Micro/Nano-engineered techniques for enhanced pool boiling heat transfer

Environmental aspects such as water treatment as well as military applications and thermal management emphasize on the need for next generation cooling technologies based on boiling heat transfer. Micro/nano enhanced surfaces have shown a great potential for the performance enhancement in the systems involving boiling phenomena. The lack of fully understanding the mechanisms responsible for the enhancement on these surfaces and scalability of these technologies for large and complex geometries over the wide range of materials are two main issues. The goals of this dissertation are to provide an understanding about the fundamentals of pool boiling heat transfer (BHT) and critical heat flux (CHF) mechanisms on engineered surfaces, to develop new techniques for surface alteration for BHT and CHF enhancement, and to propose novel, facile and scalable surfaces modification techniques for related industries. Surfaces with artificial cavities, surfaces with different wettability, and surfaces with different porosities were fabricated and tested to shed light into the fundamentals of surface/boiling interaction. In addition, 3-D foam-liked graphene and crenarchaeon Sulfolobus solfataricus P2 bio-coating surface modification techniques were proposed for BHT and CHF enhancement. For artificial cavities it was shown that CHF occurrence on the hydrophilic surfaces is mainly due to hydrodynamic instability, while dry-out is the dominant CHF mechanism on the hydrophobic surfaces. The obtained results imply that although the increase in hole diameter enhances CHF for all the fabricated samples, the effect of pitch size depends on surface wettability such that CHF increases and decreases with pitch size on the hydrophobic and hydrophilic surfaces, respectively. For biphilic surfaces, a novel and facile process flow for the fabrication of biphilic surfaces was proposed. It was shown that boiling heat transfer coefficient and CHF increased with A*=AHydrophobic/ATotal up to 38.46%. Surfaces with A*>38.46% demonstrated a decreasing trend in CHF and heat transfer coefficient enhancement, which is caused by earlier interaction of nucleated bubbles, thereby triggering the generation of vapor blanket at lower wall superheat temperatures. This ratio could serve as a valuable design guideline in the design and development of new generation thermal systems. Pool boiling on pHEMA coated surfaces with thicknesses of 50, 100 and 200 nm were used to study the effect of surface porosity and inclination angle on heat transfer and bubble departure process. According to obtained results, combination of the effects of the interaction between active nucleation sites, the increase in bubble generation frequency, and the increase in bubble interactions were presented as the reasons behind the enhancement in heat transfer on coated surfaces. It was observed that under an optimum condition for the inclination angle, the porous coating provides a suitable escape path for vapor phase, which results in space to be filled by the liquid phase thereby enabling liquid replenishment. Pool boiling experiments conducted on 3D foam-like graphene coated surfaces to show the effect of graphene coating thickness on the pool boiling heat transfer performance. According to the obtained results, 3D structure of the coating has a significant effect on pool boiling heat transfer mechanism. Factors such as pore shape and mechanical resonance of the 3D structure could be possible reasons for bubbling behavior in developed nucleate boiling. Furthermore it was found that there exists an optimum thickness of 3D graphene coatings, where the maximum heat transfer coefficient were achieved. This is mainly due to the trapped bubbles inside the porous medium, which affects the bubble dynamics involving bubble departure diameter and frequency. A novel coating, crenarchaeon Sulfolobus solfataricus P2 biocoatings, were proposed for the performance enhancement of heating and cooling devices, thermofluidic systems, batteries, and micro- and nanofluidic devices. These biocoatings have the potential for addressing high heat removal requirements in many applications involving heat and fluid flows. Pool boiling experiments were performed on biocoated surfaces with thicknesses of 1 and 2μm. The obtained results indicated that biocoated surfaces enhance boiling heat transfer by providing numerous nucleation site densities and by increasing bubble interaction on the superheated surface. Interconnected channels inside the porous coating, and capillary pumping enhance liquid transportation and reduce the liquid-vapor counter flow resistance, thereby delating CHF condition. There is a strong potential economic value of research performed in the framework of this thesis. Refrigeration, automotive/aerospace engineering, thermal management companies will benefit from the commercial development of the performed researc

Numerical and experimental studies on multi-phase flows in microchannels

Microchannels are considered as one of the key elements in thermal management of microsystems. Despite the advantages of the microchannels, understanding of the fundamental hydrodynamic and thermal transport mechanisms in multiphase flows in them is far from satisfactory. Therefore, in this thesis using numerical and experimental approaches, it is aimed to focus on the understanding of phase change phenomena in order to be able to make use of them. In the first study, convective heat transfer of alumina/water nanofluids in a microtube is presented using a numerical approach. The effects of nano-particle size and concentration on convective heat transfer are studied. Next, the effect of MWCNTs (multi-wall carbon nanotubes) on convective heat transfer was experimentally studied. The effect of MWCNT concentration on thermal performance is presented. In the second study, high mass flux subcooled flow boiling of water in microtubes is investigated. Both experimental and numerical approaches are implemented to investigate high mass flux flow boiling in micro scale. Heat transfer coefficients are obtained as a function of mass flow rate, heat flux, and vapor quality. In the third study, the effects of surface wettability and roughness on flow boiling in a rectangular microchannel are presented. Micro and nano-structured and nano-coated surfaces are integrated into the channel to investigate the effect of surface characteristics on flow map, bubble formation and release and boiling heat transfer

Numerical and experimental studies on flow condensation in hydrophilic microtubes

Microchannels have increasingly been used to miniaturize heat transfer equipment, improve energy efficiency, and minimize heat transfer fluid inventory. A fundamental understanding of condensation in microscale will yield far-reaching benefits for the different areas of industry. In this study, microtubes with inner diameters of 250, 500, 600, and 900 µm were used to investigate the effect of microtube diameter, inlet quality, and mass flux on the liquid/vapor interface near the wall boundaries in condensing flow. After validation with the experimental results, a transient numerical model (based on the Volume of Fluid approach) was developed to investigate the hydrothermal properties of condensing such as bubble dynamics, flow map transitions, transient interface shear force, and temperature on flow condensation performance in terms of heat transfer coefficient and pressure drop. The liquid film thickness, slug velocity, and location of transition from annular flow to slug flow inside the microtube were characterized for different microtubes, and the resultant alteration in condensation flow heat transfer and pressure drop is discussed in detail. Using non-dimensional analysis, a flow map was constructed and compared with the available flow maps for flow condensation in microchannels. The obtained results indicated that the interfacial characteristics of condensing flow in microtubes with hydraulic diameters lower than 500 µm are majorly different from those with D > 500 µm

Numerical investigations on the effect of fin shape and surface roughness on hydrothermal characteristics of slip flows in microchannels with pin fins

Accurate modelling of convective heat transfer in micro/nano gas flows is critical for many applications such as temperature/mass flow micro-sensors and mixing/separation analysis of gases in micro-systems. This study numerically investigates the effect of pin fin shape and wall roughness on heat transfer and flow field of gas flows in rough microchannels in the presence of the second order slip boundary condition. The hydrothermal characteristics of flows were obtained for a smooth microchannel under slip boundary conditions for the constant wall temperature (330 K) and constant wall heat flux (10 kW/m(2)) boundary conditions. Afterwards, four types of pin fin shapes (rectangular, diamond, oblong, and elliptic) and two types of surface roughness (regular and random rough elements) were taken into account. According to the obtained results, although velocity slip raises Nusselt number, temperature jump tends to reduce it. It was shown that the generated recirculating flows between the roughness elements reduce Nusselt number and increase friction factor. Furthermore; it was found that the effect of pin fin shape diminishes with surface roughness

Numerical investigation of slip flow across micro/nano pin fins

Gas flows in micro scale mostly lie in the slip flow regime, which correspond to a Knudsen number range from 0.001 to 0.1, where the velocity slip and temperature jump conditions at the walls play a major role in heat transfer and pressure drop. In this study, numerical simulations are conducted to investigate gas flow characteristics in microchannels with pin fins under velocity slip and temperature jump boundary conditions at uniform wall temperature and constant heat flux. Air is considered as the working fluid in the silicon based micro heat sink with channel cross sectional area ratio of 1:3 (minimum cross sectional area /maximum cross sectional area). The wall temperature is fixed at 303K, while the constant heat flux applied to the heat sink surface is kept as 20 kW/cm2. The computations were performed for five different Knudsen and Reynolds numbers. The flow pressure drop and heat transfer characteristics were investigated and compared to those of continuum flows with no-slip and no temperature jump conditions. Nusselt number trends in the absence and presence of temperature jump condition were revealed at isothermal and isoflux boundary conditions. The obtained results indicate that the velocity slip and temperature jump conditions have opposite effects on heat transfer. Although velocity slip condition tends increase Nusselt number, temperature jump condition tends to reduce it. The effect of surface enhancement with pin fins on heat transfer is also studied for the temperature jump boundary condition

Numerical and experimental investigation on the effects of diameter and length on high mass flux subcooled flow boiling in horizontal microtubes

High mass flux subcooled flow boiling was investigated both numerically and experimentally in horizontal microtubes. Microtubes with inner diameters of ∼600 and ∼900 μm, and outer diameters of ∼900 and ∼1100 μm, and heated lengths of 6 and 12 cm were tested in order to investigate the effects of diameter and heated length on subcooled flow boiling at high mass and heat fluxes. In the experimental part, microtubes made of stainless steel were used, and deionized water was as the working fluid. In the numerical part, the two-phase Eulerian method was adopted using the finite volume approach. Numerical results showed a good agreement with experimental results. Heat transfer coefficients were higher in the microtubes with smaller diameters, while longer microtubes resulted in higher heat transfer coefficient. The results indicated that smaller pressure drops were achieved for shorter microchannels along with higher heat fluxes. Local heat transfer coefficients were presented along the microtube to provide an understanding on local flow boiling characteristics. As the vapor quality and void fraction increased, higher heat transfer coefficients were obtained. With the increase in mass flux, an enhancement in boiling heat transfer was observed implying convective heat transfer effects on flow boiling along with nucleate boiling. Furthermore, heat transfer coefficient increased with decreasing inlet subcooling

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Surface modification is a widely utilized technique for enhancing condensation heat transfer. Two main properties of surfaces in manipulation of condensation heat transfer are the contact angle and contact angle hysteresis. This study focuses on the influence of contact angle (CA) and contact angle hysteresis (CAH) on humid air condensation. For this, hydrophilic and hydrophobic surfaces with varying CA and CAH values were fabricated and tested in a humidity-controlled climate chamber at different relative humidity levels. Three hydrophilic surfaces samples with a contact angle of approximately 70° and CAH values of 10°, 20°, and 42° were tested. Two hydrophobic surfaces with a contact angle of approximately 110° and CAH values of 21° and 39°were also prepared as well as a hydrophobic surface with a contact angle of 96° and a CAH of 43°. The role of CA and CAH in different stages of condensation cycle was investigated. Our findings show that while CA plays the main role in droplet nucleation, CAH has a significant impact on droplet coalescence and departure. Increasing CAH while keeping CA constant has a negative effect on condensation heat transfer in all wettability levels. However, the relationship between changes in CA while keeping CAH constant does not have the same trend in the condensation heat transfer performance for every case. Changing CAH for lower CAH values led to a greater impact on enhancing condensation heat transfer than higher values. Moreover, increasing CAH on hydrophobic surfaces had a more significant effect than on hydrophilic surfaces. Additionally, decreasing CAH had a more pronounced effect on improving condensation heat transfer than increasing CA. The findings emphasize on the importance of considering both the contact angle (CA) and contact angle hysteresis (CAH) in the surface design to have the optimum condensation heat transfer performance