Download or read book Investigation of Nucleation and Heat Transfer During Subcooled Flow Boiling on Augmented Surfaces written by Matthew J. Howell and published by . This book was released on 1996 with total page 338 pages. Available in PDF, EPUB and Kindle. Book excerpt: "Heat transfer characteristics of flat augmented surfaces in subcooled flow boiling with water at atmospheric pressure were obtained as functions of the surface orientation and Reynolds number. Data was collected with the heater surface temperature between 90C and 1 15C. The maximum temperature recorded by the thermocouple closest to the heater was between 200C and 220C. Five augmented surfaces and a base line plain polished aluminum surface were prepared and their performance was experimentally obtained in a 3 mm x 40 mm rectangular channel. The five augmented surfaces consisted of a sintered plain surface, microfin #1 (fin height- 132 mm, pitch - 635 mm, fin shape - round), sintered microfin #1, microfin #2 (fin height - 197 mm, pitch - 726 mm, fin shape - trapezoidal), and microfin #3 (fin height - 240 mm, pitch - 870 mm, fin shape - trapezoidal). Two microfin orientations of 0 and 20 were tested by rotating the heater surface. The heat transfer performance of the heater surface in the non-boiling (single-phase) and boiling regions were obtained over a Reynolds number range of 1565 to 7254 and compared with the plain surface performance. Microfin #3 and microfin #1 exhibited better heat transfer performance compared to the polished aluminum surface while microfin #2 transferred less heat compared to the polished aluminum surface. For microfin #1 nucleation started as soon as positive wall superheat was attained, while for microfin #2, microfin #3, and a polished aluminum surface a wall superheat of 6 to 10 C was required for inception of nucleation. A number of parameters affect the heat transfer from the augmented surfaces - subcooling, flow rate, swirl angle, surface finish, and surface geometry. The effect of these parameters was investigated for the six surfaces studied. Bubble activity was studied with the aid of video images obtained using a regular 30 frames per second video camera and a high speed video camera up to 1 000 frames per second under a magnification of up to 43 OX. As the degree of superheat increased beyond 10C, bubble activity became faster and could not be tracked by the high speed camera which only showed streaks of bubbles. It is thought that this increased bubble behavior continues with faster and smaller bubbles as the surface temperature continues to increase. Using experimental data taken by Mizo (1995), for flow boiling of water on a flat polished aluminum surface, existing pool boiling bubble growth models were investigate for possible extension into flow boiling. Extending existing pool boiling models to flow boiling proved very complex. Mikic and Rohsenow's [17] pool boiling bubble growth rate model is well accepted in literature. Limitations in applying this work to flow boiling were identified."--Abstract.

Download or read book Investigation of Separate Effects of Surface Condition on Subcooled Flow Boiling Heat Transfer written by Jee Hyun Seong and published by . This book was released on 2021 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The experimental results showed that groove pattern, average roughness, and wettability do not affect subcooled flow boiling. Instead, they suggest that the process is determined by the location, size, and shape of cavities, and that micro-scale surface modifications (e.g., porous cracks) or nano-scale structures play a crucial role in the formation of active nucleation cavities and modify the bubble dynamics. A key takeaway from this study is that, to elucidate how surface modifications affect boiling heat transfer, one should carefully examine how the surface morphology changes at both the micro- and nano-scale and how the surface preparation process affect the formation of cavities.

Download or read book Experimental Investigation of Subcooled Flow Boiling Using Synchronized High Speed Video Infrared Thermography and Particle Image Velocimetry written by Bren Andrew Phillips and published by . This book was released on 2014 with total page 206 pages. Available in PDF, EPUB and Kindle. Book excerpt: Subcooled flow boiling of water was experimentally investigated using high-speed video (HSV), infrared (IR) thermography, and particle image velocimetry (PIV) to generate a unique database of synchronized data. HSV allowed measurement of the bubble departure diameter. IR thermography allowed measurement of wall superheat (local distribution and surface-averaged values), heat transfer coefficient, nucleation site density, and bubble frequency. Particle image velocimetry allowed for the measurement of velocity profiles in the liquid phase for single bubble nucleation events. The tests were performed at pressures of 1.05, 1.5, and 2.0 bar and at subcoolings of 5, 10, and 15 °C. The mass flux values explored were 150-1250 kg/m2/s. The heat flux values explored were 100-1600 kW/m2. As expected, the heat transfer coefficients increased with increasing mass flux in the single-phase convection and partial boiling regions, and converged to a fully-developed boiling curve for high heat fluxes. The bubble departure diameter decreased with increasing mass flux and decreasing heat flux; in accordance with Sugrue's model. The nucleation site density increased with increasing superheat and decreasing mass flux, as predicted by Kocamustafaogullari and Ishii's model. The nucleation site density models under-predicted the nucleation site density for a given wall superheat. Wait time and frequency models did not reproduce the data accurately, and underestimated wait time by an order of magnitude. A new mechanistic model for calculating the wait time was developed that split the wall heat flux into the component that is transferred to the fluid, and the component that is transferred as sensible heat into the heater wall. Significant localized cooling was observed underneath bubbles sliding along the wall after departure from a nucleation site, an effect which should be considered in advanced models of subcooled flow boiling. The sliding bubble thermal effects were found to be insensitive to system conditions and were limited by the thermal conduction within the substrate. Bubble growth front velocities, and regions of flow influence of departing bubbles were measured with PIV. The database generated in this project can be used to inspire or validate mechanistic models and/or CFD simulations of subcooled flow boiling heat transfer.

Download or read book Heat Transfer and Fluid Flow in Minichannels and Microchannels written by Satish Kandlikar and published by Elsevier. This book was released on 2006 with total page 492 pages. Available in PDF, EPUB and Kindle. Book excerpt: &Quot;This book explores flow through passages with hydraulic diameters from about 1 [mu]m to 3 mm, covering the range of minichannels and microchannels. Design equations along with solved examples and practice problems are also included to serve the needs of practicing engineers and students in a graduate course."--BOOK JACKET.

Download or read book Flow Boiling in Expanding Microchannels written by Tamanna Alam and published by Springer. This book was released on 2017-05-24 with total page 89 pages. Available in PDF, EPUB and Kindle. Book excerpt: This Brief presents an up to date summary of details of the flow boiling heat transfer, pressure drop and instability characteristics; two phase flow patterns of expanding microchannels. Results obtained from the different expanding microscale geometries are presented for comparison and addition to that, comparison with literatures is also performed. Finally, parametric studies are performed and presented in the brief. The findings from this study could help in understanding the complex microscale flow boiling behavior and aid in the design and implementation of reliable compact heat sinks for practical applications.

Download or read book Convective Flow Boiling written by John C. Chen and published by CRC Press. This book was released on 2019-08-16 with total page 402 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book comprises selected papers from the First International Conference on Convective Flow Boiling. The purpose of the conference is to examine state-of-science and recent developments in technology of flow boiling, i.e., boiling systems which are affected by convective flows.

Download or read book Handbook of Phase Change written by S.G. Kandlikar and published by Routledge. This book was released on 2019-01-22 with total page 786 pages. Available in PDF, EPUB and Kindle. Book excerpt: Provides a comprehensive coverage of the basic phenomena. It contains twenty-five chapters which cover different aspects of boiling and condensation. First the specific topic or phenomenon is described, followed by a brief survey of previous work, a phenomenological model based on current understanding, and finally a set of recommended design equa

Download or read book Issues in Nuclear Energy Technologies 2013 Edition written by and published by ScholarlyEditions. This book was released on 2013-05-01 with total page 330 pages. Available in PDF, EPUB and Kindle. Book excerpt: Issues in Nuclear Energy Technologies / 2013 Edition is a ScholarlyEditions™ book that delivers timely, authoritative, and comprehensive information about Fusion Energy. The editors have built Issues in Nuclear Energy Technologies: 2013 Edition on the vast information databases of ScholarlyNews.™ You can expect the information about Fusion Energy in this book to be deeper than what you can access anywhere else, as well as consistently reliable, authoritative, informed, and relevant. The content of Issues in Nuclear Energy Technologies: 2013 Edition has been produced by the world’s leading scientists, engineers, analysts, research institutions, and companies. All of the content is from peer-reviewed sources, and all of it is written, assembled, and edited by the editors at ScholarlyEditions™ and available exclusively from us. You now have a source you can cite with authority, confidence, and credibility. More information is available at http://www.ScholarlyEditions.com/.

Download or read book Experimental and Analytical Investigation of the Bubble Nucleation Characteristics in Subcooled Flow written by Michael D. Cartwright and published by . This book was released on 1995 with total page 196 pages. Available in PDF, EPUB and Kindle. Book excerpt: "An experimental and analytical study of bubble nucleation characteristics for a polished aluminum surface under sub-cooled flow boiling of water is conducted. A high magnification (up to 1350X) microscope and an atomic force magnifier were used to visualize the aluminum surface to determine a range of cavity sizes on the heater surface as well as gain insight into the shape of the cavity. A high-speed camera was incorporated to study the actual bubble nucleation from these cavities. A review of existing theoretical models available in literature to predict bubble nucleation characteristics in flow boiling is also presented. A new bubble nucleation model is proposed which uses experimentally determined bubble geometry and results from computational fluid dynamics (CFD) modeling. Experimental data was collected through the use of the high-speed flow visualization system available in RIT's Thermal Fluid Laboratory. The data is obtained to study the effects of sub-cooling, flow rate, and wall superheat on the nucleation characteristics of different size cavities. This data is also compared with existing models as well as the one proposed in this investigation."--Abstract.

Download or read book Experimental Study of Nucleate Boiling Bubble Dynamics and Heat Transfer Enhancement on Printed Bi functional Surfaces written by Michele David and published by . This book was released on 2016 with total page 76 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book An Empirical Modification of Nucleation Theory and Its Application to Boiling Heat Transfer written by Yan Po Chang and published by . This book was released on 1961 with total page 40 pages. Available in PDF, EPUB and Kindle. Book excerpt: From the equations of momentum and of energy and by the aid of established experimental phenomena, the problem of heat transfer in nucleate boiling is reduced to one of three parameters: the first represents the Stanton number, the second bubble growth, and the third nucleation.

Download or read book Investigation of Heat Transfer in Subcooled Flow Boiling written by Jinfeng Wu and published by . This book was released on 2001 with total page 150 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Experimental Study of Heat Flux Partitioning in Pressurized Subcooled Flow Boiling written by Andrew Jonathan Richenderfer and published by . This book was released on 2018 with total page 182 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding of subcooled flow boiling and the critical heat flux (CHF) is of the utmost importance for both safety and profitability of pressurized water nuclear reactors since they are major factors in the determination of the reactor power rating. Motivated by the emergence of a new wall boiling model by Gilman [3] and previous experimental insights from Phillips [12], a first-of-a-kind experimental investigation of pressurized steady-state subcooled flow boiling was conducted using state-ofthe- art diagnostics to gain a unique insight of the relevant mechanisms, including the partitioning of the wall heat flux. Conditions up to 10 bar pressure, 2000 kg/m2s mass flux and 20 K subcooling were explored. High-speed infrared thermometry tools were developed and used to measure the local time-dependent 2-D temperature and heat flux distributions on the boiling surface. These distributions were analyzed to determine fundamental boiling heat transfer parameters such as the nucleation site density, growth and wait times, nucleation frequency, departure diameter as well as the partitioning of the wall heat flux. While established mechanistic models can capture the trends of growth time and wait time with relatively good accuracy, this work reveals current models do not accurately predict the activation and interaction of nucleation sites on the boiling surface. This is a major roadblock, since boiling curves and CHF values obtained in nominally identical environments can be significantly different depending upon the nucleation site density which in turn is determined by the surface properties. The role of evaporation in the partitioning of the heat flux increases monotonically as the average heat flux increases, up to a maximum value of 70%, and is the dominant mechanism at high heat fluxes. At low and intermediate heat fluxes single-phase heat transfer is the dominant mechanism. Traditional heat partitioning models fail to capture these physics, but newer models with a comprehensive and physically consistent framework show promise in predicting the wall heat transfer. The data and understanding produced by this work will be essential for the development and validation of these modeling tools.

Download or read book Experimental and Analytical Study of the Effects of Wettability on Nucleation Site Density During Pool Boiling written by Chunghsiung Wang and published by . This book was released on 1992 with total page 256 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Experimental Investigation on Multiscale Interactions in Intermittent Flow Boiling and a New Hybrid VOF Lagrangian Modeling Approach written by Xiang Zhang and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In flow boiling, most phase change heat transfer occurs at the smallest scales: in 1 nm -- 10 [mu]m thick liquid microlayers under nucleating bubbles and in thin thermal boundary layers around such bubbles. Nucleating bubbles evolve from microscopic cavities on heated surfaces (reaching 104 - 106 sites per cm2). Theoretical closure models have been proposed for individual bubble nucleation, growth, and departure due to hydrodynamics. However, high-fidelity integration of such microscale models and theory with full channel flow processes has proven challenging due to the large range of length and time scales. Thus, contemporary boiling engineering still relies on empirical correlations. While such models are validated by large experimental databases, they are limited to specific fluids and geometries. More rigorous descriptions are needed to predict transport with emerging configurations and fluids. The first objective of this study is to experimentally characterize the coupling between micro-scale transport and large-scale hydrodynamics in flow boiling heat transfer through high-speed photography and thermal imaging. A new hybrid volume-of-fluid (VOF)-Lagrangian multi-scale simulation approach is then proposed to directly resolve and couple these processes. Experimental data from this investigation and other publications are employed to assess this modeling approach. First, a square air-oil bubble column facility is built to track the bubble trajectories at free surface and assess the hybrid VOF-Lagrangian simulation approach. High speed videos are captured from both the front and top view to measure flow parameters including: bubble size distribution (BSD), bubble rise velocity in vertical direction, as well as bubble lifetime and propagation velocity at the free surface. A hybrid VOF-Lagrangian solver is developed to characterize the coupling between micro-scale bubble transport and macro-scale hydrodynamics in such multi-scale two-phase flows. The Lagrangian model tracks the trajectory of individual injected discrete small bubbles, accounting for effects such as buoyancy, pressure, virtual mass, drag, and turbulent dispersion. Once bubbles exceed a threshold packing density or overlap with VOF structures, they are converted to the grid-scale vapor phase. An empirical bubble-lifetime model is implemented to account for the finite coalescence times of bubbles at free surfaces. Contributions of this effort include programmable closure for bubble lifetime at the free surface (before popping/coalescence), a pinning force method for bubbles at the free surface, and Lagrangian-to-VOF transition of bubbles based on packing density. Next, a two-phase flow boiling experimental facility is developed to collect simultaneous high-speed visualization and IR temperature distribution data. The test cell channel is 420 mm long with a 10 mm square-cross section. A transparent ITO coated sapphire window serves as a heater and IR interface for measuring the internal wall temperature. The facility is charged with a low boiling point fluid (HFE7000) to reduce uncertainties from heat loss. Vertical saturated flow boiling wake-nucleation interaction experiments are performed for varying volume flow rates (0.5 -- 1.5 L min-1, laminar-to-turbulent Re) and heat fluxes (0 -- 100 kW m-2). Discrete vapor slugs are injected to explore interactions with nucleate boiling processes. By measuring film-heater power, surface temperature distributions, and pressures, local instantaneous heat transfer coefficients (HTC) are obtained. A conjugate gradient method is implemented to solve the 2-D transient inverse heat conduction problem and estimate the non-uniform heat flux distribution. For nucleate flow boiling without slug injection, visualizations show that both bubble cycle and wait time are inversely proportional to the applied heat flux. The average wall temperature displays a linear relationship with heat flux. Moreover, bubble departure diameter tends to increase with increasing the bubble nucleation temperature, and stabilizes at higher temperature. With respect to the wake-nucleation interaction experiments, it is found that small nucleate bubbles only in the core region of Taylor bubble body with a certain bubble size (1.4 to 1.6 mm) would merge into the vapor slug. IR data indicate that the wake shear effect is divided into 5 stages during the whole process. Wake heat transfer enhancement is observed due to the vortex shedding and intense mixing between in the Taylor bubble wake. In addition, the nucleate boiling is firstly suppressed in the Taylor bubble body due to the falling liquid film, then recovered after the wake region. To efficiently simulate such processes, the hybrid VOF-Lagrangian model is extended to account for heat transfer, bulk- and Lagrangian-bubble-scale phase change, and nucleation site processes. The approach tracks discrete nucleation sites on walls and simulates a contact line pinning force. Bubble growth and departure is directly predicted using grid-scale VOF velocity and temperature data rather than idealized conditions. This method is first verified with literature data for pool boiling. It is then qualitatively assessed for the Taylor flow boiling processes studied experimentally.

Download or read book An Analytical and Experimental Study of the Thermal Boundary Layer and Ebullition Cycle in Nucleate Boiling written by Yih-Yun Hsu and published by . This book was released on 1961 with total page 54 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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).

Download or read book Journal of Heat Transfer written by and published by . This book was released on 1991 with total page 1096 pages. Available in PDF, EPUB and Kindle. Book excerpt: