3D miles simulation of a hybrid rocket with SWIRL injection

3D miles simulation of a hybrid rocket with SWIRL injection Jérôme Messineo, Jean-Yves Lestrade, Jouke Hijlkema, Jérôme Anthoine To cite this version: Jérôme Messineo, Jean-Yves Lestrade, Jouke Hijlkema, Jérôme Anthoine. 3D miles simulation of a hybrid rocket with SWIRL injection. Space Propulsion 2016, May 2016, ROME, Italy. <hal- 01353580> HAL Id: hal-01353580 https://hal.archives-ouvertes.fr/hal-01353580 Submitted on 12 Aug 2016 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Powered by TCPDF (www.tcpdf.org) COMMUNICATION A CONGRES 3D miles simulation of a hybrid rocket with SWIRL injection J. Messineo, J.-Y. Lestrade, J. Hijlkema, J. Anthoine Space Propulsion 2016 ROME, ITALIE 2-6 mai 2016 TP 2016-466

SP2016_3125046 3D MILES SIMULATION OF A HYBRID ROCKET WITH SWIRL INJECTION J. Messineo (1), J.-Y. Lestrade (2), J. Hijlkema (3) and J. Anthoine (4) (1) ONERA - The French Aerospace Lab, 31410 Mauzac, France, jerome.messineo@onera.fr (2) ONERA - The French Aerospace Lab, 31410 Mauzac, France, jean-yves.lestrade@onera.fr (3) ONERA - The French Aerospace Lab, 31410 Mauzac, France, jouke.hijlkema@onera.fr (4) ONERA - The French Aerospace Lab, 31410 Mauzac, France, jerome.anthoine@onera.fr KEYWORDS: Hybrid rocket motor, oxidizer swirl injection, 3D numerical simulations, pressure oscillations, combustion efficiency. ABSTRACT Oxidizer injection plays a major role in the behaviour of hybrid rocket engines, especially regarding combustion efficiency and stability. This study aims to compare two firing tests with associated numerical simulations based on gaseous oxidizer axial or swirl injection through a catalyzer. The oxidizer and fuel couple used was H 2 O 2 (87.5 %)/HDPE. Numerical 3D MILES (Monotone Integrated Large Eddy Simulation) simulations of both configurations were performed and analyzed regarding on one side the combustion efficiency influenced by propellants mixing, and on the other side the pressure oscillations provoked by the formation of large scale vortices in the aft-combustion chamber. 1. INTRODUCTION A classical hybrid rocket motor combines a liquid oxidizer with a solid fuel. Hybrid engines provide higher theoretical specific impulses than solid rocket motors. They are safer since the propellants are stored separately and the solid fuel is inert, they can be stopped, throttled and eventually restarted. Hybrid motors are simpler than liquid motors since only one pressurized and liquid feed system is required. This technology is promising and investigated for its potentialities in some new applications. Marxman et al. [1,2] have studied the turbulent boundary layer which develops along the solid fuel in a hybrid rocket engine. The diffusion flame located in the boundary layer is fed in oxidizer by diffusion from the main flow and in fuel from the solid surface. The heat provided by the flame provokes the pyrolysis of the solid fuel which feeds the combustion in return. It is therefore a selfsustained phenomenon. During the combustion process the fuel grain geometry evolves, inducing a variation of the fuel mass flow rate and also of the oxidizer to fuel ratio (O/F). This is one of the main differences with the other chemical propulsion systems. Numerical simulations can help the understanding of internal flows in hybrid engines and have been performed for several years. Cheng et al. [3] developed a 3D RANS (Reynolds-Averaged Navier-Stokes equations) model using a Lagrangian approach for liquid droplets transport and evaporation and an Eulerian approach for the gas-phase flow calculation. Radiative heat transfers were considered in the computations realized by Sankaran and Merkel [4] and by Gariani et al [5]. Surface pyrolysis is generally calculated based on an Arrhenius law and determines the regression and mass flow rates of the fuel grain [4,6]. Sankaran [7] used a model based on the time-dependent Navier-Stokes equations including turbulence, gas-phase radiation, finite-velocity model for the combustion and a coupling between gas and solid-fuel phases. More recently, RANS or LES (Large Eddy Simulation) simulations have been employed to study the propellants mixing and improve combustion efficiency thanks to diaphragms in the combustion chamber of a hybrid engine [8,9]. One of the limiting factors regarding hybrid rockets is the low combustion efficiency with regards to other chemical propulsion technologies. Several studies have been conducted in order to improve combustion efficiency, including the use of obstacles in the combustion chamber [8]. Nevertheless, such solutions may be difficult to use for a long-time operating hybrid engines, due to high levels of temperature and heat flux in the combustion chamber. Another solution for increasing the combustion efficiency is the oxidizer swirl injection. This configuration has been studied experimentally [10,11] and numerically [8] and seems to be a promising way to make hybrid engines competitive with regards to combustion efficiency. The work of Lestrade et al. [12] combines a hybrid engine with a catalyzer and a 1

swirl injector. It has recently lead to an experimental demonstration of a specific impulse of about 272 s for a nozzle expansion ratio of 50. The use of a catalyzer, instead of an injector, ensures the oxidizer to be in a gaseous and high temperature phase directly at the inlet of the combustion chamber. The oxidizer injection plays a major role in the flow behaviour, especially for the combustion efficiency or stability. Pressure oscillations in rocket motors can be damageable for the vehicle structure or payloads and their amplitudes have to be limited. Several kinds of instabilities are possible and may be provoked by acoustics, vortex shedding or even gas-phase reactions. Carmicino [13] studied the interaction between different kinds of instabilities and revealed the importance of vortex shedding for hybrid rocket motors. He showed that the coupling between acoustics and post-chamber vortex shedding could lead to instabilities reaching over 70% of the mean pressure value. It was also experimentally shown that swirl injection could reduce instabilities amplitudes [8]. Recent work [14,18] has revealed that vortex shedding in the post-chamber was responsible for pressure oscillations in a hybrid engine combined with a catalyzer and with an axial oxidizer injection. Numerical 2D URANS (Unsteady RANS) and 3D MILES simulations were performed and showed that vortices in the post-chamber contain unburnt fuel and therefore have a major influence on the propellants mixing. Several new firing tests of a hybrid engine based on H 2 O 2 /HDPE have been performed using a catalyzer and swirl injection. This paper will present the associated 3D unsteady numerical simulations based on the MILES approach in order to get a better understanding of swirl injection effects on stability and combustion efficiency. 2. EXPERIMENTAL SETUP AND RESULTS 2.1. Description of the facility The engine is classically composed of a precombustion chamber, a combustion chamber with a cylindrical channel for the fuel grain, an aftcombustion chamber and a nozzle adapted to ambient pressure (Fig. 1). The oxidizer is stored in a pressurized tank and injected through a catalyzer combined with an axial or a swirl injection. The engine is instrumented with an oxidizer mass flow meter, thermocouples located at the outlet of the catalyzer and before the injector, pressure probes (two placed before the fuel grain and two in the post chamber) and ultrasonic sensors (one located at the head-end of the fuel grain and two at the rear-end). The ultrasonic sensors enable to follow the instantaneous regression rate of the fuel grain at these locations. The engine is placed on a thrust bench to get the propulsive performances. Figure 1: Drawing of the engine The oxidizer used is hydrogen peroxide (H 2 O 2 ) concentrated at 87.5 % in mass, which decomposes into a gaseous mixture of H 2 O and O 2 at the catalytic decomposition temperature passing through the catalyzer. The oxidizer is combined with a high density polyethylene (HDPE) fuel grain whose dimensions are reported in Tab. 1. Two firing tests have been performed using respectively the axial and swirl injector. The axial configuration was studied in a previous work [14]. The two firing tests are based on the same initial conditions and configurations, except the injector's geometry, and are given in Tab. 1. The axial firing test was performed with a catalyzer provided by Nammo [10,11] whereas the swirl firing test was conducted using a catalyzer developed by Heraeus and ONERA and based on a porous media catalytic bed [17]. Table 1: Initial parameters Initial fuel port diameter 25.0 [mm] Length of the fuel grain 240 [mm] Nozzle throat diameter 7.0 [mm] Nozzle expansion ratio 6.3 [-] 2.2. Firing tests results The firing tests results are presented in Tab. 2 and Fig. 2. In both configurations, a mono-propellant phase of 1 to 2 s preceded a hybrid phase. The oxidizer tank pressure was constant during the firing tests thanks to a pressurization system, enabling a constant oxidizer mass flow rate, mean pressure and thrust during the hybrid phases. The measured quantities have been averaged to 2

deduce mean O/F ratio, propulsive performances and efficiencies over the hybrid mode duration. The mean fuel mass flow rates were calculated based on the mass of the grain before and after the firing tests. The mean regression rates were estimated thanks to the initial and final port diameters at the rear end of the grains. Table 2: Firing tests results Injector Axial Swirl Firing test duration [s] 5.3 7.0 Oxidizer mass flow rate [g.s -1 ] 105.1 101.5 Fuel mass flow rate [g.s -1 ] 11.0 15.9 Pressure [MPa] 4.15 4.70 Thrust [dan] 22.6 26.0 Fuel regression rate [mm.s -1 ] 0.35 0.49 O/F ratio [-] 9.57 6.38 Characteristic velocity [m.s -1 ] 1367.0 1540.7 Specific impulse [s] 198.7 225.8 Combustion efficiency* [%] 88.4 97.5 Nozzle efficiency* [%] 92.8 93.6 Total efficiency* [%] 82.0 91.3 * combustion and total efficiencies are respectively calculated based on the experimental to theoretical characteristic velocities and specific impulses ratios. Nozzle efficiency is the ratio between total and combustion efficiencies. Despite similar initial conditions and oxidizer mass flow rates, the two configurations conducted to notable disparity regarding the performances. The use of a swirl injection instead of an axial one significantly increased the fuel regression rate, the mean pressure and the thrust. The combustion and total efficiencies respectively gained 9.1 % and 9.3 %. The catalyzer used for the swirl configuration resulted in a longer ignition time than the previous catalyzer. This point is under improvement and should be reduced thanks to the collaboration with Heraeus [17]. The axial injection yielded to pressure oscillations with a magnitude of 1.9 % of the mean pressure at a frequency close to 470 Hz for the main peak. These oscillations have been explained by the formation of large scale vortices at the end of the fuel grain [14,18]. The FFT (Fast Fourier Transform) of the pressure signals have been performed for the swirl configuration and is compared to the axial one (Fig. 3). Normalized Magnitude [-] 1 0.8 0.6 0.4 0.2 Axial Swirl 0 0 500 1000 1500 2000 Frequency [Hz] Figure 3: FFT of the pressure signals Figure 2-a: Axial firing test results Figure 2-b: Swirl firing test results 3

The swirl injection did not reveal a significant frequency peak in the pressure signal treated with FFT. This configuration conducted to a much more stable behaviour with regards to pressure oscillations, as shown in Fig. 2a and 2b. 3. NUMERICAL SIMULATIONS Associated 3D numerical simulations have been performed for the axial and swirl configurations. 3.1. Numerical models The CFD (Computational Fluid Dynamics) finite volume code CEDRE (Calcul d Ecoulements Diphasiques Réactifs pour l Energétique) [15] is used for the simulations which are based on a MILES approach (LES without sub-scale model). The mass, momentum and energy conservation equations are solved with an additional transport equation for the species and the flow is singlephase. The experimentally decomposed hydrogen peroxide (H 2 O 2 ) is replaced by a gaseous mixture of steam (water) and oxygen (H 2 O/O 2 ) in the computations. The experimental oxidizer is concentrated at 87.5 % in mass, which corresponds to a mixture based on 60 % - 40 % in mass of steam (water) and oxygen for the calculations. It is supposed that the pyrolysis of HDPE produces only gaseous ethylene (C 2 H 4 ). The combustion is modeled with a two-step mechanism (eq. 1) with finite velocities of chemical reactions expressed by Arrhenius laws, the coefficients being given by Westbrook and Dryer [16]. Thermal radiation is not taken into account. C 2 H 4 + 2O 2 2CO + 2H 2 O CO + 0.5O 2 CO 2 (1) The geometry of the fuel grain is fixed at the initial and known conditions. The low fuel regression rates with regards to flow velocities justifies that the constant geometry of the grain is not a limiting factor for the unsteady numerical simulations. Real injector geometries are used for the axial and swirl configurations. The walls are considered adiabatic. The oxidizer mass flow rate and injection temperature are fixed based on the experimental results. The fuel mass flow rate is determined thanks to the mean experimental value (based on the mass measurements of the grain) and the fuel injection temperature is fixed at 900 K. This value is not precisely known but it was shown that this parameter has no major effect on the computation results [8]. Boundary conditions are given in Tab. 3. Table 3: Boundary conditions Injector Axial Swirl Oxidizer mass flow rate [g.s -1 ] 105.1 101.5 Oxidier temperature [K] 900 860 Fuel mass flow rate [g.s -1 ] 11.0 15.9 Fuel temperature [K] 900 900 The axial and swirl configurations are simulated thanks to 3D mesh grids of 1/2 and 1/12 of the engine respectively, due to symmetric conditions of the injection holes, and are respectively composed of 5.10 6 and 3.10 6 elements. Grid convergence was not performed for the 3D calculations due to time constraints. Nevertheless, calculations of 2D- URANS approach were performed with a grid convergence in the axial configuration case and resulted in flow behaviour comparable to the 3D calculation. 3.2. Numerical results The numerical results are compared with the experimental firing tests based on the mean pressure of the engine. Comparisons are given in Tab. 4. Simulations found mean pressures within 3 % (axial) to 5 % (swirl) of the experimental values. Table 4: Numerical and experimental pressure [MPa] comparisons Injector Axial Swirl Experimental firing tests 4.15 4.70 Numerical simulations 4.05 4.94 Streamlines of the flows are plotted in Fig. 4. For the axial injection case, the streamlines and the engine symmetrical axis are parallel in the fuel grain channel, as expected. The main velocity component is the axial one while the other components have negligible magnitudes (Fig. 5). For the swirl injection case, the streamlines roll up around the symmetrical axis of the engine and the rotational motion of the flow is maintained from the beginning to the end of the fuel grain. In that configuration the non-axial velocity components have significant magnitudes with regards to the 4

axial one (Fig. 5). The swirl injection increases the residence time of the propellants and therefore enhances mixing and combustion efficiency of the engine. Fig. 6 presents the oxidizer and fuel mass fractions and the temperature fields in the axial case. The oxidizer is concentrated at the center of the channel while the fuel is accumulated close to the grain wall. The combustion occurs in the region where oxidizer and fuel encounter, which is a classical diffusion flame. The flame gradually moves away from the fuel grain wall as the distance from the beginning of the channel increases. In the swirl case (Fig. 7) the fuel is still concentrated close to the grain wall. The layer thickness in which the fuel is present is however lower than for the axial case at the end of the fuel grain. If the distance from the beginning of the fuel grain is low (10 %), oxidizer is mainly concentrated at the center of the channel. While this distance increases, the oxidizer is not located at a specific region in the channel. The non-axial components of the velocity are not negligible and the oxidizer and combustion products are transported away from the surface of the grain and from the reactive zone (Fig. 9). Figure 4: Streamlines in the axial (left) and swirl (right) configurations. Slices colored by velocity magnitude. 50 Vx Vy Vz 50 Vx Vy Vz 40 40 Velocity [m/s] 30 20 Velocity [m/s] 30 20 10 10 0 0 0 20 40 60 80 100 x/lfuel [%] 0 20 40 60 80 100 x/lfuel [%] Figure 5: Velocity components evolutions along the combustion chamber in the axial (left) and swirl (right) configurations. 5

x/l fuel = 10 % x/l fuel = 50 % x/l fuel = 99 % Figure 6: Fuel (left), oxidizer (middle) mass fractions and temperature (right) instantaneous fields at different positions in the fuel grain. x = distance from the beginning of the grain. Axial injection configuration. A flame is still located at the fuel/oxidizer mixing interface close to the fuel grain wall but high temperatures regions outside of the boundary layer are due to the presence of combustion products. In the axial case, these products are also formed in the flame region but are only transported through the axial direction. The experimentally and numerically observed pressure oscillations in the axial configuration are provoked by the formation of large scale and organized vortices [18]. In the swirl configuration, there was no similar pressure fluctuations and the firing test showed a more stable behavior. Fig. 8 shows iso-contours of Q criterion in the numerical simulations, allowing to locate coherent structures. In the swirl case there are multiple and nonorganized vortices in the aft-chamber. This may contribute to enhance the mixing between unburnt propellants but also drastically reduce pressure oscillations provoked by large vortices in the axial case. 6

Figure 7: Main slice of temperature field. Secondary slices of fuel and oxidizer fields at x/l fuel = 10 and 99 %. Instantaneous fields, swirl injection configuration. Figure 8: Instantaneous iso contours of Q criterion in axial (Q= 5.10 6, left) or swirl (Q = 3.10 7, right) configurations, colored by the temperature. 7

1 Y_C2H4 Y_O2 x/lfuel = 10% x/lfuel = 50% x/lfuel = 99% 1 Y_C2H4 Y_O2 x/lfuel = 10% x/lfuel = 50% x/lfuel = 99% 0.8 0.8 Mass fractions [-] 0.6 0.4 Mass fractions [-] 0.6 0.4 0.2 0.2 0 0 0.002 0.004 0.006 0.008 0.01 0.012 r[m] 0 0 0.002 0.004 0.006 0.008 0.01 0.012 r[m] Figure 9: Mean profiles of propellants mass fractions at different axial positions. Axial (left) and swirl (right) configurations. 4. CONCLUSIONS This paper presents a firing test performed with a gaseous oxidizer swirl injection through a catalyzer. This firing test is compared with a similar test realized with an axial oxidizer injection. Associated 3D MILES simulations were performed in order to get a better understanding of the swirl effects on the engine combustion efficiency and stability. The swirl injection significantly increased the combustion efficiency of the engine. The rotational motion of the flow was maintained from the beginning to the end of the fuel grain channel and enabled an enhanced mixing between the propellants. Indeed, the residence time is increased and the species transport and diffusion is more efficient due to higher non-axial velocity components. Pressure oscillations provoked by hydrodynamics were drastically reduced thanks to the use of a swirl injector. Instead of generating large and organized vortices in the aft-chamber as for the axial configuration, the swirled flow induced an important number of smaller vortices. These structures may increase mixing in the aft-chamber and do not produce significant pressure fluctuations. 5. ACKNOWLEDGMENTS This work was supported by a 2013 research grant of Région Midi-Pyrénées in France. The authors thank Heraeus and Nammo for their cooperation concerning the realization of the firing tests with the catalyzers. 6. REFERENCES 1. G. A. Marxman and M. Gilbert. 1963. Turbulent boundary layer combustion in the hybrid rocket. 9th International Symposium on Combustion. pp. 371-383. 2. G. A. Marxman. 1965. Combustion in the turbulent boundary layer on a vaporizing surface. 10th International Symposium on Combustion. pp. 1337-1349. 3. G. Cheng, R. Farmer, H. Jones and J. McFarlane. 1994. Numerical Simulation of the Internal Ballistics of a Hybrid Rocket Motor. 32nd Aerospace Sciences Meeting and Exhibit. AIAA Paper 1994-554. 4. V. Sankaran and C. Merkle. 1996. Size Scale- Up in Hybrid Rocket Motors. 34th Aerospace Science Meeting and Exhibit. AIAA Paper 96-0647. 5. G. Gariani, F. Maggi, T. Lucchini, T. Faravelli and L. Galfetti. 2009. 2D Numerical Simulation of Combustion Processes in a slab Hybrid Rocket Motor. 3rd European Conference for Aerospace Sciences (EUCASS). Versailles, France. 8

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