Critical heat flux for CuO nanofluid fabricated by pulsed laser ablation differentiating deposition characteristics

Abstract

In this study, CHF characteristics in pool boiling according to CuO nanoparticles deposition characteristics are investigated compared with that of pure water. The deposition characteristics are controlled by using in-house prepared CuO nanofluids in two ways such as one-step method of pulsed laser ablation in liquid (PLAL) and two-step method of particles dispersion. Morphology of CuO nanoparticles in shape and size is analyzed by TEM while SEM images are obtained to observe the deposition structures on heated surfaces. Also, contact angle and capillary height for deposition layer on the surfaces after pool boiling experiments are measured to investigate the surface wettability. Rayleigh–Taylor instability wavelength on the surface with respect to a unified CHF enhancement mechanism is measured indirectly by a condensation method.

Introduction

Critical heat flux (CHF) is characterized by a sudden reduction of the local heat transfer coefficient (HTC) that results from a transition from nucleate boiling to film boiling along with the heat transfer surface. With approaching CHF condition, a large abrupt decrease in the heat transfer coefficient in case of a heat flux-controlled system occurs while a sudden decrease in the heat transfer rate for a temperature-controlled system occurs. Of two, the former resulting in a jump in heated wall temperatures that may cause burnout is more important in applications such as power generation [1]. Needless to say that it is most important to be able to enhance CHF values in order to design a water-cooled device that is safe from burnout of the heated surface. Many methods to enhance the CHF have been investigated and nanofluids technology has been proposed as a new technique in recent years among these methods [2]. Nanofluids are a new class of nanotechnology-based transfer fluids engineered by dispersing and stably suspending nanoparticles in traditional heat transfer fluids such as water, ethylene glycol, and engine oil [3]. Relatively, the efficacy of use of nanofluid technology has been proven in CHF regime compared to other general heat transfer regime. Or, one of the most interesting characteristics of nanofluids is their capability to significantly enhance CHF [2], [4]. Since CHF is the upper limit of phase-change heat transfer of nucleate boiling as the most efficient heat transfer mode, such enhancement provides potential for major performance improvement in many practical applications related to thermal management problems such as electronic cooling and heat dissipation of high-thermal-load systems. Regarding CHF enhancement, however, there remain unclear points.

In particular, each level of enhancement varies in a wide spectrum depending on each kind of nanofluid and each research group. This may be due to the test duration and nanoparticle morphology, which result indifferent deposition morphology and hence differences in the enhancement ratio. On the other hand, there is a consensus that such enhancement is related to buildup of a deposition layer of nanoparticles during boiling of nanofluids [1], [4], [5], [6], [7], [8]. In order to prove this, some researchers indirectly showed that the deposition layer of nanoparticles reduced the contact angle and increased the capillary height in terms of surface wettability.

Recently, Park et al. [9] investigated the effects of newly-introduced graphene nanomaterials on pool boiling CHF. They interestingly reported that there were no surface wettability improvements in terms of contact angle and even capillary action whereas the CHF enhancement ratio was highest in graphene-oxide nanofluid when they compared that with other conventional nanofluids such as alumina. This result raised up a new question regarding CHF enhancement mechanisms. They argued that the CHF enhancement observed in the graphene-oxide nanofluid could not be explained by the well-known surface wettability improvement, but by reduction of the Taylor instability wavelength in relation to hydrodynamic instability as suggested by Liter and Kaviany [10] and Hwang and Kaviany [11] for surfaces with microporous coating [16].

Therefore, in order not to miss a true mechanism of CHF enhancement in nanofluids it is necessary to check all of the deposition characterizations such as contact angle, capillary height and instability wavelength at the same time and all together for each kind of nanofluid.

So far, evaluations about CHF enhancement for nanofluids with a variety of nanoparticles are abundant in the literature. However, there were very few efforts to investigate on the difference of CHF enhancement level according to nanoparticles deposition characteristics for a same kind of material, not in comparison with other kind of nanofluids. Therefore, we performed the pool boiling experiment with CuO nanofluids fabricated by both a pulsed laser ablation method (one-step method) and the particles dispersion method (two-step method) to make different deposition conditions. The various reasoning for difference in CHF values for nanofluids with a same kind of material, CuO (based on different preparation methods) are tried by measuring contact angle and capillary height indicating the surface wettability and even Taylor instability wavelength by a condensation method.