Experimental Study of Two-Phase Flames under Microgravity Conditions

Abstract : The mechanism of energy conversion based on liquid spray combustion is widely used in industrial applications. Practical applications of two-phase combustion cover diesel, turbofan and rocket engines, as well as furnaces. In addition, it has a major impact on the safety of industrial sites. Two-phase combustion is characterized by a complex interplay between fluid dynamics, vaporization and heterogeneous combustion. Combustion of single droplets and of arrays of droplets have been extensively experimentally and numerically studied and are now fairly understood. Therefore, more realistic aerosol combustions involving complete sprays of mono-sized fuel droplets are currently investigated. It is well-known that the presence of fuel droplets in the mixture has a major impact on the behaviour of a premixed flame. Firstly, the flame speed is highly modified, either increased or decreased, depending on the mixtures parameters (e.g. ignition pressure, equivalence ratio, droplet size and density). Secondly, the presence of cellular instabilities is enhanced. Indeed, these instabilities are triggered much earlier than for an equivalent single-phase flame. The morphology of the unstable flame, e.g. the size of the cells, is also modified. The aerosol is created through the Wilson cloud chamber method: a rapid condensation by expansion causes the nucleation of nearly mono-sized fuel droplets in the mixture. However, two-phase experiments performed under terrestrial gravity condition are characterized by the sedimentation of droplets. The fall of the droplets at their terminal velocity inside the chamber causes a stratification based on the droplet size. Other undesirable effects of gravity include coalescence between droplets and buoyancy effects. The resulting mixture is then highly heterogeneous. In order to eliminate that issue and to guarantee a uniform nucleation, experiments are performed under microgravity conditions. Gravity levels of 10-2 g achieved during 22 s aboard the Airbus A310 ZERO-G ensure a homogenous and isotropic medium and a full control of the physical parameters of the experiment. The experiments are performed in a specific high-pressure dual chamber vessel designed to guarantee operational safety and to follow the specific aircraft-related safety requirements. Several high-speed optical diagnostics are operated to characterize the aerosol and the flame propagation. A laser tomography device is used to track the droplets position and displacement while an ILIDS (Interferometric Laser Imaging for Droplet Sizing) method is used to follow the droplets size. Both diagnostics are used before and during the combustion. The flame propagation itself is recorded by chemiluminescence which allows the precise determination of the flame front. By using simultaneously two of those diagnostics, the droplets behaviour (vaporization or displacement) might be directly linked to their relative position with respect to the flame front. This method allows to investigate the interaction between the droplets and the flame front.
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Conference papers
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Submitted on : Friday, August 9, 2019 - 3:37:29 PM
Last modification on : Tuesday, August 13, 2019 - 1:18:22 AM

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G Renoux, F. Halter, C Chauveau. Experimental Study of Two-Phase Flames under Microgravity Conditions. Human Spaceflight and Weightlessness Science: An International Workshop on Science and Technology for space exploration, CNES, Sep 2018, Toulouse, France. ⟨hal-02265404⟩

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