Like Fire and Ice
Hydrogen and oxygen are the holy grail of rocket engines, but how do they burn? ZARM scientists used experiments in weightlessness to simplify the complex spray combustion process and found out that a layer of ice forms around the burning droplets.
The processes inside a rocket engine are characterized by a complex spray combustion process. In an engine powered by hydrogen and oxygen, a jet of liquid oxygen is broken up into numerous small droplets in order to burn as effectively as possible with the surrounding hydrogen. The simplest and at the same time most essential element of this spray is the single droplet.
In a study, the combustion of a single liquid oxygen droplet in a gaseous hydrogen atmosphere under microgravity was investigated. During the free fall in the drop tower, natural convection is suppressed so that the droplet assumes a spherical shape and a one-dimensional system is obtained. In an extensive experimental campaign, the combustion behavior at different pressures in the sub- and supercritical regimes was investigated. It was found that a layer of water ice forms around the burning droplet as the water produced during combustion freezes near the cryogenic droplet surface. The shape and appearance of this ice layer changes and the burning rate increases with increasing pressure. In the supercritical regime, the surface tension disappears and the droplet loses its spherical shape.
The results provide a first database for the development of fundamental numerical models and thus contribute to the development of future detailed spray combustion models.
The scientific paper of this research is published here: authors.elsevier.com/c/1ei2y2KiHabbk
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