A rectilinear droplet stream in combustion: Droplet and gas phase properties

A basic experiment has been designed to improve the understanding of certain phenomena like droplet interaction occurring in dense sprays. The interaction effect in a stream of monosized ethanol droplets on the drag coefficient (Cd) for non evaporating and reacting conditions is investigated. Also the burning rate is studied. The spacing parameter C (ratio of interdroplet distance to droplet diameter) is the main parameter retained here to represent the interaction effects. Non intrusive optical techniques have been developed to measure, simultaneously along the droplet path, the droplet size, velocity and mean temperature. The stream is first studied in non evaporating conditions. The data compiled for a large range of parameters (drag coefficient Cd, Reynolds number Re, spacing parameter C) permit to establish a new correlation for the drag coefficient taking into account the spacing parameter. The first experimental results are compared with those previously published. Then the stream is ignited by an electrically heated coil to investigate close coupled droplets (C<6) under reacting conditions. The previous optical methods are applied for those conditions. The CARS thermometry technique is used to determine the temperature fields around the droplets for different values of the spacing parameter. The detailed analysis of the two phases in the burning droplet stream allows to improve the knowledge of the fundamental mechanisms on droplet/droplet interactions. For example, the evolution of the drag coefficient with the spacing parameter is preponderant to compute the droplet trajectory in dense sprays. Particularly, the results show an important effect of droplet spacing on the burning rate and the drag coefficient Cd for this basic configuration.