Planar Droplet Sizing for the Characterizationof Droplet Clusters in an Industrial Gun-Type Burner

摘要

An important problem in spray combustion deals with the existence of dense regions f droplets, called clusters. To understand their formation mechanism, the droplet dynamics and fuel concentration profile are investigated by mean of planar laser techniques in an industrial gun-type burner. The simultaneous measurement of elastic Mie scattering and Laser Induced Fluorescence (LIF) allows the instantaneous measurement of the Sauter Mean Diameter (SMD), after proper calibration. Using two different CCDs to get the two signals requires a detailed calibration of the CCD response before getting absolute diameters. Pixels are binned 6 by 6 to obtain the final SMD map, this is a compromise between spatial accuracy and noise. Velocity field is measured on both sets of images using standard Particle Image Velocimetry (PIV) algorithms. The comparison of cross-correlation techique with PDA results shows that the velocity measured on the LIF images are close to the velocity based on D_(30), whereas the Mie scattering results are similar to D_(20). On Mie scattering images, regions of high interfacial area forming clusters can be detected. A special tracking scheme is used to characterize their dynamics in terms of velocity and diameters by ensuring that the same volume of fluid is tracked. It is shown that the clusters have a velocity similar to the velocity of droplets with the same diameter as the mean SMD of the cluster. It is also shown that an increase of pressure tends to trigger the appearance of such group of droplets, due to a smaller diameter of the droplets caused by the increase of pressure discharge. Uncertainties for the different techniques used are discussed.