Enhancing Internal Convection Heat Transfer With Nano-fluids And Nano-surfaces

The new technologies, methods and materials at micro- and nano-scales could provide some advantages that are unachievable at macro scale, and therefore a lot of innovative approaches for enhancing heat transfer were inspired. In this study, by using nano-fluids and nano-surfaces, we aim at revealing some new mechanisms of internal single phase and flow boiling heat transfer at micro- and nano-scales, providing data and theoretical guide for developing more efficient micro/nano structures for enhancing heat transfer. The main issues of this study are 1) single-phase heat transfer enhancement using nano-fluids and 2) flow boiling heat transfer enhancement using microchannel and nano-surfaces.An extensive and critical review of nano-fluids and micro/nano-engineered surfaces was conducted. Some topics are urgently needed to be settled, including mixed convection of nano-fluids, forced convection of one-step nano-fluids, effects of surfactant on flow boiling in mini-channel, and application of nano-surfaces to flow boiling in microchannel.Firstly, systematic experiments were performed to study the effects of silica nanoparticle size, volume fraction, basefluid viscosity and Prandtl number on laminar mixed convection heat transfer in a horizontal large-tube. By the inclusion of the nanoparticles, the contribution of natural convection to the overall convective heat transfer can be seriously deteriorated under the same heat flux. The criterion number and homogenous model can be used to quantify the mixed convection heat transfer characteristics of nanofluids.Secondly, plenty of silica nano-fluids made by one-step wet chemical method was utilized for forced and mixed convection heat transfer in a vertical mini-tube. The measured heat transfer coefficients are in good agreement with the traditional model predictions, provided that the loading- and temperature-dependent thermophysical properties of the nanofluids are utilized in the evaluation of the dimensionless numbers.Thirdly, the effects of SDBS surfactant on flow boiling heat transfer in mini-tube were investigated. By adding SDBS, the surface tension and the bubble departure diameter are reduced, leading to the change from confined boiling to unconfined boiling and the delay of partial dryout.Fourthly, a micro-gap flow boiling test-rig was constructed for parametric study of various micro/nano engineered surfaces. The partial dryout and rewetting phenomena in both subcooled and saturated boiling were visualized by high-speed camera. The effects of mass flux, heat flux and vapor quality on micro-scale heat transfer were investigated.Finally, by using PECVD, a super-hydrophilic surface with 100nm thick silica nano-particle layer was prepared and integrated into the micro-gap. The heating super-hydrophilic surface could be well-wetted at relatively high heat flux and vapor quality in annular flow regime. As a result, the heat transfer deterioration occurred at high vapor quality in normal micro-channel can be avoided.