Phase change heat transfer on micro/nano structured enhanced surfaces

Due to the recent trend of miniaturization of electronic devices, thermal management has become obligatory and yet challenging. There are many cooling methods such as spray cooling and passive cooling techniques involving phase change phenomena. Among phase change (liquid-vapor) phenomena, boiling is a widely used phenomenon in the industry. Owing to a large amount of heat dissipation and achievable high heat transfer coefficients, it is one of the most effective heat transfer mechanisms for cooling high power microelectronic devices. With the help of material science and nanotechnology, various types of nanostructures on surfaces are now available. Different types of enhanced surfaces (e.g. micro/nano structured surfaces) have been used by many investigators to enhance heat transfer coefficient and also reduce wall superheat. In this thesis, two different futuristic types of enhanced surfaces have been designed, fabricated and implemented. First, an effective and facile method for surface enhancement via crenarchaeon Sulfolobus Solfataricus P2 bio-coatings is presented and tested under boiling condition. Additionally, enhanced surfaces with mixed wettability (Biphilic surfaces) have been designed and fabricated to assess the effect of heterogeneous wettability on boiling heat transfer and cooling performance. In both cases, high heat removal performance has been achieved. Promising results for both ii types of coatings suggest such coatings as a novel solution for high efficiency and enhanced cooling application

Foamlike 3D Graphene Coatings for Cooling Systems Involving Phase Change

Boiling is an efficient heat-transfer mechanism because of the utilization of latent heat of vaporization and has the potential to be used for cooling high-power electronic devices. Surface enhancement is one of the widely used techniques for heat-transfer augmentation in boiling systems. Here, an experimental investigation was conducted on chemical vapor deposition-grown three-dimensional (3D) foamlike graphene-coated silicon surfaces to investigate the effect of pore structures on pool boiling heat transfer and corresponding heat-transfer enhancement mechanisms. 3D graphene-coated samples with four graphene thicknesses were utilized along with a plain surface to investigate boiling heat-transfer characteristics and enhancement mechanisms. A high-speed camera was used to provide a deeper understanding of the bubble dynamics upon departure of emerging bubbles and visualize vapor columns in different boiling regimes. On the basis of the obtained results, in addition to interfacial evaporation, mechanical resonance of the 3D structure had also a considerable effect on vapor column formation. The results indicated that there is an optimum thickness, which exhibits the best performance in terms of boiling heat transfer

Pool boiling heat transfer characteristics of inclined pHEMA-coated surfaces

New requirements for heat exchangers offered pool boiling heat transfer on structured and coated surfaces as one of the promising methods for effective heat removal. In this study, pool boiling experiments were conducted on polyhydroxyethylmethacrylate (pHEMA)-coated surfaces to investigate the effect of surface orientation on bubble dynamics and nucleate boiling heat transfer. pHEMA coatings with thicknesses of 50, 100, and 200 nm were deposited using the initiated chemical deposition (iCVD) method. De-ionized water was used as the working fluid. Experiments were performed on horizontal and inclined surfaces (inclination angles of 10 deg, 30 deg, 50 deg, and 70 deg) under the constant heat flux (ranging from 10 to 80 kW/m2) boundary condition. Obtained results were compared to their plain surface counterparts, and heat transfer enhancements were observed. Accordingly, it was observed that the bubble departure phenomenon was affected by heat flux and wall superheat on bare silicon surfaces, while the supply path of vapor altered the bubble departure process on pHEMA-coated surfaces. Furthermore, the surface orientation played a major role on bubble dynamics and could be considered as a mechanism for fast vapor removal from surfaces. Bubble coalescence and liquid replenishment on coated surfaces had a promising effect on heat transfer coefficient enhancement on coated surfaces. For horizontal surfaces, a maximum enhancement of 25% relative to the bare surface was achieved, while the maximum enhancement was 105% for the inclined coated surface under the optimum condition. iCVD was proven to be a practical method for coating surfaces for boiling heat transfer applications due to the obtained promising results

Optimum ratio of hydrophobic to hydrophilic areas of biphilic surfaces in thermal fluid systems involving boiling

Pool boiling has a high heat removal capability and is considered as one of the most effective cooling methods due to utilization of latent heat of vaporization. Surface wettability plays a key role in boiling heat transfer since it controls the contact line between liquid, gas, and solid phases. Here, surfaces with mixed wettability (biphilic) were fabricated for assessing the effect of biphilic surfaces on bubble dynamics and boiling heat transfer as well as for the determination of an optimum hydrophobic area to the total surface area (A* = AHydrophobic/Atotal) to achieve the best heat transfer performance. Pool boiling experiments were conducted on biphilic surfaces with A* ranging from 0.19% to 95%. It was shown that biphilic surfaces directly affected both the critical heat flux (CHF) and boiling heat transfer. According to the experimental results, the surface with A* of 38.46% delivered the highest CHF enhancement (197 W/cm2, and maximum boiling heat transfer enhancement of 103%) among the tested biphilic surfaces. To represent a better understanding of related heat transfer mechanisms, bubble dynamics was obtained using a high-speed camera system. Visualization results revealed that bubble formation took place sooner on biphilic surfaces with A* of higher than 38.46%, thereby triggering the generation of vapor blanket on the surfaces and CHF occurrence at lower heat fluxes

Subcooled flow boiling heat transfer enhancement using polyperfluorodecylacrylate (pPFDA) coated microtubes with different coating thicknesses

In this study, enhanced subcooled boiling heat transfer was achieved at high mass fluxes by applying a new surface enhancement method. In this method, polyperfluorodecylacrylate (pPFDA) was applied on the inner walls of the 4 cm long stainless steel hypodermic microtubes with inner diameters of 889 and 600 mu m. Initiated chemical vapor deposition (iCVD) was employed for coating inner walls of the microtubes with different coating thicknesses of similar to 50 and similar to 160 nm. iCVD could serve for a surface deposition method for closed geometries like microtubes and offered a uniform coating. The experiments were performed at high mass fluxes of 6000, 7000, and 8000 kg/m(2) s with de-ionized (DI) water (as the coolant). The Joule heating method was used for applying heat to the test section, which was located at the end (the last 2 cm) of the microtube. Temperature measurements were done at the very end of the micro tubes. The experimental results indicated that pPFDA coated microtubes could significantly enhance flow boiling heat transfer. The largest heat transfer enhancement was achieved as 61% pertinent to the coated microtube of an inner diameter of 889 mu m with the coating thickness of 160 nm, at G = 8000 kg/m(2) s, relative to its bare surface counterpart (at the same heat flux). The coatings were proven to be reliable and reproducible by analyzing the coated microtubes after performing boiling experiments with the Raman spectroscopy method

Energy Harvesting in Microscale with Cavitating Flows

Energy harvesting from thermal energy has been widely exploited to achieve energy savings and clean technologies. In this research, a new cost-effective and environment-friendly solution is proposed for the growing individual energy needs thanks to the energy application of cavitating flows. With the aid of cavitating jet flows from microchannel configurations of different sizes, it is shown that significant temperature rise (as high as 5.7 °C) can be obtained on the surface of the thin plate. The obtained heat energy could be integrated to a thermoelectric power generator, which can be used as a power resource for consumer devices, such as cell phones and laptops. To explore the difference in the temperature rise with different microtube diameters, namely, 152, 256, 504, and 762 μm, and also with different upstream pressures of 10, 20, 40, and 60 bar, the cavitation flow patterns are captured and analyzed using an advanced high-speed visualization system. The analysis of the captured data showed that different flow patterns exist for different diameters of the microtubes, including a pattern shift from micro- to macroscale, which accompanied the pattern of temporal results very well

Hysteresis in cavitating flows within transparent microchips

Cavitation phenomenon has attracted much attention in engineering applications so the industry has provided considerable funding during recent years. Despite the simplicity and rather low price of small devices generating cavitation bubbles, the physics behind the creation and collapse of these bubble is still not well understood particularly in micro/nano scale. The assessment of size effects is vital for the design and development of new generation microfluidic devices involving phase change. Additionally, as the length scale decreases, surface nuclei dominate and dictate cavitation events. The modifications in the microchip geometry and enhancement in the micro device performance in applications involving cavitation will lead to increased cavitation bubbles number, reduced noise, improved bubbles collapse and increased energy sustainability. This study aims to investigate the creation of cavitation bubbles and classify the cavitating flow patterns in a novel roughened microchannel configuration. Cavitating flows are characterized in a transparent microchannel configuration in order to achieve a comprehensive understanding of cavitation inception and collapse in micro scale, which are crucial in the development of new-generation energy harvesting systems. In this device, a restrictive element and a big channel downstream of the restrictive element are mainly considered. The microchip consists of two main wafers, namely silicon and glass, which are anodically bonded together to withstand high pressures. The flow rate and discharge are evaluated at the outlet of the channels to characterize the chocking flow conditions in micro scale. The flow characteristics are determined to recognize differences in flow physics between smooth and roughened micro channels. Moreover, cavitation number, which is a major parameter for flow patterns, is considered in order to have valuable insights to the inception, development and collapse of the cavitation phenomenon in micro scale. Furthermore, the surface characteristics are also considered in detail in the microchip, and the effect of surface roughness on cavitating flows is investigated

Experimental and numerical investigation of inlet temperature effect on convective heat transfer of γ-Al2O3/water nanofluid flows in microtubes

Nanofluids are the combination of a base fluid with nanoparticles with sizes of 1-100 nm. In order to increase the heat transfer performance, nanoparticles with higher thermal conductivity compared to that of base fluid are introduced into the base fluid. Main parameters affecting single-phase and two-phase heat transfer of nanofluids are shape, material type and average diameter of nanoparticles, mass fraction and stability of nanoparticles, surface roughness and fluid inlet temperature. In this study, the effect of inlet temperature of deionized (DI) water/alumina (Al2O3) nanoparticle nanofluids was both experimentally and numerically investigated. Nanofluids with a mass fraction of 0.1% were tested inside a microtube having inner and outer diameters of 889 and 1067 micrometers, respectively, for hydrodynamically developed and thermally developing laminar flows at Reynolds numbers of 650, 1000, and 1300. According to the obtained numerical and experimental results, the inlet temperature effect was more pronounced for the thermally developing region. The performance enhancement with nanoparticles was obtained at rather higher Reynolds numbers and near the inlet of the microtube. There was a good agreement between the experimental and numerical results so that the numerical approach could be further implemented in future studies on nanofluid flows

Experimental studies on ferrofluid pool boiling in the presence of external magnetic force

The past decade has witnessed rapid advances in thermal-fluid applications involving nanoparticles due to existing heat transfer enhancements. The main challenges in working with nanoparticles are clustering, sedimentation and instability encountered in many studies. In this study, magnetically actuated Fe3O4 nanoparticles were coated with a fatty acid and dispersed inside a base fluid (water) in order to avoid clustering, sedimentation and instability as well as to improve the thermal performance. Boiling heat transfer characteristics of the ferrofluids were experimentally investigated with magnetic actuation and compared to the results without magnetic actuation. Nanoparticle mass fraction was the major parameter. Boiling heat transfer coefficient of the magnetically actuated system was found to be significantly higher compared to the case without magnetic actuation. The results showed that boiling heat transfer coefficient was not sensitive to the nanoparticle mass fraction