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
