Spray cooling characteristics of nanofluids for electronic power devices

Hsieh et al. Nanoscale Research Letters Spray cooling characteristics of nanofluids for electronic power devices Shou-Shing Hsieh 0 1 Hsin-Yuan Leu 0 1 Hao-Hsiang Liu 0 1 0 Department of Mechanical and Electromechanical Engineering, National Sun Yat-Sen University , Kaohsiung 80424 , Taiwan 1 Authors' information SSH is a professor at the Department of Mechanical and Electro Mechanical Engineering, National Sun Yat-Sen University , Kaohsiung, Taiwan , Republic of The performance of a single spray for electronic power devices using deionized (DI) water and pure silver (Ag) particles as well as multi-walled carbon nanotube (MCNT) particles, respectively, is studied herein. The tests are performed with a flat horizontal heated surface using a nozzle diameter of 0.5 mm with a definite nozzle-to-target surface distance of 25 mm. The effects of nanoparticle volume fraction and mass flow rate of the liquid on the surface heat flux, including critical heat flux (CHF), are explored. Both steady state and transient data are collected for the two-phase heat transfer coefficient, boiling curve/ cooling history, and the corresponding CHF. The heat transfer removal rate can reach up to 274 W/cm2 with the corresponding CHF enhancement ratio of 2.4 for the Ag/water nanofluids present at a volume fraction of 0.0075% with a low mass flux of 11.9 104 kg/cm2s. Spray heat transfer; Ag/MCNT nanofluids; Transient boiling; Cooling enhancement - Background Spray cooling is an efficient way to remove high heat flux from heated surfaces. Frequently, the essential requirements for many electronic power devices are a small surface superheat and a low mass flow rate. It has long been recognized [1] that spray cooling with phase change has been demonstrated to be a powerful method to remove high heat flux from modified surfaces, using water as a coolant with a higher mass flux. Three different regimes have been termed in boiling spray cooling: nucleate boiling from surface and secondary sites, convection heat transfer, and direct evaporation from the liquid film over the surface [2]. Many studies were conducted on the influence of the spray parameters on the cooling heat flux. It was found that the volumetric spray flux has a major effect on the heat transfer [2,3] compared to those of hydrodynamic parameters of spray [1]. Still, investigators believe that spray cooling performance and critical heat flux (CHF) usually depend on a number of parameters, including the following: nozzle type, nozzle-tosurface distance, heated surface condition, working liquid, and droplet dynamics [4,5]. Applications exist in a wide range of industrial processes, including rapid cooling and quenching in metal foundries, emergency core cooling systems, cooling of microelectronics, and the ice chiller in air-conditioning systems. The physical process of spray cooling, due to the impact of in-flight droplet impingings onto a heated surface, consequently may lead to splashing, spreading, or rebounding [6]. Obviously, the rebound process would result in decreased liquid cooling capacity and efficiency. The impinging droplets spread on the surface and can form a continuous liquid film. At high wall superheat, a thin vapor layer can form under the droplets or the thin liquid films due to boiling [7]. Advances in nanofabrication processes have led to many innovations in spray and atomization technologies. Nanofluids are fluids that contain nanoparticles, such as metals, oxides, carbides, and nitrides, with sizes less than 100 nm. They are known to have higher thermal conductivity compared to that of the base fluid; hence, the enhancement of their thermal conductivity at room temperature was considered in the majority of the research [8]. In addition, the application of nanofluids in spray cooling for electronic devices is an emerging area of research [9]. In fact, some metals and non-metals, like gold, silver, copper, aluminum, and carbon, have been found to have quite high thermal conductivity compared to cooling liquids like water, engine oil, and ethylene. Therefore, small amounts of these materials with high thermal conductivity added to base fluids like water would increase the thermal conductivity of the base fluids without the problems encountered in common slurries, such as clogging, erosion, sedimentation, and a large increase in pressure drop. As stated previously, the addition of metal/or metal oxide nanoparticles to a liquid coolant is one of the notable examples proffered to increase the mixtures thermal conductivity and possibly increase the heat transfer. Although several investigators [10,11] have proven this concept, quite a few results show an opposite trend [12-14] due to nanoparticle deposition on the surface impeding heat transfer performance. In addition, inconsistency in the heat transfer performance by nanofluids with spray cooling can also be found [9,15]. Based on the findings above, it may be concluded that the heat transfer coefficient increase/or decrease from the addition of nanoparticles depends on either the base fluid used or the target surface temperatures and the spray duration time on the surface/or the nanofluid impact velocity. Although the results are inconsistent with respect to boiling enhancement, both results may be true in their respective particle concentration range, because these two ranges may be dominated by different phenomena which result in different heat transfer characteristics. Moreover, it has been shown [4] that the CHF is enhanced for the pool boiling because the deposition of nanoparticles on the heated surface results in a change in the surface properties including capillarity and coatability. The contact angle, therefore, decreases for a nucleate boiling in nanofluids. Although there are plenty of advantages of spray cooling over existing cooling techniques, it appears that there is a very limited knowledge base with contrary experimental data on spray impingement cooling of surfaces for situations when the coolant of nanoparticle and liquid mixtures has a very low volume concentration (0.0075%) of nanopowder, especially for metal (like Ag) and MCNT nanoparticles. In fact, Ag/water nanofluid spray has not been seen in publications. In view of the foregoing discussion, this paper presents a relatively detailed study on the spray impingement heat transfer, both steady and transient, to broaden our fundamental understanding of the two-phase spray cooling of nanofluids. In order to accomplish this goal, experiments were performed with Ag/MCNT nanoparticles and a deionized (DI) water mixture, respectively, with different particle volume fractions. Furthermore, the influence of the liquid mass flux on the heat transfer performance was examined. In the following section, the experimental setup is described; in Section Results and discussion, experimental results are presented and discussed. Methods The preparation of Ag/MCNT nanoparticles and deionized wate (...truncated)