FirePhysChem (Dec 2021)
On the liquid layer instability process in hybrid rocket fuels
Abstract
This work is focused on the study of the combustion mechanism of a class of fast-burning fuels, the so-called liquefying fuels, in hybrid rocket engines. The typical diffusive flame of this kind of propulsion technology brings to poor regression rate performance with conventional pyrolyzing fuels. On the other hand, the liquefying fuels, characterized by low viscosity and surface tension, are able to form a thin liquid layer on the fuel surface during the combustion, thus enabling an additional mass transfer by entrainment of liquid droplets. Unfortunately, this phenomenon has not been well understood yet and still needs to be fully characterized. The main goal of this research is to experimentally characterize the hybrid combustion process and, therefore, facilitate and help the preliminary design process of this kind of engines. Particular attention is paid to understanding the relation between the unstable waves that enable the droplets entrainment process and the regression rate. Optical investigations on the combustion behavior of paraffin-based hybrid fuel slabs burning with GOX (Gaseous Oxygen) have been performed at the Institute of Space Propulsion of the German Aerospace Center (DLR) in Lampoldshausen. The results show that the fuel liquid layer is dominated by periodic, wave-like structures for all the investigated operating conditions. Frequencies and wavelengths characterizing the liquid melt layer depend on the identified experimental parameters, especially on fuel viscosity and oxidizer mass flow. A dependency of the entrainment rate, which partially influences the regression rate, on the most excited frequencies and longitudinal wavelengths is also found.
