Research Papers: Evaporation, Boiling, and Condensation

Fundamental Roles of Nonevaporating Film and Ultrahigh Heat Flux Associated With Nanoscale Meniscus Evaporation in Nucleate Boiling

[+] Author and Article Information
Shalabh C. Maroo

Department of Mechanical and Aerospace Engineering,
Syracuse University,
Syracuse, NY 13244
e-mail: scmaroo@syr.edu

J. N. Chung

Department of Mechanical and Aerospace Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: jnchung@ufl.edu

1Corresponding author.

Manuscript received October 8, 2012; final manuscript received January 17, 2013; published online May 16, 2013. Assoc. Editor: Leslie Phinney.

J. Heat Transfer 135(6), 061501 (May 16, 2013) (10 pages) Paper No: HT-12-1550; doi: 10.1115/1.4023575 History: Received October 08, 2012; Revised January 17, 2013

The three-phase moving contact line present at the base of a bubble in nucleate boiling has been a widely researched topic over the past few decades. It has been traditionally divided into three regions: nonevaporating film (order of nanometers), evaporating film (order of microns), and bulk meniscus (order of millimeters). This multiscale nature of the contact line has made it a challenging and complex problem, and led to an incomplete understanding of its dynamic behavior. The evaporating film and bulk meniscus regions have been investigated rigorously through analytical, numerical and experimental means; however, studies focused on the nonevaporating film region have been very sparse. The nanometer length scale and the fluidic nature of the nonevaporating film has limited the applicability of experimental techniques, while its numerical analysis has been questionable due to the presumed continuum behavior and lack of known input parameters, such as the Hamaker constant. Thus in order to gain fundamental insights and understanding, we have used molecular dynamics simulations to study the formation and characteristics of the nonevaporating film for the first time in published literature, and outlined a technique to obtain Hamaker constants from such simulations. Further, in this review, we have shown that the nonevaporating film can exist in a metastable state of reduced/negative liquid pressures. We have also performed molecular simulations of nanoscale meniscus evaporation, and shown that the associated ultrahigh heat flux is comparable to the maximum-achievable kinetic limit of evaporation. Thus, the nonevaporating film and its adjacent nanoscale regions have a significant impact on the overall macroscale dynamics and heat flux behavior of nucleate boiling, and hence should be included in greater details in nucleate boiling simulations and analysis.

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Grahic Jump Location
Fig. 1

Schematic of bubble growth, (a) overall picture at macroscale (shaded region depicts the nonevaporating film region), and (b) zoomed in nano- and microscale regions at the three-phase contact line

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Fig. 2

Constant wall-temperature boundary condition [75]

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Fig. 3

Evaporation sequence of liquid film of argon on Pt wall in the Y-Z plane along the computational domain. The liquid film does not completely evaporate and a nonevaporating film is obtained [58].

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Fig. 4

(a) 3D view of MD simulation domain of thin liquid argon films in a nanochannel. Vapor pressure-density plot of initial and final states for (b) varying nanochannel height, and (c) varying film thickness. Initial and final vapor states lie on the saturation curve [57].

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Fig. 5

Simultaneous evaporation of argon thin film on nanochannel upper wall with condensation on the lower wall. (a) Variation of the vapor pressure with time, and (b) snapshots of the domain at different time steps. Nonevaporating film does not form on the upper wall since the vapor pressure reduces due to vapor condensation [57].

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Fig. 6

Liquid argon meniscus, surrounded by argon vapor, in an opening constructed of platinum wall atoms. (a) 3D view of the simulation domain where the liquid-vapor interface can be clearly noticeable, and (a) 2D view along the x-z plane depicting the boundary conditions and dimensions. Heat is transferred to the meniscus from the platinum wall region shown in red, while the region shown in blue is maintained at the lower initial temperature [80].

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Fig. 7

Snapshots of x-z plane at different time steps of evaporating nanoscale meniscus. Nonevaporating film is obtained at the center of the meniscus, while the interline region is present in the adjacent regions [80].

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Fig. 8

(a) Average heat and evaporation flux along the length of the nanoscale meniscus [80], and (b) comparison of the average heat flux obtained from MD simulations [80] with that of the maximum theoretical possible heat flux

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Fig. 9

Variation in liquid pressure along the meniscus at t = 2500 ps for the nanoscale meniscus. High negative pressure values are seen at the center of the meniscus. A normalized function log(Π/δne) is plotted in the region of negative liquid pressure for Π=RC=-2γ/PL and Π=δ(x), which nullifies the possibility of cavitation in this region as the meniscus thickness is smaller than the critical cavitation radius [90].

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Fig. 10

Heat flux distribution along the nano- and microlayers at the base of the bubble [80]

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Fig. 11

Meniscus evaporation on a surface with nanostructured ridges can possibly rupture the nonevaporating film and lead to enhancement in nucleate boiling heat transfer [93]




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