Multi-physics thermal characterization of rocket combustion chambers

This thesis is devoted to the numerical modeling and thermal characterization of technological devices based on combustion and operating under severe thermodynamic conditions. Several approaches are proposed throughout the thesis, sharing the same tabulated chemistry approach for the turbulent combustion modeling and the implementation in Unsteady Reynolds Averaged Navier-Stokes settings. First, single-region solvers will be discussed, dealing only with the modeling of the fluid domain. This approach is selected to collect a significant amount of data at a reasonable computational cost, allowing therefore parametric investigations and the development of data-driven models. Such analyses will be mainly devoted to the assessment of the effect of geometrical and injection parameters over the flow field and thermal load in a combustion chamber. The information gathered through the parametric analyses is then used as stepping stone for the preliminary implementation of a data-driven model for the thermal characterization of complex multi-injector geometries. The fidelity of the approach will be then increased including also the description of the heat transfer across different continua, through the simulation of multi-region test cases and hence introducing the coupled simulation of fluid and solid domains. In order to do so, a multi-region solver for Conjugate Heat Transfer is developed, featuring an interface boundary condition that guarantees temperature and heat flux continuity across the interfaces between an arbitrary number of solid and fluid domains. Also, an efficient coupling strategy aimed to the further reduction of the computational costs in convection-dominated phenomena is developed, based on the alternation between the two major approaches for coupled simulations found in the literature: directly coupled approach with Conjugate Heat Transfer and thermally chained simulations. Such a strategy will allow for the description of laboratory-scale test cases for the entire duration of the experimental runs. An extension of the solver will be also presented, accounting for pressure-dependence effects and allowing therefore the simulation of low-to-high Mach number flows such as the nozzle expansion downstream a combustion chamber. The main applications to which the thesis work is devoted are non-premixed, turbulent flames in combustion chambers under high pressure operating conditions, representative of the conditions encountered in Liquid Rocket Engines, simulated in both two-dimensional and three-dimensional settings. Nonetheless, a brief excursus on premixed injection modeling, relevant to gas turbine and aeronautical applications, will be given in the final section of the thesis, with particular reference to the peculiar challenges raised by this type of flames. More specifically, the topic of premixed hydrogen combustion will be tackled. Hydrogen flames present several modeling challenges, starting from the higher flame temperatures and laminar flame speeds compared to other conventional fuels, to the intrinsic instabilities that characterize premixed flames, that can be generated by either thermo-diffusive or hydrodynamic effects. A data-driven tabulated chemistry approach for premixed hydrogen combustion modeling will be proposed

Thermal characterization in LRE: a parametric analysis on injector arrangement

The injector layout influence on a gaseous-methane/gaseous-oxygen (GCH4/GOX) flame emanating from a single shear coaxial Liquid Rocket Engine (LRE) injector is presented. More specifically, in this contribution we investigate the flame-flame interaction, by collecting a database of two-dimensional axis-symmetric simulations, modeled with a symmetry boundary condition to represent the presence of a neighbouring identical injector. The geometrical feature varied as parameter is the lateral confinement, representative of the mutual distance between adjacent injectors. The differences between such a flame and a flame bounded by an isothermal wall, on equal geometry and injection conditions, is also presented. The numerical framework chosen for the high-pressure, turbulent, non-premixed flame description is based on a unsteady Reynolds-Averaged Navier Stokes (uRANS) approach, coupled to a flamelet model accounting for non-equilibrium and non-adiabatic effects

Development and validation of an efficient numerical framework for Conjugate Heat Transfer in Liquid Rocket Engines

In this contribution, the development and validation of a multi-region, multi-physics solver for turbulent combustion and Conjugate Heat Transfer in Liquid Rocket Engine relevant conditions are presented. The solver tackles simultaneously multiple solid and fluid domains, either mono-species or single-species, and enforces an interface condition guaranteeing temperature and heat flux continuity. Two different coupling strategies are developed and validated, making the solver amenable to the numerical simulation of industrial and laboratory time-scales. Results of the validation process are presented, based on several test cases each one devoted to a specific solver feature

Theoretical and numerical modeling of multicomponent transcritical diffuses interfaces

In this study, we present a theoretical and numerical framework for simulating transcritical flows under a variety of conditions of interest for aerospace applications. A real multi-component and multi-phase thermodynamic model, based on a cubic equation of state and vapor-liquid equilibrium assumptions, is used to describe transcritical mixtures. The versatility of this model is reported since it can represent at the same time the supercritical states and subcritical stable two-phase states at equilibrium. We emphasize the difference in the mixing behaviors conducted with and without the VLE assumptions. The well-known numerical challenges that arise with the coupling between thermodynamics and governing equations under transcritical conditions are addressed by comparing a fully conservative to a quasi-conservative scheme in the context of density-based solvers while discussing the possibility of employing a pressure-based approach given the typically low-Mach number at play for the cases of interest

Dataset of wall-resolved large-eddy simulations turbulent pseudoboiling in cryogenic hydrogen pipe flows

In this paper, a dataset of wall-resolved large-eddy simulations of cryogenic hydrogen at supercritical pressure and different values of wall heat flux is presented. The aim is to provide a reference dataset for wall-function development under trans- and supercritical conditions, such as those found in liquid rocket engine applications. The employed numerical framework is a pressure-based segregated low-Mach-number approach based on an equation-of-state independent formulation. The wall-adapting local eddy-viscosity subgrid model is used for turbulence closure. Real-gas effects are taken from the National Institute for Standards and Technology database and stored as a function of a nondimensional temperature at the considered pressure. A validation and a grid-convergence analysis are first performed on an incompressible case without imposed heat flux. The effect of axial, radial, and azimuthal refinements on first- and second-order velocity statistics is discussed and compared with direct numerical simulation data from the literature. A parametric analysis at different wall heat fluxes is then performed by keeping the inlet mass flux, temperature, and Reynolds number constant. Particular attention is devoted to turbulent pseudoboiling and its effect on the wall temperature. The latter shows a more pronounced increment as the heat flux increases, which is attributed to the pseudochange of the phase in the core flow. Correspondingly, a flattening of the probability density function of the temperature is observed, and it is associated with the pseudoboiling interface forming close to the wall and causing a more intense stratification. First- and second-order statistics for velocity and selected scalars are then presented, and the effect of pseudoboiling is discussed. The effect of the wall heat flux on the viscous and thermal resolution of the computational grid is also assessed, and considerations on the relation between turbulent pseudoboiling and near-wall gradients is finally provided

Dataset of wall-resolved large eddy simulations for the investigation of turbulent pseudo-boiling and wall-functions in cryogenic hydrogen pipe flows

In this contribution a database of wall-resolved LES of cryogenic hydrogen at 5 MPa and different values of wall heat flux is presented. The aim is to provide a reference dataset for wall-functions development under trans- and supercritical conditions, such as those found in Liquid Rocket Engine applications. The employed numerical framework is a pressure-based segregated low-Mach number approach based on an equation-of-state independent formulation. The WALE sub-grid model is used for turbulence closure. Real-gas effects are taken from the NIST database and stored as a function of a non-dimensional temperature at the considered pressure. A validation and a grid-convergence analysis is first performed on an incompressible case without imposed heat flux. The effect of axial, radial and azimuthal refinements on first and second order velocity statistics is discussed and compared with DNS data from literature. A parametric analysis at different wall heat fluxes is then performed by keeping the inlet mass flux, temperature and Reynolds number constant. Particular attention is devoted to turbulent pseudo-boiling and its effect on the wall temperature. The latter shows a more pronounced increment as the heat flux increases attributed to the pseudo-change of phase in the core flow. Correspondingly, a flattening of the probability density function of temperature is observed, associated to the pseudo-boiling interface forming close to the wall and causing a more intense stratification. First and second order statistics for velocity and selected scalars are then presented and the effect of pseudo-boiling discussed. The effect of the wall heat flux on the viscous and thermal resolution of the computational grid is also assessed and considerations on the relation between turbulent pseudo-boiling and near wall gradients finally provided