The Effect of Dissolving Gases or Solids in Water Droplets Boiling on a Hot Surface

Abstract

We conducted experiments on the effect of dissolving either a gas (carbon dioxide) or a solid salt (sodium carbonate or sodium bicarbonate) in water droplets boiling on a hot stainless steel surface. Substrate temperatures were varied from 100°C to 300°C. We recorded the boiling of droplets with a video system, and photographed droplet impact using short-duration flash photography. At surface temperatures that were too low to initiate nucleate boiling, dissolved salts were found to reduce the evaporation rate since they lower the vapor pressure of water. Dissolved gas had the opposite effect: it came out of solution and formed bubbles in the liquid, enhancing evaporation. In the nucleate boiling regime dissolved carbon dioxide enhanced heat transfer by a small amount. However, sodium carbonate prevented coalescence of vapor bubbles and produced foaming in the droplet, greatly enhancing heat transfer and reducing the droplet lifetime to approximately half that of a pure water drop. Sodium bicarbonate, which decomposes to give carbon dioxide and sodium carbonate when heated, produced an even larger enhancement of heat transfer. When the surface temperature was raised above the Leidenfrost temperature of water, droplets went into film boiling and bounced off the surface following impact. Dissolved carbon dioxide was found to suppress heterogeneous bubble formation in the droplet during impact. However, dissolved salts promoted bubble formation and led to droplet break-up during impact.

Hydrodynamics and boiling phenomena of water droplets impinging on hot solid

“…This can be explained as follows: bubble nucleation is dependent on the temperature of the solid, the surface profile of the solid substrate, and the combination of liquid and solid materials. Moreover, the gas (air) dissolved in the liquid affects the boiling phenomena (Cui et al, 2001). Although the temperatures of the solid surfaces during the collisions could have been slightly different in the two experiments due to the bulk motion of liquid, the other parameters were the same.…”

The Effect of an Alcohol Resistant Aqueous Film Forming Foam (AR-AFFF) on the Evaporation, Boiling, and Collision Dynamics of a Water Droplet on a Heated Solid Surface

“…For AR-AFFF droplets containing simulated seawater, as the droplet evaporated, a residue of salt was produced. It is known that the presence of the salt residue may provide more nucleation sites (28), resulting in vigorous miniaturization compared to AR-AFFF mixed with distilled water.…”

Experimental investigation of aircraft spray cooling system with different heating surfaces and different additives

“…Cui [19] investigated the influence of soluble salt and gas to spray cooling performance, and found that adding soluble salt (NaCl, Na 2 SO 4 , MgSO 4 ) to water would enhance the heat transfer performance of spray cooling in non-boiling condition and nucleate boiling condition. Kim [20] summarized the results of Cui [19] and considered that dissolved gases and solids could enhance the heat transfer performance of spray cooling system. When mass fraction is 1%, NaHCO 3 in cooling medium could be resolved to Na 2 CO 3 and CO 2 , which could promote the expansion and boiling of droplets.…”