Turbochargers are widely used to help reduce the environmental impact of automotive engines. However, a limiting factor for turbochargers is compressor surge. Surge is an instability that induces pressure and flow oscillations that often damages the turbocharger and its installation. Most predictions of the surge limit are based on low-order models, such as the Moore-Greitzer model. These models tend to rely on a characteristic curve for the compressor created by extrapolating the constant speed lines of a steady-state compressor map into the negative mass flow region. However, there is little validation of these assumptions in the public literature. In this paper, we develop further the first-principles model for a compressor characteristic presented in Powers et al. [1], with a particular emphasis on reverse flow. We then perform experiments using a 58mm diameter centrifugal compressor provided by Cummins Turbo Technologies, where we feed air in the reverse direction though the compressor while the impeller is spinning in the forwards direction in order to obtain data in the negative mass flow region of the compressor map. This demonstrated experimentally that there is a stable operating region in the reverse flow regime. The recorded data showed a good match with the theoretical model developed in this paper We also identified a change in characteristic behaviour as the impeller speed is increased which, to the authors knowledge, has not been observed in any previously published experimental work.
Highlights• Inclination only makes a small difference to thermal performance of ACHE• Increasing plenum depth only is slightly more effective -1% benefit over baseline• The flow tends to move to one side as the heat exchanger is inclined• This side is a good location for a performance limiting heat exchanger Investigation of heat exchanger inclination in forced-draught air-cooled heat exchangersIan J. Kennedy a * ikennedy03@qub.ac.uk +44 (0) 28 9097 AbstractThe purpose of this study is to determine the influence of inclining the heat exchanger relative to the fan in a forced draught air cooled heat exchanger. Since inclination increases plenum depth, the effect of inclination is also compared with increasing plenum depth without inclination. The experimental study shows that inclination improves thermal performance by only 0.5%, when compared with a baseline noninclined case with a shallow plenum. Similarly, increasing plenum depth without inclination has a thermal performance benefit of approximately 1%. The numerical study shows that, as the heat exchanger is inclined, the low velocity core at the centre of the heat exchanger moves to one side.
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.
Turbochargers are widely used to help reduce the environmental impact of automotive engines. However, a limiting factor for turbochargers is compressor surge. Surge is an instability that induces pressure and flow oscillations that often damages the turbocharger and its installation. Most predictions of the surge limit are based on low-order models, such as the Moore-Greitzer model. These models tend to rely on a characteristic curve for the compressor created by extrapolating the constant speed lines of a steady-state compressor map into the negative mass flow region. However, there is little validation of these assumptions in the public literature. In this paper, we develop further the first-principles model for a compressor characteristic presented in Powers et al. [1], with a particular emphasis on reverse flow. We then perform experiments using a 58mm diameter centrifugal compressor provided by Cummins Turbo Technologies, where we feed air in the reverse direction though the compressor while the impeller is spinning in the forwards direction in order to obtain data in the negative mass flow region of the compressor map. This demonstrated experimentally that there is a stable operating region in the reverse flow regime. The recorded data showed a good match with the theoretical model developed in this paper. We also identified a change in characteristic behaviour as the impeller speed is increased which, to the authors knowledge, has not been observed in any previously published experimental work
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