A model to predict the effect of surface wettability on critical heat flux ☆ Hai Trieu Phan a, b , Rémi Bertossi a, b , Nadia Caney a, b , Philippe Marty a, b, ⁎, Stéphane Colasson b a UJF-Grenoble 1/Grenoble-INP/CNRS, LEGI UMR 5519, Grenoble, F-38041, France b CEA, LITEN/DTS/LETH, 17 rue des Martyrs, 38054 Grenoble cedex 9, France abstract article info Available online 24 October 2012 Keywords: Pool boiling Critical heat flux Contact angle Wettability Critical heat flux (CHF) in pool boiling experiments corresponds to the heat flux at which a vapor film is formed on the heated surface resulting from the replacement of liquid by vapor adjacent to this surface. Poor thermal conductivity of vapor can severely deteriorate heat transfer. It is important that systems operate below this limit which is a strong limitation to heat transfer due to the huge increase of the thermal resis- tance near the wall. The concept of macro- and micro-contact angles has been introduced in a previous paper (Phan et al., 2010 [28]) to describe the bubble growth processes. In this paper, an explicit relation be- tween the bubble departure diameter and the contact angle has been presented. Based on these results, we propose a model of critical heat flux, taking into account the effects of the wettability of the fluid, whose property is known to strongly influence boiling heat transfer. A new correlation for CHF, dependent on the contact angle, is proposed. It is found in fair agreement with existing experimental results concerning subcooled boiling to describe the variation of CHF with wettability. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction The critical heat flux (CHF) is the maximum heat flux at which nucleate boiling heat transfer sustains high cooling efficiency. When a surface is submitted to CHF, evaporation of the liquid close to the heated wall occurs. The consequence is the augmentation of the tem- perature leading to a deterioration of the material. It is then of prima- ry interest to delay the CHF occurrence in order to enhance heat transfer efficiency of the system. Many recent studies proved that critical heat flux can be enhanced by modifying surface wettability [1–6]. Over the past several years, studies on pool boiling have demonstrated that the addition of nanoparticles in a fluid can significantly increase CHF. A number of investigations show that this CHF enhancement can be related to the modification of the heated surface properties due to nanoparticle deposition. A number of nanofluid boiling studies have reported up to 100% enhancements in pool boiling CHF [1,2]. Many models of CHF have been developed. Most of them do not take into account the wettability of the fluid. Bonilla and Perry [7] and Cichelli and Bonilla [8] first presented basic correlations based on their experimental data using organic liquids. Kutateladze [9,10] proposed a correlation (similar to Eq. (1)) for CHF based on a critical velocity of bubbles and gave a correlation dependent to a parameter C (equal to κ -1/2 in Eq. (1)) that can be determined thanks to experimental data. Borishanskii [11] developed an analytical expression of the constant C only dependent on fluid physical properties. Zuber [12], who argued that CHF is caused by Taylor and Helmholtz instabilities, proposed a new value for C equal to 0.131. The theory of Chang [13] which con- siders that CHF is reached when the Weber number We reaches a criti- cal value provided that another value for C is equal to 0.098. In all these models, C is never linked to the contact angle. Rohsenow and Griffith [14] presented another correlation for CHF considering that increased packing of the heating surface with bub- bles at higher fluxes inhibited the liquid flow to the heating surface. Haramura and Katto [15] based their analysis on assuming that Kelvin–Rayleigh instabilities can occur in a macrolayer under the bubble, resulting in coalescence of vapor stems; they finally found an equation similar to the one of Kutateladze [9,10]. Guan et al. [16] proposed a new mechanistic model for predicting CHF in horizontal pool boiling systems based on the critical vapor velocity in the bubble. Chung and No [17] also developed a model of CHF, based on the direct observation of the physical boiling phenomena and using a nucleate boiling limitation model which can predict a heat transfer in a nucle- ate boiling region including CHF. In all these models, the influence of the fluid wettability is never taken into account. Kirishenko and Cherniakov [18] developed a correlation with the contact angle as a parameter. Diesselhorst et al. [19] noticed that this equation gives higher values of CHF for high contact angles. This correlation was found to be inaccurate for water. Ahn et al. [20] proposed a new concept of flow boiling model based on the wetting zone fraction which is given as a function of the wettability. Kim et al. [21] presented an analytical model associating the wettability and the nucleation site density. Wang and Dhir [22] developed a International Communications in Heat and Mass Transfer 39 (2012) 1500–1504 ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author at: CEA, LITEN/DTS/LETH, 17 rue des Martyrs, 38054 Grenoble cedex 9, France. E-mail address: philippe.marty@legi.grenoble-inp.fr (P. Marty). 0735-1933/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2012.10.019 Contents lists available at SciVerse ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt