A compressible flow theory for regenerative compressors with aerofoil blades

Regenerative flow compressors (RFCs) are rotodynamic machines capable of producing high heads at very low flowrates. They have very low specific speed and share some of the characteristics of positive displacement machines such as a roots blower, but without the problems of lubrication and wear. They can produce heads equivalent to that of several centrifugal stages from a single rotor with comparable tip speed. The compression process is usually not regarded as efficient. Typically they produce efficiency of less than 50 per cent but still they have found many applications because they allow the use of fluid dynamic compressors in place of positive displacement compressors for duties requiring high heads at low flowrates. There are very few mathematical models in the literature that explain the behaviour of regenerative turbomachines and predict the performance. Most of these models assumed incompressible flow, thus limiting their use to only pumps and blowers. Moreover, they needed extensive experimental support for performance prediction. Hence, it is very interesting from an industrial point of view to find efficient theoretical means that are able to forecast regenerative compressor performances, using easy to find geometric and fluid dynamic parameters. A compressible flow theory is thus presented for the first time in this paper to describe complex three-dimensional corkscrew flow patterns in regenerative compressors. Conventional RFC were designed with radial, non-radial or semicircular impeller blades. In the present investigation, the authors have discussed RFCs with aerofoil blades and an annular flow channel containing a core to direct circulating flow to the blades with a minimum amount of losses. The effects of various geometric elements on the performance of RFCs are studied. All the major sources of losses in blade and channel region are identified. Governing equations for the flow in the compressor are derived and a performance prediction code based on governing equations and loss models is developed. Theoretical performance results are compared with published test data on aerofoil blade RFCs. Based on sensitivity analysis from the code, design changes are suggested for performance improvement.