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The compressor flow is fully described by the mass averaged equations of continuity, momentum, energy, species (oil and liquid phase) and space equations, which are closed by Stockes, Furiers and Frick’s laws and  accompanied by the equations of k-e turbulence model and equation of state to form closed system. In the slide, equations are written in their integral form in which the governing laws are best expressed and which are easiest to solve.
•Despite a large number of publications in the CFD field, little has been written on the use of this technique for the analysis of flow through screw machines. This is mainly due to the complexity of both their geometric configuration and the flow through them.
•Some CFD codes are already available which seem to be able to cope with screw compressor geometry, but they need considerable development in order to obtain useful results with them.
•The aim of this study was to develop method for the flow and thermodynamic calculation with any commercial CFD code which fulfils specified requirements:
- Finite volume method is used for solution of governing equations of mass, momentum, energy, species and space,
- Can conduct calculations on block structured hexahedral mesh with moving domains and sliding boundaries with implicit cell connectivity,
- Has open and accessible user subroutines for source terms in transport equations, initial and boundary conditions. •Novel features are then introduced and the numerical grid is made by:
- Advanced analytical grid generation process with Hermite transfinite interpolation and multidimensional stretching functions,
- Multiparameter boundary adaptation and orthogonality, smoothnes and regularity check procedures. •Developed is stand alone program which creates, input files and user subroutines for a CFD code that:
- Reads vertices, cells and regions,
- Calculate sources of liquid phase and oil in mass, momentum and energy equations
- Reads initial and boundary conditions,
- Keep constant pressures at the inlet, outlet and oil ports through the sources in energy equations,
- Calculate thermodynamic properties of real fluids.
•Although this approach sound simple and easy there are some problems associated with difficulties in operation as well as with numerical analysis of screw compressor: •Working chambers with different pressure in the screw compressor are separated by clearances which in order to seal adequately have to be as small as possible. Geometry ratio between size of the working chamber and clearances is therefore high which is visible on these three figures where the numerical mesh in clearances is enlarged for the reasons of comparison. •Depending on the position in the compressor flow regimes become either laminar or turbulent which is caused by different velocity distribution. Velocities in some places can easily become transonic as the consequence of high pressure differences. •High pressure gradients can appear in the screw compressor especially in the later phase of the compression process. As the consequence of high pressure and reduced volume high temperatures appear in specific parts of screw compressor. •High temperatures and pressures cause the distortion of rotors which further on make clearance gaps smaller. This further increase velocities but in some extreme cases can cause direct contact of rotors in which case compressor can easily seize causing unwanted compressor failure. •All this together with possibility of appearance of multiphase flow in case of oil injection or when processes of phase change occur, make numerical simulation difficult and cause compressor to fail in proper operation.
Some additional features, which are not usually incorporated in CFD code are presented in this slide. IN order to solve multiphase flow additional equations for species should be solved. There are two of them, one for each oil and liquid phase.
Some additional features, which are not usually incorporated in CFD code are presented in this slide. IN order to solve multiphase flow additional equations for species should be solved. There are two of them, one for each oil and liquid phase.
•The first requirement for its use is therefore to develop an interface, which will enable screw compressor geometry to be used within standard codes.
•Grid generation is extremely important part of FV CFD analysis. Overall result is dependent on topology, orthogonality and smoothness. Grid topology strongly affects accuracy, efficiency and ease to generate proper solution. According to FV method requirements the most accurate  and the fastest solutions are obtained with full structured block generated hexahedral 3D mesh.
•To generate such mesh screw compressor domain has been divided into a number of sub-domains:
- Male rotor with radial clearances,
- Female rotor with radial clearances on it,
- End clearances,
  Such mesh contains all connections between the rotors and the housing and include the interlobe, tip and blow-hole leakage paths.
•The suction port is divided into five subdomains while discharge chambers consist of two subdomains.
•Inlet and outlet receivers are then associated to the compressor grid.
•The mesh calculation was based on an algebraic transfinite interpolation procedure. This included stretching functions, to ensure grid orthogonality and smoothness. An iterative procedure was used to obtain a satisfactory grid.
•
•The first requirement for its use is therefore to develop an interface, which will enable screw compressor geometry to be used within standard codes.
•Grid generation is extremely important part of FV CFD analysis. Overall result is dependent on topology, orthogonality and smoothness. Grid topology strongly affects accuracy, efficiency and ease to generate proper solution. According to FV method requirements the most accurate  and the fastest solutions are obtained with full structured block generated hexahedral 3D mesh.
•To generate such mesh screw compressor domain has been divided into a number of sub-domains:
- Male rotor with radial clearances,
- Female rotor with radial clearances on it,
- End clearances,
  Such mesh contains all connections between the rotors and the housing and include the interlobe, tip and blow-hole leakage paths.
•The suction port is divided into five subdomains while discharge chambers consist of two subdomains.
•Inlet and outlet receivers are then associated to the compressor grid.
•The mesh calculation was based on an algebraic transfinite interpolation procedure. This included stretching functions, to ensure grid orthogonality and smoothness. An iterative procedure was used to obtain a satisfactory grid.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
•The rack generation procedure is a starting point for the discretization process of the screw compressor working chamber. •The rack, which is in fact the rotor with infinite radius and with the shortest lobe length, always lies between rotors. While the rotor has a rotation during the working process, imaginary rack has a motion of translation dividing the working chamber, which is positioned between rotors and a housing, into two parts. That phenomenon allowed discretization of the complicated working chamber which rotates, slides and stretches in time. •This procedure is described in literature and incorporated in SCORPATH – Screw Compressor Optimal Rotor Profiling And Thermodynamics •For the process of the rotor working chamber discretization only few parameters are necessary. These are: the rack file with x an y coordinates of the rack lobe, rotor configuration (number of teeth) and the rotor axis distance. •Additionally, clearance distribution between rotor and rack as well as the radial distribution between rotor and outer circle are applied to make possible calculation of the entire working chamber. Clearance distribution file is the next required item for grid generation. •Rack should be enclosed with the outer rotor circle that forms one edge of the cross section perpendicular to the rotor axis. The other edge is the rotor profile produced by the rack generating procedure.
•
Grid generated by use of SCORG is shown in the slide. 2D numerical mesh in the compressor cross section is shown in the right bottom corner. In the top left corner, 3D moving mesh of the male rotor working domain is shown.
Grid generated by use of SCORG is shown in the slide. 2D numerical mesh in the compressor cross section is shown in the right bottom corner. In the top left corner, 3D moving mesh of the male rotor working domain is shown.
Grid generated by use of SCORG is shown in the slide. 2D numerical mesh in the compressor cross section is shown in the right bottom corner. In the top left corner, 3D moving mesh of the male rotor working domain is shown.