Centre for Compressor Technology

Supervisor: Dr Matthew Read

Bearings are a key component of rotary positive displacement machines, where both axial and radial loads must be supported.  There are a range of bearing types available, with rolling element and hydrodynamic most commonly used.  Other technologies used in higher specific speed applications such as turbomachinery include hydrostatic, magnetic and foil gas bearings.  This project will aim to characterise the speed, load, and space requirements of bearings for screw machines.  The working fluid/lubricant is an important consideration, and the use of bearing types is constrained due to cooling and lubrication requirements.

Objectives:

• Review state of the art for bearings across a range of rotating machinery applications
• Characterise constrains for screw compressor bearings across a range of scales, working fluids and operating conditions
• Investigate influence of bearing selection on design and operating parameters such as operating speed, clearance gaps and drag losses

Supervisor: Prof Ahmed Kovacevic

The aim of the of the proposed research project is to set criteria for defining new carbon neutral lubricants for positive displacement machines and develop requirements and tools for selecting lubricants for new generation of environmentally friendly positive displacement machines. The objectives are:

• Review state-of-the-art literature on lubrication and lubricants in positive displacement machines and specify critical parameters which should be investigated for carbon neutral lubricants for PDM’s
• Develop analytical and numerical tools to evaluate manufacturability and application of novel lubricants for manufacturers and users of these lubricants for carbon neutral oil injected positive displacement machines.
• Validate the developed tools by extensive experimental investigation on a typical oil injected air screw compressor.

Supervisor: Prof Ahmed Kovacevic

Screw compressors are used in variety of applications which include compression of aggressive gasses, high pressure and high temperature applications, dry running conditions, water vapour compression etc. Rotors of such machines are mostly manufactured using classical machining methods including hobbing, profile grinding and milling.  They are mostly produced from carbon steel, in some cases covered by protective coats. Materials such as stainless steel or aluminium are occasionally used to protect material from degradation. Despite considered to as mature technology, little research has been invested in exploring materials and manufacturing techniques for future efficient and reliable positive displacement machines. With the advent in manufacturing methods and material science, there are possibilities to make rotors of positive displacement machines, in particular screw compressors, lighter and more resistant to environments and operating conditions. In addition, it would be interesting to find possible methodologies to introduce microscopic surface features and new rotor profiles using suitable modern manufacturing techniques and materials. The success of such application will depend upon economical and technical feasibility for rotating positive displacement machines such as screw compressors and expanders. The focus of the research should be towards applications of water injected screw machines and protection from corrosion caused by applications of aggressive gasses and vapours. Materials which could be evaluated should include polymers, ceramics, materials suitable for sintering, printing, dye casting and extrusion manufacturing methods.

The objectives of the proposed pre-competitive research project are:

• Review available literature and the state-of-the-art materials and manufacturing methods for application in screw compressor and expanders
• Develop analytical and numerical tools to evaluate manufacturability and application of novel manufacturing methods and materials for intended applications of screw compressors.
• Design and manufacture of prototype rotors and casings for advanced efficient and reliable screw machines and validate developed analytical and numerical tools.
• Experimentally investigate possibilities and advantages and disadvantages of new materials for a new generation of screw machines

Supervisor: Dr Sathiskumar Anusuya Ponnusami

Analysis and design of compressor systems involve tools ranging from conservative analytical methods to advanced computational techniques. Currently available tools have matured over years, leading to optimal designs of compressor and expander systems. In terms of the design, significant improvements have been made in the last couple of decades, leaving very narrow room for further improvements. In this context, the proposed project aims to exploit machine learning tools to identify further optimal compressor designs, unexplored before. One of the key design considerations in such systems is the rotor profile design and optimisation. Multiple research efforts have made to optimise the profile using iterative optimisation techniques, however, there is a limitation in terms of the parameters explored in rotor shapes.  In this context, the PhD project aims to develop a set of generative design techniques for rotor profiles using a class of AI algorithms, namely Conditional Generative Adversarial Networks (CGAN). CGAN is an architecture using deep convolutional neural networks (CNN) which generates images of relevance encoded with conditions. In the current context, the images are the rotor profiles, and the conditions are the thermodynamic performance metrics of the blade profiles. Upon training the CGAN using sufficiently detailed dataset obtained using thermodynamic calculations, the deliverable will be a handy tool that can yield rotor profiles for specified metrics. The underlying vision of this first step of a series of explorative studies is to close any gap in the optimality of current state-of-the-art blade profiles, while opening a new avenue of AI application in engineering systems.

Objectives: The overall objective is to realise a smart tool that yields new blade profiles with specified performance metrics and operating and manufacturing constraints. The PhD research would involve a series of research modules to realise the above objective:

• Establish an automated framework for data generation involving existing state-of-the-art analysis tools (profile-thermodynamic relationships).
• Develop an intelligent design platform for designing new profiles using CGAN without any recourse to usual optimisation procedures.
• Train, test and numerically validate the CGAN tool, followed by experimental validation/demonstration.
• Manufacturing of CGAN-derived blades and performance assessment to showcase the capability.

Supervisor: Prof Ahmed Kovacevic

Leakage flows play critical role on the performance of rotary positive displacement compressors. Such compressors today are used in refrigeration, air-conditioning, oil and gas, process industries and air compression and consume more than 20% of electrical energy generated in industrialised countries. Even small reduction in leakage flows will make significant savings and reduction of carbon footprint. To develop the high efficient screw compressor, the leakage characteristic needs to be acquired. However, the leakage characteristic is very difficult to obtain due to the complexity of geometry and process.

This PhD study will contribute to better understanding of the leakage characteristics of screw compressor by means of numerical and experimental methods. It will utilise existing compressor test rig and the numerical methods previously developed in the Centre.

Supervisor: Prof Ahmed Kovacevic, Dr Matthew Read

Unsteady Thermo-Fluid-Solid Interaction is a common fundamental physical phenomenon in leakage flows among various rotary machines. The effect of unsteadiness and conjugation have not been extensively studied previous and potentially this could lead to inaccurate or misleading engineering design strategies. In the open literature, there is a lack of consensus in fundamental understanding of the unsteady CHT analysis methods. Multi-physics modelling tools are currently available, but still in development stage. The remaining challenges being faced include the disparity in the time scales, computational efficiency, reliability of turbulence models, etc. In most cases, there are little experimental data to validate the accuracy of modelling methodology. This research will provide a series of benchmark experimental data using LDV and PIV techniques in optical Roots blower. These data will be used to understand the fundamental physics of heat transfer in leakage flows and support further development of the multi-physics modelling and multi-disciplinary design strategy for improved energy efficiency.

Supervisor: Prof Ahmed Kovacevic, Dr Sham Rane

Single and multiphase screw pumps usually have very large helix angles which makes it very difficult to use Computational fluid dynamics for estimation of their performance. The numerical procedures explained in literature are based on generation of the numerical mesh in a cross sectional area which follows the rotor helix and gives a good conformal mesh. However, if the rotor helix is large, the cell skewness becomes prohibitively large which introduces error in numerical simulation. This research is focused on method to develop numerical grid generation for screw machines with large helix angles to enable reliable and fast solution using generic CFD solvers. The developed methods should be validated by experimental results. The project will allow development of methodologies for evaluation of system dynamic performance, and the system nonlinear fluid-structure coupling.

Supervisor: Dr Matthew Read

There are a huge number of theoretical mechanisms which can be employed to achieve compression or expansion via positive displacement of the working fluid.  Significant academic and industrial resources are spent on analysing and developing many of these to a stage where the performance can be tested and compared to the relatively small number of established machine types.  There are however a small number of key geometrical performance metrics that can be used to investigate how novel concepts compare to conventional machines, thus allowing the potential benefits and drawbacks to be established at an early stage. This project will develop a robust methodology for comparing positive displacement configurations using non-dimensional groups to characterise the key performance metrics, while allowing operating constraints such as dimensions, load, and tip speed to be specified for different compressor types.  A further benefit of this approach is that initial multi-objective optimisation of novel configurations can be performed using a fitness function approach based on these non-dimensional groups.  This ensures a suitable basis for comparison between different machines configurations.

Objectives:

• Apply dimensional analysis to identify key machine characteristics and allow comparison on a normalised basis
• Use non-dimensional groups to achieve initial optimisation of a particular configuration within practical constraints such as maximum suction fluid velocity and bearing DN factor.
• Verify approach using conventional machine examples (where extensive performance data is available), and apply to novel configurations

Supervisor: Prof Ahmed Kovacevic, Dr Sham Rane

Twin screw machines are today widely used as compressors, expanders, pumps or motors in different industrial applications which either require or permit coexistence of gas and liquid in the system. These are known to be very reliable for handling clean fluids with relatively small content of liquid. To improve designs of such machine for multiphase flows of solids, fluids and gases it is necessary to fully understand flows in the inlet and internal passages. This research study will focus on experimental investigation of the suction and internal flows of such compressors by use of Laser Doppler Velocimetry in order to fully understand suction and internal multiphase flows. The study will help to develop experimental methods suitable for such investigation and will provide computational suite which will allow automated integration of measured results with the results of performance prediction obtained by numerical means which will further allow optimisation of machines for multiphase fluid handling.

Supervisor: Prof Ahmed Kovacevic, Dr Matthew Read

Cylindrical helical gearing profiles can allow an externally lobed inner gear to rotate inside an internally lobed outer gear while maintaining continuous lines of contact between the gears. The continuous contact between the ‘inner’ and ‘outer’ rotors (analogous to the ‘main’ and ‘gate’ rotors in a conventional screw machine) creates a series of separate working chambers. Ported end plates can be used to control the period during which fluid is allowed to enter or leave the working chambers of the internally geared screw machine. This novel configuration can be used as either a compressor or expander and has many potential advantages compared to conventional screw machines including smaller leakage paths, lower sliding velocities and more uniform thermal expansion. Research in this area can focus on a wide range of topics including:

• generation of rotor profiles
• investigation of viscous losses between co-rotating rotors
• effect of inter-rotor and end-face leakage flows
• rotor manufacturing methods
• deformation of the rotors due to temperature and pressure variations of the working fluid
• optimisation of machine geometry

Supervisor: Dr Matthew Read

Recent estimates suggest that the amount of heat rejected in industrial processes is greater than all renewable resources combined. The use of organic fluids in Rankine cycles has potential advantages for maximising the power generated in waste heat recovery applications. The cost of these systems is however high due to the low conversion efficiencies possible from such sources (typically only 10% or less). Recently, interest in power recovery from such heat sources has increased, particularly for smaller units where the number of potential heat sources is greatest. The use of Rankine Cycles with organic working fluids (hydrocarbons or refrigerants) instead of steam is well established for low temperature power generation, and the expansion of liquid or 2-phase fluid (rather that superheated vapour) has been shown to improve performance. Depending on the application, waste heat recovery can be achieved using single cycles, or as cascaded systems where the heat from the source is used to heat fluid in a high temperature cycle, the condenser of which provides a heat input to a low temperature cycle which uses a different fluid. This project will investigate the effect that the choice of single or cascaded cycles has on the performance and the specification of components in the system for particular applications. A range of expander, pump and heat exchanger technologies will be considered in order to identify suitable system configurations to enable greater uptake of this technology.