Cavity Purge Flows inside axial turbines

Turbomachinery in its various applications form the principal prime mover in the energy and aviation industries. Any improvement to this vast fleet of machines has the potential of significant impact on global emissions. Areas identified to benefit from continued research are the topics of flow mixing and cooling.
These are topics inherent in stationary gas turbines and jet engines due to the hot gas flows utilized. Cooling is achieved through injection of cold air in critical areas and thereby ensuring safe operation. The cooling however comes at a cost. On the cycle level this flow requires power to be compressed to the appropriate pressure, but does not contribute to the cycle output. In addition, the injection itself reduces the output power due to the losses associated with the mixing process. The study is centered to a turbine testing facility allowing detailed flow measurements in a rotating turbine stage under the influence of the cavity purge flow and also develop CFD models which can simulate the flow physics accurately.

Funded by:

SIEMENS TURBOMACHINERY AB, Finspång

Time period:

01-11-2018 to 01-11-2020

Project partners:

KTH & SIEMENS TURBOMACHINERY

Background

The study is centered to a turbine testing facility allowing detailed flow measurements in a rotating turbine stage under the influence of the cavity purge flow. General performance is quantified by measurement of the output torque. Flow details are quantified through pneumatic probes, and cooling performance is predicted through gas concentration measurements. The CFD model is based on the design of the SIEMENS Test rig design on which both steady and unsteady simulations are being conducted

Aim and objectives

Understand the impact of varying cavity purge flow rate on the aerodynamic performance of the turbine.
Use concentration measurements to trace the mixing of the purge flow with the annular flow. Also comprehend the variation of the flow radially and in pitch (stator) to check for zones of ingress and egress. Furthermore, validate this with the CFD predictions as well.
Visualize the change in the flow structures within the cavity with change in purge flow rates and reynold’s number. Also attempting to identify the instabilities and finding an optimal flow rate to avoid instabilities.
Understand the effect of changing pressure ratio across the stage on the purge mixing and flow behavior. Additionally, conduct unsteady analysis of the purge mixing and rotor wake to identify frequencies of the instabilities and their sub-harmonics especially in relation to tip vortices and the leakage flow from rotor tip gap.
Validate the results with findings from University of Bath who have an identical test rig and then conducting the same experiments at higher operating points of the turbine.

Outcomes

So far:

Global performance analysis have revealed maximum efficiencies achieved at the design operating conditions and identified parameters that can help improve this further.
Concentration measurements revealed similar behavior to Bath results at high radial positions, however low radial positions have slightly different flow phenomenon which will be investigated.
Steady initial CFD simulations show similar behavior to experimental data, but the Torque is a little undermined which is also under investigation and improvement.