Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux

https://doi.org/10.1016/j.ijheatmasstransfer.2007.02.002Get rights and content

Abstract

The pool boiling characteristics of dilute dispersions of alumina, zirconia and silica nanoparticles in water were studied. Consistently with other nanofluid studies, it was found that a significant enhancement in critical heat flux (CHF) can be achieved at modest nanoparticle concentrations (<0.1% by volume). Buildup of a porous layer of nanoparticles on the heater surface occurred during nucleate boiling. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. A review of the prevalent CHF theories has established the nexus between CHF enhancement and surface wettability changes caused by nanoparticle deposition. This represents a first important step towards identification of a plausible mechanism for boiling CHF enhancement in nanofluids.

Introduction

Many important industrial applications rely on nucleate boiling, to remove high heat fluxes from a heated surface. These include cooling of high-power electronics, nuclear reactors, chemical reactors and refrigeration systems, to mention a few. Nucleate boiling is a very effective heat transfer mechanism, however it is well known that there exists a critical value of the heat flux at which nucleate boiling transitions to film boiling, a very poor heat transfer mechanism. In most practical applications it is imperative to maintain the operating heat flux below such critical value, which is called the critical heat flux (CHF). Obviously, a high value of the CHF is desirable, because, everything else being the same, the allowable power density that can be handled by a cooling system based on nucleate boiling is roughly proportional to the CHF. Therefore, an increase of the CHF can result in more compact and efficient cooling systems for electronic devices, nuclear and chemical reactors, air conditioning, etc., with significant economic benefits in all these applications.

Addition of solid nanoparticles to common fluids such as water and refrigerants is an effective way to increase the CHF. The resulting colloidal suspensions are known in the literature as nanofluids [1]. Materials used for nanoparticles include chemically stable metals (e.g., gold, silver, copper), metal oxides (e.g., alumina, zirconia, silica, titania) and carbon in various forms (e.g., diamond, graphite, carbon nanotubes, fullerene). Nanoparticles are relatively close in size to the molecules of the base fluid, and thus, if properly prepared, can realize very stable suspensions with little erosion and gravitational deposition over long periods of time. As such, nanofluids lend themselves well to ‘real world’ applications, contrary to the milli- and microsize particle slurries explored in the past, which quickly settle and often clog the flow channels. At MIT we are conducting research to assess the feasibility of water-based nanofluids for nuclear applications [2].

As of today (7/06), over 10 studies of CHF and nucleate boiling in nanofluids have been reported in the literature [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The findings can be summarized as follows:

  • Significant CHF enhancement (up to 200%) occurs with various nanoparticle materials, including silicon, aluminum and titanium oxides.

  • The CHF enhancement occurs at relatively low nanoparticle concentrations, typically less than 1% by volume.

  • During nucleate boiling some nanoparticles precipitate on the surface and form a layer whose morphology depends on the nanoparticle materials.

  • Some studies report no change of heat transfer in the nucleate boiling regime [3], [5], some report heat transfer deterioration [4], [9] and others heat transfer enhancement [6], [11].

Researchers have carefully reported the experimental data, but they have made few attempts at and little progress in explaining the CHF enhancement mechanism. The main objective of this paper is to start developing an insight of the CHF enhancement mechanism in nanofluids. The structure of the paper is as follows. Preparation and characterization of our nanofluids is reviewed in Section 2. New CHF and surface wettability data are presented in Section 3. The data are discussed in light of the CHF theories in Section 4. The conclusions are provided in Section 5.

Section snippets

Preparation and characterization of nanofluids

We have selected three nanoparticle materials for this study, i.e., alumina (Al2O3), zirconia (ZrO2) and silica (SiO2). Water-based nanofluids of these three materials were purchased from Sigma–Aldrich (alumina and zirconia) and Applied Nanoworks (silica). The vendor-specified concentration of the nanofluids was 10% by weight, which we verified with thermo-gravitometric analyses. The as-purchased nanofluids were then diluted with deionized water to the low concentrations of interest for the CHF

CHF experiments with wires

The CHF of deionized pure water and nanofluids was measured in the apparatus shown in Fig. 4, which basically consists of a wire heater horizontally submerged in the test fluid at atmospheric pressure, surrounded by an isothermal bath. The wire, made of stainless steel grade 316, has a 0.381-mm diameter and 12-cm length. The wire is soft soldered with a silver–lead solder to the copper electrodes and heated by resistance heating with a DC power supply of 20-V and 120-A capacity. Voltage and

Data interpretation

The experiments presented in Section 3 have shown that nanofluids exhibit enhanced CHF at low nanoparticle concentrations, that during nanofluid boiling the heater surface becomes coated with a porous layer of nanoparticles, and that such layer significantly increases surface wettability. In this section we address some key questions related to the above observations. Why do nanoparticles deposit on the surface during nucleate boiling? Why does the nanoparticle layer enhance wettability? What

Conclusions

The main findings of this study are as follows:

  • Dilute dispersions of alumina, zirconia and silica nanoparticles in water exhibit significant CHF enhancement in boiling experiments with wire heaters.

  • During nucleate boiling some nanoparticles deposit on the heater surface to form a porous layer.

  • This layer improves the wettability of the surface considerably, as measured by a marked reduction of the static contact angle.

  • A review of the prevalent CHF theories demonstrates that the higher

Acknowledgments

The following sponsors are gratefully acknowledged. AREVA (PO CP04-0217), the Idaho National Laboratory (Contract no. 063, Release 18), the Nuclear Regulatory Commission (NRC-04-02-079), the DOE Innovation in Nuclear Infrastructure and Education Program (DOE-FG07-02ID14420), the Korea Science and Engineering Foundation for Mr. Kim’s doctoral fellowship and the Korea Research Foundation (KRF-2005-214-D00400) for Dr. Bang’s post-doctoral fellowship.

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