The current trend in the area of highly energetic storable liquid rocket propellant research is to develop environmentally friendly gelled/metallized systems and to explore the feasibility of their application in rocket engines. The idea stems from the fact that the conversion of a conventional liquid propellant to a gelled state and its subsequent metallization has the potential to significantly enhance the performance and density-specific impulse. The gelation of liquid fuels could be induced at a critical gellant concentration of as low as 8 wt% for the pure ethanol case and as low as 4 and 6 wt% for metallized ethanol depending on the metal type. Furthermore, the gel formed should be thixotropic. Metallized gels using 20 wt% Al and B metal powders could also be formulated. These metallized (Al and B) ethanol gel systems showed a reduction in the critical gellant concentration depending on the degree of metallization. The rheological properties of metallized and nonmetallized ethanol gels using methyl cellulose (MC) as a gelling agent at different ambient temperatures (283.15, 293.15, 303.15, 313.15, and 323.15 K) were experimentally investigated in this study. The gel fuels were rheologically characterized using a rheometer at shear rates ranging from 1 to 12 s⁻¹ and 1 to 1,000 s⁻¹. Metallized and nonmetallized ethanol gels were found to be thixotropic in nature. The apparent viscosity and yield stress (for shear rate range 1 to 12 s⁻¹) of gels were observed to significantly decrease at higher ambient temperatures and as the gellant and metal particle concentrations decreased. The thixotropic behavior was found to be a strong function of the Al and B metal particle concentration for all test temperatures at shear rate ranges from 1 to 12 s⁻¹ and 1 to 1,000 s⁻¹. It was also a function of the MC concentration at a shear rate range of 1 to 1,000 s⁻¹.

Ethanolamine is considered to be an environmentally friendly propellant system because it has low toxicity and is noncarcinogenic in nature. In this article, efforts are made to formulate and prepare ethanolamine gel systems, using pure agarose and hybrids of paired gelling agents (agarose + polyvinylpyrrolidine (PVP), agarose + SiO₂, and PVP + SiO₂), that exhibit a measurable yield stress, thixotropic behavior under shear rate ranges of 1–1,000 s⁻¹ and a viscoelastic nature. To achieve these goals, multiple rheological experiments (including flow and dynamic studies) are performed. In this article, results are presented from experiments measuring the apparent viscosity, yield stress, thixotropy, dynamic strain, frequency sweep, and tan δ behaviors, as well as the effects of the test temperature, in the gel systems. The results show that the formulated ethanolamine gels are thixotropic in nature with yield stress between 30 and 60 Pa. The apparent viscosity of the gel decreases as the test temperature increases, and the apparent activation energy is the lowest for the ethanolamine-(PVP + SiO₂) gel system. The dynamic rheology study shows that the type of gellant, choice of hybrid gelling materials and their concentration, applied frequencies, and strain all vitally affect the viscoelastic properties of the ethanolamine gel systems. In the frequency sweep experiment, the ethanolamine gels to which agarose, agarose + PVP, and agarose + SiO₂ were added behave like linear frequency-dependent viscoelastic liquids, whereas the ethanolamine gel to which PVP + SiO₂ was added behaves like a nearly frequency-independent viscoelastic solid. The variation in the tan δ of these gelled propellants as a function of frequency is also discussed.

Understanding the combustion behavior of gel fuel droplets is pivotal for enhancing burn rates, lowering ignition delay and improving the operational performance of next-generation propulsion systems. Vapor jetting in burning gel fuel droplets is a crucial process that enables an effective transport (convectively) of unreacted fuel from the droplet domain to the flame zone and accelerates the gas-phase mixing process. Here, first we show that the combusting ethanol gel droplets (organic gellant laden) exhibit a new oscillatory jetting mode due to aperiodic bursting of the droplet shell. Second, we show how the initial gellant loading rate (GLR) leads to a distinct shell formation which self-tunes temporally to burst the droplet at different frequencies. Particularly, a weak-flexible shell is formed at low GLR that undergoes successive rupture cascades occurring in same region of the droplet. This region weakens due to repeated ruptures and causes droplet bursting at progressively higher frequencies. Contrarily, high GLRs facilitate a strong-rigid shell formation where consecutive cascades occur at scattered locations across the droplet surface. This leads to droplet bursting at random frequencies. This method of modulating jetting frequency would enable an effective control of droplet trajectory and local fuel-oxidizer ratio in any gel-spray based energy formulation.

Inverse thermal problem is applied to natural convective flow with radiative heat transfer. The bottom wall temperature in the 2-D cavity domain is estimated by using gas temperature measurements in the flow field. The inverse problem is solved through a minimization of an objective function using the conjugate gradient method with adjoint problem. The effects of functional form of bottom wall temperature profile, the number and the position of measurement points, and the measurement errors are investigated and discussed. The conjugate gradient method is found to work well in estimating the bottom wall temperature, even when natural convection with radiation phenomena is involved.

Diesel fuel exhibits excellent combustion characteristics and stability. However, diesel use is becoming restricted because of its associated environmental problems. Fuel emulsification, which increases efficiency and reduces pollution, became the solution of environmental problem. In this study, five water:diesel emulsions with mass ratios (0.3, 0.6, 1.0, 1.2, and 1.5) via ultrasonication were synthesized with and without surfactant. The optimal water:diesel ratio (=1:1) of an emulsion containing the surfactant was found by analyzing fuel concentration, mixing time, and viscosity. The combustion characteristics of single-droplet optimal emulsions were studied through ignition delay, burning rate, and total droplet lifetime at high temperature (400–700 °C) and pressure (1–15 bar), and micro-explosion phenomenon was observed. Although the ignition delay of emulsion increased, the total lifetime of the emulsion droplet was lower than that of diesel under 5 bar, 600 °C condition.