Author: Alex Mesny -
In a gas turbine, air is drawn in, compressed to high pressures, and the addition of combusted fuel raises the temperature of this gas to extreme levels. Components of a gas turbine, just downstream of where the fuel is added have to survive temperatures close to 1000K above that of the melting point of even the best superalloys. The high-power requirements of modern gas turbines used in aircraft engines and power generation necessitates high rotational speeds in the order of 10,000 RPM.
The hot, high-pressure and high RPM environment is not exactly hospitable for precise measurement techniques, nor is the cost associated with running a gas turbine at operational conditions. The turbomachinery research centre would also be even louder than it already is if we had actual gas turbine engines in the basement of 4East; and apparently this is considered less than ideal.
Modelling the problem
Given that it is impractical to learn any detailed information from an operational gas turbine, it is necessary to model the problem with scaled down flows and rotational speeds. Through the use of ambient temperature compressed air and geometrically similar features such as the blade profile, detailed information about the aerodynamics inside of the stage. But then of course, the conditions of the experiment are now very dissimilar to that of the engine. The information is still useful, or else I’d be out of a job!
By getting the ratio between the rotational speed and mainstream mass flow rate correct, a scaled down gas turbine can represent a very similar aerodynamic problem to the “real thing”. The main problem is that the interactions between the gas, stationary vanes, rotating blades and the cavities between them is highly complex and unsteady. Computational Fluid Dynamics (CFD) is a powerful tool, if you can be certain that the answer it gives is correct. Particularly in this complicated flow field, the experimental data helps to validate the CFD models.
The goal of the turbomachinery research centre is to increase the efficiency of gas turbines. A source of inefficiency is found with the turbine blade passage. The arrangement of aerofoils in close proximity leads to the formation of vortices. These structures, known as secondary flows, are a loss of energy into kinetic energy that does not contribute to the power generation of the turbine. The secondary flows are further enhanced by the presence of coolant air that intensifies the vortices, further sapping the efficiency.
Taking a snapshot of the flow
Did you know that olive oil particles of the right size have the exact aerodynamic properties required to track the streamlines in a flow? I didn’t, so imagine my surprise when I find bottles of the stuff strewn about a turbomachinery lab. By injecting the research turbine with particles of olive oil, and illuminating them with a high-power laser, pictures can be taken that snapshot the flow. Using three cameras and two sets of images taken with a very small time delay, it is possible to trace out the exact path that the air takes through the turbine. This allows me to visualise the secondary flows in unprecedented detail for an experimental turbine and gives a great understanding into the mechanisms that govern the loss of efficiency in gas turbines.
This technique, known as Volumetric Velocimetry will be used to investigate new turbine designs featuring endwall contours, a shaping of a previously under-utilised surface in the design of turbine blades. The effect of these complicated three-dimensional shapes have on the secondary flows will inform the design of the next-generation of gas turbine engines.
These engines mark a transition away from traditional fossil fuels, with the Siemens Energy goal to transition to hydrogen as fuel. Gas turbines will remain the most efficient means of burning a combustible fuel in air. The hydrogen burned will be used as energy storage to provide flexibility in a renewable-energy based grid.
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