The "Vortex Reloaded" project: experimental investigation on fully tangential vortex injection in N2O - paraffin hybrid motors

Abstract

This paper deals with an experimental and numerical project intended to study fully tangential vortex injection in a hybrid motor of 1kN class. Due to the knowledge of the CISAS Hybrid Team, the choice for oxidizer has been pressurized nitrous oxide, while paraffin wax has been used as fuel. This investigation follows a previous project where a mixed axial/vortex device has been tested: also if an increase in performance has been observed, a non-uniform grain consumption showed an issue of such device. The work starts with a mission scenario that gives the design drivers for the subsequent preliminary design, performed with an iterative, transient and 0D numerical code. A successive step is the detailed design of the combustion chamber and test bed. The aim of the study was to investigate three objectives: vortex injection and a comparison with axial; throttling behavior at fixed mass flows (reduction of 75% and 50% of oxidizer flow); combustion chamber performance changing its configurations, in particular using inside a mixer. Performance parameters taken into account were chamber pressure oscillations, combustion efficiency and regression rate. In the preliminary phase two different issues have been discovered and solved: the first regards a chamber pressure behavior variation, linked to a too long postchamber; the second is referred to pressure pikes in the ignition phase, solved using a longer prechamber and a different ignition configuration. It has been shown that vortex injection lowers the chamber pressure oscillations respect to axial case from more than 7% down to 4%. Moreover, regression rate has been increased of 41%, and the a coefficient of its law up to 67% from axial. This last value, indeed, shows a constant behavior throttling down the oxidizer mass flow. The increase is due to the higher wall heat flux in the grain surface, given by the higher velocity and thermal gradient of the fluid. Combustion efficiency has been increased with vortex injection given by the higher turbulence flow that enhances the mixing of the reactants. Axial case showed a value of 76% of this last parameter, while vortex case went up to 90%. Coupling of injection and mixer (a diaphragm-like device inside combustion chamber) increases this value up to 96%. A cost analysis has been performed for this project, showing that hybrid propulsion is a low cost technology. NomenclatureO/F = oxidizer to fuel mass ratio a = multiplication coefficient for regression rate n = exponential coefficient for regression rate ρ f , ρ o ,ρ f = density (general oxidizer and fuel) r = radial coordinate z = axial coordinate u r , u θ , u z = radial, tangential and axial velocity ϕ i , ϕ e ,ϕ m = initial, final and mean diameter of the grain μ = dynamic viscosity M f = mass of burned fuel G ox = mass flux in combustion chamber L g = grain length ω = vortex angular velocity T = temperature t = time M m = molar mass R u = Universal gas constant p, p c = pressure, mean chamber pressure A t = nozzle throat area ṁ tot = mean total mass flow

CFD Simulations of Self-pressurized Nitrous Oxide Hybrid Rocket Motors

“…In a similar fashion to this thesis, the analysis have been validated with the test results provided by an experimental campaign [12]. This research expanded further adding new elements as Liquid Injection [13] [14] and Vortex Injection [15] [16] [17] [18].…”

“…A closer look on the vorticity in the precombustion chamber is given in figure 5. 16, where the plot of the velocity vectors in the plane highlights the hot eddies.…”

Small-Scale Hybrid Rocket Test Stand & Characterization of Swirl Injectors

“…The 18 tests performed as part of this experiment showed that, as Bellomo said, "…for the regression rate performance parameter the most important factor is the injection type," (Bellomo, et al, 2012). The ANOVA results discussed in Section 4.2…”

“…Additional research focusing on the effects of swirl injectors was completed by a group at the University of Padua in Padova, Italy (Bellomo, et al, 2012 (Bellomo, et al, 2012). After an extensive study, consisting of 66 tests, they proposed the following statement.…”

“…After an extensive study, consisting of 66 tests, they proposed the following statement. "It can be said, then, that for the regression rate performance parameter the most important factor is the injection type," (Bellomo, et al, 2012).…”