Modelling and computer simulation of rotor chatter and oscillating bearing forces in twin screw compressors
Abstract
A model of the rotor interaction in twin screw compressors is developed, and implemented via a computer simulation. Geometric parameters of the rotors are used to develop a kinematic analysis of the rotor motion, and to compute the loads associated with the compression process. The main objectives are to provide the ability to predict backlash type rotor vibrations (chatter) and compute the bearing forces. The rotor surfaces are defined by the 2-dimensional rotor profiles and the helix angle associated with each rotor. This geometry is used to identify the kinematic constraints which apply to the rotor motion based on gear theory. The nature of the contact between the rotors and the resulting force transmission is investigated. The rotor profiles are designed to obey the laws of conjugacy. This fact is used in the development of an iterative procedure for generating mating rotor profiles. The forces and moments due to gas compression are computed using vector calculus principles to integrate the chamber pressure over the rotor surfaces. The 3-dimensional surface of each rotor is mapped to a 2-dimensional region and the pressure integrated over the region. The general method is presented, with the details for a specific compressor configuration given. Of primary interest is the accurate computation of the moment load, due to compression, about the axis of rotation for each rotor. Through these computations, it is demonstrated that the moment load induced on the female is approximately 10% of that induced on the male. Therefore, the female rotor motion approaches that of an idler gear. The bearing forces are computed for a specific compressor configuration for various operating conditions. The total bearing force is separated into components caused by the contact force between the rotors and components caused by compression loads. The component due to compression loads dominate the bearing forces. A frequency analysis demonstrates that the frequency content of the bearing forces does not decrease substantially until beyond approximately the 8$\sp{th}$ harmonic of the fundamental screw frequency.
Degree
Ph.D.
Advisors
Soedel, Purdue University.
Subject Area
Mechanical engineering|Aerospace materials
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