A study has been conducted to examine the effects of macroscale, microscale, and nanoscale surface modifications in water pool boiling heat transfer and to determine the effects of combining the multiple scales. Nanostructured surfaces were created by acid etching, while microscale and macroscale surfaces were manufactured through a sintering process. Six structures were studied as individual and/or collectively integrated surfaces: polished plain, flat nanostructured, flat porous, modulated porous, nanostructured flat porous, and nanostructured modulated porous. Boiling performance was measured in terms of critical heat flux (CHF) and heat transfer coefficient (HTC). Both HTC and CHF have been greatly improved on all modified surfaces compared to the polished baseline. The CHF and HTC of the hybrid multiscale modulated porous surface have achieved the most significant improvements of 350% and 200% over the polished plain surface, respectively. Nanoscale, microscale, and macroscale integrated surfaces have been proven to have the most significant improvements on HTC and CHF. Experimental results were compared to the predictions of a variety of theoretical models with an attempt to evaluate both microscale and nanoscale models. It was concluded that models for both microscale and nanoscale structured surfaces needed to be further developed to be able to have good quantitative predictions of CHFs on structured surfaces.

With the rapidly increasing demand for efficient, small, light, low cost, and reliable electronic devices, the nano-scale thermal transport properties of semiconductors has become a prominent focus of research. Molecular dynamic simulation has emerged as a powerful compliment to experiment and is beginning to play a crucial role in tailored material properties for improved thermal management of electronic devices. The thermal conductivity of semiconductor has been investigated using the Stillinger-Weber (SW) and the Modified Embedded Atom Method (MEAM) potential model, the dependence of temperature, system size, and simulation time has been investigated. We find this MEAM model to quantitatively reproduce bulk thermal conductivity predictions using the linear extrapolation method comparable to previous literature using the well studied Stillenger-Weber (SW) potential.

The critical heat flux values of copper substrates were increased from 87 to 125 W/cm 2 by using a simple chemical process resulting in growth of micro and nano-scale copper structures on the surface. Pre- and post-test surface analysis revealed that the morphology of the micro and nano-scale features of these copper structures changed during the boiling process accompanied by a change in oxide layer composition. Boiling performance of the micro and nano-structured samples was repeatable when testing at lower heat fluxes.

An experimental study of the pool boiling two-phase heat transfer on a sintered Cu microparticle porous structure module surface is conducted. Enhanced heat transfer capacity of this module surface has been reported, and the boiling characteristics have been investigated. The bubble dynamics and nucleate size distribution have been compared to the theoretical predictions, and the speculated mechanisms have been discussed.

An experimental study of the pool boiling two-phase heat transfer on a sintered Cu microparticle porous structure module surface is conducted. Enhanced heat transfer capacity of this module surface has been reported, and the boiling characteristics have been investigated. The bubble dynamics and nucleate size distribution have been compared to the theoretical predictions, and the speculated mechanisms have been discussed.

Spherical glass and copper beads have been used to create bead packed porous structures for an investigation of two-phase heat transfer bubble dynamics under geometric constraints. The results demonstrated a variety of bubble dynamics characteristics under a range of heating conditions. At low heat flux of 18.9 kW/m 2 , a single spherical bubble formed at nucleation sites of a heating surface and departed to the interstitial spaces of porous structure. When heat flux increased to 47 kW/m 2 , a single bubble grew into a Y shape between beads layers and connected with others to generate a horizontal vapor column. As heat flux reached 76.3 kW/m 2 , vertical vapor columns obtained strong momentum to form several major vapor escaping arteries, and glass beads were pushed upward by the vapor in the escaping arteries. According to Zuber’s hydrodynamics theory, choking will take place when the size of vapor columns reaches a certain value that is comparable to the critical hydrodynamic wavelength of the vapor column in plain surface pool boiling. The experimental and simulation results of this investigation illustrated that, under the geometric constrains of bead packed porous structures, similar characteristics had been induced to trigger the earlier occurrence of vapor column chocking inside porous structures. The bubble generation, growth, and detachment during the nucleate pool boiling heat transfer have been filmed, the heating surface temperatures and heat flux were recorded, and theoretical models have been employed to study bubble dynamic characteristics. Computer simulation results were combined with experimental observations to clarify the details of the vapor bubble growth process and the liquid water replenishing the inside of the porous structures. This investigation has clearly shown, with both experimental and computer simulation evidence, that the millimeter scale bead packed porous structures could greatly influence pool boiling heat transfer by forcing a single bubble to depart at a smaller size as compared to that in a plain surface situation at low heat flux situations, and could trigger the earlier occurrence of critical heat flux (CHF) by trapping the vapor into interstitial space and forming a vapor column net. The results also proved data for further development of theoretical models of pool boiling heat transfer in bead packed porous structures.

A combined experimental and analytical investigation of single proton exchange membrane (PEM) fuel cells, during cold start has been conducted. The temperature influence on the performance of a single PEM fuel cell and the cold start failure of the PEM fuel cell was evaluated experimentally to determine the failure mechanisms and performance. The voltage, current and power characteristics were investigated as a function of the load, the hydrogen fuel flow rate, and the cell temperature. The characteristics of cold start for a single PEM fuel cell were analyzed, and the various failure mechanisms explored and characterized. In an effort to better understand the operational behavior and failure modes, a numerical simulation was also developed. The results of this analysis were then compared with the previously obtained experimental results and confirmed the accuracy of the failure mechanisms identified.

Experimental evidence exists that the addition of a small quantity of nanoparticles to a base fluid, can have a significant impact on the effective thermal conductivity of the resulting suspension. The causes for this are currently thought to be due to a combination of two distinct mechanisms. The first is due to the change in the thermophysical properties of the suspension, resulting from the difference in the thermal conductivity of the fluid and the particles, and the second is thought to be due to the transport of thermal energy by the particles, due to the Brownian motion of the particles. In order to better understand these phenomena, a theoretical model has been developed that examines the effect of the Brownian motion. In this model, the well-known approach first presented by Maxwell, is combined with a new expression that incorporates the effect of the Brownian motion and describes the physical phenomena that occurs because of it. The results indicate that the enhanced thermal conductivity may not in fact be due to the transport of energy by the particles, but rather, due to the stirring motion caused by the movement of the nanoparticles which enhances the heat transfer within the fluid. The resulting model shows good agreement when compared with the existing experimental data and perhaps more importantly helps to explain the trends observed from a fundamental physical perspective. In addition, it provides a possible explanation for the differences that have been observed between the previously obtained experimental data, the predictions obtained from Maxwell’s equation and the theoretical models developed by other investigators.