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Author: Michele David Publisher: ISBN: Category : Bubbles Languages : en Pages : 76
Book Description
Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface enhancement with increased surface area, varied geometry, wettability contrast and micro/nano-structures can further enhance boiling heat transfer performance through bubble nucleation augmentation. Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. Bi-functional surfaces for enhanced boiling heat transfer were fabricated and studied through investigation of bubble dynamics and pool boiling experiments. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of surfaces with distinct wettability regions. Bi-functional surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water at atmospheric pressure. Resulting boiling curves of enhanced surfaces showed up to 3X enhancement in heat transfer coefficients at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface. The surfaces show promising results for energy savings in two-phase change applications.
Author: Michele David Publisher: ISBN: Category : Bubbles Languages : en Pages : 76
Book Description
Boiling heat transfer is studied for its ability to dissipate high fluxes and achieve heat transfer coefficients two orders of magnitude greater than single-phase heat transfer systems. Heater surface enhancement with increased surface area, varied geometry, wettability contrast and micro/nano-structures can further enhance boiling heat transfer performance through bubble nucleation augmentation. Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. Bi-functional surfaces for enhanced boiling heat transfer were fabricated and studied through investigation of bubble dynamics and pool boiling experiments. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of surfaces with distinct wettability regions. Bi-functional surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. A pool boiling experimental facility has been designed and built to investigate nucleate pool boiling in water at atmospheric pressure. Resulting boiling curves of enhanced surfaces showed up to 3X enhancement in heat transfer coefficients at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface. The surfaces show promising results for energy savings in two-phase change applications.
Author: Yih-Yun Hsu Publisher: ISBN: Category : Ebullition Languages : en Pages : 54
Book Description
The ebullition cycle of nucleate pool boiling from a heating strip was studied through the use of high-speed motion pictures of schlieren and shadowgraph images. The effects of bubble agitation on thermal layer and neighboring bubbles were observed. An over-all model of ebullition based upon experimental observation was proposed that included an analytical method for predicting bubble growth rate and an analysis of the factors that influence the magnitude of the waiting period (time interval between bubbles at a particular site). Thermal-layer thickness, cavity size, and area of influence of a growing bubble are considered. A film supplement is made available. (Author).
Author: Samuel Siedel Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Since boiling heat transfer affords a very effective means to transfer heat, it is implemented in numerous technologies and industries ranging from large power generation plants to micro-electronic thermal management. Although having been a subject of research for several decades, an accurate prediction of boiling heat transfer is still challenging due to the complexity of the coupled mechanisms involved. It appears that the boiling heat transfer coefficient is intimately related to bubble dynamics (i.e. bubble nucleation, growth and detachment) as well as factors such as nucleation site density and interaction between neighbouring and successive bubbles. In order to contribute to the understanding of the boiling phenomenon, an experimental investigation of saturated pool boiling from a single or two neighbouring artificial nucleation sites on a polished copper surface has been performed. The bubble growth dynamics has been characterized for different wall superheats and a experimental growth law has been established. The interaction between successive bubbles from the same nucleation site has been studied, showing the bubble shape oscillations that can be caused by these interactions. The forces acting on a growing bubble has been reviewed, and a complete momentum balance has been made for all stages of bubble growth. The curvature along the interface has been measured, and indications concerning the mechanism of bubble detachment have been suggested. The rise of bubble after detachment has been investigated, and the maximum velocity reached before a change of direction has been estimated and compared to existing models from the literature. The interaction between bubbles growing side by side has been studied: the generation and propagation of a wave front during the coalescence of two bubbles has been highlighted. As boiling heat transfer enhancement techniques are being imagined and developed, this study also focuses on the electrohydrodynamic enhancement technique. Boiling experiments have been performed in the presence of electric fields, and their effects on heat transfer and bubble dynamics have been characterized. Although the volume of the bubbles at detachment and the relationship between the bubble frequency and the wall superheat were not affected, the bubble growth curve was modified. The bubbles were elongated in the direction of the electric field, and this elongation was estimated and compared to other studies from the literature. The rising velocity of the bubble was reduced in the presence of electric field, and the behaviour of bubbles growing side by side was modified, the electric field causing the bubbles to repeal each other. These results, obtained in a fully controlled environment, provide compelling evidence that electric fields can be implemented to alter the bubble dynamics and subsequently heat transfer rates during boiling of dielectric fluids.
Author: Jian-Fu Zhao Publisher: ISBN: Category : Biotechnology Languages : en Pages : 19
Book Description
A series of experimental studies on bubble dynamical behaviors and heat transfer in pool boiling on thin wires in different gravity conditions have been performed in the past years, including experiments in long-term microgravity aboard the 22nd Chinese recoverable satellite RS-22, in short-term microgravity in the drop tower Beijing, and in normal gravity on the ground. Steady pool boiling of degassed R113 on thin platinum wires has been studied using a temperature-controlled heating method. A voltage-controlled heating method has also been used in normal gravity. A slight enhancement of nucleate boiling heat transfer is observed in microgravity, while dramatic changes of bubble behaviors are very evident. Considering the influence of the Marangoni effects, the different characteristics of bubble behaviors in microgravity have been explained. A new bubble departure model including the influence of the Marangoni effects has also been proposed, which can predict the whole observation both in microgravity and in normal gravity. The value of CHF (critical heat flux) in microgravity is lower than that in normal gravity, but it can be predicted well by the Lienhard-Dhir correlation, although the dimensionless radius, or the square root of the Bond number, in the present case is far beyond its initial application range. A further revisit on the scaling of CHF with heater radius in normal gravity, which is focused on the case of a small Bond number, has also been performed in our laboratory using different kinds of working fluids at different subcooling conditions. Interactions between the influences of the subcooling and heater radius will be important for the case of a small Bond number. In addition to the Bond number, there may exist some other parameters, which may be material-dependent, that play important roles in the CHF phenomenon with a small Bond number.
Author: Shailesh Malla Publisher: ISBN: Category : Heat Languages : en Pages : 124
Book Description
The present work consists of two major studies. The first study investigates the effects of surface energy or wettability on nucleate pool boiling and the second study investigates the thin-film evaporative cooling for near junction thermal management. For the first study, effects of surface energy or wettability on critical heat flux (CHF) and boiling heat transfer (BHT) of smooth heated surfaces was studied in saturated pool boiling of water at 1 atm. For this purpose hydrophilic and hydrophobic surfaces were created on one side of 1cm x 1cm double-side polished silicon substrates. A resistive heating layer was applied on the opposite side of each substrate. The surface energies of the created surfaces were characterized by measuring the static contact angles of water sessile drops. To provide a wide range of surface energies, surfaces were made of Teflon (hydrophobic), bare silicon (hydrophilic) and aluminum oxide (most hydrophilic). The measured contact angles on these surfaces were ~108, ~57 and ~13 degrees respectively. The results of pool boiling tests on these surfaces clearly illustrate the connection between surface energy and CHF. CHF was shown to linearly decrease with contact angle increase, from ~125 W/cm2 on aluminum oxide (most hydrophilic) to nearly one tenth of this value on Teflon (hydrophobic). The most hydrophilic surface also produced increasingly better BHT than plain silicon and Teflon as heat flux increased. However, below ~5 W/cm2 the hydrophobic surface demonstrated better heat transfer due to earlier onset of nucleate boiling, reducing surface superheats by up to ~5 degrees relative to the other two surfaces. Above ~5 W/cm2 the BHT of the hydrophobic surface rapidly deteriorated as superheat increased towards the value at CHF. To further understand the effect of surface energy on pool boiling performance, the growth and departure of bubbles from single nucleating sites on each surface were analyzed from high-speed video recordings. A distinct bubble behavior was observed in the hydrophobic surface where bubble growth and departure period was extremely long compared to plain silicon and aluminum oxide surfaces. This study also investigated the performance of thin-film evaporative cooling for near-junction thermal management. A liquid delivery system capable of delivering water in small volumes ranging 20~75 nl at frequencies of up to 600 Hz was established. On one side of the silicon chip, a resistive heating layer of 2 mm x 2 mm was fabricated to emulate the high heat flux hot-spot, and on the other side a superhydrophilic nanoporous coating (SHNC) was applied over an area of 1 cm x 1 cm. With the aid of the nanoporous coating, delivered droplets spread into thin films of thicknesses less than 10[mu]m. With this system, evaporative tests were conducted in ambient in an effort to maximize dryout heat flux and evaporative heat transfer coefficient. During the tests, heat flux at the hot spot was varied to values above 1000 W/cm2. Water was delivered at either given constant frequency (constant mass flow rate) or at programmed variations of frequency (variable mass flow rate), for a given nanoliter dose volume. Heat flux and hot spot surface temperatures were recorded upon reaching steady state at each applied heat flux increment. Relative to bare silicon surface, dryout heat flux of the SHNC surface was found to increase by ~5 times at 500~600 Hz. Tests were also conducted at various system pressures and temperatures in a micro-gap to emulate the actual embedded thermal management system. The micro-gap was made by positioning a top cover plate 500 [mu]m above the test surface. System temperature did not influence the hotspot temperature. This was due to the formation of near saturation temperature inside the micro-gap for all cases as a result of vapor accumulation. Increase in system pressure increased the hotspot temperature. At 1500 W/cm2, hotspot temperature increased by 6 C and 24 C by increasing the system pressure by 7.32 and 14.7 psi respectively. This was due to increase in saturation point as a result of increase in pressure. On the SHNC surface a mixed mode of heat transfer comprising of thin-film boiling and thin-film evaporation was observed particularly at moderate heat flux (~700 W/cm2). To further enhance the heat transfer coefficient, aluminum microporous coating was developed that increased the number of nucleation sites for thin-film boiling and also maintained the wettability for thin-film evaporation at higher heat fluxes. Test results showed a marginal improvement in dry-out heat flux compared to SHNC, however, significant reduction was achieved in hot-spot temperature at all heat flux levels. A net reduction of ~ 58oC was obtained at ~1600 W/cm2 by using aluminum based microporous coating.
Author: Michele David
Author: Michele David
Author: Thomas H. Cochran
Author: Yih-Yun Hsu
Author: Samuel Siedel
Author: R. A. Rockow
Author: 
Author: Edward G. Keshock
Author: Jian-Fu Zhao
Author: Shailesh Malla
Author: Kurt Walter Forster