Average droplet vaporization rates in partially prevaporized turbulent spray flames
Résumé
Lean, partially prevaporized and premixed turbulent spray flames of n-heptane/air mixtures are studied experimentally at ambient conditions of temperature and pressure. The effect of three important parameters for the structure of this kind of flame is investigated: the global equivalence ratio, the droplet residence time in the prevaporization tube, and the initial spray droplet mean diameter. Measurements in the flame are performed for three equivalence ratios (U 0.72, 0.79, and 0.87), for two residence times (21 and 49 ms), and for three initial droplet mean diameters (D32 10, 20, and 25 lm). For all flames, phase Doppler anemometry (PDA) is used to characterize the droplet velocities and diameters. The mean progress variableC¯ is obtained from planar laser-induced fluorescence measurements. From the PDA results, the local average vaporization rates K of the droplets within the flame are estimated. The centerline variation of K is discussed versus the normalized axial position Z/D and versus the mean progress variable C¯ in terms of the three parameters previously cited. It is observed that K depends mainly on the initial mean droplet diameter of the spray and on the spray turbulence intensity. The influence of the global equivalence ratio is explained in terms of the flame height and thus can be eliminated by using the mean progress variable as parameter. The average vaporization rates within the spray flame are compared to those obtained for the vaporization of single droplets under various conditions of surrounding temperature and turbulence intensity.