TY - JOUR

T1 - Mixing and wall heat transfer during vertical coalescence of drops placed over a superhydrophobic surface

AU - Somwanshi, Praveen M.

AU - Muralidhar, K.

AU - Khandekar, Sameer

AU - Vyacheslav, Cheverda

N1 - Funding Information: Financial support for colleagues from India was provided by the Department of Science & Technology, India from an Indo-Russian project (Grant No. DST/ME/2018193) and is gratefully acknowledged. Financial support for colleagues from Russia was received from the Russian Foundation for Basic Research (RFBR Grant No. 18-58-45016). Measurement of the wetting angle of the substrate is performed under state contract with IT SB RAS.

PY - 2020

Y1 - 2020

N2 - 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.

AB - 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.

KW - Coalescence

KW - Conjugate heat transfer

KW - Copper

KW - Cs-alloy

KW - Glycerin

KW - Heat flux

KW - Liquid drops

KW - Superhydrophobic surface

KW - Teflon

KW - Vertical orientation

KW - Wall temperature

KW - Water

KW - vertical orientation

KW - coalescence

KW - superhydrophobic surface

KW - CONTACT LINE

KW - liquid drops

KW - water

KW - conjugate heat transfer

KW - FLOW

KW - wall temperature

KW - glycerin

KW - copper

KW - heat flux

UR - http://www.scopus.com/inward/record.url?scp=85098864901&partnerID=8YFLogxK

U2 - 10.1615/InterfacPhenomHeatTransfer.2020035034

DO - 10.1615/InterfacPhenomHeatTransfer.2020035034

M3 - Article

AN - SCOPUS:85098864901

VL - 8

SP - 207

EP - 224

JO - Interfacial Phenomena and Heat Transfer

JF - Interfacial Phenomena and Heat Transfer

SN - 2169-2785

IS - 3

ER -