Applications of Additive Manufacturing to High-Temperature Combustion Chambers for Micro-Gas Turbines

Abstract

With the current demand for cleaner, more efficient, and durable sources of power in almost every market, research into multiple types of small-scale power sources is growing fast. Gas turbines are especially interesting to many researchers due to their proven low emissions, high power density and reliability. Efficiencies are directly affected though when attempting to apply gas turbine technology to smaller applications such as unmanned aerial vehicles, range extenders and decentralised heat and power units. This is due to the energy losses that occur within them (e.g., due to friction, heat dissipation, and leakage) being proportional to the size and power levels produced. Meaning at kilowatt ranges the losses are significant enough to negate the benefits of the technology for most applications.
The research reported in this thesis demonstrates how designing micro gas turbine combustion chambers for additive manufacturing can improve operating limits, which in turn increases overall system efficiency, increase cooling to lengthen service life, and improve air/fuel mixing to reduce emissions. The initial background research indicated that lean premixed combustion has the greatest potential to achieve the challenging goals laid down at the conception of this project, and also provided an excellent platform to demonstrate how additive manufacturing could be implemented to improve the key drawbacks, which include flashback, high-pressure losses, and the need for efficient passive cooling.
The architecture of the baseline additively manufactured combustor is a tubular type with reverse airflow. For the flame stabilisation and fuel injection, a radial swirler with in-vane fuel injectors was decided upon. A major design alteration to the traditional swirler was the reshaping of the swirler vanes to follow a conical shape. This was theorised to improve pressure losses by eliminating the flow separation which commonly occurs in flat radial swirlers. For the liner cooling, augmented backside cooling was decided upon. From the initial testing, it was noted that the baseline combustor was capable of single-digit ppm NOx, CO and THC emissions.
In parallel to the baseline swirler/injector development, the augmented backside cooling surfaces were developed and tested. The two cooling fin geometries include a rotating plain fin and an offset strip fin. Experimental results showed a reduction of approximately 150 °C at all operating conditions with pressure losses of around 1% of the inlet pressure. This would significantly improve the lifetime of the liners and allow for hotter inlet temperatures if desired.
Further, additively manufactured combustor features were also developed and tested with the main objective of reducing emissions by improving air-fuel mixing. The features included in-vane lattice structures and upstream liner fuel injection rings. Although testing of the lattice structures did not provide the desired reduction in emissions, upstream fuelling, in combination with the liner cooling surfaces, made it possible to ignite the combustor at maximum air flow irrespective of operating temperature, and without over-fuelling and the consequent emissions.
Finally, an investigation into additively manufactured vaporisation injectors found significant benefits when compared to traditionally manufactured examples. Experiments demonstrated a change in flame colour, from which a reduction in soot emissions may be inferred, in theory due to increased vaporisation levels. This was corroborated by experimental and CFD results that compared the additively and traditionally manufactured designs.

Date of Award	25 May 2022
Original language	English
Awarding Institution	University of Bath
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Aaron Costall (Supervisor) & Sam Akehurst (Supervisor)