advantages of PumpLinx: 1. Templates for twin-screw pumps/compressors, setup of dynamic meshes for the rotor area, calculation of model parameters, setting of boundary conditions, efficient post-processing outputs. 2. Handling of compressible fluids and heat transfer, prediction of pressure, flow rate, efficiency, etc., prediction of thermodynamic properties. 3. Spectrogram analysis to help identify vibration and noise; animations, traces, contour plots, and velocity vectors for summary. SCORG can automatically generate structural meshes for the twin-screw area, taking into account micro-level clearance gaps, and it integrates seamlessly with PumpLinx. PumpLinx can automatically set the moving mesh in the rotor area of twin-screw compressors, reducing the difficulty of such setup. The PumpLinx solver is stable and fast, with good convergence stability. The calculation results can be exported to structural or noise software for further coupling analysis. Key features of PumpLinx VOF technology · Strict adherence to mass conservation · Volume conservation for incompressible terms · Ability to consider the compressible/incompressible nature of any term · Option to choose between implicit and explicit time formats · High-resolution method for capturing interfaces · Surface tension model · Wall contact model · Suitable for two-phase flow problems with distinct gas-liquid boundaries (gearbox lubrication, liquid ring pumps, agitators, dam failures, pump self-priming, etc.)