Effect of viscosity on droplet-droplet collision outcome: Experimental study and numerical comparison

Abstract

The influence of viscosity on droplet-droplet collision behavior at ambient conditions was studied experimentally and numerically. N-decane, monoethyleneglycol (MEG), diethyleneglycol (DEG), and triethyleneglycol were used as liquid phase providing viscosities in the range from 0.9to48mPas. Collision Weber numbers ranged approximately from 10 to 420. A direct numerical simulation code, based on the volume-of-fluid concept, was used for the simulations. Experimentally, observations of two droplet streams using a modified stroboscopic technique (aliasing method) were used to investigate the whole range of impact parameters during one experimental run. The experimental method has previously been verified for the water/air system [C. Gotaas et al., Phys. Fluids 19, 102105 (2007)]. In the present work, it was tested and validated for the n-decane/air system. Measured data agree well with those published in the literature. Well-defined regions of stretching separation and coalescence were identified, while reflexive separation regions were not found by using a single sinusoidal disturbance. However, the onset of reflexive separation was identified for MEG and DEG using an amplitude modulation technique. The results show that the criteria for onset of reflexive separation for viscous fluids provided by Y. I. Jiang et al. [J. Fluid Mech. 234, 177 (1992)] are not valid. This is consistent with the results given by K. D. Willis and M. Orme [Exp. Fluids 34, 28 (2003)]. A new empirical correlation for the onset of reflexive separation for high viscosity fluids is presented. The borders between coalescing and stretching separation were shifted toward higher Weber numbers with increasing viscosity. The lack of occurrence of reflexive separation for the single sinusoidal disturbance (small droplets), as well as the stretching separation boundary shift, can be explained by dissipation of collision kinetic energy in viscous flows inside the merged droplet after collision. Results from numerical simulations for MEG, DEG, and TEG correlated well with experimental data for the same fluids.

Hypergolic ignition by head-on collision of N,N,N′,N′ −tetramethylethylenediamine and white fuming nitric acid droplets

“…[28] and Qian and Law [23] , and further confirmed and characterized in subsequent studies [29][30][31] . The Volume-of-Fluid simulation of Dai and Schmidt [29] on the head-on collision of equal-size droplets shows that the dependence of the dissipated energy and the maximum deformation on the collision Reynolds number decreases with increasing the Reynolds number up to 200.…”

“…Their results suggest that the viscosity effect on the maximum deformation becomes insignificant at sufficiently high Reynolds number. The experiments of Gotaas et al [30] on the collision of equal-size droplets of wide ranges of viscosity from 0.9 ×10 −3 kg/(m ·s) to 48 ×10 −3 kg/(m ·s) and the Weber number from 10 to 420 show that the transition Weber number for the droplet separation linearly increases with the Ohnesorge number (Oh) for the droplets with Oh < 0.04 and exponentially increases with the Oh for the highly viscous droplets with Oh > 0.04.…”

Numerical analysis of sprays with an advanced collision model

“…Since the correlation of Jiang et al (1992) includes properties of the liquid (i.e. dynamic viscosity, surface tension and density) as well as the small droplet diameter it could be successfully used for liquids with increasing viscosity with the above set of model constants (Gotaas et al 2007). In the work of Sommerfeld and Kuschel (2016) Finally the boundary line between coalescence and reflexive separation is considered.…”

“…Hence, this is the only correlation which may be used to consider viscosity effects, as for example done by Gotaas et al (2007) and Sommerfeld and Kuschel (2016).…”

“…The analysis of Gotaas et al (2007) suggested to use for the involved constants the values Ca = 2.33 and Cb = 0.41 (m 2 /s 2 ). Since the correlation of Jiang et al (1992) includes properties of the liquid (i.e.…”