Photographic study of hydrodynamics of drops of aqueous polymer solution impinging on hot solid

Spray-cooling methods are utilized in quench hardening and throughout the metal-forming industry. Aqueous solutions of water-soluble polymers are the typical quenchant. Although the impact behavior of the droplets has a great influence on the heat transfer between the hot metal surface and quenchant, the hydrodynamics of drops of aqueous polymer solutions are rarely studied. In this study, the collision of drops of an aqueous solution of polymer with a hot sapphire solid surface was investigated using strobe photography. A solution of 10 wt% polyoxyethylene polyoxypropylene glycol, with an average molecular weight of approximately 20,000, was used as the test polymer quenchant. Experiments were conducted with drop diameters in the range 2.09-2.42 mm, impact velocities from 0.83 to 1.25 m/s, and surface temperatures of 200, 350, and 500 °C. The effect of varying the temperature and the dimensionless Weber number on the collision behavior and drop evolution was investigated. Microscopic observations revealed that polymer residue remained on the surface when the temperature of the solid was equal to or less than 350 °C. At 500 °C, drops impacting on the surface at low Weber numbers deformed into a thin disc, recoiled, and finally rebounded off the solid in a spray of mist. No polymer residue remained. The residence times of the rebounding drops after impact increased with the Weber number. In addition, the measured residence times were slightly longer than some experimentally determined formulae for simple compound liquid drops predict.