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Effects of High Frequency Droplet Train Impingement on Spreading-Splashing Transition, Film Hydrodynamics and Heat Transfer

[+] Author and Article Information
Taolue Zhang

Texas A&M University, College Station, Texas, USA
surfztl@tamu.edu

Jorge Alvarado

Texas A&M University, College Station, Texas, USA
jorge.alvarado@tamu.edu

J. P. Muthusamy

Texas A&M University, College Station, Texas, USA
jayaveera@tamu.edu

Anoop Kanjirakat

Texas A&M University at Qatar, Education City, Doha, Qatar
anoop.baby@qatar.tamu.edu

Reza Sadr

Texas A&M University at Qatar, Education City, Doha, Qatar
reza.sadr@qatar.tamu.edu

1Corresponding author.

J. Heat Transfer 138(2), 020902 (Jan 18, 2016) Paper No: HT-15-1699; doi: 10.1115/1.4032230 History: Received November 05, 2015; Revised December 03, 2015

Abstract

The objective of this study is to investigate the effects of droplet-induced crown propagation regimes (spreading and splashing) on liquid film hydrodynamics and heat transfer. In this work, the effects of high frequency droplet train impingement on spreading-splashing transition, liquid film hydrodynamics and surface heat transfer were investigated experimentally. HFE-7100 droplet train was generated using a piezo-electric droplet generator at a fixed flow rate of 165 mL/h. Optical and IR images were captured at stable droplet impingement conditions to visualize the thermal physical process. The droplet-induced crown propagation transition phenomena from spreading to splashing were observed by increasing the droplet Weber number. The liquid film hydrodynamics induced by droplet train impingement becomes more complex when the surface was heated. Bubbles and micro-scale fingering phenomena were observed outside the impact crater under low heat flux conditions. Dry-out was observed outside the impact craters under high heat flux conditions. IR images of the heater surface show that heat transfer was most effective within the droplet impact crater zone due to high fluid inertia including high radial momentum caused by high-frequency droplet impingement. Time-averaged heat transfer measurements indicate that the heat flux-surface temperature curves are linear at low surface temperature and before the onset of dry-out. However, a sharp increase in surface temperature can be observed when dry-out appears on the heater surface. Results also show that strong splashing (We = 850) is unfavorable for heat transfer at high heat flux conditions due to instabilities of the liquid film, which lead to the onset of dry-out. In summary, the results show that droplet Weber number is a significant factor in the spreading-splashing transition, liquid film hydrodynamics and heat transfer.

Copyright © 2016 by ASME
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