7th International Conference on Compressors and their Systems 2011

IMechE

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN:
  9780857092083
• eBook ISBN:
  9780857095350
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

This book contains the papers presented at the 7th International Conference on Compressors and their Systems at City University London in conjunction with the IMECHE. This conference is the ultimate global forum for reviewing the latest developments and novel approaches in compressor research. It features contributions from equipment manufacturers, suppliers, users and research organisations; these papers present developments in air, gas and refrigeration compressors; vacuum pumps; expanders; and related systems and components. Papers cover the design, development and operation of a wide range of compressors and expanders. Equipment manufacturers, suppliers, users and research organisations are all represented. Aspects covered include: present and future developments in scroll compressors; design and optimisation of screw compressors; latest thinking in oscillating and vane compressors; improving the function of valves; latest research in dynamic compressors; detailed analysis of reciprocating compressors; improved accuracy and usefulness of modelling techniques; developing better control of centrifugal compressors; and reducing unwanted noise and vibration. - Presents all the papers of the International Conference on Compressors and their Systems 2011 - Up to date papers on compressor technology improvements - The latest prediction modelling techniques are presented

Screw Compressors

Evaluation of various turbulence models in predicting screw compressor flow processes by CFD

M. Kethidi, A. Kovacevic, N. Stosic and I.K. Smith, Centre for Positive Displacement Compressor Technology, City University London, UK

ABSTRACT

Computational Fluid Dynamics (CFD) plays an important role in modelling and design of positive displacement screw machines. It is used mainly to identify and quantify effects of internal flows which cannot be modelled by use of other methods. Although significant improvements have been made to achieve accurate prediction of the overall performance, there are still aspects which have not been fully evaluated, such as the influence of turbulence modelling on local velocities. The usual practice is to use k-ε model but it sometime fails to predict local velocity values in the compression chamber and ports measured by Laser Doppler Velocimetry (LDV).This paper evaluates and compares various readily available turbulence models in screw compressor performance predictions. Modelling of the screw compressor with three different turbulence models on three different mesh sizes is performed by use of commercial CFD software ANSYS CFX v11. These predictions are then compared with the measurements obtained by LDV. It is shown in the paper that the flexibility of the developed procedures together with appropriate turbulence modelling creates further opportunities for a broader use of CFD for analysis of twin screw machines in a range of new applications.

1 INTRODUCTION

Constant need for high pressure gases and vapours in various industrial applications has increased the demand for improvements in the design and manufacture of screw compressors. Numerous research activities undertaken during the past decade have increased both reliability and efficiency of the machine. Some of the attributions include 3D numerical analysis of screw compressors [1], grid-generation in screw compressors [2], 3D numerical performance estimation [3], solid-fluid interaction in screw machines [4], prediction of flow generated noise in screw machines [5], cavitation modelling in gear pumps [6], and flow in multiphase pumps.

In recent years there has been a steady growth in the use of Computational Fluid Dynamics (CFD) as a means of calculating 3-D internal and external flow fields [7], [8], [3]. It is widely used today for estimating flow in screw compressors. Specialized codes have been developed for this to increase the speed of calculation. But there is still a need to reduce computational time and increase the accuracy of the results. This requires the development of even more specific procedures. Previous study [9] in which the flow measurement results obtained by Laser Doppler Velocimetry (LDV) in screw compressor flow domains were compared with predicted values, suggested that errors in modelling exist and that improving the turbulence modelling can provide more accurate and faster CFD calculations.

In spite of the numerous publications in the field of screw machines, very few authors have analyzed the effects of turbulence on accuracy of predictions. Examples are the work of Vimmr 2006 [7], [10], who analyzed the flow of a single leakage path through a static mesh at the male rotor tip to conclude that the rotor relative velocity in that region does not affect flow velocities significantly and that none of the turbulence models used change the modelling outcome significantly. This agreed with the findings of Kovacevic et al [1], who later confirmed that further validation of full 3-D CFD calculation results could not be obtained by the use of simplified numerical or experimental methods. However, it was also shown that the use of alternative differencing schemes and turbulence methods influences local velocity and pressure values in certain machine regions. Although these differences have a low impact upon the overall performance [1], their influence upon the flow development needs further investigation. For this, a full understanding of the effects of turbulence in the machine suction, compression and discharge chambers is needed. The material presented in this paper is a part of the long term research project aimed to investigate, develop and validate suitable turbulent models for accurate CFD calculation in screw compressors.

2 CFD ANALYSIS OF SCREW COMPRESSOR

Following the validation of the CFD results presented in [10], a study was performed to evaluate differences in results of well known turbulence models for CFD calculation readily available for use with commercial CFD software ANSYS CFX. These initial results will then be used to compare with the measurements of internal velocities by use of Laser Doppler Velocimetry (LDV) made by Guerrato et al [9]. This study should help to decide which of the more complex turbulence model should be evaluated further and give clear directions for further investigation into the effects of turbulence modelling on the results of CFD calculations in screw compressors.

The experiment was performed on an oil-injected screw air compressor with 5/6 lobe configuration of ‘N’ rotor profile rotors. The compressor was operated and measured in a near oil free mode in order to avoid building up of the oil film deposit on the window for the laser beam [9], [12]. By studying an oil free compressor the effects of different turbulence models can be analyzed avoiding other effects, such as oil injection. The male and female rotor outer diameters are 128 and 101 mm respectively with centre distance of 90 mm. The rotor length to diameter ratio is 1.65. The compressor working conditions are, the shaft speed is 1000rpm and pressure ratio of 1.0.

The numerical mesh for the case study is shown in Figure 1. The flow paths around the rotors are obtained using the in-house software SCORG. The...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Sponsors
• Foreword
• Introductory Papers
• Turbo Compressors
• Heat Pump and Refrigeration
• Reciprocating Compressors (Valves)
• Refrigeration
• Reciprocating and Variable Speed Compressors
• Manufacturing
• Screw Compressors
• Scroll and Turbo Compressors
• Rotary Compressors
• Author Index