City Research Online

Abstract

An effective and efficient CFD simulation is of high importance to accelerate the design activities of twin screw machines. However, mesh generation for screw machines with large helix angles can produce highly skewed numerical cells which can make simulation unreliable. From the established literature, the two main approachesto generate structured deforming mesh for CFD analysis of twin screw machines are algebraic and differential. The purpose of this thesis is to explore grid generation techniques suitable for 3D numerical modelling of rotatory positive displacement machines with large helix angles. Both cut-cell cartesian and body-fitted grid generation methods are investigated for analysis of machines with different profiles and helix angles.

The conservation and simulation accuracy of cut-cell cartesian method was first evaluated using a simple piston cylinder example and a hook and claw rotor profile used in vacuum pump which is difficult to set by a body fitted mesh. It was shown that even with coarse initial mesh, such profiles could have been analysed using a cut-cell cartesian mesh, but it required special model to account for leakage flows. Secondly, screw machines with low helix angle are used to examine the capability of both grid generation methods. ANSYS CFX was used for analysis of body fitted mesh produced by SCORG grid generator, while ANSYS Forte was used for evaluation of cut-cell cartesian method. Thirdly, oil-injected twin screw compressor with intermediate helix angle was explored using body-fitted method. In addition, a case study of a twin-screw vacuum pump with higher helix angle of 62 degree was studied using SCORG and FLUENT to further explore the body-fitted method. Lastly, a twin-screw vacuum pump with variable pitch and cusp points was analysed using cut-cell cartesian method. These five case studies demonstrated advantages and disadvantages of body-fitted and cut-cell cartesian methods for low, intermediate, and large helix angles.

This thesis then proposes development of an alternative method called a normal grid generation method which is expected to improve mesh quality of a body fitted mesh for screw machine with large helix angles and in turn could improve accuracy of flow calculation.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QA Mathematics > QA75 Electronic computers. Computer science T Technology > T Technology (General)
Departments:	School of Science & Technology > Computer Science School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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