Research Article: A numerical investigation of the effect of surface wettability on the boiling curve

Date Published: November 10, 2017

Publisher: Public Library of Science

Author(s): Hua-Yi Hsu, Ming-Chieh Lin, Bridget Popovic, Chii-Ruey Lin, Neelesh A. Patankar, Yogendra Kumar Mishra.

http://doi.org/10.1371/journal.pone.0187175

Abstract

Surface wettability is recognized as playing an important role in pool boiling and the corresponding heat transfer curve. In this work, a systematic study of pool boiling heat transfer on smooth surfaces of varying wettability (contact angle range of 5° − 180°) has been conducted and reported. Based on numerical simulations, boiling curves are calculated and boiling dynamics in each regime are studied using a volume-of-fluid method with contact angle model. The calculated trends in critical heat flux and Leidenfrost point as functions of surface wettability are obtained and compared with prior experimental and theoretical predictions, giving good agreement. For the first time, the effect of contact angle on the complete boiling curve is shown. It is demonstrated that the simulation methodology can be used for studying pool boiling and related dynamics and providing more physical insights.

Partial Text

Boiling occurs in a variety of industrial applications such as high heat flux electronic devices [1], chemical processes [2], power plants [3], etc. During boiling, a heated surface is adjacent to a liquid, which vaporizes. The large latent heat of vaporization makes it an efficient mode of heat transfer in the nucleate boiling mode. Boiling is quantified in terms of a plot of heat flux versus the wall superheat defined as the temperature of the wall minus the saturation temperature (i.e. the boiling point) at the pressure of the liquid. The heat flux divided by the wall superheat is the heat transfer coefficient (HTC). Large HTC is an indication of efficient heat transfer.

In this work, the VOF method is chosen since the fluid mass can be conserved appropriately and it can be applied on a larger scale with any grid compared with the LBM and MD method. The continuing work can be extended to a multi-bubble problem to predict real industrial phase change applications.

The primary goal is to qualitatively investigate how the contact angle influences the boiling curve, i.e., the heat transfer during various phases of boiling. Fig 2 shows the configuration used in this study. Computations were performed for three and two-dimensional, unsteady, incompressible flow with heat transfer. For the VOF method, an interface tracking method, small amount of vapor phase has to be specified at the initial of the computation [16, 25]. This is the reason why the VOF method cannot be used to study the nucleation process. Vapor phase was initially placed at the bottom wall. If the film boiling mode is not stable, then the vapor wraps into a bubble and the dynamics proceeds in the nucleate or transition boiling regime. The governing equations were solved using the finite volume method. The governing equations are solved using the software ANSYS® Fluent Academic Research, Release 16.2.

The numerical results are presented in three-dimensional cylindrical and two-dimensional planar simulations, respectively.

In this work, numerical simulations of evolving liquid-vapor interfaces during pool boiling on a horizontal smooth surface have been performed to study the surface wettability effect and related dynamics. Instead of a saturated temperature constrained interface, an interfacial heat flux exchange model at the interface is adapted. The simulation results based on the volume-of-fluid method and static contact angle model have been carried out and compared with some prior theoretical and experimental predictions, demonstrating good agreement. The effect of surface contact angle and superheat on the complete boiling heat transfer curve is obtained for the first time and the corresponding dynamics has been qualitatively captured. It is verified this approach can be used for investigating boiling phenomena and providing more physical insights into the corresponding dynamics. In addition, it can provide some guidance for more time consuming three dimensional cylindrical numerical simulations. In a near future, specific boiling regime will be focused and more physics behind the dynamics can be assured.

 

Source:

http://doi.org/10.1371/journal.pone.0187175