CFD Analysis and Optimisation of Triple Volute Exit Diffuser: Vaned Vs Vaneless and Vane Design

Abstract

At the heart of any high performing centrifugal compressor design is a well designed, and well matched, impeller and volute. Connecting these two pieces of geometry is the diffuser. Diffuser size is a constant trade off: increase diffuser size and stand to recover more static pressure, albeit with a larger total pressure loss and, perhaps more importantly in some applications, larger physical size and material cost. On the other hand, a small diffuser can mean less total pressure loss and less physical size, particularly important in applications where space is at a premium, such as in automotive turbochargers. An issue with this is there is less space and time for the flow to adequately diffuse (lower Mach number), this can lead to large compressible losses especially at the diffuser – volute interface where the flow area suddenly expands. To try to achieve high pressure recovery in a smaller space vaned diffusers are used. Vaned diffuser allow for increased pressure recovery/higher static pressure output but come at the expense of operational flexibility and often perform poorly away from their intended design point. This work begins by examining the relationship between static pressure recovery, total pressure loss and vaneless diffuser radius size. It can be shown that there is a point at the flow is able to sufficiently diffuse without incurring the larger total pressure losses that come with a much larger diffuser. It also shows the effect of compressible losses at the diffuser – volute interface for smaller radius value diffusers. Vaned diffuser design is then explored in a diffuser of smaller radius value in an attempt to replicate or even exceed the performance of the vaneless diffuser. Vane parameters such as vane thickness, chord length, maximum thickness point along vane chord point, vane numbers and angular offset of vanes from key points, such as the volute tongue. The test subject for this work is a high speed centrifugal compressor using air as it’s working fluid. Perhaps to add novelty the volute is a triple exit volute and so the interaction between impeller blade numbers (main and splitter), vane number and volute exit number will be examined. It will be shown that vane angular offset from the tongue(s) is key even if the vane number and size is held constant. This work is conducting using Ansys CFX and key geometric drafting, meshing and solution method will be provided.