Understanding Kelvin–Helmholtz instability in paraffin-based hybrid rocket fuels

Abstract

Liquefying fuels show higher regression rates than the classical polymeric ones. They are able to form, along their burning surface, a low viscosity and surface tension liquid layer, which can become unstable (Kelvin-Helmholtz instability) due to the high velocity gas flow in the fuel port. This causes entrainment of liquid droplets from the fuel surface into the oxidizer gas flow. To better understand the droplets entrainment mechanism, optical investigations on the combustion behaviour of paraffin-based hybrid rocket fuels in combination with gaseous oxygen have been conducted in the framework of this research. Combustion tests were performed in a 2D single-slab burner at atmospheric conditions. High speed videos were recorded and analysed with two decomposition techniques. Proper orthogonal decomposition (POD) and independent component analysis (ICA) were applied to the scalar field of the flame luminosity. The most excited frequencies and wavelengths of the wave-like structures characterizing the liquid melt layer were computed. The fuel slab viscosity and the oxidizer mass flow were varied to study their influence on the liquid layer instability process. The combustion is dominated by periodic, wavelike structures for all the analysed fuels. Frequencies and wavelengths characterizing the liquid melt layer depend on the fuel viscosity and oxidizer mass flow. Moreover, for very low mass flows, no wavelength peaks are detected for the higher viscosity fuels. This is important to better understand and predict the onset and development of the entrainment process, which is connected to the amplification of the longitudinal waves. List of symbols

Ballistic and thermomechanical characterisation of paraffin-based hybrid rocket fuels loaded with light metal hydrides

“…The thin melted layer formed on the grain surface is susceptible to the shear-driven instability caused by the oxidiser flow injection. It is characterised by lowviscosity and surface tension which results in increased fuel mass flow rate due to the lift-off and entrainment of droplets into the flow-stream [26]. In case of paraffin-based fuels, it is also noted that heat transfer is improved due to increased surface roughness while the blockage effect is decreased as entrained liquid droplets do not contribute to the heat-transfer blocking phenomenon, leading to high regression rates [16,18].…”

Nonequilibrium and morphological characterizations of Kelvin–Helmholtz instability in compressible flows

“…The Kelvin-Helmholtz instability (KHI) occurs at a perturbed interface between two fluids or two parts of the same fluid with different tangential velocities [1]. As an efficient and important initiating mechanism of turbulence and mixing of fluids [2][3][4][5][6][7], it plays crucial roles in various fields, ranging from high-energy-density physics [8], geophysics and astrophysics [9][10][11][12][13][14], inertial confinement fusion (ICF) [15][16][17], combustion [18][19][20], to Bose-Einstein condensate [21,22] and graphene [23], etc. Concretely, in geophysical and astrophysical situations, on the one hand, the fully developed KH billows are responsible for the formation of large-scale vortical structures in systems such as hurricane [9], galaxy spiral arms [10], heliopause [11,12], and solar wind interaction with the Earth's magnetosphere [13,14], leading to violent intermixing across shear layers; on the other hand, the significantly suppressed KH roll-ups contribute to the sufficiently long, stable and highly collimated supersonic astrophysical jets [16,[24][25][26] with length-to-width ratios as high as 100 or more, emanated from young stellar objects or active galactic nuclei [27], and jet-like long spikes observed in the high-energy-density laboratory astrophysics experiments [28].…”

Local anomaly detection in hybrid rocket combustion tests

“…A test sequence is programmed before the test and is run automatically by the test bench control system. More details about the test bench and test settings are given in Kobald et al (2015), Petrarolo and Kobald (2016), Petrarolo et al (2018).…”