Pressure drop across micro-pin heat sinks under boiling conditions

Two-phase pressure drop was studied in four different micro pin fin heat sinks. Micro pin fin heat sinks used in the current studies were operated under boiling conditions using water and R-123 as working fluids. It was observed that once boiling was initiated severe temperature fluctuations and flow oscillations were recorded for three of the micro pin fin heat sinks, which was characterized as unstable boiling. Pressure drop signals were presented just before and after the unstable boiling conditions. Flow images and FFT (fast Fourier Transform) profiles of pressure signals were used to explain experimental results and unstable nature in flow boiling observed in the three of the devices. Stable boiling conditions where the temperature and pressure drop had a steady and stable profile could be only obtained from one micro pin fin heat sink at high mass velocities. The two-phase pressure drop in this hydrofoil-based micro pin fin heat sink has been investigated using R-123 as the working fluid. Two-phase frictional multipliers have been obtained over mass fluxes from 976 to 2349 kg/m2. It has been found that the two-phase frictional multiplier is strongly dependent on flow pattern. The theoretical prediction using Martinelli parameter based on the laminar fluid and laminar gas flow represented the experimental data fairly well for the spray-annular flow. For the bubbly and wavy-intermittent flow, however, large deviations from the experimental data were recorded. The Martinelli parameter was used successfully to determine the flow patterns, which were bubbly, wavy-intermittent, and spray-annular flow in the current study

Localized radiative energy transfer from a plasmonic bow-tie nanoantenna to a magnetic thin film stack

Localized radiative energy transfer from a near-field emitter to a magnetic thin film structure is investigated. A magnetic thin film stack is placed in the near-field of the plasmonic nanoantenna to utilize the evanescent mode coupling between the nanoantenna and magnetic thin film stack. A bow-tie nano-optical antenna is excited with a tightly focused beam of light to improve near-field radiative energy transfer from the antenna to the magnetic thin film structure. A tightly focused incident optical beam with a wide angular spectrum is formulated using Richards-Wolf vector field equations. Radiative energy transfer is investigated using a frequency domain 3-D finite element method solution of Maxwell’s equations. Localized radiative energy transfer between the near-field emitter and the magnetic thin film structure is quantified for a given optical laser power at various distances between the near-field emitter and magnetic thin film

Thermoelectric-coupled hydrodynamic cavitation energy harvesting system

The ever growing energy demand has led to the advent of different energy harvesting systems. This study investigates the performance of a thermoelectric coupled hydrodynamic cavitation system as an energy harvesting device. The effect of changing the working fluid from water to Titania-water nanofluid on the heat generation of the cavitation system is discussed in this study and also the coupling of the cavitation system with one of the micro thermoelectric generators in the literature is included. At the end, the device performance is quantified by comparing its power generation with the required power for the daily used miniature electrical devices

Changing bubble dynamics in subcooled boiling with TiO₂ nanoparticles on a platinum wire

Bubble dynamics in pool boiling provides a suitable platform for researchers to understand the mechanisms of subcooled boiling heat transfer. Besides, the effects of nanoparticles on this phenomenon have not been fully understood yet. In this study, the effect of TiO2 nanoparticles (with two weight fractions, 0.002% and 0.005%) mixed in de-ionized water during subcooled pool boiling on a thin platinum wire with a diameter of 250 μm was experimentally investigated for working bulk fluid temperatures between 30 and 50 °C at atmospheric pressure, and new bubble dynamics phenomena were reported. Applied heat fluxes varied from onset of nucleate boiling point to higher heat fluxes up to nucleation jet flow. The experiments were visualized with a high speed camera system, and acquired videos and images were utilized for analysing prevalent phenomena, such as bubble-bubble and nucleation site-bubble interactions, surface tension and Marangoni convection, structural disjoining pressure, pinning and nanoparticles deposition effects as well as the main mechanisms. In general, migration, coalescence, leaping and detaching were recorded for nanofluids with weight fractions of 0.002 wt% and 0.005 wt%, while oscillation, dancing and stick processes were exclusively observed only in nanofluids with a weight fraction of 0.005 wt%. The images, results, and related discussion provide new knowledge and physics for pool boiling phenomena on platinum fine wires in the presence of nanoparticles

On Bubble Dynamics in Subcooled Nucleate Boiling on a Platinum Wire

While much attention has been given to the phenomenon of boiling, the underlying physics and mechanisms are still not fully understood due to its complicated and illusive nature. In this study, an experimental investigation of subcooled pool boiling of de-ionized (DI) water, at a bulk temperature of 30 °C and atmospheric pressure, was performed on a platinum wire with a diameter of 250 μm, starting from the onset of nucleate boiling (ONB) point and rising to higher heat fluxes. A high speed camera system was utilized for visualization. Images of bubbles growth, migration, coalescence, leaping and detachment were recorded. Each of these events was investigated separately, and a comprehensive discussion undertaken. High speed camera images were analyzed by taking the most important parameters, such as surface tension, bubble-bubble interactions, nucleation site-bubble interactions, Marangoni convection and accumulation of non-condensable gases inside the bubbles. Lower applied heat fluxes led to bubble nucleation from the nucleation sites and slow growth of bubbles to a certain diameter, which were ready to migrate along the wire. Increasing the heat flux made the bubbles grow faster and favored coalescence, leaping and detaching that be observed one after another due to an increase in the temperature gradient and perturbation of the temperature field near the nucleation sites. Another important mechanism considered was the momentum of the non-condensable gases inside the bubbles, especially while the bubble tail disappeared and the bubble shrank whilst stopping near an immobile bubble or near the nucleation sites

Biphilic Functional Surfaces for Frost Prevention andEfficient Active Defrosting

The present work addresses the systematic accurate fabrication and design of biphilic surfaces having superhydrophobic circular islands surrounded by a hydrophilic background by investigating their condensation frosting and defrosting behavior. A significant delay in frost formation is observed on samples with higher superhydrophobicity ratio A*, defined as superhydrophobic area to total area ratio. As the superhydrophobic island diameter D increases from D = 500 µm to D = 700 µm (A* from 19.62% to 38.46%), a 50% improvement/delay is observed in terms of frost formation or densification. Besides delaying icing/frosting, the presence of superhydrophobic areas empowers the formation of porous and nonuniform frost structure, which facilitates ice removal during the defrosting process. To this end, as the surface is recovered the ambient temperature, almost complete passive cleaning performance within only 23 s is observed on the biphilic design having superhydrophobic islands with the diameter of D = 500 µm, that is, a superhydrophobicity ratio A* of 19.62%. This work concludes on the optimum biphilic ratio, which is not only effective as a passive method by hindering frosting but also leads to a slush/water free surface after defrosting eased by the Laplace pressure gradient which is imposed by the different biphilic wettability patterns

Upcycled graphene nanoplatelets integrated fiber-based Janus membranes for enhanced solar-driven interfacial steam generation †

The increasing demand for drinking water and environmental concerns related to fossil fuels have given rise to the use of solar energy in water desalination. Solar-driven interfacial steam generation is a promising method for water purification, particularly in remote areas. Janus membranes, featuring bilayer hydrophobic/hydrophilic structures, offer high functionality and have attracted significant interest in this field. This study explores the integration of novel graphene nanoplatelets (GNP) derived from waste tire pyrolysis through upcycling as a photothermal source in Janus membranes. The membranes consist of polyacrylonitrile (PAN) nanofibrous membranes for water supply and polymethyl methacrylate (PMMA)/graphene nanoplatelets (GNP) nanofibrous membranes for light harvesting. The effects of GNP content and layer thicknesses on photothermal activity, water transport, and overall evaporation rate were analyzed experimentally and numerically. The results showed that a decrease in membrane thickness led to a 19% to 63% enhancement in evaporation rate, highlighting the importance of optimizing membrane design for efficient water desalination

Investigation of Single Air Bubble Dynamics and the Effect of Nanoparticles in Rectangular Minichannels

Bubble dynamics and understanding related mechanisms based on force analysis are necessary for better understanding two-phase flow phenomena in small channels. To address this subject, experiments were conducted with injected single air bubbles into rectangular minichannels containing flows of pure water, pure ethanol and TiO2-nanoparticle-based nanofluids, which had a nanoparticle mass fraction of 0.005 wt% for both water and ethanol base fluids. For a range of fluid flow rates, bubble movement and temperature profiles were captured along the channel using high-speed and infrared (IR) cameras, respectively. Upon heating, when using nanofluids, deposition of TiO2 nanoparticles occurred. The results in the channels with cross sectional dimensions of 2 mm × 4 mm and heated length of 7 cm were compared with their counterparts on plain surfaces. Heat fluxes were applied by means of a tantalum film heater on the outer surface of the channel. Bubble dynamics and forces acting on the bubbles were quantitatively analyzed in relation to the fluid type, heat flux, flow rate and deposition. This study highlights the effects of TiO2-nanoparticles (dispersed in two different base fluids) on single-bubble dynamics in minichannels. The nanoparticle deposition was found to have a retarding effect on the bubble movement and led to a more elliptical shape rather than a spherical bubble shape. The bubble behavior is comprehensively assessed in the light of the visualization data and acting forces

Examining the Entrepreneur University Model in the Context of Higher Education System of Turkey: A Qualitative Study

Yükseköğretimde yaşanan küreselleşme, kitleselleşme, profesyonel yönetim, hesap verebilirlik akımları ve yükseköğretime yönelik artan talebe dayalı olarak şirket üniversitesi, sanal üniversite ve küçük üniversite gibi farklı üniversite modelleri kavramsal düzeyde tartışılmaktadır. Bu noktada tartışma konusu edilen modellerden biri de girişimci üniversite modelidir. Girişimci üniversiteler gerek yapı, işleyiş, yönetim ve finansman gibi üniversitenin temel boyutları gerekse öğretim, araştırma ve topluma hizmet gibi üniversitenin temel amaçları noktasında diğer üniversite modellerinden ayrışan birtakım özelliklere sahiptir. Ancak girişimci üniversite kavramına ilişkin çalışmalar genellikle kavramsal düzeyde kalmaktadır. Girişimci üniversitenin ne olduğu, olumlu ve olumsuz yanları ve diğer üniversite modellerinden farklı yanları ile ilişkili çalışmalar oldukça sınırlıdır. Bununla birlikte, girişimci üniversite modelinin Türkiye’de yükseköğretim sistemi içinde nasıl bir görünüm arz ettiği ve Türk eğitim sistemine nasıl katkı sunabileceği konusunda daha çok inceleme yapılması gerekmektedir. Bu itibarla alanyazına katkı sağlamaya yönelik akademik bir çaba olarak bu çalışma, girişimci üniversite modelinin temel özelliklerini, olumlu ve olumsuz yanlarını, diğer üniversitelerden ayrışan yönlerinin neler olduğunu ve Türk yükseköğretim sistemine nasıl katkı sunabileceğini öğretim üyelerinin görüşlerine dayalı olarak ortaya koymayı amaçlamaktadır. Nitel araştırma yöntemi ve temel betimleyici desende tasarlanan araştırmanın katılımcılarının belirlenmesinde amaçlı örnekleme yöntemlerinden ölçüt örnekleme kullanılmıştır. Buna göre, araştırmanın katılımcılarının belirlenmesinde kullanılacak ölçüt, genelde yükseköğretim sistemi özelde ise farklı üniversite modelleri ve girişimci üniversite modeli üzerine çalışmalar yapmış; seminer, kongre ya da konferans gibi çeşitli etkinliklere iştirak etmiş olmaktır. Araştırmanın çalışma grubu, yukarıda belirlenen ölçüte uyan altı ö ğretim üyesinden oluşmaktadır. Çalışmanın verileri, yazarlar tarafından ilgili alanyazın ve araştırmanın amaçları doğrultusunda hazırlanan yarıyapılandırılmış bir görüşme formuyla toplanmış; içerik analizi tekniği kullanılarak ve tematik bir yöntem izlenmek suretiyle çözümlenmiştir.Depending upon the globalization, massification, professional management, accountability and increasing amount of higher education demands, such university models as online and small-sized have been discussed theoretically in the higher education field. In this regard, one of the university models which has been discussed in the related literature is entrepreneur university. Entrepreneur universities have a line of distinct features in terms of its structure, functioning, management and finance as the fundamental processes of a university and of teaching, research and community services as basic purposes of a university. However, research on entrepreneur university focused mainly on examining its theoretical background. Research focusing on discovering its meaning, its advantages and disadvantages or its basic qualities that distinguish it from other models is scarce. Furthermore, research is needed to find out how entrepreneur universities may function and how they may well contribute to the higher education system of Turkey. As an academic effort and attempt to fill this gap in the higher education literature, therefore, this study aims at revealing the basic features of entrepreneur university, its advantageous and disadvantageous aspects, the points that separate it from other university models and its possible contributions to the Turkey’s higher education system according to the opinions of faculty members. The participants of this qualitative study designed in descriptive model were selected through criterion under purposeful sampling method. The criteria determined to select the participant of the current study is to be faculty member who has conducted studies mainly in higher education area and specifically on various university models including entrepreneur universities; and who has participated in various seminars, conferences, symposiums on university models. Therefore, the participants of the study included six faculty members that meet the criteria described above. The data of the study were gathered through a semi-structured interview form developed by the researchers depending on the related literature and the purposes of the present study and the data were analysed through content analysis and by following a thematic method