Scaling Parameters of Swirling Oxidizer Injection in Hybrid Rocket Motors Enrico Paccagnella, * Francesco Barato, † and Daniele Pavarin ‡ Università degli Studi di Padova, 35131 Padova, Italy and Arif Karabeyoğlu § Stanford University, Sunnyvale, California 94305 DOI: 10.2514/1.B36241 Hybrid rockets present some disadvantages, mainly low regression rate and combustion inefficiencies. A promising technology to solve both is swirling oxidizer injection, which enhances the wall heat flux and the mixing of the combustion reactants and thus increases the regression rate and the combustion efficiency. A numerical investigation is carried out with a commercial computational fluid dynamics code. This type of analysis can really help with the comprehension of the physical phenomena hidden behind the experimental measurement, and so it can be a powerful aid in the preliminary development and testing of hybrid motors. The first step of this numerical investigation is to study the initial motor geometry, increasing the complexity of the system with the addition of each component one by one to better understand which parameters influence the swirling flowfield inside the combustion chamber. Afterward, a comparison between the axial and swirl injection is done, analyzing the qualitative differences in the flowfields and the quantitative ones in the performance. The central and most important part of this numerical study is focused on the inspection of the motor performance related to several scaling parameters. Nomenclature A = Arrhenius multiplicative coefficient, area a = speed of sound, coefficient c d = discharge coefficient c p = specific heat at constant pressure c v = specific heat at constant volume c  = characteristic velocity D = mass diffusion coefficient Da = Damköhler number d = diameter e = internal energy e 0 = total energy G = mass flux h v = effective heat of vaporization k = turbulent kinetic energy M m = molecular mass _ m = mass flow rate N hol = number of injector inlet holes O∕F = oxidizer to fuel mass ratio p = pressure _ Q = heat flux q = heat generation rate R = molar rate of creation or destruction R u = universal gas constant r = radius _ r = regression rate  _ r = average regression rate S = strain rate T = temperature v = velocity Y = mass fraction γ = specific heat ratio Δ = percentage change ε = turbulent dissipation η = combustion efficiency λ = thermal conductivity μ = dynamic viscosity ν = kinematic viscosity, stoichiometric coefficient σ = Prandtl number ϱ = density τ = shear stress, timescale ω = specific rate of dissipation, angular velocity Subscripts av = average ax = axial injection c = chemical, combustion chamber f = fuel, final g = geometric hol = injector inlet holes i = internal, initial inj = injector outlet hole max = maximum mod = modified o = oxidizer P = product p = port R = reactant ref = reference sim = simulation sw = swirling injection t = turbulent, throat th = theoretical w = wall z = axial direction ϑ = tangential direction 0 = initial conditions, reference value Presented as Paper 2015-3833 at the 51st AIAA/SAE/ASEE Joint Propulsion Conference, Orlando, FL, 27–29 July 2015; received 9 March 2016; revision received 18 December 2016; accepted for publication 21 December 2016; published online 9 March 2017. Copyright © 2016 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the ISSN 0748-4658 (print) or 1533- 3876 (online) to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/randp. *Ph.D. Student, CISAS G. Colombo; enrico.paccagnella.1@phd.unipd.it. † Research Fellow, CISAS G. Colombo; francesco.barato@unipd.it. ‡ Associate Professor, CISAS G. Colombo; daniele.pavarin@unipd.it. § President/CTO, Space Propulsion Group, Inc., Consulting Professor; arif@spg-corp.com. Article in Advance / 1 JOURNAL OF PROPULSION AND POWER Downloaded by UNIVERSITA DEGLI STUDI DI PADOVA on August 2, 2017 | http://arc.aiaa.org | DOI: 10.2514/1.B36241