Study of Arterial Gas Emboli

In an attempt to better understand the behavior of gas emboli, a bench top vascular bifurcation model was designed and manufactured. The model represented physiological parameters of arteries and arterioles, resulting in an accurate portrayal of likely bubble behavior in the body. To further achieve realistic properties, a 60% glycerin-water solution was prepared to match relevant physiological Reynolds and Capillary numbers. By investigating various flow rates, roll angles, and bubble geometries, the findings indicate that all of these factors influence the emboli transport

Three-Dimensional Liquid-Vapor Interface Reconstruction from High-Speed Stereo Images during Pool Boiling

A technique for reconstruction of liquid-gas interfaces based on high-speed stereo-imaging is applied to the liquid-vapor interfaces formed above a heated surface during pool boiling. Template matching is used for determining the correspondence of local features of the liquid-vapor interfaces between the two camera views. A sampling grid is overlaid on the reference image, and windows centered at each sampled pixel are compared with windows centered along the epipolar line in the target image to obtain a correlation signal. The three-dimensional coordinates of each matched pixel are determined via triangulation, which yields the physical world representation of the liquid-vapor interface. Liquid-vapor interface reconstruction is demonstrated during pool boiling for a range of heat fluxes. Textured mushroom-like vapor bubbles that are fed by multiple nucleation sites are formed close to the heated surface. Analysis of the temporal attributes of the interface distinguishes the transition with increasing heat flux from a mode in which vapor is released from the surface as a continuous plume to one dominated by the occurrence of intermittent vapor bursts. A characteristic morphology of the vapor mushroom formed during vapor burst events is identified. This liquid-vapor interface reconstruction technique is a time-resolved, flexible and non-invasive alternative to existing methods for phase-distribution mapping, and can be combined with other opticalbased diagnostic tools, such as tomographic particle image velocimetry. Vapor flow morphology characterization during pool boiling at high heat fluxes can be used to inform vapor removal strategies that delay the occurrence of critical heat flux during pool boiling

Adipic Acid Sonocrystallization in Continuous Flow Microchannels

Crystallization is widely employed in the manufacture of pharmaceuticals during the intermediate and final stages of purification and separation. The process defines drug chemical purity and physical properties: crystal morphology, size distribution, habit and degree of perfection. Particulate pharmaceuticals are typically manufactured in conventional batch stirred tank crystallizers that are still inadequate with regard to process controllability and reproducibility of the final crystalline product. Variations in crystal characteristics are responsible for a wide range of pharmaceutical formulation problems, related for instance to bioavailability and the chemical and physical stability of drugs in their final dosage forms. This thesis explores the design of a novel crystallization approach which combines in an integrated unit continuous flow, microreactor technology, and ultrasound engineering. By exploiting the various benefits deriving from each technology, the thesis focuses on the experimental characterization of two different nucleation systems: a droplet-based system and a single-phase system. In the former, channel fouling is avoided using a carrier fluid to segment the crystallizing solution in droplets, thus avoiding the contact with the walls. In the latter channel blockage is prevented using larger channel geometries and employing higher flow rates. The flexibility of the developed setup also allows performing stochastic nucleation studies to estimate the nucleation kinetics under silent and sonicated conditions. The experiments reveal that very high nucleation rates, small crystal sizes, narrow size distributions and high crystal yields can be obtained with both setups when the crystallizing solution is exposed to high pressure field as compared to silent condition. It is concluded that transient cavitation of bubbles and its consequences are a significant mechanism for enhancing nucleation of crystals among several proposed in the literature. A preliminary study towards the development and design of a growth stage is finally performed. Flow pulsation is identified as a potential method to enhance radial mixing and narrow residence time distribution therefore achieving optimal conditions for uniform crystal growth. The results suggest that increasing values of Strouhal number as well as amplitude ratio improve axial dispersion. Helically coiled tubes are identified as potential structures to further improve fluid dynamic dispersion

Surface engineering for phase change heat transfer: A review

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoverie

非金属粒子を縣濁させた水ベースナノ流体のプール沸騰熱伝達に関する研究

Heat transfer characteristics in boiling systems are significantly important, especially in high-density cooling, for instance, the application of In-Vessel Retention (IVR) during the Loss of Coolant Accident (LOCA) in a Nuclear Reactor. In the present study, the performance of nanofluids in a boiling system, namely, non-metallic water-based nano fluids has been explored. The parametric effects of the nanofluids in nucleate pool boiling with various configurations have been tested. Three main experimental setups were prepared separately to investigate the effect of heater orientations:material, concentration, and dispersion as well as heat flux density, respectively. Additional research was performed by using a separate experimental apparatus in order to elucidate a possible nucleate boiling mechanism occurring in nanofluids. Conclusively, the Critical Heat Flux(CHF) was improved significantly in the nanofluid nucleate boiling, compared to pure water. The orientation effects showed similar magnitudes of enhancement, up to 200 percent in both upward-facing and downward-facing heaters. Several parameters related to the CHF enhancement rate, such as concentration and boiling time in nanofluids, were simultaneously investigated. The CHF enhancement rates are considerably high in a higher concentration of TiO2 nanofluid and vice versa. In addition, the CHF enhancement for the downward-facing heater orientation is only half of that for the upward-ward facing heater. Surface wettability measurements were also being conducted to explore the relationship between surface properties and the CHF enhancement. Separately, the effects of nanoparticle materials, concentrations and dispersion conditions on the heat transfer coefficient and CHF were elucidated. The boiling heat transfer characteristics observed were significantly different depending on the nanoparticles’material as well as on the difference in the concentration. The higher concentration of TiO2 and Al2O3 showed higher heat transfer enhancements (except for the low concentration of TiO2), whereas for SiO2 the heat transfer deteriorated for all concentrations in the time-variation of wall superheat. However, no noticeable effects of the dispersion condition was observed. Some peculiar boiling curves (BCs) were observed in TiO2and SiO2 at the high heat flux compared to the simple BCs in Al2O3. The CHF enhancement was found to be within the range of 1.7 up to 2.1 MW/m2 for all materials. The effects of different heat flux density on the CHF enhancements were also investigated. The enhancement rate of CHF greatly depended on the heat flux density; the heat flux at the higher densities had shown considerably higher CHF enhancements rate to compare to lower heat flux density. The CHF enhancement still did not reach the asymptotic CHF value after boiling for 1 hour at the lowest heat flux in the present experimental investigation. Both the dimensionless CHF enhancement value respective to the dimensionless heat flux, concentration and boiling time were correlated. The trend showed a linearity in the high heat flux, especially for 450 and 600 kW/m2. Nevertheless, for lower heat flux, non-linear trends were observed especially at heat flux densities of 300 kW/m2 and more obvious at 150 kW/m2.In conclusion, nanofluids showed an enhanced CHF both for upward-facing and downward-facing conditions. However, the heat transfer characteristic (HTC) performances was stochastic depending on materials and concentration of nanofluids, and nearly no noticeable dispersion condition was observed. The heat flux density affected the rate of CHF enhancements, where the high heat flux resulted in high enhancement rates, but nominal enhancements in the lowest heat flux.　非金属ナノ粒子を縣濁させた水ベースのナノ流体中における沸騰伝熱特性を系統的に調べた。3種類の実験装置を用いて、伝熱面姿勢、ナノ粒子材料、ナノ粒子濃度、ナノ粒子分散状態、ナノ粒子層形成時の熱流束の影響を検討した。伝熱面姿勢としては、上向き面と下向き面で実験を実施し、限界熱流束(CHF)の絶対値は伝熱面姿勢によって異なるが、いずれの条件においてもナノ流体中のCHFは純水中の値の約2倍となることを示した。次に、ナノ流体中の沸騰熱伝達率は、ナノ粒子の材質および濃度により大きく異なり得ることを示した。ナノ粒子の材質として、本研究ではTiO2、Al2O3、SiO2を使用したが、Al2O3では伝熱促進、SiO2では伝熱劣化が生じるのに対して、TiO2では低粒子濃度で劣化、高粒子濃度で向上する結果となった。一方、ナノ流体中における粒子の分散状況は、本実験で調べた範囲内において、沸騰熱伝達に及ぼす影響は顕著ではなかった。また、各実験条件で沸騰曲線を描いたところ、Al2O3ナノ流体では水の場合と類似の沸騰曲線が得られたのに対して、TiO2とSiO2では、高熱流束条件で壁面過熱度が大きく増加するという独特の振る舞いを呈する場合があった。ただし、計測されたCHF値は1.7～2.1MW/m2の範囲にあり、純水中のCHFよりも顕著に増大するものの、ナノ粒子の材質、濃度、分散状態による明確な影響は認められなかった。これに対して、伝熱面上にナノ粒子層を形成する際の熱流束は、CHF値に多大な影響を及ぼした。すなわち、高熱流束条件では、ナノ粒子層を形成する際の沸騰時間が短くても顕著なCHF増大を実現できるのに対して、低熱流束条件では十分なCHF向上を達成する伝熱面状態とするのにきわめて長いナノ粒子層形成時間を要した。特に、本研究で用いた最低熱流束条件では、ナノ流体中で沸騰状態を1時間継続した場合でも、十分なCHF向上効果を発現するには至らなかった。本研究では、純水中にナノ粒子を添加した後の熱伝達率の時間変化を様々な条件で調べたが、ナノ粒子の添加直後では、熱伝達率が向上する場合が多かった。そこで、ナノ流体中における熱伝達率変化のメカニズムについて知見を得るため、透明容器を用いた可視化実験を実施して、ナノ粒子天下の前後における沸騰気泡の生成状況の差異を検討した。この結果、ナノ粒子を加えた直後、より多数の発泡核で気泡生成が生じることが観察された。これより、伝熱面上にナノ粒子層が形成される際に、そのいくつかの部分が気泡生成核となり、核沸騰熱伝達の促進に寄与することを示した。電気通信大学201

Response to fire of pressure vessels for the storage and transportation of hazardous materials

Pressure vessels present a serious hazard when exposed to fires. Boiling liquid expanding vapour explosions (BLEVEs) and other associated vessel failure events are known to have significant consequences, and be key propagators of industrial accident scenarios. Understanding the response of pressure vessels to fire, and especially the rate of pressurisation, remains a significant challenge. This study reviews the ability of existing models to capture the physical processes that drive vessel pressurisation, and the existing fire test evidence used to validate such models. The scope of existing test evidence is found to be inadequate to validate complex numerical models. This study defines and describes a set of test conditions and a novel piece of experimental apparatus that can provide detailed and reproducible test evidence in an economic manner for the purpose of numerical model validation. The equipment included a 2.6 m3 vessel with a full cross-section (Ø1 m) glass window. A pressure compensation system maintained the window integrity, allowing combined temperature and velocity field measurements, using thermocouples and particle imaging velocimetry (PIV), to be made during fire exposure. Initial studies using ANSYS CFX indicate that the Eulerian-Eulerian multiphase model and the RPI wall boiling model are capable, when used together, of providing a good basis for simulation of the pressurisation rate, given the use of appropriate bubble-related parameters obtained by experiment

Membraneless Electrolyzers for Solar Fuels Production

Solar energy has the potential to meet all of society’s energy demands, but challenges remain in storing it for times when the sun is not shining. Electrolysis is a promising means of energy storage which applies solar-derived electricity to drive the production of chemical fuels. These so-called solar fuels, such as hydrogen gas produced from water electrolysis, can be fed back to the grid for electricity generation or used directly as a fuel in the transportation sector. Solar fuels can be generated by coupling a photovoltaic (PV) cell to an electrolyzer, or by directly converting light to chemical energy using a photoelectrochemical cell (PEC). Presently, both PV-electrolyzers and PECs have prohibitively high capital costs which prevent them from generating hydrogen at competitive prices. This dissertation explores the design of membraneless electrolyzers and PECs in order to simplify their design and decrease their overall capital costs. A membraneless water electrolyzer can operate with as few as three components: A cathode for the hydrogen evolution reaction, an anode for the oxygen evolution reaction, and a chassis for managing the flows of a liquid electrolyte and the product gas streams. Absent from this device is an ionically conducting membrane, a key component in a conventional polymer electrolyte membrane (PEM) electrolyzer that typically serves as a physical barrier for separating product gases generated at the anode and cathode. These membranes can allow for compact and efficient electrolyzer designs, but are prone to degradation and failure if exposed to impurities in the electrolyte. A membraneless electrolyzer has the opportunity to reduce capital costs and operate in non-pristine environments, but little is known about the performance limitations and design rules that govern operation of membraneless electrolyzers. These design rules require a thorough understanding of the thermodynamics, kinetics, and transport processes in electrochemical systems. In Chapter 2, these concepts are reviewed and a framework is provided to guide the continuum scale modeling of the performance of membraneless electrochemical cells. Afterwards, three different studies are presented which combine experiment and theory to demonstrate the mechanisms of product transport and efficiency loss. Chapter 3 investigates the dynamics of hydrogen bubbles during operation of a membraneless electrolyzer, which can strongly affect the product purity of the collected hydrogen. High-speed video imaging was implemented to quantify the size and position of hydrogen gas bubbles as they detach from porous mesh electrodes. The total hydrogen detected was compared to the theoretical value predicted by Faraday’s law. This analysis confirmed that not all electrochemically generated hydrogen enters the gas phase at the cathode surface. In fact, significant quantities of hydrogen remain dissolved in solution, and can result in lower product collection efficiencies. Differences in bubble volume fraction evolved along the length of the cathode reflect differences in the local current densities, and were found to be in agreement with the primary current distribution. Overall, this study demonstrates the ability to use in-situ HSV to quantitatively evaluate key performance metrics of membraneless electrolyzers in a non-invasive manner. This technique can be of great value for future experiments, where statistical analysis of bubble sizes and positions can provide information on how to collect hydrogen at maximum purity. Chapter 4 presents an electrode design where selective placement of the electrocatalyst is shown to enhance the purity of hydrogen collected. These “asymmetric electrodes” were prepared by coating only one planar face of a porous titanium mesh electrode with platinum electrocatalyst. For an opposing pair of electrodes, the platinum coated surface faces outwards such that the electrochemically generated bubbles nucleate and grow on the outside while ions conduct through the void spacing in the mesh and across the inter-electrode gap. A key metric used in evaluating the performance of membraneless electrolyzers is the hydrogen cross-over percentage, which is defined as the fraction of electrochemically generated hydrogen that is collected in the headspace over the oxygen-evolving anode. When compared to the performance of symmetric electrodes – electrodes coated on both faces with platinum – the asymmetric electrodes demonstrated significantly lower rates of cross-over. With optimization, asymmetric electrodes were able to achieve hydrogen cross-over values as low as 1%. These electrodes were then incorporated into a floating photovoltaic electrolysis device for a direct demonstration of solar driven electrolysis. The assembled “solar fuels rig” was allowed to float in a reservoir of 0.5 M sulfuric acid under a light source calibrated to simulate sunlight, and a solar to hydrogen efficiency of 5.3% was observed. In Chapter 5, the design principles for membraneless electrolyzers were applied to a photoelectrochemical (PEC) cell. Whereas an electrolyzer is externally powered by electricity, a PEC cell can directly harvest light to drive an electrochemical reaction. The PEC reactor was based on a parallel plate design, where the current was demonstrated to be limited by the intensity of light and the concentration of the electrolyte. By increasing the average flow rate of the electrolyte, mass transport limitations could be alleviated. The limiting current density was compared to theoretical values based off of the solution to a convection-diffusion problem. This modeled solution was used to predict the limitations to PEC performance in scaled up designs, where solar concentration mirrors could increase the total current density. The mass transport limitations of a PEC flow cell are also highly relevant to the study of CO2 reduction, where the solubility limit of CO2 in aqueous electrolyte can also limit performance