Performance enhancement of two-phase closed thermosyphon via carbon-based surface functionalization

Abstract

Formidable challenges have been posed to the thermal management community in developing effective heat dissipation devices due to the exponential growth in heat generation of miniaturized electronics devices. With the continuous development of advanced microprocessors, convectional phase-change heat transfer devices are insufficient to overcome the aggressive growth in heat generation in the coming future. Addressing the problem, extensive studies have been conducted to enhance the phase-change heat transfer via surface functionalization which facilitates effective evaporation and condensation. In general, hydrophilic surface is well-recognized to promote filmwise evaporation while dropwise condensation is observed across hydrophobic surface. In this thesis, experimental studies were conducted with the objective of enhancing the thermal performance of a two-phase closed thermosyphon (TPCT) by exploring various innovative techniques through surface functionalization. The study begins with the circulation enhancement of the condensates by coating a hydrophobic thin film (Teflon AF-1600) on the condenser surface. Imperative information was obtained from the exposition of the size and distribution of condensate droplets in the analysis of the circulation effectiveness. The coupled effects of high acceleration induced vibration with the hydrophobic layer on the thermal performance of a TPCT were also investigated. Interestingly, we observe that the formation of elongated liquid jets and the entrainment of droplets induced by the high-acceleration vibration counteract the enhancing effect from the increased body force of the condensed droplets. In a separate study, graphene-oxide (GO) nanofluid was utilized as working fluid in TPCT to enhance its strength of evaporation. Surface morphology of GO layer inherently deposited on the evaporator surface was investigated. The consensus of high thermal conductivity nature of GO was demonstrated to be inadequate to explain the anomalous performance enhancement of TPCT. Water molecules are able to permeate through the nanocapillaries in GO deposition with ultralow friction. The thermal performance enhancement is primarily attributed to the water permeation across GO deposition which extends the effective evaporating region, leading to a significant increase in the evaporator heat transfer coefficient. To gain further insights in the water permeation effect, we analysed the dynamic behaviour of a droplet vaporizing on a GO-coated surface under various boiling regimes. The nanocapillaries embedded within the GO-coating provide effective dissipation channels for the vapour water molecules. Rapid transition of boiling regimes from transition boiling to contact boiling of a droplet vaporizing on GO-coated surface was observed. At high surface temperature, the Leidenfrost state was suppressed due to the water permeation effect across GO-coating. A significant increase of Leidenfrost point was obtained. The underlying physical mechanism of rapid water permeation was scrutinized to elucidate the outstanding capability of GO-coating in suppression of Leidenfrost state. An unprecedented approach in enhancing the thermal performance of TPCT was also demonstrated with the introduction of graphene nanoplatelets (GNPs) thin film. GNPs-coating with high water permeability was coated on the inner wall of TPCT, forming a continuous layer across the evaporator and the condenser sections. Exceeding the water permeability of GO-coating, GNPs-coating with its implicit characteristic of rapid water permeation significantly augments the evaporation, condensation and circulation processes which govern the operation of TPCT. The optimized ratio of non-oxidized to oxidized regions endows GNPs-coating with excellent water permeability. Water molecules intercalating through the layered structure of GNPs-coating experiences more frictionless flow to a greater extent compared to its counterpart of GO-coating. Evaporation is enhanced with the induced thin film evaporation and the effective distribution of water molecules across GNP-coated evaporator surface for suppression of dryout. GNPs-coating is endowed with unique surface characteristics. The circulation of condensates is enhanced through rapid water permeation while the nucleation of condensed droplets is promoted by its hydrophilic surface characteristic. The unique fast water permeation of functionalized graphene-based coating is appealing for practical implementation in effective phase-change thermal management systems. This study paves the way for a promising start of employing fast water permeation property of graphene-based coatings in thermal applications.