Evaluation of Paraffin–Polyethylene Blends as Novel Solid Fuel for Hybrid Rockets

Abstract

A novel paraffin-based blends fuel prepared from pure paraffin wax of the alkane family with polyethylene of the alkene family is proposed and tested in a slab motor to visualize the droplet entrainment mechanism followed by a hybrid rocket motor test to analyze its combustion characteristics. The mechanical strength of the proposed blended fuel is investigated by increasing the polyethylene weight percent. The overall regression rate of 5 wt % of polyethylene with 95 wt % of pure paraffin is found to be 3.9 factors higher than that of a pure polyethylene. Improved combustion efficiency under the condition of a similar oxidizer mass flow rate range is achieved with respect to pure paraffin fuel in which performance gain is comparable to that of the SP-1a fuel of Stanford University. Spectrum analysis of the chamber pressure shows no critical instability mode for the range of this study. The blend fuel is found to be comparatively effective for the hybrid rocket fuel in terms of mechanical strength, combustion efficiency, combustion instability, and combustion performance. Nomenclature A f = cross-sectional area of final chamber, m 2 A i = cross-sectional area of initial chamber, m 2 A pf = cross-sectional area of final grain port, m 2 A pi = cross-sectional area of initial grain port, m 2 A t = exhaust nozzle throat area, m 2 a = regression rate coefficient c exp = experimental characteristic velocity, m∕s c theo = theoretical characteristic velocity, m∕s D pf = diameter of final grain port, m D pi = diameter of initial grain port, m G o = time-space-averaged oxidizer mass flux, kg∕m 2 -s H f = height of final slab fuel grain, m H i = height of initial slab fuel grain, m L f = fuel grain length, m _ m ent = entrained mass flux from fuel surface, kg∕m 2 -s _ m o = time-averaged oxidizer mass flow rate, g∕s n = oxidizer mass flux exponent P c = chamber pressure, MPa _ r = time-space-averaged fuel regression rate, mm∕s t b = burning time, s W f = fuel grain width, m α = dynamic pressure exponent β = thickness exponent γ = viscosity exponent η c = efficiency of characteristic velocity μ l = liquid layer viscosity, Pa-s π = surface tension exponent ρ f = fuel grain density, kg∕m 3 σ = surface tension, N∕m

Mechanical Modifications of Paraffin‐based Fuels and the Effects on Combustion Performance

“…Compared with the pure paraffin, the regression rates of formulation No.1 increased while the regression rates of formulations No.2~No.6 decreased. Since the melting points of formulations doped with the stearic acid were reduced while the liquid viscosities were essentially unchanged, this was beneficial to yield droplets which can promote the mass transfer during combustion process [15]. In contrast, adding AÀC6A, EVA, LDPE, PP and HDPE boosted the liquid viscosities significantly but changed the melting points slightly, thereby the mass transfer was negatively affected due to the increased viscosity can augment the shear force during the droplet detachment [6].…”

“…Blended additives also can affect the combustion performance of paraffin-based fuel. Kim [15] investigated the combustion characteristics of paraffin-based fuels mixed LDPE, and found that blended LDPE can increase surface tension and melted viscosity leading to a decrease of the amount of fuel droplets during the combustion, thus the regression rate reduced. Kumar [16] fired paraffin grains containing 20 mass % EVA and found that the addition of EVA decreased the regression rate due to the increase of the melting point and viscosity of the melted layer.…”

Regression Rate Characterization of HTPB‐Paraffin Based Solid Fuels for Hybrid Rocket

“…A more interesting result is that pure paraffin fuel has displayed a considerable amount of instability in chamber pressure. These low‐frequency pressure oscillations associated with pure paraffin could be attributed to the unstable thin liquid layer formed over the fuel surface during the combustion [32]. These low‐frequency pressure oscillations may be ascribed to the entrainment of liquid fuel droplets from the fuel surface to the combustion zone.…”

Role of Precombustion Chamber Design in Feed-System Coupled Instabilities of Hybrid Rockets

“…(4) was considered equal to 0.5 (n=0.5). This is a reasonable assumption since the oxidizer mass flux exponent has been observed to range from 0.5 to 0.75 regardless of the type of propellant for most hybrid propulsion systems [1,19,20]. And the time rate of change of the chamber gas volume was neglected because the fuel regression rate is on the order of a few mm/s, much slower than combustion time scales.…”