Experimental study of rheology and combustion characteristics of solidified ethanol fuel for hybrid rocket engine

Abstract

The study reports first findings on the solidication 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. Molecular interaction studies using fourier transform infrared spectroscopy (FTIR) reveals that the intra- and inter-molecular hydrogen bonding increases with increase in MC/HPMC. However, from the thermal studies using dynamic scattering calorimetry (TMDSC) and Thermogravimetric analysis (TGA, the melting temperature (Tm) and onset of decomposition temperature ($T_{(d,onset)}$) lies close for all the samples (~ $60^\circ C $). The apparent activation energy (Ea) of the solidified ethanol is determined using is-conversional Friedman method from TGA. Ea is found to be in the range of 9.06-14.01 kJ/mol and found to decrease with increase in the gellant concentration. The rheological properties of solidified ethanol (SE) 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'($\omega$) and G"($\omega$) 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". 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. The hypergolic ignition delays of the SE fuels with rocket grade hydrogen peroxide (90 %RGHP

$H_2O_2$) are investigated using manganese (III) acetylacetonate (Mnacac) as a catalyst and found that the ID value lies in the range of 41 -307 ms from the drop-test. A key finding from the hypergolic study is that a minimum catalyst concentration ($[C]_{Mn3+

L}$) is required to ignite SE fuel using $H_2O_2$ and ID decreases with increase in the $[C]_{Mn3+}$. Similarly, there exist an upper catalyst loading ($[C]_{Mn3+

L}$) above which ID is increased or not improved. For MC case $[C]_{Mn3+

U}$=18 wt.% and HPMC case $[C]_{Mn3+

U}$=16 wt.%. Additionally, ID is found to be directly depended on the type and concentration of the gellant ([C]g). Finally, SEM-EDS analysis of the residue indicates the formation of oxides of Manganese with a small amount of Carbon (~1.66 to 2.53 wt.%) hinting a complete combustion of the gellant particles.The reaction-driven SE fuel is formulated using metal salt $FeCl_3$ with HPMC and predicted the possible solidication mechanism using FTIR. It is found that the solidication of ethanol using $FeCl_3$ is the interplay between hydrogen bonding and ionic bond formed between HPMC and $Fe^{3+}$ ion. The ignition delay for reaction-driven fuels are in the range of 588.0-787.6 ms and observed to be higher in comparison with catalytically promoted SE fuels. Furthermore, when the temperature of the fuel was raised from room temperature to $80^\circ C $, the ID was lowered by ~71 % indicating a strong dependence of temperature on reaction rate. Finally the regression rate ($\dot{r}_b$) of the unloaded SE fuels is determined using in-house designed opposed flow burner with gaseous oxygen (GOX) at varying oxidizer mass flux ranges from 0.97 to 11.7 kg/$m^2$-s. The $\dot{r}_b$ is found to increase with increasing oxidizer mass flux, a typical hybrid solid fuel behavior. $\dot{r}_b$ value lies in the close range with the maximum value observed to be 0.22 mm/s at 11.7 kg/$m^2$-s for the HPMC 10 fuel. Furthermore, an increase of 67% in the $\dot{r}_b$ is observed when HPMC 10 fuel was loaded with 4 wt.% of nano-Al particles at 11.7 kg/$m^2$-s. From the hypergolic and regression rate experiments, it is observed that the solidication technique using organic gellants for liquid fuels have a potential to create eco-friendly, high-regressive hypergolic fuel for the next generation hybrid rocket engines.