Development and Three-dimensional Numerical Simulation of a Double-tube Hybrid Rocket Motor

Abstract

This paper presents a new concept of Hybrid Rocket Motor (HRM) called Double-tube.In this configuration, the gaseous oxidizer is injected using a head end injector and an inner tube with injector holes distributed along the motor longitudinal axis. The inner tube is located inside the combustion chamber, is coaxial to the motor case and goes from the motor head end to the aft end of the fuel grain. In order to test the potential advantages of this design, a series of three-dimensional numerical simulations have been carried out with gaseous oxygen (GOX) as oxidizer and hydroxyl terminated polybutadiene (HTPB) as solid fuel. The simulation model considers realizable k-ε turbulence model combined with eddydissipation combustion model. Furthermore, the solid fuel pyrolysis is computed through customized user-defined functions. This numerical model has been validated for previous conventional HRM designs by comparing the computational results against experimental data. The results obtained for the Double-tube have shown that the regression rate can be increased over 50% with respect to classical hybrid rockets. Indeed, the characteristic velocity, combustion efficiency and species mixing are improved by injecting the oxidizer where it is most needed thanks to an appropriate distribution of the inner tube injector holes. Finally, the Double-tube design allows for a higher control of the oxidizer-to-fuel ratio along the grain because the inner tube provides a customized distribution of oxidizer. NomenclatureA = Arrhenius preexponential constant, mm/s a = preexponential factor of the regression rate law c* = characteristic exhaust velocity, m/s D = port diameter, mm d = diameter of the injector holes, mm E a = activation energy, J/mol e = energy, J/kg G ox = oxidizer mass flux, kg/m 2 s H = enthalpy, J/kg k = kinetic energy of turbulent fluctuations L = grain length, mm ṁ ox = total oxygen mass flow rate, kg/s N = total number of chemical species n = exponent of the oxidizer mass flux O/F = oxidizer-to-fuel ratio p c = chamber pressure, atm or Pa p t = effective pressure, Pa R 2 = coefficient of determination R u = universal gas constant, J/kg-K 1 Master's Degree candidate, School of Astronautics, Beihang University, arnau.pons.lorente@gmail.com, and AIAA Student Member. 2 r = regression rate, mm/s T = temperature, K t = time, s u = velocity, m/s Y m = mass fraction of the mth species α = inner tube injection angle ε = turbulence dissipation rate η = c* efficiency, ratio between the simulated and the theoretical characteristic velocity λ = ratio of inner tube oxidizer mass flow rate to total oxidizer mass flow rate λ' = thermal conductivity, W/m-K μ = viscosity, kg/s-m ρ = density, kg/m 3 Subscripts f = fuel g = gas HE = head end IT = inner tube n = normal direction ref = reference s = fuel surface

Testing and Evaluation of a Double-Tube Hybrid Rocket Motor

“…This characteristic is caused by the high impact of the inner tube injection jets on the inner fuel grain combustion. Nevertheless, in case of the double-tube with inserts, the regression rates of the inner and outer fuels are very similar as it was predicted in the previous CFD study 12 for an injection angle of 135º.…”

“…The experimental tests have confirmed the performance trends and characteristic features obtained in the CFD study 12 . Mainly, the regression rate of the solid fuel in the double-tube configuration is significantly larger (over twice) than in a conventional axial showerhead injector.…”

“…Note that the former velocity is given by: (12) where c p is the effective mean pressure of the combustion chamber, and A t is the nozzle throat area. To sum up, the theoretical characteristic exhaust velocity has been estimated utilizing the CEA chemical equilibrium code 13 at the effective mean pressure of the combustion chamber, oxidizer-to-fuel ratio and fuel vapor temperature at the grain wall calculated in the CFD simulations.…”

“…Bearing in mind the great influence of the injector design on hybrid rockets, a new configuration named doubletube HRM was proposed in a previous work with the aim of increasing the regression rate as well as the overall performance 12 . The layout of this new design consists of using two coaxial tubes, in which the inner tube is the main oxidizer injection source and the outer one is the motor case (see Fig.…”

“…Note that the injection angle α is defined as the angle between the motor longitudinal axis and the axis of the inner tube injector holes defined positive counterclockwise 12 . In addition, λ is established as the ratio of the inner tube oxidizer mass flow rate, ṁ ox_IT , to the total oxidizer mass flow rate, ṁ ox :…”