Transient Computational Thermofluid-Dynamic Simulation of Hybrid Rocket Internal Ballistics

Abstract

A computational thermofluid-dynamic model of hybrid rocket internal ballistics is developed. Numerical simulations of the flowfield in a laboratory small-scale hybrid rocket motor, operated with gaseous oxygen and high-density polyethylene propellants, are carried out with the aim of predicting the solid fuel regression rate experimentally achieved with two different oxidizer injectors. The fuel regression rate is the main parameter for the hybrid rocket design. Here, it is calculated with a detailed gas/surface interface characterization based on local mass and energy balances. The combustion of oxygen and gaseous ethylene injected from the fuel wall is modeled by means of the probability-density-function approach coupled to chemical equilibrium. Two oxidizer-injection configurations, which generate either a two-dimensional axially symmetric or three-dimensional flowfield, are analyzed. The local regression rate is evaluated along both the fuel grain axis and inner circumference in the three-dimensional case, as well as at several stages in the firing, by updating the local port diameter, which is thus not assumed constant. Data retrieved from three firing tests are compared with the numerical results, revealing good agreement between both the average regression rates (maximum deviation less than 5%) and the fuel consumption axial profiles (with maximum deviation of 14%

Numerical Study of Fuel Regression in Hybrid Rocket Engine

“…According to literature, several approaches have been adopted for radiation modelling. In most studies on fuel regression rate hybrid rockets, radiation is neglected ( [14][15][16][17][18][19]) assuming that this phenomenon would be significant only for metallized fuels. However Chiaverini et al [7] have shown that for relatively low oxidant flows, radiation could have a significant effect on the fuel regression rate even though the importance of this effect is particularly controverted in the literature.…”

Modeling of High Density Polyethylene Regression Rate in the Simulation of Hybrid Rocket Flowfields

“…Indeed, the extension of those models to new motors that can be different in geometry, scale, etc., is hardly possible without the availability of existing experimental data for each motor. For these reasons, there is a renewed interest in the development of more accurate and advanced models based on Computational Fluid Dynamics (CFD) [3][4][5][6][7][8][9][10][11][12][13][14][15] that are capable of representing more accurately the physico-chemical phenomena involved. The numerical modeling of the fluid dynamics and the combustion process in the fuel port area and nozzle of a hybrid rocket is a challenging task as it involves strongly-interacting multiphysics processes such as fluid dynamics, fuel pyrolysis [16,17], atomization and vaporization of the oxidizer, mixing and combustion in the gas phase [11][12][13]18], thermochemical erosion of the nozzle [3,19], particulate formation, and the radiative characteristics of the flame.…”

“…Commercial CFD tools are generally not optimized to this task, as they are typically less flexible for the treatment of fluid/solid boundary conditions, which are typically prescribed as constant temperature or heat flux with no feedback with the mass transfer mechanisms (pyrolysis, sublimation, etc.). To obtain an adequate tool for the analysis of the flowfield of hybrid rocket burning classical non-liquefying fuels, CFD codes should take into account spatially-varying heat flux, surface temperature, and fuel regression rate, realistic surface multispecies mass and energy balances, thermal soak into the fuel grain, radiative energy exchange, and finite-rate Arrhenius kinetics for fuel pyrolysis modeling or, in the case of commercial solvers, a number of user-defined functions need to be built and integrated in the numerical framework [8,14,20]. In the most general case, an in-house code has to be used.…”

“…The in-house computational tool used for the simulations, and its gas-surface interaction capability has been validated for high-speed re-entry flows [24], for the analysis of Hydroxyl-Terminated Polybutadiene (HTPB) fuel grains [3,4,25], and for hybrid rocket nozzle thermal protection systems' ablation [3,19,26]. Recent literature works focusing on HDPE fuel grains in hybrid rockets either used commercial CFD tools with user-defined functions to compute the fuel mass flux as a function of the wall heat flux [8,14] or in-house CFD code [15] with imposed fuel mass flux using the mean regression rates measured during the firing test. Thermal radiation was not taken into account in [14,15].…”

“…Recent literature works focusing on HDPE fuel grains in hybrid rockets either used commercial CFD tools with user-defined functions to compute the fuel mass flux as a function of the wall heat flux [8,14] or in-house CFD code [15] with imposed fuel mass flux using the mean regression rates measured during the firing test. Thermal radiation was not taken into account in [14,15]. Interestingly, the work in [8] showed that the regression rate was underestimated by 30% for high-density polyethylene grains when radiative heat exchange was not accounted for.…”