Pool Boiling on Structured Surfaces: Heat Transfer and Critical Heat Flux : -Experiments and Mechanistic Modelling

Abstract: In this thesis, pool boiling heat transfer was experimentally studied on structured surfaces with dielectric liquids (HFE-7200, NOVEC-649, FC-72), organic liquids (Acetone, Pentane) and deionized water.In the first step, nanoparticle coatings on copper surfaces were prepared by an electrophoretic deposition method, with Cu-Zn nanoparticles (100 nm) and Cu nanoparticles (150 nm). Two types of nanoparticle-coating surfaces were prepared, namely nanoparticle coatings uniformly deposited on smooth surfaces and nanoparticle coatings partially deposited on smoothsurfaces. Pool boiling of HFE-7200 and acetone was tested on the coating surfaces. It is found that pool boiling heat transfer coefficients are significantly enhanced by nanoparticle coatings. However, the uniform coating cannot enhance the critical heat flux, while the partially-deposited coating can enhance critical heat flux. Mechanistic heat transfer models were developed to predict the heat transfer coefficients, considering natural convection, transient heat conduction, microlayer evaporation and micro convection, while the critical heat flux was analyzed from the point of wickability and hydrodynamic instability.In the following step, microporous coatings on copper surfaces were generated by an electrochemical deposition method, with electrolyte solutions (CuSO4+H2SO4). Pool boiling of HFE-7200, NOVEC-649 and water was tested. The results show that heat transfer coefficients and critical heat flux are enhanced, and the heat transfer coefficients are obviously dependent on deposition-relevant parameters, like deposition time and electrolyte concentration. Heat transfer coefficients were discussed mechanistically and empirically by a mechanistic model and correlations, while the critical heat flux was predicted by a modified force balance model which considers the forces exerted on vapor and assumes occurrence of the critical heat flux when the vapor expands on surfaces.Finally, hybrid micro/nano structures were fabricated on copper surfaces by femtosecond laser machining and electrophoretic deposition, and on silicon wafers by dry etching and electrostatic deposition. Pool boiling of acetone and FC-72 was investigated on the copper surfaces and the silicon wafers, respectively. It is found that the hybrid structures induce higher heat transfer coefficients than sole structures and wickability plays an important role on enhancement of the critical heat flux.