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 10 3 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-dimensional 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 Re x = Reynolds number, U s hlvi, 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 We s = 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, p r + //3, 77 = disturbance amplitude 6 = impingement half-angle A = wavelength ju = dynamic viscosity v = kinematic viscosity TT = pi, 3.14159 p = density a = surface tension </> = angular coordinate on sheet Subscripts b D e § i i L I m nd 0 r s sw 10 30 = breakup = drop = edge = gas = imaginary = jet = ligament = liquid = maximum = nondimensional = orifice or initial = real = sheet = surface wave = arithmetic = volume

Characterization of Liquid Impinging Jet Injector Sprays for Bi-Propellant Space Propulsion: Comparison of PDI and High-Magnification Shadowgraphy

“…The spray is studied in steady operation. Its wavy behaviour is reported in the literature [2], [6] (see Figure 5 for visualization) and the ligament wavelength is less than 1.0 mm, hence a characteristic frequency about 10 kHz for a typical velocity of 10 m/s. Our diagnostics are compatible with these space and time scales: the shadowgraphy field of view (2.5 x 3.2 mm) corresponds to several wavelengths of the spray (2-3 wavelengths), while PDI is performed over a large number of time-periods (6 kHz in average during 1-2 s).…”

“…Additional spray pictures highlight the influence of pressure (ΔP) that drives the Reynolds and Weber numbers through jet momentum. As ΔP increases, the spray generates smaller droplets because of this increase in impingement force, which is a common result in like-doublets sprays [2], [5], [7]: this effect is evidenced by PDI and shadowgraphy measurements performed on the same water spray (see Figure 6a). Both diagnostics show that the drop size distribution is mainly composed of small droplets (D < 0.20 d0); this is due to the measurement location situated far from the impingement point (Z/d0 = 100) i.e.…”

“…The liquids of interest are sprayed using a classical setup for impinging jets under inert gas pressurization, as described in former studies [2], [5]. The two jets of the like-doublet are generated from 2 symmetrical orifices manufactured in a single stainless steel injector designed for combustion experiments; they are drilled with a diameter d0 = 0.…”

“…Following the initial development of Bachalo, the Phase-Doppler Interferometer (PDI) has proved to stand as a powerful standard laser-based diagnostic instrument for simultaneous and accurate measurements of the size and velocity of individual spherical particles in polydisperse particle flow environment [2], [5]- [6]. A modular PDI system (Artium 200 MD) is used for the real-time, non-intrusive measurement of individual drop size and 2 velocity components in this application.…”

“…at elevated Reynolds and Weber numbers (10 3 -10 4 ) leading to ligaments and fully developed breakup; a review of such spray phenomena can be drawn from detailed experiments [1]. Like-impinging doublets are investigated through theoretical studies [2] and numerical simulation [3]- [4], highlighting the dynamics of the liquid sheet breakup that generates wavy ligaments and then droplets. However in this complex situation, the reference knowledge concerning the spray shape, the distribution of droplet size and velocity, is provided by experiments [5]- [8]; for this purpose, most studies are based on PhaseDoppler Interferometer (PDI) and direct visualization of the spray.…”