Atomization characteristics of impinging liquid jets

Abstract

The atomization characteristics of sheets formed by both laminar and turbulent impinging jets were experimentally studied as a function of flow and injector geometric parameters. In particular, sheet breakup length along the sheet centerline, distance between adjacent waves apparent on the sheet, and drop-size distributions were measured over a Weber number range between 350-6600 and a Reynolds number range between 2.8 x 103 to 2.6 x 10 4. A linear stability-based model was used to determine the most unstable wave number and the corresponding growth rate factor on two-dimension al thinning inviscid and viscous sheets. These wave characteristics were used to predict both the sheet breakup length and the resulting drop sizes. A second model, applicable for a low Weber number regime, in which sheet disintegration is controlled by stationary antisymmetric waves, was used to predict the shape of the sheet formed by two impinging liquid jets. The linear stabilitybased theory predictions of breakup length did not agree in trend or magnitude with experimental measurements. However, for Weber numbers less than 350, the measured breakup length for laminar impinging jets was within 50% of that predicted by the stationary antisymmetric wave-based model. Finally, drop-size predictions based on linear stability theory agreed in trend, but not in magnitude, with the measured drop sizes. The contrast between the sheet atomization characteristics of laminar vs turbulent impinging jets suggest that the initial conditions of the impinging jets significantly influence the sheet breakup mechanism. Also, the comparison between experimental results and theoretical predictions indicates that the impact wave generation process at the jet impingement point needs to be incorporated in the theoretical models for sheet atomization. Nomenclature d = diameter F = thickness distribution h = sheet thickness k = wave number L = length of injection element / = length r = radial distance from impingement point Re = Reynolds number, Ujdjv/, based on liquid properties, jet velocity, and orifice diameter Rex = Reynolds number, Ushlvi, based on liquid properties, sheet velocity, and sheet thickness 5 = ratio of gas density to liquid density t = time U = velocity W = maximum width of sheet We = Weber number, piUjdJcr, based on liquid properties, jet velocity, and orifice diameter Wes = Weber number, p,£/;/z/cr, based on liquid properties, sheet velocity, and sheet thickness x = axial distance from impingement point y = coordinate perpendicular to x in the plane of the sheet a = fan inclination angle ft = complex growth rate factor, pr + //3, 77 = disturbance amplitude 6 = impingement half-angle A = wavelength ju = dynamic viscosity