Experimental and Simulation Study of Advanced Hybrid Surfaces on Boiling and Condensation Heat Transfer Enhancement


Student thesis: Doctoral Thesis

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Award date6 Nov 2023


Since phase transition absorbs/releases a substantial quantity of latent heat, boiling and condensation processes have been omnipresent in today's energy fields. Enhancing the thermal performances of phase-change processes can meet the urgent needs of various emerging applications. Sustainable dropwise condensation is widely utilized in water harvesting, power generation, and seawater desalination. Microchannel flow boiling can be adopted in high heat dissipation electronic cooling. Among various methodologies, surface modification provides a passive method for phenomenal heat transfer enhancement by optimizing the liquid-gas dynamic behaviors. On the other hand, Lattice Boltzmann Method (LBM), a mesoscopic simulation ideal for capturing the critical physical information at the micro/nanoscale, has recently been developed for boiling and condensation simulation, providing a vital supplementary for enhancement mechanism elucidation. Hence, in this thesis, surface modification on condensation enhancement adopting experimental and LBM simulation is conducted. An improved boundary scheme for LBM simulation is developed to evaluate the dropwise condensation enhancement performance and provide enhancement mechanisms analysis. Furthermore, the developed LBM model is adopted in multiscale surfaces on flow boiling enhancement, including single-channel flow boiling and multichannel flow boiling coupling flow instability mitigation. Conclusions of this thesis provide insights for modified surface on phase change heat transfer enhancement and assist us in heightening the efficiency for various engineering fields by proposing advanced hybrid surface fabrication methods.

Firstly, an easy-to-fabricate method under regular environment is proposed for sustainable dropwise condensation enhancement. Polytetrafluoroethylene is infused on chemically-formed silver-coated surfaces to create a multi-material hydrophobic surface. Surface flooding is avoided at high subcooling and various non-condensable gas concentrations. The polytetrafluoroethylene (PTFE)-infused layer can enhance porosity and form heterogeneous conductivity distribution, leading to controlled nucleation and bubble growth on the silver bumps regions to prevent pre-flooding. The microscale grooves array further achieves efficient droplet sliding by adopting this surface modification method. To elucidate the enhancement mechanisms and determine the optimum microgroove geometry design, a 3D LBM model is developed with a three-layered improved boundary scheme with better numerical stability. The effect of surface wettability, groove geometry, and orientation are studied for mechanism analysis. Results show suction and bridging modes are two primary droplet behaviors on microgroove surfaces. The suction mode can accelerate the droplets’ sliding-off process and lead to a smaller departure radius. In contrast, the bridging mode could deteriorate the enhancement performance by causing strip-like liquid films. Based on theoretical and simulation results, optimum geometry based on the critical sliding-off radius under plain surface conditions is proposed. These conclusions provide a valuable guide for surface modification design and unraveling the enhancement mechanisms. Based on the suction and bridging modes, the experimental study of hybrid-coated groove-etched surfaces is conducted. It is found that heat transfer enhancement of 177.3% up to the subcooling of 35 K is achieved. The proposed surface modification method can benefit a broad range of water-harvesting and condensation heat transfer applications.

Furthermore, a multi-functional surface is proposed by simultaneously improving the droplet nucleation, merging, and sliding behaviors. The hybrid coating technique is adopted, macroscale hydrophobic conical bumps are designed for efficient droplet nucleation and merging, and Tesla-valve shape channels are carved to facilitate droplet sliding and anti-flooding. A fourth-fold increment of HTC is obtained compared with filmwise condensation (FWC) results. The enhancement mechanisms are explained by close-up high-speed optic recording at micron length and millisecond time scales. This design and enhancement mechanism are helpful for condensation heat transfer enhancements under the background of nuclear safety, energy consumption reduction, and liquid harvesting.

As for flow boiling enhancement studies using LBM simulation, a pillar-decorated surface is conducted for single microchannel flow boiling enhancement. The effects of surface wettability and pillar geometry are analyzed through bubble dynamic behaviors, Nusselt number, heat flux, and pressure drop. Design-based suggestions are proposed, and the enhancement mechanisms are explained. Results show that a hydrophobic surface is preferred for temperature-sensitive devices with low superheat requirements, while a hydrophilic surface is preferred for devices with large heat dissipation requirements. The micropillar surfaces with the geometric factor of 7 can yield the optimum heat transfer performance under a wide range of superheat conditions. Finally, biphilic patterns with superhydrophobic regions located at the top of the pillars and other regions remain hydrophilic surfaces are simulated. An excellent heat transfer enhancement of 105.8% is achieved even compared with a pure hydrophilic micropillar surface. The enhancement is attributed to the superhydrophobic top regions efficiently blocking the bubbles’ merging process, which leads to more intense bubbles’ departure. These results provide a valuable guide for microchannel heat sink design and unravel the enhancement mechanism of the flow boiling process.

Finally, I adopt our LBM model to multichannel flow boiling process under the background of battery and chip cooling. The effects of surface wettability, channel number, input heat flux, and inlet velocity on the bubble dynamics, heat transfer coefficient enhancement, dimensionless pressure drop, and flow instability are analyzed. Hydrophilic coating is found to prevent the film boiling transition at high input heat flux and reduce the maximum temperature for safer electronic operation. Designing a dense channel array can enhance the overall heat transfer coefficient (HTC) but also significantly increase flow instability on the inlet, leading to shorter pumping operation life. To solve this, a hybrid design of multichannel with downstream microgap region is proposed, and results indicate great mitigation ability with the increase of gap length and inlet velocity. These findings offer an invaluable blueprint for multichannel heat sink design and reveal the mechanisms of flow boiling enhancement.