The current study is an experimental and analytical investigation of JIB within the nucleate and transition boiling regimes. This study focuses on studying JIB within the stagnation zone of a free water jet. An experimental setup has been designed and built at the Thermal Processing Laboratory (TPL) with the capability of carrying out boiling experiments at heat fluxes up to 12 MW/m2. The JIB curves have been obtained under steady-state conditions for a wide range of jet conditions, higher than those considered during previous JIB studies. The effect of jet velocity, up to 3.8 m/s, and degree of subcooling, up to 49 °C, on the JIB curve has been studied. The results showed that both jet velocity and degree of subcooling have a weak effect on the nucleate boiling regime and significantly affect the transition boiling regime. Bubble dynamics under the impinging jet within the nucleate boiling regime and the stability of the vapor layer within the transition boiling regime have been investigated. An analytical mechanistic model, based on force balance and thermal balance equations, has been developed to predict the bubble growth rate and the BDD. The developed model was validated using current experimental data. The model gave a relative deviation of 17.8 %. Results of the mechanistic model within the stagnation zone showed that, amongst the three heat transfer mechanisms that affect bubble growth (i.e., the microlayer evaporation, the heat from the superheated layer, the convection heat loss to subcooled liquid), the microlayer evaporation is the most significant contributor to the rate of bubble growth. The current work conducted within the transition boiling regime was focused on the determination of the total wall heat flux within the stagnation zone, both experimentally and analytically. Steady-state experiments have been carried out during which the vapor layer stability was examined. The vapor layer breakup frequency was measured using a fiber-optic probe. Experiments were conducted at a jet velocity of 1 m/s and degrees of subcooling between 11 and 49 ºC. / Thesis / Doctor of Philosophy (PhD)

The effects of a carbon nanotube (CNT) coating on bubble departure diameter and frequency in pool boiling experiments was investigated and compared to those on a bare silicon wafer. The pool boiling experiments were performed at liquid subcooling of 10 degrees Celsius and 20 degrees Celsius using PF-5060 as the test fluid and at atmospheric pressure. High-speed digital image acquisition techniques were used to perform hydrodynamic measurements. Boiling curves obtained from the experiments showed that the CNT coating enhanced critical heat flux (CHF) by 63% at 10 degrees Celsius subcooling. The CHF condition was not measured for the CNT sample at 20 degrees Celsius subcooling. Boiling incipience superheat for the CNT-coated surface is shown to be much lower than predicted by Hsu's hypothesis. It is proposed that bubble nucleation occurs within irregularities at the surface of the CNT coating. The irregularities could provide larger cavities than are available between individual nanotubes of the CNT coating. Measurements from high-speed imaging showed that the average bubble departing from the CNT coating in the nucleate boiling regime (excluding the much larger bubbles observed near CHF) was about 75% smaller (0.26 mm versus 1.01 mm)and had a departure frequency that was about 70% higher (50.46 Hz versus 30.10 Hz). The reduction in departure diameter is explained as a change in the configuration of the contact line, although further study is required. The increase in frequency is a consequence of the smaller bubbles, which require less time to grow. It is suggested that nucleation site density for the CNT coating must drastically increase to compensate for the smaller departure diameters if the rate of vapor creation is similar to or greater than that of a bare silicon surface.

<p>To improve the prediction of two-phase local structure and heat transfer in subcooled boiling flow, the wall nucleation phenomenon was studied to accurately model the wall source term in the interfacial area transport equation (IATE) for the use with the two-fluid model. The existing experimental datasets and modeling works of departure diameter, departure frequency and active nucleation site density were comprehensively reviewed. Since these parameters are coupled in the bubble ebullition cycles, simultaneous measurements of departure diameter, departure frequency and active nucleation site density were performed in a vertical annular test section. The ranges of the existing experimental database were extended to high pressure and high heat flux conditions. The stochastic characteristics of the departure diameter and departure frequency measured from a single nucleation site and over multiple nucleation sites were investigated. Significant variations between different nucleation sites were observed. A parametric study of departure diameter, departure frequency and nucleation site density were conducted at varying system pressure, heat flux, flow rate and subcooling conditions. The existing models of these parameters were evaluated with the experimental dataset of the existing and the present works. Significant discrepancies were observed between model predictions and experimental data, which indicates that the mechanism of nucleate boiling is not fully understood. The heat flux partitioning model was also evaluated. The results show that the heat flux at high pressure or low flow rate conditions was significantly underestimated. This may suggest that major heat transfer mechanisms are missing in the heat flux partitioning model.</p>