In aero-engines, fuel is injected as a liquid which subsequently atomizes, evaporates and finally burns. Investigating such a two-phase flow requires the description and understanding of many different phenomena like atomization, droplet dispersion by turbulence, spray evaporation and combustion. To this end, a specific test rig named PROMETHEE has been developed at ONERA [1]. The design of this setup aims at reproducing partially the flow conditions met inside the combustion chamber of a turbo-reactor and enabling optical measurements. Therefore, the geometry of the test rig is as far as possible two- dimensional. Experimental campaigns have been conducted in non-reactive and reactive conditions to obtain an exhaustive experimental database. The present paper will focus on an estimation of the nearest-neighbour droplet distance under non-reactive and reactive conditions. Indeed, we think that this parameter is important for studying and modelling spray behaviour and, in particular, collective effects between droplets are believed to have a large influence on their evaporation.

The flow geometry corresponds to a 120 x 120 mm square-section channel. A trapezoidal bluff-body is located in the span-wise direction and its blockage ratio β is equal to 43%. A flat-fan atomizer is fixed in the middle of the rear face for fuel injection. Optical measurements have been performed at atmospheric pressure with air heated at 450 K. The mean flow velocity is equal to 5.8 m/s. The Reynolds number based on the height of the obstacle (50 mm) and the mean flow velocity is equal to 22 000. Air and fuel mass flow rates are respectively equal to 58 g/s and 1 g/s. Liquid fuel is n-decane.

The analysis is based on thousands of Mie scattering images recorded during the experiments [2]. In order to define local values of the nearest-neighbour droplet distance, specific image processing algorithms have been developed to retrieve geometrical parameters of interest from these large data bases.

Furthermore, since a von Karman vortex street appears under non-reactive conditions, we also propose a phase average to describe the nearest-neighbour droplet distance with respect to the vortex shedding. The results show that, even under non-reactive conditions, the nearest-neighbour droplet distance varies linearly with the inverse of the droplet density number square-root. Comparisons with the theoretical Hertz-Chandrasekhar law or the regular arrangement law demonstrate that the experimental distribution lies between the theoretical ones.

[1] Vicentini, M., PhD Thesis, Toulouse, 2016.
[2] Rouzaud, O. et al., 27th ILASS Conference, Brighton, 2016.