Droplet impingement dynamics: effect of surface temperature during boiling and non-boiling conditions

Abstract

This study investigates the hydrodynamic characteristics of droplet impingement on heated surfaces and compares the effect of surface temperature when using water and a nanofluid on a polished and nanostructured surface. Results are obtained for an impact Reynolds number and Weber number of approximately 1700 and 25, respectively. Three discs are used: polished silicon, nanostructured porous silicon and gold-coated polished silicon. Seven surface temperatures, including single-phase (non-boiling) and two-phase (boiling) conditions, are included. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. Results of water and a water-based single-wall carbon-nanotube nanofluid impinging on a polished silicon surface are compared to determine the effects of nanoparticles on impinging dynamics. The nanofluid results in larger spreading velocities, larger spreading diameters and an increase in early-stage dynamic contact angle. Results of water impinging on both polished silicon and nanostructured silicon show that the nanostructured surface enhances the heat transfer for evaporative cooling at lower surface temperatures, which is indicated by a shorter evaporation time. Using a nanofluid or a nanostructured surface can reduce the total evaporation time up to 20% and 37%, respectively. Experimental data are compared with models that predict dynamic contact angle and non-dimensional maximum spreading diameter. Results show that the molecular-kinetic theory's dynamic contact angle model agrees well with current experimental data for later times, but over-predicts at early times. Predictions of maximum spreading diameter based on surface energy analyses indicate that these models over-predict unless empirical coefficients are adjusted to fit the test conditions. This is a consequence of underestimates of the dissipative energy for the conditions studied.

Wetting kinetics of water nano-droplet containing non-surfactant nanoparticles: A molecular dynamics study

“…This excess pressure in turn alters the force balance near the contact line and enhances the spreading of nanofluids. [10][11][12][13] The super-spreading behavior of nanofluids was also reported in single-wall carbon nanotubes suspended in water 14 and aluminum-ethanol nanofluids. 15 The selfassembly of nanoparticles and the structural disjoining pressure induced super-spreading behavior of nanofluids were widely used to explain the enhancements in drop-wise evaporation 16,17 and critical heat flux of nanofluids.…”

Effects of Injection Pressure and Ambient Gas Density on Fuel - Ambient Gas Mixing and Combustion Characteristics of D.I. Diesel Spray

“…Single droplet-wall impact process. Shen et al [2009] studied the effect of wall temperature and different wall structures on the droplet's evolution by using high speed video camera. Arifin et al…”

An experimental study on the effect of nanoparticle shape on the dynamics of Leidenfrost droplet impingement

“…Some experimental studies and theoretical explanations are found in literature addressing these issues (Duursma et al, 2009;Shen et al, 2009;Murshed and Nieto de Castro, 2011;Kahani et al, 2016). Interestingly, the nanofluid droplets have larger spreading diameters than that of its base fluid (Shen et al, 2009). Furthermore, it is reported that the spreading diameter increases with increase in volume fraction while the droplet rebounding height increases with rise in temperature (Murshed and Nieto de Castro, 2011).…”

“…It becomes further complicated when the impinging surface is heated and the evaporation of the base fluid happens during impingement. Some experimental studies and theoretical explanations are found in literature addressing these issues (Duursma et al, 2009;Shen et al, 2009;Murshed and Nieto de Castro, 2011;Kahani et al, 2016). Interestingly, the nanofluid droplets have larger spreading diameters than that of its base fluid (Shen et al, 2009).…”

“…The droplet impingement of nanofluid prepared from single-walled carbon nanotubes (SWCNT) having 0.5-10 nm diameter and 100-2000 nm length dispersed in water, on a gold coated nanostructured porous silicon substrates at different substrate temperatures showed interesting results. It is observed that the nanofluid causes larger spreading diameters as surface temperature is increased; in addition, it gives lower equilibrium contact angles at high temperatures (Shen et al, 2009). On the other hand, experimental study on the droplet impingement and spreading characteristics of nanofluid having spherical TiO 2 particles (15 nm in diameter) in ethylene glycol on aluminum substrate demonstrated that the spreading diameter is decreased when surface temperature is increased (Murshed and Nieto de Castro, 2011).…”