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An Experimental Study on Micro-Hydrodynamics of Evaporating/Boiling Liquid FilmGong, Shengjie January 2011 (has links)
Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process. The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process. The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm. To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture. A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (<60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux. In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions. A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%. A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime. / QC 20111205 / VR-2005-5729, MSWI
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Extraction of Metal Values : Thermodynamics of Electrolyte Solutions and Molten Salts Extraction ProcessGe, Xinlei January 2009 (has links)
Over the past centuries, a number of process routes for extraction of metal values from an ore or other resources have been developed. These can generally be classifiedinto pyrometallurgical, hydrometallurgical or electrometallurgical routes. In the caseof the latter two processes, the reaction medium consists of liquid phase electrolytesthat can be aqueous, non-aqueous as well as molten salts. The present dissertationpresents the work carried out with two aspects of the above-mentioned electrolytes.First part is about the electrolyte solutions, which can be used in solvent extractionrelevant to many hydrometallurgical or chemical engineering processes; the secondpart is about the molten salts, which is often used in the electrometallurgical processesfor production of a variety of many kinds of metals or alloys, especially those that arehighly reactive.In the first part of this thesis, the focus is given to the thermodynamics ofelectrolyte solutions. Since the non-ideality of high concentration solution is not wellsolved, a modified three-characteristic-parameter correlation model is proposed,which can calculate the thermodynamic properties of high concentration electrolytesolutions accurately. Model parameters for hundreds of systems are obtained foraqueous as well as non-aqueous solutions. Moreover, a new predictive method tocalculate the freezing point depression, boiling point elevation and vaporizationenthalpy of electrolyte solutions is also proposed. This method has been shown to be agood first approximation for the prediction of these properties.In the second part, a process towards the extraction of metal values from slags,low-grade ores and other oxidic materials such as spent refractories using molten saltsis presented. Firstly, this process is developed for the recovery of Cr, Fe values fromEAF slag as well as chromite ore by using NaCl-KCl salt mixtures in the laboratoryscale. The slags were allowed to react with molten salt mixtures. This extraction stepwas found to be very encouraging in the case of Cr and Fe present in the slags. Byelectrolysis of the molten salt phase, Fe-Cr alloy was found to be deposited on thecathode surface. The method is expected to be applicable even in the case of V, Mnand Mo in the waste slags.Secondly, this process was extended to the extraction of copper/iron from copperore including oxidic and sulfide ores under controlled oxygen partial pressures.Copper or Cu/Fe mixtures could be found on the cathode surface along with theemission of elemental sulphur that was condensed in the cooler regions of the reactor.Thus, the new process offers a potential environmentally friendly process routereducing SO2 emissions.Furthermore, the cyclic voltammetric studies of metal ions(Cr, Fe, Cu, Mg, Mn)in (CaCl2-)NaCl-KCl salt melt were performed to understand the mechanisms, such asthe deposition potential, electrode reactions and diffusion coefficients, etc. In addition,another method using a direct electro-deoxidation concept(FFC Cambridge method),was also investigated for the electrolysis of copper sulfide. Sintered solid porouspellets of copper sulfide Cu2S and Cu2S/FeS were electrolyzed to elemental Cu, S andCu, Fe, S respectively in molten CaCl2-NaCl at 800oC under the protection of Argongas. This direct electrolysis of the sulfide to copper with the emission of elementalsulfur also offers an attractive green process route for the treatment of copper ore. / QC 20100714
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Ammonia - water desorption in flooded columnsGolden, James Hollis 10 July 2012 (has links)
Refrigeration systems employing the NH3-H2O absorption cycle provide cooling using a thermal energy input. This cycle relies on the zeotropic nature of the refrigerant - absorbent pair: because of the difference in boiling temperatures between NH3 and H2O, they can be separated through selective boiling in the desorber. Desorbers with counter-current flow of the solution and generated vapor enable efficient heat and mass transfer between the two phases, reducing the absorbent content in the generated vapor.
Flow visualization experiments at temperatures, concentrations and pressures representative of operating conditions are necessary to understand the heat and mass transfer processes and flow regime characteristics within the component. In this study, a Flooded Column desorber, which accomplishes desorption of the refrigerant vapor through a combination of falling-film and pool boiling, was fabricated and tested. Refrigerant-rich solution enters the top of the component and fills a column, which is heated by an adjacent heated microchannel array. The vapor generated within the component is removed from the top of the component, while the dilute solution drains from the bottom.
Flow visualization experiments showed that the Flooded Column desorber operated most stably in a partially flooded condition, with a pool-boiling region below a falling-film region. It was found that the liquid column level was dependent on operating conditions, and that the pool-boiling region exhibits aggressive mixing between the vapor and solution phases.
Heat transfer coefficients were calculated from the data for the pool-boiling region, and were compared with the predictions of several mixture pool-boiling correlations from the literature. The correlations from the literature were in general unable to predict the data from this study adequately. It was found that the Flooded Column desorber yielded higher heat transfer coefficients within the pool-boiling region than those predicted by these correlations. Therefore, modifications to existing mixture boiling correlations are suggested based on the findings of this study. The resulting modified correlation predicts 33 of the 35 data points from this study within ±40%, with an average absolute error of 19%.
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Detailed B-10 depletion in control rods operatingin a Nuclear Boiling Water ReactorJohnsson, John January 2011 (has links)
In a nuclear power plant, control rods play a central role to control the reactivity ofthe core. In an inspection campaign of three control rods (CR 99) operated in theKKL reactor in Leibstadt, Switzerland, during 6 respectively 7 consecutive cycles,defects were detected in the top part of the control rods due to swelling caused bydepletion of the neutron-absorbing 10B isotope (Boron-10). In order to correlatethese defects to control rod depletion, the 10B depletion has in this study beencalculated in detail for the absorber pins in the top node of the control rods.Today the core simulator PLOCA7 is used for predicting the behavior of the reactorcore, where the retrievable information from the standard control rod follow-up isthe average 10B depletion for clusters of 19 absorber holes i.e. one axial node.However, the local 10B depletion in an absorber pin may be significantly differentfrom the node average depletion that is re-ceived from POLCA7. To learn more, the 10B depletion has been simulated for each absorber hole in the uppermost node usingthe stochastic Monte Carlo 3D simulation code MCNP as well as an MCNP- based2D-depletion code (McScram). It was found that the 10B depletion is significantly higher for the uppermost absorberpins than the node average. Furthermore, the radial depletion in individual absorberpins was found to be much higher than expected. The results are consistent with theexperimental data on control rod defects.
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Experimental and Numerical Study of Dual-Chamber ThermosyphonPal, Aniruddha 18 May 2007 (has links)
An experimental and numerical investigation was conducted to study boiling and condensation - the two most important phenomena occurring in a dual-chamber thermosyphon. Boiling experiments were carried out using water at sub-atmospheric pressures of 9.7, 15 and 21 kPa with a three-dimensional porous boiling enhancement structure integrated in the evaporator. Sub-atmospheric pressure boiling achieved heat fluxes in excess of 100 W/cm2 with negligible incipience superheat, for wall temperatures below 85 oC. Reduced pressures resulted in reduction of heat transfer coefficient with decrease in saturation pressure. The boiling enhancement structure showed considerable heat transfer enhancement compared to boiling from plain surface. Increased height of the structure decreased the heat transfer coefficient and suggested the existence of an optimum structure height for a particular saturation pressure. A parametric study showed that a reduction in liquid level of water increased the CHF for boiling with plain surfaces. For boiling with enhanced structures, the liquid level for optimum heat transfer increased with increasing height of the enhanced structure.
A numerical model was developed to study condensation of water in horizontal rectangular microchannels of hydraulic diameters 150-375 µm. The model incorporated surface tension, axial pressure gradient, liquid film curvature, liquid film thermal resistance, gravity and interfacial shear stress, and implemented successive solution of mass, momentum and energy balance equations for both liquid and vapor phases. Rectangular microchannels achieved significantly higher heat transfer coefficient compared to a circular channel of similar hydraulic diameter. Increasing the inlet mass flow rate resulted in a higher heat transfer coefficient. Increasing the inlet temperature difference between wall and vapor led to a thicker film and a gradually decreasing heat transfer coefficient. Increasing the channel dimensions led to higher heat transfer coefficient, with a reduction in the vapor pressure drop along the axial direction of the channel.
The unique contributions of the study are: extending the knowledge base and contributing unique results on the thermal performance of thermosyphons, and development of a analytical model of condensation in rectangular microchannels, which identified the system parameters that affects the flow and thermal performance during condensation.
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Design and Performance Analysis of a Miniature Spray Cooling SystemLu, Chin-Yuan 27 August 2012 (has links)
The aim of this study is to design and build a miniature spray cooling system, in which the manufactured and adopted chamber, pump and heat exchanger are smaller than the conventional ones. An experiment was conducted to explore the cooling performance of the spray cooling system after its size has been minimized. In the experiment, copper was used to make the heated surface and different working media, such as DI water, as nanofludics with silver and multi-walled carbon nanotubes
powder were sprayed on the heated surface to enhance the heat dissipation efficiency of the system. The experiment in this study was set according to two conditions: transient and steady state, with Weber number as the main parameter, to observe the boiling phenomenon of different working media on heated surface and to record the temperature changes of the heated surface. The results were shown in boiling curve and cooling curve. The ultimate goal of this study was to obtain a better understanding of the cooling performance of the miniature spray cooling system in order to apply it to micro-electronic cooling devices, thereby solving the problem of the sharp increase in heating power per unit area on electronic components.
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Experimental Two-Phase Flow Characterization of Subcooled Boiling in a Rectangular ChannelEstrada Perez, Carlos E. 16 January 2010 (has links)
On the efforts to provide a reliable source of experimental information on turbulent
subcooled boiling ow, time resolved Particle Tracking Velocimetry (PTV) experiments
were carried out using HFE-301 refrigerant ow through a vertical rectangular
channel with one heated wall. Measurements were performed at liquid Reynolds numbers
of 3309, 9929 and 16549 over a wall heat flux range of 0.0 to 64.0 kW=m2. From
the PTV measurements, liquid two dimensional turbulence statistics are available,
such as: instantaneous 2-D velocity fields, time-averaged axial and normal velocities,
axial and normal turbulence intensities, and Reynolds stresses. The present results
agree with previous works and provide new information due to the 2-D nature of
the technique, for instance, this work shows that by increasing heat ux, the boiling
bubbles influence on the liquid phase is portrayed as a persistent increase of axial
velocity on regions close to the heater wall. This persistent increase on the axial
velocity reaches a maximum value attributed to the terminal bubble velocity. These
new observed phenomena must be considered for the development and improvement
of two-phase ow turbulence models. To this end, an extensive error analysis was also
performed with emphasis on the applicability of the PTV measurement technique on
optically inhomogeneous flows. The error quantification exhibited negligible optically
induced errors for the current conditions, making the data acquired in this work a
vast and reliable source.
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Effects of Carbon Nanotube Coating on Bubble Departure Diameter and Frequency in Pool Boiling on a Flat, Horizontal HeaterGlenn, Stephen T. 2009 August 1900 (has links)
The effects of a carbon nanotube (CNT) coating on bubble departure diameter and frequency in pool boiling experiments was investigated and compared to those on a bare silicon wafer. The pool boiling experiments were performed at liquid subcooling of 10 degrees Celsius and 20 degrees Celsius using PF-5060 as the test fluid and at atmospheric pressure. High-speed digital image acquisition techniques were used to perform hydrodynamic measurements. Boiling curves obtained from the experiments showed that the CNT coating enhanced critical heat flux (CHF) by 63% at 10 degrees Celsius subcooling. The CHF condition was not measured for the CNT sample at 20 degrees Celsius subcooling. Boiling incipience superheat for the CNT-coated surface is shown to be much lower than predicted by Hsu's hypothesis. It is proposed that bubble nucleation occurs within irregularities at the surface of the CNT coating. The irregularities could provide larger cavities than are available between individual nanotubes of the CNT coating.
Measurements from high-speed imaging showed that the average bubble departing from the CNT coating in the nucleate boiling regime (excluding the much larger bubbles observed near CHF) was about 75% smaller (0.26 mm versus 1.01 mm)and had a departure frequency that was about 70% higher (50.46 Hz versus 30.10 Hz). The reduction in departure diameter is explained as a change in the configuration of the contact line, although further study is required. The increase in frequency is a consequence of the smaller bubbles, which require less time to grow. It is suggested that nucleation site density for the CNT coating must drastically increase to compensate for the smaller departure diameters if the rate of vapor creation is similar to or greater than that of a bare silicon surface.
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Heat transfer enhancement in single-phase forced convection with blockages and in two-phase pool boiling with nano-structured surfacesAhn, Hee Seok 17 September 2007 (has links)
The first study researched turbulent forced convective heat (mass) transfer down-
stream of blockages with round and elongated holes in a rectangular channel. The
blockages and the channel had the same cross section, and a distance equal to twice
the channel height separated consecutive blockages. Naphthalene sublimation experiments were conducted with four hole aspect ratios (hole-width-to-height ratios) and
two hole-to-blockage area ratios (ratios of total hole cross-sectional area to blockage
area). The effects of the hole aspect ratio, for each hole-to-blockage area ratio, on the
local heat (mass) transfer distribution on the exposed primary channel wall between
consecutive blockages were examined. Results showed that the blockages with holes
enhanced the average heat (mass) transfer by up to 8.5 and 7.0 times that for fully
developed turbulent flow through a smooth channel at the same mass flow rate, respectively, in the smaller and larger hole-to-blockage area ratio (or smaller and larger
hole diameter) cases. The elongated holes caused a higher average heat (mass) transfer and a larger spanwise variation of the local heat (mass) transfer on the channel
wall than did the round holes.
The second study explored the heat transfer enhancement for pool boiling on
nano-structured surfaces. Experiments were conducted with three horizontal silicon surfaces, two of which were coated with vertically aligned multi-walled carbon nanotubes (MWCNT) with heights of 9 and 25 ùm, respectively, and diameters between
8 and 15 nm. The MWCNT arrays were synthesized on the two silicon wafers using
chemical vapor deposition. Experimental results were obtained over the nucleate boiling and film boiling regimes under saturated and sub-cooled (5ñC and 10ñC) boiling
conditions. PF-5060 was the test fluid. Results showed that the MWCNT array with
a height of 25 ùm enhanced the nucleate and film boiling heat fluxes on the silicon
surface by up to 380% and 60%, respectively, under saturated boiling conditions, and
by up to 300% and 80%, respectively, under 10ñC sub-cooled boiling conditions, over
corresponding heat fluxes on a smooth silicon surface. The MWCNT array with a
height of 9 ùm enhanced the nucleate boiling heat flux as much as the taller array,
but did not significantly enhance the wall heat flux in the film boiling regime.
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Pretreatment of wheat straw with superheated steam and boiling water, its effect on cellulose structure, and fermentation by Clostridium thermocellumMirhosseini, Shayan 12 September 2015 (has links)
The focus of this study was to determine the effects of pretreatment of wheat straw by superheated steam (SS) alone or in combination with boiling water (BW) on biomass structure and yields of fermentation products (cell mass and fermentation end-products) by Clostridium thermocellum. Different cultivars of wheat straw were ground to a particle size less than 355 µm, and exposed to the following methods of pretreatment: i) 15 min soaking in 119 °C boiling water under absolute pressure of 193 kPa, followed by processing with SS at atmospheric pressure at different temperatures and retention times; ii) 15 min processing with SS at atmospheric pressure; and iii) 15 min soaking in 119 °C boiling water under absolute pressure of 193 kPa. Processing with SS was conducted at a variety of temperatures in the range of 180-220 °C. The severity of pretreatment was expressed through a treatment severity factor as a measure of harshness of treatment. Pretreatment combinations of boiling water with superheated steam at different retention times inside the SS chamber were also investigated. Wheat straw samples were then used as substrates in fermentation reactions with C. thermocellum. The most noticeable effects on biomass structure and fermentation were observed at the highest severity factor of 6.5, corresponding to 15 min pretreatment with boiling water followed by 15 min treatment with SS at 220˚C. This pretreatment provided the maximum increase in percentage of contribution of amorphous cellulose (% CAC), and the highest fermentation yield in terms of hydrogen, carbon dioxide, and ethanol production. / October 2015
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