The adoption of hybrid propulsion systems has been hampered in the past by the low regression rate associated with classical polymeric fuels, which required a complex, multi-port fuel grain geometry. The discovery of liquefying hybrid rocket fuels (e.g. paraffin) in recent years has generated a renewed interest in hybrid propulsion. This kind of fuels enables higher regression rates due to the presence of an unstable melt layer on the fuel surface during combustion, which causes entrainment of liquid droplets into the oxidizer gas flow. In order to have a sufficient droplet entrainment, the liquid layer must be characterized by low viscosity and surface tension. The higher regression rate enables a simple, single-port fuel grain geometry, which can be used in hybrid propulsion systems for many space applications. However, the mechanism responsible for the droplets entrainment still needs to be investigated and fully understood. Therefore, the liquid layer combustion mechanism of paraffin-based hybrid rocket fuels in combination with gaseous oxygen (GOX) has been analysed with different optical techniques in the framework of this research. A 2D slab burner with windows on two sides has been used for performing combustion tests at atmospheric conditions. High-speed videos have been recorded and analysed in detail with an automated video evaluation routine. Two different decomposition techniques (Proper Orthogonal Decomposition and Independent Component Analysis) were applied to the scalar field of the flame luminosity. The fuel composition and configuration and the oxidizer mass flow have been varied in order to study the influence of these parameters on the liquid layer instability process. In this paper the influence of the oxidizer mass flow and fuel viscosity is presented and discussed. The results show that the combustion is dominated by a periodic, wave-like structure and that the most excited frequencies and wavelengths characterizing the liquid melt layer depend on the oxidizer mass flow and on the liquid layer viscosity. Moreover, for very low mass flows, no distinct wavelength peaks are detected with both decomposition methods. This is important to better understand the onset of the entrainment process, which is connected to the amplification of longitudinal unstable waves caused by the high velocity gas flow over the fuel surface
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