Validation and development of wall-function models for Liquid Rocket Engine applications
Creators: name:  G. INDELICATO 
name:   A. REMIDDI 
name:   P. LAPENNA 
name:   F. CRETA 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
LIQUID ROCKET ENGINE
COMBUSTION
MODELIZATION

description: Nowadays high pressure turbulent combustion is a topic of interest for a number of applications, from industrial burner up to diesel and rocket engines. In this context the numerical investigation of high pressure, turbulent and reactive flows by means of Computational Fluid Dynamics (CFD) is an attractive tool both for the design of realistic combustion chambers and characterization of thermal loads, and both from a more fundamental perspective, such as the analysis of mixing dynamics and flame structures at high pressure. Progressively more reliable and affordable multi-dimensional simulations are nowadays performed, especially prompted by the development of High Performance Computing (HPC) algorithms and infrastructures. However, the realization of such complex simulations is still associated to numerical challenges far from being resolved. Among the many, one is the computational bottleneck associated to boundary layer resolution of high-Reynolds number flows, such as those found in LRE applications. These kind of flows are generally characterized by extreme thermodynamic conditions and substantial variation of properties during nominal operative conditions. In this context an example is given by propellant injection, which generally occurs at high pressure and cryogenic temperatures and is then followed by mixing and combustion processes, or by the regenerative cooling system of a LRE, where the fluid enters the system under cryogenic conditions and is then heated up by the heat flux from the combustion chamber. The mentioned situations induce substantial variation of properties which impacts the near-wall region of a wall-bounded flow, making its numerical modeling even more complex. In this context wall-modeled simulations are conceived in order not to solve the turbulent boundary layer down to the wall. This results in relevant saving in terms of computational power, but requires huge modeling efforts of the wall and of the near-wall field. In this kind of simulations, wall functions are generally employed to provide algebraic expressions for wall shear stress and wall heat flux as boundary conditions of the flow. In particular two main categories can be identified: analytical and numerical wall functions. The objective of the present work is to develop and validate wall-function models for LRE applications under stratified conditions. To this aim, 2D and 3D simulations in both unsteady Reynolds Averaged Navier Stokes (u-RANS) and Large Eddy Simulation (LES) frameworks are presented. The reference configurations are two experimental combustors developed at TUM. First, a categorization of thermal wall-function models from a literature survey is proposed. Selected wall-function models are then implemented in OpenFOAM and tested on typical stratified wall-bounded flows encountered in LRE applications. The investigated configurations envisage a flame parallel to an isothermal wall, such as that depicted in Fig. 1 representative of the reactive mixture inside a LRE combustion chamber; and a cryogenic flow through an heated pipe, the latter being representative of the regenerative cooling channels of a LRE or of the propellant inlets, where the thermal break-up mechanism has been observed to play an important role. The effect of the constant and variable property assumptions in the modelization of the near-wall turbulent quantities affecting the wall-heat flux is examined and discussed. The selected models are then consistently corrected and improved to deal with stratified conditions. In particular, a newly proposed algebraic wall-function model for cryogenic and high pressure flows is presented and examined both in an a-priori and a-posteriori fashion.