the growing interest in spray cooling for high-heat flux electronics has led researchers at villanova university and the university of leeds to study the basic physics of droplet impingement experimentally and in computer simulations to see what happens when a microdroplet impacts a dry surface.

this new research gives a greater understanding of the impact of microdroplets
on a dry surface. (wikimedia commons)

according to a report from the american institute of physics (aip), “two-phase spray cooling, in particular, has been shown to cool heat fluxes that are orders of magnitude higher than traditional cooling methods like fans and heat sinks. the complex physics of two-phase spray cooling, in which droplets are atomized with a secondary pressurized gas phase, demands deeper understanding.”

researchers used the lattice-botzmann method (lbm) algorithms to simulate the impact of a single microdroplet. lbm is used to model fluid flow in complex geometries and multiple flows. this allowed the researchers to observe the dynamics of the spreading phase at a microscopic level.

“for single-phase spray cooling,” the article noted, “a liquid is sprayed in ambient air without significant air pressure or forces acting on the droplet surface. the researchers were able to develop a correlation for the system that can reasonably predict the instantaneous droplet diameter after the low-impact regimes.”

in two-phase spray cooling, an atomizing gas forms smaller droplets (three magnitudes of order smaller) that impact the surface underneath a stagnation jet. previous theories indicated that the stagnation jet would impact spreading, but lbm showed no significant effect.

according to researchers, “the jet had no such effects for capillary number ratios below 0.35, and thus defined a new dimensionless metric (ca*) as the ratio of jet-to-droplet capillary numbers.”

this demonstrated a distinct point of difference between macro- and microscopic droplets.

the research was recently published in physics of fluids. the abstract stated:

“spray cooling is one of the most promising methods of cooling high heat flux electronics. depending on the type of the nozzle, spray cooling can be categorized as single-phase or two-phase. in the latter, which is known to be more effective, a secondary gas is used to further pressurize the liquid and form smaller droplets at higher velocities.

“the gas is also assumed to assist the spreading phase by imposing normal and tangential forces on the droplet free surface which adds to the complicated hydrodynamics of the droplet impact. moreover, the order of magnitude of droplet size in spray cooling is 10⁻⁶ m, thereby introducing a low weber and reynolds numbers’ impact regime which heretofore has not been well understood.

“a 3d lattice boltzmann method was implemented to simulate the impact of a single micro-droplet on a dry surface both in ambient air and under a stagnation gas flow. two cases were closely compared and correlations were proposed for the instantaneous spreading diameter.

“contrary to recent findings at higher impact weber and reynolds numbers, it was found that a stagnation flow only significantly affects the spreading phase for ca∗ ≥ 0.35ca* ≥ 0.35 but has little influence on the receding physics.”