Boiling heat transfer enhancement on structured surfaces by using active and passive methods

Official URL: https://risc01.sabanciuniv.edu/record=b2759648_(Table of contents)

Thermal management is becoming increasingly important for new electronic device applications. Boiling is a very effective phase change phenomenon capable of dissipating a large amount of heat. This thesis investigates the effect of surface structures on boiling heat transfer by using both active and passive approaches. In the first part, comprehensive series of pool boiling tests were carried out on surfaces having microchannels with different spacings and artificial cavities to investigate the mutual effect of surface microstructures, microstructure spacing, and artificial nucleation sites on the boiling heat transfer (BHT) performance and critical heat flux (CHF). The BHT performance of different samples could be significantly improved for samples containing artificial cavities. While microchannel configurations had no significant effect at low wall superheats, further increases in wall heat flux revealed the effect of surface structure on BHT and bubble dynamics. For samples with 50 and 100 artificial cavities, the largest spacing exhibited the best performance at high heat fluxes. Furthermore, the visualization results revealed that for surfaces with artificial cavities, dry-out was the main CHF mechanism, whereas hydrodynamic instability was responsible for the occurrence of CHF for surfaces without artificial cavities, which proved the dependence of the CHF mechanism on the surface configuration. The second part aims to experimentally investigate the effect of magnetic nanoparticles on saturated flow boiling heat transfer in a narrow microchannel by using microstructured silicon surfaces and comparing the heat transfer performance in the absence and presence of an external magnetic field. BHT results show that due to the orientation of the heating blocks, the heat transfer coefficients (HTC) on the bottom surface were higher than those on the top surface. Moreover, adding nanoparticles enhanced BHT and resulted in an increase in up to 21.5% in BHT coefficient. The presence of the external magnetic field decreases the bubble departure size and increases HTC on the top surface at high heat fluxes. The maximum HTC enhancement in the presence of a magnetic field was 25%. Bottom surface exhibited higher ferrofluid BHT enhancement in the absence of a magnetic field compared to the case with external magnetic field.