Experimental analysis of droplet spatial distribution in a spray burner
Résumé
A new experimental setup PROMETHEE has been built to study spray combustion. The design corresponds to a square-duct channel with a bluff body spanning over the entire width of the channel. Turbulent flow circulates around the obstacle and liquid fuel is injected at the rear of the bodyvthrough a flat-fan nozzle. The two-dimensional configuration has been chosen to facilitate the optical measurements, and the operating conditions partially reproduce those encountered inside a turboreactor. A procedure based on Mie scattering images is proposed to characterize the interdroplet distance to the nearest neighbor and to describe the spray spatial distribution. Such a procedure could benefit spray combustion modeling, providing a more precise length scale. First, the droplet spatial distribution under reacting conditions was investigated with respect to the interdroplet distance and the spatial distribution law at six locations downstream from the bluff body for the PROMETHEE experiments. For the distribution law, comparisons between the experimental, theoretical uniform, and uniformly random distributions, and numerical results from an in-house Monte-Carlo solver show that the distribution is close to the uniformly random one and that the interdroplet distance and its standard deviation evolve linearly with respect to the inverse of the density number square root. In addition, the numerical approach was used to estimate the projection and restriction errors associated with the interdroplet distance. Second, the procedure has been assessed through six Direct Numerical Simulation—Direct Particle Simulation (DNS/DPS) isotropic homogeneous turbulent simulations to highlight the clustering effects on the dimensionless mean and standard deviation of the nearest-neighbor interdroplet distance when decreasing the Stokes number. The results show that the droplet distribution is similar to the uniformly random distribution when the particle Stokes number is high enough while the mean (standard deviation) decreases (increases) with the reduction of the Stokes number due to the clustering effect. Such an observation demonstrates that preferential segregation is probably not important in the present experimental data.
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