Turbulence modelling of cavitating flows in cryogenic turbopumps

Robust turbulence modelling of cavitating flows is essential for physically accurate predictions of the performance characteristics of cryogenic rocket turbo-pump inducers. Numerical modelling of such flows complement experiments and are comparatively inexpensive, but they lack accurate predictive capabilities. This work focuses on the characterisation of the choice of turbulence models on cavitation predictions for the modelling of cryogenic turbo-pump inducer. To isolate the influence of the turbulence modelling effects on the concurrent physical phenomena arising in the inducer, four canonical problems are abstracted and studied individually to separately consider rotation, adverse pressure gradients, blade passage and bluff body cavitation. Additionally, cavitation bubble dynamics in a bubbly flow in a 3D inducer are numerically simulated and studied so as to understand the effects of such on flow turbulence. It has been found that the type of cavitation model and the choice of turbulence model play a major role in the prediction of phase-distribution in the flow and concomitantly, on the cavitation induced instabilities predicted from computational fluid dynamics. The influence of the turbulence model is strongly dependent on the type of flow and a universal model which can be used to accurately predict cavitation in all flow domains seems to be unavailable among the existing classical turbulence closure models (both for two-equation and second-order closures). Stark differences in the predictions of cavitation are observed among non-bounded, bounded, and rotating flows based uniquely on the selection of turbulence models. The influence of other parameters such as the thermo-physical fluid characteristics, Reynolds number, or turbulence intensity have a minor effect for non-bounded flows. On the other hand, cavitation arising within the blade passage (bounded flows), the selection of the thermo-physical characteristics has a great effect on the cavitation prediction. Simulations of bubble dynamics using the Rayleigh-Plesset equation in cryogenic bubbly flows, with and without thermal effects, clearly depict the cavitating regions in a 3D inducer and yield corresponding bubble oscillation frequencies and time scales. The thermal effects attenuate bubble oscillations and distinct dominant frequencies of oscillations are observed for changing initial bubble radii. The information obtained from the turbulence model influence on cavitation and bubble oscillation influence on flow turbulence is used in the analysis of turbulence-cavitation interaction. A set of considerations are provided to turbo-pump designers as they need to carefully consider the choice of turbulence model and cavitation physics in order to use CFD tools to predictively model cryogenic inducers. These considerations will in turn help to improve dependability of the CFD tools, which will lead to reduced computational and monetary costs.

Subject

Turbopump
Cavitation
Turbulence Modelling

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master thesis

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