Investigation of droplets impinging on a deep pool: transition from coalescence to jetting

Abstract

An experimental investigation of droplets impinging vertically on a deep liquid pool of the same fluid was conducted. Coalescence and jetting as two of the main regimes were identified and studied. Five fluids, distilled water, technical ethanol, n-pentane, methanol and 1-propanol were used for providing different liquid-phase physical properties with density from 600 to 1,000 kg/m 3 , viscosity from 0.20 to 2.00 mPa s, and surface tension from 13.7 to 72.0 mN/m. Except for the experimental run of n-pentane, which was carried out in n-pentane saturated vapor, the ambient gas for the other experiments was air. The impact processes of micro-level (diameter below 1 mm) droplets were captured using a high-speed camera with a backlight. The observations, velocity and diameter ranges of the experimental runs were described, and based on them, the effects of the liquid-phase properties were studied. It was found that both low viscosity and low surface tension can increase the instability during impact processes. By curve-fitting, the transition from coalescence to jetting was characterized by using two models, one employing the Weber number (We) and the Ohnesorge number (Oh), and one employing the Froude number (Fr) and the Capillary number (Ca). Both models characterize the coalescence-jetting threshold well. The We-Oh model was based on a commonly used model from Cossali et al. (in Exp Fluids 22:463-472, 1997) for characterizing coalescence-splashing. For the small droplet diameters (below 1 mm) considered in this study, it was required to modify the We-Oh model with a diameter-dependent term to fit the sharp change in thresholds for fluids with relatively high viscosity. The Fr-Ca model has not previously been presented in the literature. A comparison of the two models with literature data (Rodriguez and Mesler, J Colloid Interface Sci 106(2): [347][348][349][350][351][352] 1985) indicates that they are also valid for impacts of droplets with diameters above 1mm. Calculation methods to generalize the two models were proposed.

Transition between coalescence and bouncing of droplets on a deep liquid pool

“…(4) is close to the exponent (−0.57) for coalescence and jetting threshold found by Zhao et al (2011). Figure 13(a) shows the We-Re diagram with logarithmic coordinates, where the thresholds characterized using Eq.…”

“…A generalized correlation for the transitions between both the high-energy-collision and lowenergy-collision coalescence and bouncing was found by employing the approach outlined by Zhao et al (2011). Herein, a threshold line is characterized with the least number of uncertain points 7 .…”

“…and Zhao et al (2011) listed some of the values of a and b used in the investigations for the high inertia collision. There are, however, very few investigations dealing with the characterization of the transitions between the flow regimes associated with relatively low inertia collisions, such as the transitions between coalescence and bouncing.…”

From Droplets to Process: Multilevel Research Approach to Reduce Emissions from LNG Processes

“…In our work, we have progressed systematically to conditions that more realistically simulate heat exchangers, going from droplets falling on deep pools [2,3,4] to studies of droplets on flowing films at various angles [5]. The latter study showed that the threshold between coalescence and splashing of a flowing film can be well-characterized by using a modification of a model suggested by Cossali et.…”

Two jets during the impact of viscous droplets onto a less-viscous liquid pool

“…When the crater collapses, a jet rises out of the centre of the crater, i.e., the wellknown Worthington jet [2,15]. The generation of the Worthington jet by surface waves not only occurs during the impact of a droplet onto a deep liquid pool [16][17][18][19][20][21][22][23][24], but also in other situations such as the collapse of a bubble at a liquid-gas interface [25]. A different kind of Worthington jet is induced by bubble pinch-off, which occurs in several conditions, such as the impact of a circular disk onto a liquid surface [26], and the impact of a droplet on a superhydrophobic solid surface [27].…”