Radial Turbine-Diffuser Interaction

Abstract

This document contains the final Master Thesis Report to obtain the Master of Science on Aerospace Engineering (Flight Performance & Propulsion - Propulsion & Power) at the Delft University of Technology.

The research work focuses on characterizing the interaction of a radial inlet turbine with a downstream diffuser through the size of the rotor tip gaps. In the implementation of power turbines it is common to add a diffuser downstream of it to lower the rotor exhaust pressure and thus increase power extraction for the same inlet conditions. However, diffusersare bulky and take a lot of space in the assembly. This lowers the power density of the machine, increases installation weight in transport applications, and installation costs in ground based operations. In the quest to obtain compact diffusers, researchers noticed that the non-dimensional static pressure recovery (Cp) of this device was higher when operating downstream of a turbine than with uniform inlet conditions. The publications in this field are relatively scarce, and thus there is a knowledge gap with immediate practical application.

Some researchers have focused on the interaction of turbine vortical structures with the boundary layer of the diffuser. They have found out that under this conditions the boundary layer can support steeper pressure gradients without detaching. This is only applicable in very steep diffusers that would stall in isolation. Notably, all the publications in this field deal with axial machines, and as the text will show the picture changes considerably when applying the theory to radial turbines.

This work studies another side of the problem. The text will focus on stable diffusers, and thus there is no boundary layer that needs reinforcement. Turbine rotor tip gaps generate an increase of entropy and reduce turbine power generation. However, these gaps also cast powerful vortexes that affect the diffuser flowfield. This project will study the effect of the tip gap configuration on the pressure recovery of the diffuser, in order to better understand the trade-offs and support the design process. Tip gap sizes are
usually determined from mechanical constraints. This work provides insight into the real cost of a tip gap by analyzing the assembly turbine + diffuser, and thus it guides the designer into where to focus his efforts when working with these mechanical constraints.

The general conclusion of the project is that there is a range of tip gap sizes where the performance of the diffuser in enhanced. In this range, the performance cost in the rotor for increasing the tip gaps is partially compensated by a better pressure recovery in the diffuser. Furthermore, it has been discovered that in a radial turbine it is more influential to close the leading edge gap (axial gap), even when this implies opening the trailing edge (radial) one. This information will guide engineers when choosing the bearings for the machine and dealing with design trade-offs.

As a byproduct of this research, a novel study in radial turbine flow structures is carried out. Not a lot is known about the vortical structures in these machines, and this work provides a first qualitative description about origin and behaviour of this vortexes. Furthermore, different flow modes are identified and correlated to a simple geometrical parameter that is readily available in 0D designs. The combination of this information with previous tip gap flow models, and diffuser-vortex interaction models, will allow the
obtention of improved losses models including the effect of the diffuser from the conceptual design stage. This data could also feed into noise models for radial turbines, a novel field of interest. This work not only provides useful design guidelines, but also sets down the basis for future research leading to a better
understanding and modeling of radial inflow turbines.