Numerical Investigation of Various Modelling Strategies for Throttling a Transonic Compressor Rig Diffuser

Abstract: The concern of this study is modeling of the flow field in the diffuser of a 1.5 stage compressor test rig in which the mass flow is constrained at the exit of the diffuser. Constraining method is is applied through, throttling by means of a slab at the exit of the diffuser, where the flow enters the exit volute. Mass flow constraining enables achieving different mass flow rates through the machine. For this reason, modeling of the flow field in the diffuser during the throttling stages and how the compressor will be affected by that, is the question this study is answering. In attempt to answer the research questions, Ansys CFX platform has been used for numerical investigation. Various geometries of the diffuser are studied to examine the sensitivity of outflow to modelling strategies of the upstream diffuser and how the flow parameters at the outlet of the upstream compressor stage would change accordingly. In this study various geometry designs are considered including simplification of geometry according to flow behaviour in the diffuser, horizontal nozzle diffuser and annular diffuser design including slab with three sizes of outlet area.. From the model setup it is realized that extending the height of the outlet of diffuser has a significant effect on the model stability although changing the outlet area does not impact the results in terms of mass flow rate and inlet pressure profile. In addition transient studies were carried out to assess how the outlet flow is changing with the variations in the flow field at the inlet. The results indicate that the fluctuations die out before reaching the slab and do not affect the outlet flow.