Stéphane Bernard

SFGP 2007 - Pyrolysis of Supercritical Endothermic Fuel: Evaluation for Active Cooling Instrumentation

Hypersonic flight is expected to be achieved in the coming years by use of Supersonic Combustion RAMJET (SCRAMJET). One of the main issues is the thermal management of the overall vehicle and more specifically the cooling of the engine. In order to simulate the behaviour of an actively cooled SCRAMJET by use of supercritical endothermic fuel, a one-dimensional transient numerical model has been developed with heat and mass transfer, fluid mechanics and detailed pyrolysis chemistry. A dedicated experimental test bench is now available since 2006 at the LEES laboratory of Bourges to study supercritical fuel pyrolysis under steady-state and transient conditions. It aims to provide understanding of coupled phenomena, validation data for the numerical code and evaluation of onboard and real-time measurement methods for industrial use. A brief overview of the numerical code and a presentation of the experimental bench are proposed in this paper. Experimental results are discussed and a comparison is provided between numerical and experimental data. Discrepancies are shown to be lower than a few percent in terms of molar chemical compositions. This is due to uncertainties on experimental temperature measurement and to 2-D effects, which are not taken into account by the modelling. The numerical code appears to be of great importance in accessing unmeasured data and providing new understanding of coupled phenomena. Experimental and numerical tools are proved to be efficient to test future measurement methods under extreme conditions, especially at supercritical states.

Comparison of Combustion Characteristics of Magnesium and Aluminum Powders

ABSTRACT This work presents an experimental study of the combustion characteristics of micron-sized aluminum and magnesium powders under constant volume combustion experiments. Burning velocities were estimated from the measured pressure traces using both a simplified model for combustion on closed spherical bombs and a semi-empirical correlation for dust explosions, and compared to previous literature. Flame temperatures were measured by bi-color pyrometry and indicate that, for aluminum powders with a mean particle diameter smaller than 12 μm, the flame moves closer to the particle’s surface. However, emission spectra obtained during combustion indicate that vapor-phase oxidation exists for all studied powders. Analysis of the combustion products further supported the presence of a vapor-phase reaction. For aluminum, the residue is composed by partially crystallized nanometric spheres as fine as 200 nm. MgO was found in crystallized cubic structures of different sizes, the finest ones also about 200 nm.

Numerical and experimental validation of transient modelling for Scramjet active cooling with supercritical endothermic fuel

One of the main issues of hypersonic flight is the thermal management of the overall vehicle and more specifically the cooling of the engine. In order to simulate the behaviour of a complete actively cooled scramjet, a one-dimensional transient numerical model has been developed with heat and mass transfer in a cooling channel for supercritical fuel under pyrolysis. This model is called RESPIRE (French acronym for Scramjet Cooling with Endothermic Fuel., Transient Reactor Programming). A supplementary step by step validation of the model, based on 2-D numerical data from CFD-ACE, is presented in this paper. On stationary cases, fluid temperature profiles are in good agreement and values are comprised between those of centre fluid and those of near-wall fluid. Effects of one-dimensional semi-empirical correlations are shown and the boundary layer impact at the channel entrance is of great importance on wall temperatures. Heat fluxes conservation is verified and hydraulic behaviour too. On transient cases, temperature and velocity evolutions are well followed. Values after the change in mass flow rate are exactly those of stationary test case. RESPIRE is quantitatively validated under stationary and transient conditions and can be used to compare with experimental data. Qualitatively good agreement is found with experimental results on a chemical aspect.

Experimental characterization of combustion regimes for micron-sized aluminum powders

This work presents an experimental study of combustion characteristics of micron-sized aluminum particles in the transition regime under constant volume combustion experiments. Burning velocities were estimated from the measured pressure traces using both a simplified model for aerosol combustion on closed spherical bombs and a semi-empirical correlation, and compared to previous literature. Flame temperatures were measured by bi-color pyrometry and indicate that for particles smaller than 12 μm, the flame moves closer to the particle’s surface, since flame temperatures were close to aluminum boiling point. For 17.9 μm particles, flame temperatures were close to predicted adiabatic flame temperature and alumina vaporization-dissociation temperature, indicating a classical vapor phase flame under a diffusion-controlled mechanism. However, spectroscopy measurements did not detect significant reductions on molecular AlO emissions for finer particles. This indicates a still very significant presence of vapor phase reaction for powders with a Sauter mean diameter at least as large as 7 μm, which is further supported by the presence of nanometric spheres in the combustion residues, since alumina formed under a vapor phase reaction is expected to condensate into nanometric droplets.