Vaporization injectors have been in existence for decades and are a well-proven method of preparing liquid fuels for combustion by heating them above the boiling point of their heaviest hydrocarbon ingredient. By doing so, it converts the fuels into a vapour prior to combustion.
When attempting to apply this method of fuel vaporization to micro gas turbines, manufacturing difficulties arise, due to the small complex passages that are required to direct the fuel closer to the high-temperature zone in the combustion chamber and then back to a favourable injection location. This is where the use of additive manufacturing (AM) can prove advantageous due to the complex designs that can be achieved at much smaller scales and potentially at cheaper costs when compared to traditional subtractive manufacturing.
The motivation behind the research is to improve the overall efficiency of micro-gas turbines, so they can be applied as range extenders in electric vehicles. Due to the increasing adoption of vehicle electrification.
This paper covers the comparison of experimental results for two traditionally manufactured injectors and a third selective laser melted injector (SLM), which were tested in a swirl stabilised micro gas turbine can type combustor on the University of Baths gas stand. The operating range of the tests was 1–4 Bar and 30 to 630 °C inlet air. To the authors knowledge, this is the first such comparison to be made for a gas turbine in open literature, despite wide reports of AM being used in large gas turbines.
From the tests, it was found that the 3 and 8 hole machined injectors could not produce stable combustion at the desired operating condition of 4 Bar and 630 °C. The SLM 8 hole injector, however, was able to sustain a stable and constant burn at this design point with low NOx, CO and THC emissions. It was also noticed that the flame colour changed from a yellow flame when testing the first two injectors, to a blue flame when testing the SLM injector suggesting more complete combustion was being achieved due to the lack of soot in burned products, this was assumed to be due to the fuel reaching its saturation conditions within the injector.
A number of measurements were taken at various points around the combustor, which included temperatures, pressures and emissions readings. These results were then used to create and validate a non-premixed steady diffusion flamelet model in ANSYS Fluent for the AM injector case. The CFD results were found to overpredict the temperature by approximately 10% when compared to the thermocouple values. This was found to be similar to other studies with similar experimental and computational setups, so it was deemed acceptable.
From the validated CFD model, the heat flux at the front surface of the injector was extracted, to be used in a simple heat balance model. Based on a conservative estimate of fuel temperature, the model found that the SLM injectors should have created very near saturation conditions in the nozzle. As this was a conservative analysis, it confirms the experimental findings that partially vaporized fuel was exciting the injector. The model also showed that the fuel in the traditionally machined, 8 hole injector would most likely exit as a liquid.