Experimental investigation of the effects of metal oxides and nucleating agents on nano-emulsions heat transfer performance in mini-channels

Abstract

Recently, thermal conductivity, even and stable suspension of particles, and agglomeration of particles have been studied comprehensively. These properties are especially important parameters for nanofluids in small-size heat transfer devices like mini-channels, which are prone to block due to the small size of the channels. In the present study, a method was proposed to increase heat transfer using composite nano-phase change emulsions (NPCEs) slurries, utilizing the latent heat of PCM during melting. In this experimental study, a series of composite nano-phase change emulsions (NPCEs) with high thermal conductivity, high thermal storage, and low supercooling were developed and prepared from hexadecane, octadecane, hexadecanol, octadecanol, and nano-Al2O3. Heat transfer experiments were conducted in a mini-channels using the prepared NPCEs as working fluids. The effects of the nanoparticle concentration, NPCE type, and flow rate on multiple parameters, including the heat transfer coefficient, flow resistance, and heat transfer performance of composite NPCEs were investigated. As a result, the heat transfer experiments demonstrated that the proposed NPCEs as heat transfer fluids could enhance the heat transfer performance in mini-channels in comparison with deionized water. Compared with deionized water, the convective heat transfer coefficient was increased by a maximum of 1.28 %, 4.07 %, and 4.19 % with an octadecane concentration of 5 wt%, 10 wt%, and 20 wt%, respectively. The nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3 achieved a maximum increase of 7.01 % in the heat transfer enhancement factor. In addition, the nano-emulsion with a combination of 5 wt% hexadecane, 5 wt% octadecane, and 1 wt% nano-Al2O3, in addition to 1.25 wt% hybrid nucleating agents allowed a maximum increase of 26.49 % in the heat transfer enhancement factor. The obtained results and performed analyses demonstrate that metal particle-composite alcohol hybrid nanofluid has high stability, high thermal conductivity, low subcooling, high durability, and a convenient preparation method, and is an appropriate choice as a working fluid, especially in small-scale heat transfer devices.