Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Modelling of wall-bounded high pressure flows in Liquid Rocket Engines

In this thesis an efficient numerical framework for the prediction of wall heat loads in Liquid Rocket Engine applications is presented. The proposed framework is based on a multi-dimensional flamelet-tabulated approach and on wall functions for turbulent boundary layer modelling in order to minimize the computational efforts and stiffness at run-time. Wall function models selected from the literature are applied to 2D and 3D wall heat flux simulations of typical wall-bounded flows in Liquid Rocket Engine in both Unsteady Reynolds Averaged Navier Stokes and Large Eddy Simulation contexts. A systematic analysis and categorization of these models is carried out and a comprehensive comparison performed. The implementation in a flamelet-based, pressure based solver is discussed and improvements to deal with rocket engine-relevant conditions proposed. Particular emphasis is given to the modellization of near-wall turbulent quantities under constant or variable properties assumption. The impact of these assumptions on the determination of the wall heat flux is assessed and the resulting friction velocity scaling investigated. Based on this analysis a density-corrected near-wall modelling framework is proposed and applied to the simulation of single- and multi-element gaseous oxygen/gaseous methane combustors, showing a substantial agreement in terms of wall heat flux with both experimental data and higher fidelity simulations results from literature. Concurrently, a database of wall-resolved Large Eddy Simulation of cryogenic hydrogen in a heated cylindrical pipe at 5 MPa and different values of wall heat flux is presented. The goal is twofold: on one hand, to provide a high fidelity database of wall-resolved Large Eddy Simulations for further investigation behind the mechanism of heat transfer to supercritical fluids, given the still limited studies on the subject; on the other, to provide a reference benchmark solution aimed at improving wall and near-wall turbulence modelling for cryogenic, high pressure flows. Particular attention is devoted to turbulent pseudo-boiling and its effect on wall temperature and near wall modelling. A-priori analysis of wall-function models are then led revealing poor performances in reproducing the wall-resolved reference solution in terms of wall quantities, especially when pseudo-boiling takes place inside the pipe. A newly developed model is therefore proposed in order to extend validity of algebraic wall functions to trans- and supercritical boundary layers.