Effect of extreme wetting scenarios on pool boiling conditions

TLDR: In this paper, a detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity, is presented.

About: This article is published in Applied Thermal Engineering.The article was published on 2017-03-25 and is currently open access. It has received 60 citations till now. The article focuses on the topics: Wetting transition & Bubble point.

Figures

Table 2 Surface characteristics

Table 1 Thermo-physical properties of water, taken at saturation at 1.013 x 105 Pa.

Table 4 Bubble departure diameter as a function of the bubble macro-contact angle: comparison between the experimental results obtained in the present study and those provided by the expression proposed by Phan et al. [17].

Table 3 Uncertainties in bubble dynamics parameters

Fig. 4. Boiling curves obtained for water on hydrophilic and superhydrophobic stainless steel surfaces.

Fig. 5. Effect of surface topography (quantified by the parameters Ra and Rz depicted in Table 2 on the boiling curves of water on: a) hydrophilic surfaces, b) superhydrophobic surfaces.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Effect of extreme wetting scenarios on pool boiling conditions in a quiescent medium" ?

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. This behaviour is in agreement with the so-called `` quasi-Leidenfrost '' regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in the present study ; furthermore here a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle ( or its supplemental value ), for the hydrophilic surfaces, even if they can not accurately predict bubble sizes. 

Q2. What is the common term used for water attractive surfaces?

The terms hydrophilic/hydrophobic, which are commonly used for liquid attractive/repellent surfaces, derive from the specific attraction/repellence of water. 

Q3. What is the characterization of the nucleation mechanisms and bubble dynamics?

The characterization of the nucleation mechanisms and bubble dynamics is based on high-speed visualization and image post-processing. 

Q4. What is the role of the contact angle in bubble growth?

Recognizing the importance of hysteresis and the use of quasi-static advancing and receding angles for the bubble growth scenario as previously reported for instance by [18], Phan et al [3] argue that as the convex vapour appears at the cavity shape, the contact angle is the equilibrium angle (at saturated temperature), which is kept when the bubble forms at the cavity mouth. 

Q5. Why is the bubble growth argued to be slower?

The bubble growth is argued to be slower due to the fact that the pressure difference between the bubble and the liquid is not constant during growth, as simplified by the previously mentioned authors, but instead the vapor pressure inside the bubble should decrease as the bubble grows, so its actual growth rate should be slower. 

Q6. What are the average values of parameters quantifying bubble dynamics?

The average values of parameters quantifying bubble dynamics, namely the departure diameter and frequency, as usually presented in the literature, are useful to identify general trends and to perform rough evaluations of the heat transfer. 

Q7. What causes the bubble to be affected by pressure variations?

This instability is attributed to the large bubble size, to the vapour layer resultingfrom the “quasi-Leidenfrost” phenomenon and to the slowness of the growth process, which allows the bubble to be affected by pressure variations, occurring within minutes. 

Q8. What is the evolution of micro- and nano-fabrication techniques?

The evolution observed in micro-and-nano-fabrication techniques within the last decade provides the researchers the opportunity to test a wide range of surface treatments, which quickly evolved from the micro-patterned surfaces [1] to nano-coatings [2,3]. 

Q9. How is the temperature and pressure controlled?

The temperature and the pressure inside the boiling chamber are accurately controlled with a precision of 1oC and 1.6 mbar, respectively. 

Q10. What is the atypical boiling curve for superhydrophobic surfaces?

a quite atypical boiling curve is obtained: the heat flux increases almost linearly with the superheat, until reaching a maximum value, after which it does not further increase. 

Q11. What is the trend for the bubbles on the hydrophilic surfaces?

This trend is naturally contrary to that typically reported for the hydrophilic surfaces, as expected, since the bubble formation and release process occurs over the vapor layer, so the entire growth and departure process mainly depend on the amount of vaporization.