Effect of Cu–Al₂O₃ nanocomposite coating on flow boiling performance of a microchannel

Abstract

Convective heat transfer performance of Cu–Al₂O₃ nanocomposite coated surface has been investigated experimentally for its potential use in heat transfer equipment operating in corrosive environment. Experimental studies were carried out in a bottom surface heated single microchannel of 672 μm hydraulic diameter using de-ionized (DI) water as the coolant. Thin nanocomposite coating was developed on the bottom surface of the microchannel using electrocodeposition technique. Both single-phase and two-phase convective heat transfer experiments were performed at different mass flux. The results from microchannel with bare Cu surface are used as the baseline data. Cu–Al₂O₃ nanocomposite coating has been found to enhance single-phase heat transfer rate marginally, whereas in the two-phase regime, the enhancement was ∼30% to ∼120% depending on the flow rate and surface temperature with an additional pressure drop penalty of less than ∼15%. CHF also improves for the coated surface by ∼35% to ∼55%.

Introduction

Multifunctional coating is essential for the heat transfer equipment operating at corrosive or harsh environment such as seawater evaporator, boiler and boiler tube, heat exchanger for the marine applications, exhaust heat recovery, etc. [1]. Corrosion and high temperature wear are some of the main challenges for equipments under such harsh environment. Commonly used heat exchanger materials such as Cu, Al have very excellent thermal properties but their corrosion and wear resistivity are noticeably poor. Bulk modifications of these materials cause increased production costs and adversely affect thermal properties which necessitates the development of a surface coating that will enhance mechanical properties of the surface and at the same time enhance heat transfer related properties or at worst do not adversely affect heat transfer performance. Incorporation of nano-sized particles in the surface metal matrix which is called as nanocomposite coating offers opportunities for generating multifunctional coating [1]. Metal oxide (i.e. Al₂O₃, CuO, etc.) or metal carbide (i.e. SiC) nanoparticles embedded in metal (i.e. Cu, Ni, etc.) matrix presents promising potential and has been studied previously to modify targeted surface properties such as dispersion hardening, self-lubricity, wear resistivity, chemical compatibility, corrosion resistivity, etc. [2], [3].

Thickness, costs and thermal properties of the coating play a dominating role in selecting its application in heat transfer equipment. Previous research has demonstrated that, using electrocodeposition technique a thin composite coatings (1–100 μm) can be developed on a solid surface from nano-sized particles to enhance surface corrosion resistivity without adversely affecting its thermal conductivity [2]. Electrocodeposition technique is relatively simple; can produce uniform surface coatings; production parameters can be tailored easily for mass production and can be applied to any complex geometry. A large number of metal nanoparticle combinations have been studied previously for different possible applications. Cu–Al₂O₃ coating has been studied recently for its potential application in electrical contacts [4] and has been reported to enhance surface hardness; surface wear resistivity and corrosion resistivity [2], [5]. An alternate application of this coating is potentially in heat exchanger or heat sink surfaces.

An important aspect of this coating is surface morphology modification of the base metal [6]. From all the previous morphological studies, it has been observed that this nanocomposite coating introduces nanostructures on the base surface [2]. These nanostructures can act as a nanofin on the solid heat transfer surface which is in contact the liquid. Nanostructures present on a solid surface also increase surface wicking characteristics, surface wettability and nucleation site density. These morphological modifications can impact heat transfer characteristics significantly, particularly boiling heat transfer [7]. Boiling heat transfer has been studied extensively for many years due to its high heat removal capability and extensive use in industrial applications. Very recently researchers have demonstrated that by altering the surface morphology with nanostructures such as Si or Cu nanowires [7], [8] or Cu nanorods [9] or carbon nanotubes [10], silicon nanopillars [11], etc. the boiling performance of a solid surface can be enhanced significantly. In view of the above it is concluded that nanocomposite coating may serve multifunctional purposes for the heat transfer equipment: (i) enhance surface resistivity to wear and corrosion and also (ii) enhance heat transfer performance, this has not been studied before therefore it has not been justified yet. Therefore, the study of single-phase and two-phase convective heat transfer performance of such coating is necessary to assess its potential use in the heat transfer equipment operating in harsh environment.

In this present study, convective heat transfer performance of the Cu–Al₂O₃ nanocomposite coating has been investigated experimentally; specific emphasis has been given to the flow boiling. The coating was applied on the bottom surface of a single side heated Cu microchannel having dimension 5 × 0.372 × 26 mm. Microchannel has the advantages of high surface area to volume ratio and has been studied extensively for the last decade for its potential application in the next generation heat transfer and cooling equipment [12], [13] which motivated us to select microchannel for this study. Results of the nanocomposite coated microchannel are compared with that of the bare surface channel. Single-phase heat transfer coefficient, onset of nucleation boiling (ONB), two-phase heat transfer coefficient, and onset of unstable boiling (OUB) or critical heat flux (CHF) has been focused particularly to assess the performance of this coating compared to the bare surface. Results of this investigation will be useful to assess applicability of this nanocomposite coating in the next generation heat transfer equipment operating in extreme environment.