An experimental design is presented for studying fuel droplet combustion at elevated pressures. The combustion configuration considered is one where both buoyancy and forced convection is minimized, and gas transport is entirely due to fuel evaporation at the droplet surface. The near spherically symmetric burning process that results is amenable to detailed numerical modeling with capabilities to predict formation of combustion products such as greenhouse gases and particulates (soot) that are important for mitigating global pollution concerns. The experimental design incorporates a high pressure combustion chamber that can withstand pressures up to about 40 atm. The pressure vessel contains hardware to form, deploy and ignite test droplets. The chamber is placed in free-fall to minimize buoyancy effects over a distance of 7.6 m for an observation time of about 1.2 s. Hardware within the chamber includes an assembly generator for deploying droplets (diameter of about 0.5 mm) onto two SiC (14 mm) fibers and an arrangement for positioning Kanthal coils around the droplet for ignition. An electronic circuit design is described for controlling the duration of the coil heating times including the energy and time of retraction through a Raspberry Pi controller interfaced with a local computer. The droplet burning history is recorded using a Nikon D5600 camera for color flame images at 2 MP/frame and 60 fps, and a black and white (BW) Photonics MS-80K camera operated at 200 fps with 3.9 MP resolution for droplet images with backlighting provided by a 630 nm collimated LED light source. Preliminary experimental results are presented for several organic liquids (e.g., an alcohol and alkane) to show how the flame structure evolves as pressure is increased. The effect of diluting the ambient gas with an inert (e.g., helium) is also presented to show its influence on formation of particulates and flame luminosity.

Design of a high pressure isolated droplet combustion apparatus

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