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Experimental and numerical investigation of droplet evaporation on heated surfaces: saltwater and the role of Marangoni effect.

Number: 2442

Author(s) : JOY T. I., JACOBI A. M.

Summary

Except for uncommon surfaces manifesting superhydrophilicity, water forms droplets on heat transfer and air-handling surfaces in air conditioning, heat pumping, and refrigeration systems. Droplet evaporation is ubiquitous in these systems, and it is especially important when water is present on the air side of a condenser, during defrosting when liquid water is present on a refrigerator or heat-pump evaporator, and when water is present on air-handling surfaces of a ventilation system. Droplet evaporation is also important in related applications, such as desalination, power generation, etc. Often the evaporating droplet is impure, whether due to a contaminant—say acquired from the heated surface—or to a naturally occurring salt or mineral. The evaporation of a water droplet with a solute is complex, involving phase change, simultaneous heat and mass transfer, wetting behavior, and thermosolutal convection with Marangoni effect.
We have experimentally investigated the evaporation of ASTM D1141-98 artificial seawater droplets on a heated stainless surface. After the seawater droplet is completely evaporated, a ring-like deposition is formed on the surface. The flow pattern inside the evaporating seawater droplet is measured using particle image velocimetry (PIV) technique. To better understand the physics, a numerical simulation of droplet evaporation is performed using COMSOL and the numerical results are compared with PIV results. The surface-tension gradient (Marangoni stress) at the liquid-vapor interface changes the flow patterns inside the evaporating droplets. During evaporation, the contact angle decreases, and secondary eddies form near the contact line. The formation of these eddies increases the salt concentration near the pinned contact line, because salt is ‘trapped’ in the recirculating eddies. As the process continues, the higher evaporation rate near the contact line causes the salt concentration to increase inside the eddies, leading to supersaturation and the formation of ring-like deposition. When Marangoni stress is neglected, the formation of eddies near the contact line occurs earlier and the flow pattern is in completely opposite direction compared to the case where Marangoni stress is considered. The general flow pattern inside the evaporating droplet obtained from PIV is similar to the numerical results where Marangoni stress is not considered. The effects of initial salt concentration on flow pattern evolution and salt concentration distribution are also examined.

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  • Original title: Experimental and numerical investigation of droplet evaporation on heated surfaces: saltwater and the role of Marangoni effect.
  • Record ID : 30033121
  • Languages: English
  • Subject: Technology
  • Source: 2024 Purdue Conferences. 20th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2024/07/17

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