Mixing and wall heat transfer during vertical coalescence of drops placed over a superhydrophobic surface

Praveen M. Somwanshi, K. Muralidhar, Sameer Khandekar, Cheverda Vyacheslav

Research output: Contribution to journalArticlepeer-review

Abstract

The configuration of a pair of liquid droplets of unequal temperatures with one placed initially above the other is encountered in thermal spray applications and investigated in the present study. The lower drop is initially cold and placed above a horizontal superhydrophobic surface of equilibrium contact angle 150 deg. The second drop placed above the first is initially at a higher temperature before coalescence. The two drops merge and spread over the substrate. Three liquids of interest are Cs-alloy, water, and glycerin with Prandtl numbers of 0.036, 6.64, and 7188.6, respectively. Coalescence process takes place under atmospheric conditions while thermal interaction between the liquid medium and the substrate (copper, α ∼ 10−4 m2 /s and Teflon, α ∼ 10−7 m2 /s) makes it a conjugate heat transfer process. With reference to the volume of the combined drop, Bond number is close to 0.2. Flow and heat transfer simulations are performed for the coalescence process of the two drops as they exchange energy with the substrate and approach thermal equilibrium. The interfacial shapes generated in time, temperature distribution, and the wall heat flux are primary quantities of interest. An axisymmetric coordinate system has been adopted for numerical simulations with the Kistler’s model representing contact line motion. Water and Cs-alloy show drop recoil followed by oscillatory spreading over the surface. Thermal convection is visible in water while it is suppressed in glycerin and Cs-alloy owing to high viscosity and thermal diffusivity, respectively, in these media. The instantaneous surface-averaged wall heat flux is initially zero, increases quickly to a maximum, and then gradually decreases to small values. The evolution of wall heat flux in water shows time-dependent oscillations while it is monotonic in glycerin as well as Cs-alloy. Among the three liquids, Cs-alloy displays the highest instantaneous peak in wall heat flux.

Original languageEnglish
Pages (from-to)207-224
Number of pages18
JournalInterfacial Phenomena and Heat Transfer
Volume8
Issue number3
DOIs
Publication statusPublished - 2020

Keywords

  • Coalescence
  • Conjugate heat transfer
  • Copper
  • Cs-alloy
  • Glycerin
  • Heat flux
  • Liquid drops
  • Superhydrophobic surface
  • Teflon
  • Vertical orientation
  • Wall temperature
  • Water
  • vertical orientation
  • coalescence
  • superhydrophobic surface
  • CONTACT LINE
  • liquid drops
  • water
  • conjugate heat transfer
  • FLOW
  • wall temperature
  • glycerin
  • copper
  • heat flux

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