Rheology of solid-like ethanol fuel for hybrid rockets: Effect of type and concentration of gellants

This study reports first findings on the solidification of eco-friendly ethanol fuel by organic gellant, namely, Methylcellulose (MC) and Hydroxypropyl methylcellulose (HPMC) for use as a solid fuel for the hybrid propulsion system. Specifically, the rheological properties of solidified ethanol are determined using both shear flow tests and dynamic oscillation tests for the gellant concentration varying in the range of 5 wt% to 17 wt% and nanoparticle loading varying in the range of 2 wt% to 6 wt% for HPMC and 2 wt% to 4 wt% for MC samples respectively. It is observed that over the range of applied shear rate (0.1-1000 s−1) solidified ethanol fuels exhibit a strong shear thinning, high yield thixotropic behavior. The yield stress of the fuel sample ranges from 424.20-1252.40 Pa, and found to the direct function of type, concentration of gellant and nanoparticle loading. Below the yield stress, the solid samples exhibit an elastic dominant behavior (G′>G″) and found to be independent of applied stress in the linear viscoelastic region. In dynamic tests, the spectra of G′(ω) and G″(ω) indicates that solidified ethanol forms a covalently cross-linked network between ethanol-water blend and gellant material. A key finding of this study reveals that all ethanol fuel formulations display solid-like characteristics under test conditions as G′ and G″ are nearly independent of the frequency and the magnitude of G′ is 4.1-5.4 times higher than G″. Finally, the effect of gellant type on rheological behavior is studied where it is observed that the relative thixotropic area and creep strain of HPMC laden fuels is significantly higher compared to their MC counterparts which imparts them a viscous dominant character.