Design And Development Of A Mixed Flow Compressor Impeller For Industrial Applications

Abstract

The requirement of compressed industrial air/gas has always been a costly affair. Centrifugal and axial type of compressors are commercially available which meets the demands of pressure ratio or mass flow rate per stage according to their flow regime. However, if the flow path is so designed (modified) as to accommodate the advantages of both of these discrete family of compressors to a limited extent than, such machine would have a diagonal flow at exit and run at specific speeds higher than conventional centrifugal types to accommodate a fair amount of mass flow rate while developing a suitable pressure ratio. A higher specific speed would thus lead to a direct reduction in size of the compressor. A favourable future of such a family would be a drastic reduction in overall size of multi-stage compressors for industrial applications. The research available in the design of such compressors is very limited in public domain and hence, a thorough review of the available literatures is discussed as a first basis in this thesis. Subsequently a brief overview of the different compressors used presently at commercial level is discussed as a motive to implicate the need of improvising the selection of compressor for a particular application. In present work, to explore the possibility of such a mixed flow compressor, a turbocharger application is selected whose turbine shaft is running at 1,50,000 rpm and delivers compressed air at a total pressure ratio of 3 against an available shaft power of 25 kW. A 1D approach is then utilised to obtain preliminary dimensions of the mixed flow impeller by modifying its velocity triangle for different exit flow angles viz., 45°, 60° and 90°. The flow through the meridional path is analysed initially for each exit flow angle using the streamline curvature method available as VistaTF module of Ansys. Necessary changes are then implemented in geometry to give a preliminary decision of the geometry for undergoing full 3D solution. Impeller with 60° exit flow angle is found to give the good results and its flow field is thus analysed numerically using Ansys CFX and TurboGrid as a meshing module. An attempt has been made then to numerically assess the internal flow field of the passage at design point which gives the initial idea to control flow separation and thus improve its efficiency. By changing the blade curvature improvement in efficiency from 81% to 88% is obtained which is noteworthy but mechanical considerations may impose strict limitations. Effect of constant tip clearance (0.1 mm to 1 mm) and variable tip clearance (5% to 1% of the blade height) at design point is also carried out and it is seen that an increase in tip gap reduces the machine performance. A comparison of different variables as predicted by design, VistaTF and CFD results is drawn and it is found that the deviation between CFD and design is quite high while that between VistaTF and CFD are low and thus necessitates improvement in design strategy. Overall, the analysis dealt herein has revealed the possible potential of a diagonal flow compressor impeller to outperform conventional centrifugal compressor for the given design conditions.