Liquid oxygen droplet combustion in hydrogen under microgravity conditions

Abstract

The combustion of hydrogen and oxygen is important in the field of liquid rocket propulsion and in the near future also in aeronautical and automotive propulsion as well. In a rocket engine, a spray combustion process with gaseous hydrogen and liquid oxygen is formed as a result of the vaporization of the liquid hydrogen before it enters the combustion chamber. To optimize engine performance by improving combustion efficiency and aiming for smaller combustion chambers as well as to avoid combustion instabilities, a fundamental understanding of the vaporization and combustion processes at relevant ambient pressures is necessary. In order to study these fundamental processes focusing on the most central element of the technical spray combustion process, the single droplet, an experimental apparatus was developed which allows the combustion of single liquid oxygen droplets in a hydrogen atmosphere. It is shown that the dimensions of a single droplet in a technical spray (Ø5-150 μm) can be scaled up in reduced gravity conditions. The experimental setup was integrated into a drop capsule to perform experiments under microgravity conditions at the drop tower in Bremen to allow for the application of sophisticated diagnostics on large stationary and mostly spherical droplets. The main component of the setup is the cryogenic combustion chamber, which is surrounded by a liquid nitrogen jacket. The temperature of 77 K allowed gaseous oxygen to condense and to generate a liquid oxygen droplet at the tip of a quartz suspender. The droplet with a diameter of about 0.7 mm was ignited by a laser-induced plasma breakdown at different ambient pressures in the sub- and supercritical regime (0.1 to 5.7 MPa), and the combustion during the free-fall phase was investigated by shadowgraph imaging and OH∗ chemiluminescence diagnostics (hydroxyl radical). Despite a wide flammability range of the hydrogen/oxygen system, the ignition had to take place very close to the droplet surface due to the high diffusion rate of hydrogen. As a consequence of the ignition, the burning droplet detached from the suspender in all experiments and burned free-floating next to the suspender. This was initially seen as a problem but the detachment meant that the droplet was no longer affected by the suspender. At low pressures (< 0.8 MPa), it was observed that the shape of the droplet changed significantly during combustion, which is attributed to the formation of a (water) ice crust in close vicinity to the droplet surface. In a parametric study, it was found that the burning rate in the subcritical regime increases significantly with increasing pressure, whereas the flame standoff ratio decreases only slightly. In the supercritical regime, the measured data indicate a slight decrease in the burning rate. However, due to the vanishing of the surface tension, no defined detachment of the droplet from the suspender occurred, so that the subcritical experiments are not directly comparable with the experiments under supercritical conditions. An alternative evaluation method indicates a continuous increase of the burning rate in the transition to the supercritical regime. So far, only numerical simulations on single liquid oxygen droplet combustion have been performed and reported in the literature. By means of the developed experimental apparatus it has become possible to provide a first eing rate constant, which can be used for validation and future development of numerical models. These fundamental models form the basis for advanced simulations that xperimental database (exploiting microgravity conditions). These data provide key combustion research parameters, such as the flame standoff ratio or the burnalso include the interactions in a spray and can lead to an improvement in the volumetric heat release and thus of the efficiency of an engine.