This doctoral thesis addresses heat transfer in nucleate boiling and its enhancement through the development of structured microchannel heat sinks intended for high heat-flux thermal management, typical of advanced electronics and energy conversion systems. Conventional smooth metallic surfaces often reach the critical heat flux (CHF) at relatively moderate heat fluxes; once local dryout occurs, the boiling regime shifts towards film boiling, accompanied by a sharp increase in wall superheat. The central objective of this work was to clarify how surface properties (topography and wettability) govern bubble dynamics, boiling curves, and operational limits (HTC and CHF), and to develop microchannel-based concepts that mitigate the inherent trade-off between high heat transfer coefficients and high CHF. First, the influence of laser texturing (texture line spacing) and surface chemistry modification (hydrophobization via self-assembled monolayers) on the boiling curve was systematically investigated. The results show that hydrophobization typically promotes earlier nucleation and higher heat transfer coefficients (lower wall superheats at a given heat flux), whereas its impact on CHF strongly depends on the characteristic length scale of the texture and on the efficiency of surface rewetting. Next, open microchannel surfaces and hierarchical designs with laser-induced microcavities were developed and tested. This combination enables simultaneous improvement of HTC and CHF and demonstrates that both microchannel geometry and wettability are critical for controlling bubble coalescence and delaying dryout. Finally, microchannel concepts were integrated with local forced convection (submerged jet impingement) to further intensify liquid supply to achieve substantially higher CHF compared to a flat reference surface and provided a clear path towards robust high-performance two-phase cooling solutions. Based on experimental results, high-speed visualization, and comparison with the literature, the thesis provides practical design guidelines for microchannel evaporators and related phase-change thermal management devices.
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