An experimental investigation was performed to study the confined boiling heat transfer characteristics over a saturated porous structure using distilled water as the working fluid. A thin stainless steel resistive foil stretched between two copper electrodes was used to heat a saturated porous plate with an effective pore size of 50 gm. The temperature distribution on the foil heater was measured using a high speed thermal imaging camera. The effect of the gap height between the heater and the porous plate on the heat transfer was investigated for gap heights ranging from 0 um to 1000 um and for heat fluxes ranging from 11.7 kW/m2 to 58.3 kW/m2. It was observed that the highest heat transfer rate was obtained at a gap height of approximately 600 pm. The main heat transfer mechanism is thought to be confined boiling in the small gap between the heating surface and the saturated porous structure. It was observed that the effect of the subcooled liquid temperature did not have a significant effect on the heat transfer. The effect of the pore size in the porous plate was investigated by repeating the measurements with a porous plate of 200 gm pore size. It was observed that the thermal resistance for the plate with a 200 gm pore size was significantly higher than the plate with 50 gm pores for gaps less than 300 gm. At a larger gap height of 600 gm, similar heat transfer performances were obtained for the two porous media. / Thesis / Master of Applied Science (MASc)

<p> With new technologies taking advantages of the rapid miniaturization of devices to microscale across emerging industries, there is an unprecedented increase in the heat fluxes generated. The relatively low phase-change thermal resistance associated with boiling is beneficial for dissipating high heat flux densities in compact spaces. However, for boiling heat transfer, a high degree of geometrical confinement significantly alters two-phase interface dynamics which affects the flow pattern, wetting dynamics, and moreover, the heat transfer rate of the boiling processes. Hence, it is crucial to have a deeper understanding of the mechanistic effects of confinement on two-phase heat dissipation and carefully examine the applicability of boiling correlations developed for unconfined pool boiling to predict and optimize design of extremely compact two-phase thermal management solutions. This dissertation develops and demonstrate a fundamental understanding of the impact of confinement on pool boiling. To elucidate the mechanisms that impact confined boiling, this study experimentally evaluates boiling characteristics through the quantification of boiling curves and high-speed visualization across a range of gap spacing smaller than the capillary length of the working fluid. </p> <p><br></p> <p> This work reveals the existence of two distinct boiling regime uniquely observed in boiling in confined configurations (namely, intermittent boiling and partial dryout). In contrast to pool boiling where the maximum heat transfer coefficient occurs below the critical heat flux limit, the intermittent boiling regime demonstrates the highest heat transfer coefficient in confined boiling. Then, this study provides a mechanistic explanation for the enhanced heat transfer rate due to geometrical confinement. Mainly, small residual pockets of vapor, termed ‘stem bubbles’ herein, remain on the boiling surface through a pinch-off process. These stems bubbles act as seeds for vapor growth in the next phase of the boiling process without the need for active nucleation sites. Furthermore, this dissertation develops a more accurate, mechanistic-based model for the phenomena that occur at CHF in confined configurations. The newly developed mechanistic understanding and model provides guidance on new directions for designing extremely compact two-phase thermal solutions.</p>