The Simultaneous Simplicity and Complexity of Supersonic Turbines and their Modern Application

Supersonic axial turbines have attracted interest in the industry since the 1950s due to the high power they  provide, allowing a reduction in the number of low-pressure stages, and thus leading to lighter turbines as well as lower manufacturing and operational costs. Due to these valuable features, supersonic axial turbines are currently widely used in different power generation and mechanical drive fields such as rocket engine turbopumps [1, 2, 3, 4], control stages in high pressure multi-stage steam turbines, standalone single stage and 2-row velocity compound steam turbines [5, 6], ORC turbo-generator including geothermal binary power stations [7, 8, 9, 10], turbochargers for large diesel engines [11] and other applications. Therefore it is not forgotten, but instead a very important field in turbomachinery when highest specific power, compactness, low weight, low cost and ease of maintenance are dominant requirements. Especially nowadays, when development of small capacity reusable low-cost rocket launchers, compact and powerful waste heat recovery (WHR) units in the automotive industry, distributed power generation, and other fields are in extreme demand.

Meanline Results of Supersonic Turbine in AxSTREAM
Meanline Results of Supersonic Turbine in AxSTREAM

Typically, supersonic turbine consists of supersonic nozzles with a subsonic inlet and one or two rows of rotating blades. The turbine usually has partial arc admission. The total flow could go through either a single partial arc or several ones. The latter is typical for a steam turbine control stage or standalone applications. The inlet manifold or nozzles chests, as well as exhaust duct, are critical parts of the turbine as well. Due to the very frequent application of partial admission, it is not possible to implement any significant reaction degree. Thus, this kind of turbine is almost always an impulse type. However, some reaction degree could still be applied to full admission turbines. The influence of  the rotor blades profile designed for high reaction degree on rotor-stator supersonic interaction and turbine performance is not well studied at the moment.

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What parallels exist between traditional Gas Turbines with SCO2 turbine of the future?

At the beginning of my studying of the peculiarities of supercritical CO2 (S-CO2) cycle I was wondering: why do scientists involved in this area state that highest temperature limit for the cycle is about 650-700 ˚C. In turn, the inlet temperature in the first stages of gas turbines handles the temperatures about 900 ˚C without cooling at similar pressure levels as for supercritical CO2 Turbines. As a result the following question rose in my mind – why the temperature magnitudes of 900 ˚C are not achievable in S-CO2 turbines?

As a next step, some investigations were performed with the aim to reveal the essence of such a temperature limit. Eventually the result was quite obvious but rather interesting. The density of S-CO2 is significantly higher than the density of combustion products at the same pressure and temperature magnitudes. This fact means that stresses at static vanes and rotating blades are significantly higher than in gas turbines vanes and blades at the same conditions. Therefore the maximum allowable temperature for S-CO2 turbine will be respectively less with the same high temperature material. However, you might say that there is another way to solve the problem with stresses, namely, increasing the chords of blades, leading edge thickness, trailing edge thickness, fillets etc. This approach would lead to such blades shape and turbine cascade configuration that their aerodynamic quality becomes very low so the gain in efficiency at cycle level will be leveled.

Interested in learning more about our research, and how using the AxSTREAM turbomachinery platform, we were able to study these phenomena?

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