Direct numerical simulation of a subcritical coaxial injection in fiber regime using sharp interface reconstruction

The numerical simulation of space launchers combustion chambers is a topic of increasing interest, as it may help the development of safer and more efficient designs. Understanding fuel injection is a particularly severe challenge. The liquid oxygen is injected by a round orifice surrounded by an annular gaseous stream of fuel, leading in subcritical conditions to a two-phase assisted atomization process. The result of this process is a very dense and polydisperse two-phase flow, which strongly influences the behavior of the chamber. Experimental investigation of this flow is difficult due to the axisymmetric geometry and the dense characteristic of the spray. Neither RANS nor Large Eddy Simulation (LES) possess reliable models able to reproduce the smallest scales of atomization, one of the reasons being the lack of relevant experimental data. Therefore, this work aims to provide detailed information on the atomization process using Direct Numerical Simulation. This paper presents a Direct Numerical Simulation (DNS) of a coaxial liquid–gas assisted atomization in the typical fiber regime encountered in cryogenic injectors. This study aims to better understand the evolution of liquid topology and extract relevant information that may help develop larger-scale models. The flow was first analyzed in terms of topology statistical data, using a dedicated detection and classification algorithm that could characterize the individual liquid structures. These include the central liquid core, the ligament created during primary atomization, and the spherical droplet obtained at the end of the atomization process. Subsequently, a more global statistical topology indicator was investigated, namely the interface area density distribution. This quantity is used in larger-scale RANS or LES models to predict the smallest scales of atomization. Therefore, understanding its behavior in a realistic case is of utmost importance. The interface area density distribution was correlated to the global jet behavior and the liquid topology data obtained by the detection algorithm. The results showed, in particular, a strong correlation between the initial increase of liquid–gas interface area density with the generation of ligaments and between the continuous decrease of the interface area density during droplet formation and stabilization.