EXPERIMENTAL DETERMINATION OF THE WETTED AREA OF A FALLING FILM ON THE OUTSIDE OF HORIZONTAL TUBES USING IMAGE PROCESSING AND HEAT TRANSFER ANALYSIS

2017 ◽  
Vol 48 (6) ◽  
pp. 529-548
Author(s):  
Cenk Onan ◽  
Derya B. Ozkan
Keyword(s):  
Heat Transfer ◽  
Image Processing ◽  
Experimental Determination ◽  
Heat Transfer Analysis ◽  
Falling Film ◽  
Horizontal Tubes ◽  
Wetted Area

Wettability Effects on Falling Film Flow and Heat Transfer Over Horizontal Tubes in Jet Flow Mode

2020 ◽  
Vol 142 (12) ◽  
Author(s):  
Avijit Karmakar ◽  
Sumanta Acharya
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Nusselt Number ◽  
Film Flow ◽  
Jet Flow ◽  
Equilibrium Contact Angle ◽  
Flow Mode ◽  
Falling Film ◽  
Tube Surface ◽  
Horizontal Tubes

Abstract The performance of a falling-film heat exchanger is strongly linked to the surface characteristics and the heat transfer processes that take place over the tubes. The primary aim of this numerical study is to characterize the influence of surface wettability on the film flow behavior and its associated surface heat transfer in the jet-flow mode. Volume of fluid (VOF) based simulations are carried out for horizontal tubes with different surface wettabilities. The wettability of the tube surfaces is represented using the Kistler's dynamic contact angle model. Surface wettability effects ranging from superhydrophilic to superhydrophobic are studied by varying the equilibrium contact angle from 2 deg to 175 deg. Two different liquid mass flow rates of 0.06 and 0.18 kg/m-s corresponding to the inline and staggered jet flow modes are studied. Results are presented in terms of the liquid film thickness, the contact areas between the different phases (solid–liquid and liquid–air), and the heat transfer coefficient or Nusselt number. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences the liquid film thickness, and the wetted area of the tube surface. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 deg to 175 deg. The local distributions of the Nusselt number over the tube surface are strongly influenced by the flow recirculation in the liquid bulk.

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