Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Boiling heat transfer is a critical process in large-scale industrial applications such as steam engines and heat exchangers in power plants, and in microscopic heat transfer devices such as heat pipes and microchannels for cooling electronic chips. Enhancing boiling heat transfer thus has great significance on lots of energy transportation and utilization systems. Recent studies has suggested that micro/nano structured surfaces can produce considerably different boiling heat transfer curves than normal plain surfaces, resulting in different values of the critical heat flux (CHF) and heat transfer coefficient (HTC). In this thesis, pool boiling on several new micro/nano structured surfaces was experimentally investigated to further understand the mechanism of boiling heat transfer and increase boiling performance. We first evaluated enhanced boiling heat transfer on three kinds of micro/nano structured super-hydrophilic surfaces: 1) nanowire coated super-hydrophilic surfaces, 2) hybrid microscale cavity and nanowire structured surfaces and 3) hybrid microscale pillar and nanowire structured surfaces. All three surfaces showed significant enhancement of CHF and HTC compared to plain silicon surfaces. Combined micro/nano structured surfaces presented better performance than nanowire coated surfaces suggesting that both active nucleation density and surface roughness significantly affect boiling heating transfer. Experimental investigations indicate an optimum design both in size (~ 20μ𝑚) and density (between 0 and 10000=cm^2) of cavities for microscale cavity/nanowire structured surfaces. The highest CHF and peak HTC values were obtained on microscale pillar/nanowire structured surfaces. Among the test surfaces, the largest enhancements of CHF and peak HTC were 228% and 298%, respectively, compared to plain silicon surfaces. For a better understanding of the boiling phenomena, pool boiling on super-hydrophobic surfaces was also studied. We found that, for super-hydrophobic surfaces, the major heat transfer mechanism at the initial boiling regime is natural convection of liquid water. In conclusion, micro/nano structured surfaces can greatly influence nucleate boiling heat transfer. The various physical attributes employed with the structured surfaces further revealed the profound influence of surface topography on enhancing boiling heat transfer. / 2031-01-01

An experimental investigation was performed to study the confined boiling heat transfer characteristics over a saturated porous structure using distilled water as the working fluid. A thin stainless steel resistive foil stretched between two copper electrodes was used to heat a saturated porous plate with an effective pore size of 50 gm. The temperature distribution on the foil heater was measured using a high speed thermal imaging camera. The effect of the gap height between the heater and the porous plate on the heat transfer was investigated for gap heights ranging from 0 um to 1000 um and for heat fluxes ranging from 11.7 kW/m2 to 58.3 kW/m2. It was observed that the highest heat transfer rate was obtained at a gap height of approximately 600 pm. The main heat transfer mechanism is thought to be confined boiling in the small gap between the heating surface and the saturated porous structure. It was observed that the effect of the subcooled liquid temperature did not have a significant effect on the heat transfer. The effect of the pore size in the porous plate was investigated by repeating the measurements with a porous plate of 200 gm pore size. It was observed that the thermal resistance for the plate with a 200 gm pore size was significantly higher than the plate with 50 gm pores for gaps less than 300 gm. At a larger gap height of 600 gm, similar heat transfer performances were obtained for the two porous media. / Thesis / Master of Applied Science (MASc)

Two-phase heat transfer in mini/micro-channels is capable of meeting the high cooling demands of modern high heat flux applications. The phase change process ensures better temperature uniformity and control for local hot spots. Furthermore, these compact channels could be helpful in reducing the required charge and material inventories.Environmental concerns—mainly ozone depletion and global warming—have instigated a search for new alternatives in refrigeration industry. While new compounds are being developed to address stringent legislative demands, natural alternatives are also coming into prominence. A limited number of investigators have reported on thermal performance of such alternatives. The current study is therefore focused on saturated flow boiling heat transfer, pressure drop and dryout characteristics for three low global warming potential (GWP) refrigerants (R152a, R600a and R1234yf) in a vertical mini-channel.In this study experiments were carried out by uniformly heating a test section (stainless steel tube with 1.60 mm inside diameter and 245 mm heated length) at 27 and 32 oC saturation temperature with 50-500 kg/m2s mass velocities. The effects of various parameters of interest (like heat flux, mass flux, system pressure, vapor quality, operating media) on flow boiling heat transfer, frictional pressure drop and dryout characteristics were recorded. R134a, which has been widely used in several applications, is utilized as a reference case for comparison of thermal performance in this study.Experimental results for saturated boiling heat transfer showed strong influence of heat flux and system pressure with insignificant contributions from mass flux and vapor quality. Two phase frictional pressure drop increased with mass flux, vapor quality and with reduced operating pressure. The dryout heat flux remained unaffected with variation in saturation temperature, critical vapor quality in most cases was about 85%. The experimental results (boiling heat transfer, two-phase pressure drop and dryout heat flux) were compared with well-known macro and micro-scale correlations from the literature. / <p>QC 20141124</p>

The current study is a part of a long term experimental project devoted to investigate flow boiling heat transfer, pressure drop and flow visualization of R134a in small to mini/micro-diameter tubes. The experimental facility was first designed and constructed by X. Huo (2005) with the contribution of L. Chen (2006). In the present study, the experimental facility was upgraded by changing the heating system from AC to DC heating and also upgrading the logging system through using a faster data logger and developing a new Labview program. The objectives of the current study include (i) contribute in identifying the reasons behind the wide scatter in the published flow boiling heat transfer results, (ii) contribute in understanding the fundamentals of flow boiling heat transfer in mini/micro-diameter tubes and (iii) evaluation of the existing heat transfer and pressure drop prediction methods. Two sizes of stainless steel tubes were investigated in the current study; 0.52 mm and 1.1 mm diameter. In the current study, the 0.52 mm tube was roughly called a “micro-tube” whilst the 1.1 mm tubes were called “mini-tubes”. The present study proposes two possible reasons for the scatter in the published heat transfer results. The first reason is the variations in the heated length from one study to another–there is no criterion for choosing the heated length. The second reason is the variations in the inner surface characteristics of the channels from one study to another. These two important parameters were not taken into consideration by researchers in the past studies. Accordingly, the effect of the heated length was investigated in the current study using a seamless cold drawn tube with diameter of 1.1 mm and heated length ranging from 150 to 450 mm. The effect of the tube inner surface was also tested here by conducting the test in two stainless steel tubes with diameter of 1.1 mm and manufactured by two different processes. The first tube was manufactured by welding technique whilst the second tube was a seamless cold drawn tube. Both tubes were identical in design and dimensions. The inner surface of each tube was examined first using SEM analysis and demonstrated that, the surface morphology is completely different. The local heat transfer coefficient was determined through measuring the local wall temperature using 14 K-type thermocouples attached to the wall using thermally conducting but electrically insulating epoxy supplied by Omega. Pressure drop was measured directly across the heated section and a high speed camera was used for the flow visualization at 1000 frames/s. All measurements were recorded after the system attained steady state. The experimental conditions include mass flux range of 100 – 500 kg/m2 s, system pressure range of 6 – 10 bar, inlet sub-cooling of about 5K and exit quality up to about 0.9. The most frequently observed flow regimes in the 0.52 mm tube were found to be slug (elongated bubble), transition to annular and annular flow regimes. In the 1.1 mm tube, the observed regimes were found to be slug, churn and annular. The transition from slug flow to annular flow in the 0.52 mm tube occurred smoothly with little disturbances at the liquid vapour interface compared to the 1.1 mm tube. Additionally, increasing the heated length of the 1.1 mm tube was found to shift the transition to annular flow to occur at lower vapour quality. The heat transfer results demonstrated that the behaviour of the local heat transfer coefficient in the 0.52 mm diameter tube is different compared to that in the 1.1 mm tubes. Also, the tube inner surface characteristics and the heated length were found to strongly influence the local behaviour of the heat transfer coefficient. Flow boiling hysteresis was investigated and the results indicated that hysteresis exists only at very low heat fluxes near the boiling incipience. Existing heat transfer and pressure drop correlations were examined using the results of the 0.52 and 1.1 mm seamless cold drawn tubes. The pressure drop data were predicted very well using the Muller-Stienhagen and Heck (1986) correlation, the homogeneous flow model and the correlation of Mishima and Hibiki (1996). On the contrary, all macro and microscale heat correlations failed to predict the current experimental data. The mechanistic models failed to predict the data of all tubes with the same accuracy. Accordingly, two heat transfer correlations were proposed in the current study. The first correlation is based on dimensionless groups whilst the second is based on the superposition model of Chen (1966). Both correlations predicted the current experimental data and the data of Huo (2005) and Shiferaw (2008) very well.

<p>Despite worldwide interest in a number of applications involving cryogenic fluids that are crucial to future space exploration, there is presently a lack of a large, reliable cryogenic pool boiling critical heat flux (CHF) database that can be used for assessment of accuracy of available predictive tools - model and correlations – or development of new tools. This shortcoming is a primary motivation for the present study, prompting compilation of a new consolidated cryogenic pool boiling CHF database from world literature. The database is used to assess accuracy of previous models and correlations, which are segregated according to ability to predict key operating parameters, such as pressure, surface orientation, and subcooling. A new correlation is constructed which shows very good predictive accuracy, evidenced by a mean absolute error of 16.95%, based on Earth gravity data which comprise a large fraction of the consolidated database. Using a limited subset of datapoints for three cryogens and a reduced gravity range of 0 to 0.7466, the new correlation is further modified with a reduced gravity multiplier to tackle reduced gravity conditions. The modified correlation has a mean absolute error of 17.47%, slightly higher than for Earth gravity alone. Overall, the new correlations are proven far more accurate than all prior models and correlations and therefore constitute new powerful tools for design of cryogenic space systems. It is shown CHF is very sensitive to pressure, increasing with increasing pressure up to maximum before decreasing appreciably toward critical pressure. CHF is also shown to be strongly influenced by surface orientation, being highest for horizontal surfaces and decreasing monotonically with increasing orientation angle, and increasing fairly linearly with increased subcooling.</p><p>Additionally, CHF models and correlations are assessed using amassed quenching CHF data that showed overpredictions of data. A new correlation is formulated which includes the effects of surface material and heater thickness to achieve high predictive accuracy for complied quenching CHF database. The new correlation has a mean absolute error and root mean square error of 10.79% and 16.12%, respectively, based on a compiled database. Analysis of complied quenching data showed that CHF is sensitive to the surface material, increasing with increasing thermal conductivity but, the influence of surface material becomes weak with increasing thermal conductivity. CHF is also strongly influenced by heater thickness, increasing with increased heater thickness till it reaches the asymptotic thickness. </p>

<p> Benzene and water films were passed over an inclined, hot copper plate. The boiling heat transfer to the turbulent liquid films has been measured with local heat-flux meters. These meters have been adopted from a design suggested by Gardon (G10), and could be used to measure boiling heat fluxes with an accuracy of about -20,000 BTU/hr.sq.ft. The results indicate that the heat transfer to the liquid films compares well with that of forced convection subcooled boiling. </p> <p> During boiling the liquid film is destroyed by the growing vapour bubbles and by the separation of the liquid from the hot plate. The break-up of the liquid films has been investigated with high-speed photography but an analysis of the hydrodynamics of this break-up has not been made. </p> / Thesis / Master of Engineering (ME)

No description available.

Master of Science / Department of Mechanical and Nuclear Engineering / Donald L. Fenton / Increase in demand for energy necessitates nuclear power units to increase their peak power limits. This increase implies significant changes in the design of the nuclear power unit core in order to provide better economy and safety in operations. A major hindrance to the increase of nuclear reactor performance especially in Pressurized Water Reactors (PWR) is the so called ‘Axial Offset Anomaly (AOA)’. An Axial Offset Anomaly (AOA) is the unexpected change in the core axial power distribution during the operation of a PWR from the predicted distribution. This problem is thought to be occurring because of precipitation and deposition of lithiated compounds such as lithium metaborate (LiBO[subscript]2) on the fuel rod. Due to its intrinsic property, the deposited boron absorbs neutrons thereby affecting the total power distribution in the reactor. AOA is thought to occur when there is sufficient build up of crud deposits on the cladding during subcooled nucleate boiling. Predicting AOA is difficult because there is little information regarding the heat and mass transfer during subcooled nucleate boiling. This thesis describes the experimental investigation that was conducted to study the heat transfer characteristics during subcooled nucleate boiling at prototypical PWR conditions. Pool boiling tests were conducted with varying concentrations of LiBO[subscript]2 and boric acid (H[subscript]2BO[subscript]3) solutions along with deionized water. The experimental data collected includes the effect of coolant concentration, degree of subcooling, system pressure and heat flux on pool boiling heat transfer coefficients. An analysis of deposits formed on the fuel rod during subcooled nucleate boiling is also included in the thesis. The experimental results reveal that the pool boiling heat transfer coefficient is degraded by the presence of boric acid and lithium metaborate in water. At concentration of 5000 ppm in water, the boric acid solution reduced the heat transfer coefficient by 23% and lithium metaborate solution reduced the heat transfer coefficient by 26%.

Orientador: Elaine Maria Cardoso / Resumo: A intensificação da transferência de calor por meio de alterações na morfologia da superfície aquecida vem sendo estudada no meio científico, a fim de suprir a crescente demanda de resfriamento de dispositivos com alta capacidade de processamento e dimensões cada vez menores. O presente trabalho apresenta o estudo experimental do efeito de superfícies nanoestruturadas e do espaçamento do canal de confinamento durante a ebulição em piscina da água deionizada, à temperatura de saturação na pressão atmosférica, sobre o coeficiente de transferência de calor, HTC, e fluxo crítico de calor, CHF. As superfícies nanoestruturadas foram obtidas pelo processo de ebulição do nanofluido de Al2O3-água deionizada em duas diferentes concentrações más-sicas: 0,03 g/l (“baixa” concentração, LC) e 0,3 g/l (“alta” concentração, HC). Foram realizados testes livres, com espaçamento, entre a superfície aquecida e a superfície adiabática, de 30 mm (correspondendo a Bo = 12), e testes sob condições de confinamento, com espaçamento de 1,0 mm (Bo = 0,4). As superfícies de teste foram caracterizadas por meio de medição da rugosidade média (Ra), do ângulo de contato estático (molhabilidade), e imagens MEV. Foi observado um aumento médio de 45% no HTC do teste com superfície lisa nanoestruturada em baixa concentração de nanofluido, em relação à superfície lisa sem deposição. Esse ganho está relacionado com o aumento do número de sítios ativos de nucleação causado pela deposição das nanopartículas sobre a ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The intensification of heat transfer through changes in the heated surface morphology has been studied in the scientific community to meet the increase demand for cooling of devices with high processing power and smaller dimensions. This work presents the experimental study of the effect of nanocoated surfaces and gap size during nucleated boiling of deionized water, in saturation temperature at atmospheric pressure, about heat transfer coefficient, HTC, and critical heat flux, CHF. The pool boiling process of Al2O3-water based nanofluid at two different mass concentrations: 0.03 g/l (“low” concentration, LC) and 0.3 g/l (“high” concentration, HC), produced nanostructured surfaces. Unconfined tests were analyzed, with gap size between the heated surface and the adiabatic surface of 30 mm (corresponding to Bo = 12), and tests under confinement conditions, with gap size of 1.0 mm (Bo = 0.4). The tested surfaces were characterized by means of surface roughness (Ra) measurement, static contact angle (wettability), and SEM images. An average increase of 45% in HTC of the test with nanocoated smooth surface in low nanofluid concentration was observed in relation to smooth surface without deposition. This enhancement is related to the increase in the number of active nucleation sites caused by the nanoparticle’s deposition on the smooth surface. For all tests with rough nanocoated surfaces and nanocoated smooth one with high nanofluid concentration, there was degradation of the HTC ... (Complete abstract click electronic access below) / Mestre

Submitted by Jessica Martha Nunes (je.nunes25@gmail.com) on 2018-10-04T01:31:03Z No. of bitstreams: 1 Dissertação_NunesJM.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) / Approved for entry into archive by Cristina Alexandra de Godoy null (cristina@adm.feis.unesp.br) on 2018-10-08T14:23:35Z (GMT) No. of bitstreams: 1 nunes_jm_me_ilha.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) / Made available in DSpace on 2018-10-08T14:23:35Z (GMT). No. of bitstreams: 1 nunes_jm_me_ilha.pdf: 7307066 bytes, checksum: 4cd6a3e84b8e10900f14961caf7df4e5 (MD5) Previous issue date: 2018-08-10 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A intensificação da transferência de calor por meio de alterações na morfologia da superfície aquecida vem sendo estudada no meio científico, a fim de suprir a crescente demanda de resfriamento de dispositivos com alta capacidade de processamento e dimensões cada vez menores. O presente trabalho apresenta o estudo experimental do efeito de superfícies nanoestruturadas e do espaçamento do canal de confinamento durante a ebulição em piscina da água deionizada, à temperatura de saturação na pressão atmosférica, sobre o coeficiente de transferência de calor, HTC, e fluxo crítico de calor, CHF. As superfícies nanoestruturadas foram obtidas pelo processo de ebulição do nanofluido de Al2O3-água deionizada em duas diferentes concentrações más-sicas: 0,03 g/l (“baixa” concentração, LC) e 0,3 g/l (“alta” concentração, HC). Foram realizados testes livres, com espaçamento, entre a superfície aquecida e a superfície adiabática, de 30 mm (correspondendo a Bo = 12), e testes sob condições de confinamento, com espaçamento de 1,0 mm (Bo = 0,4). As superfícies de teste foram caracterizadas por meio de medição da rugosidade média (Ra), do ângulo de contato estático (molhabilidade), e imagens MEV. Foi observado um aumento médio de 45% no HTC do teste com superfície lisa nanoestruturada em baixa concentração de nanofluido, em relação à superfície lisa sem deposição. Esse ganho está relacionado com o aumento do número de sítios ativos de nucleação causado pela deposição das nanopartículas sobre a superfície lisa. Para todos os testes com superfícies rugosas nanoestruturadas e lisa nanoestruturada com alta concentração, houve degradação do HTC, devido ao efeito de preenchimento das cavidades e formação de uma resistência térmica adicional. Para baixos fluxos de calor, houve um aumento no HTC para os casos confinados em comparação aos livres, como consequência da evaporação do filme líquido presente entre a superfície aquecida e a bolha de vapor. Porém com o aumento do fluxo de calor, o fenômeno do dryout é antecipado em relação aos testes livres, o que compromete o desempenho de componentes sob essas condições. Nos testes sob confinamento foram observados ganhos no fluxo de calor de início do dryout para todas as superfícies nanoestruturadas testadas, chegando a 52% para a superfície lisa nanoestruturada em alta concentração, em comparação à superfície lisa sem nanoestrutura. Isso mostra que a nanoestruturação, apesar de não promover ganho no HTC, auxilia no ganho do fluxo de calor de início do dryout, que é o limite operacional de sistemas que trabalham sob confinamento. / The intensification of heat transfer through changes in the heated surface morphology has been studied in the scientific community to meet the increase demand for cooling of devices with high processing power and smaller dimensions. This work presents the experimental study of the effect of nanocoated surfaces and gap size during nucleated boiling of deionized water, in saturation temperature at atmospheric pressure, about heat transfer coefficient, HTC, and critical heat flux, CHF. The pool boiling process of Al2O3-water based nanofluid at two different mass concentrations: 0.03 g/l (“low” concentration, LC) and 0.3 g/l (“high” concentration, HC), produced nanostructured surfaces. Unconfined tests were analyzed, with gap size between the heated surface and the adiabatic surface of 30 mm (corresponding to Bo = 12), and tests under confinement conditions, with gap size of 1.0 mm (Bo = 0.4). The tested surfaces were characterized by means of surface roughness (Ra) measurement, static contact angle (wettability), and SEM images. An average increase of 45% in HTC of the test with nanocoated smooth surface in low nanofluid concentration was observed in relation to smooth surface without deposition. This enhancement is related to the increase in the number of active nucleation sites caused by the nanoparticle’s deposition on the smooth surface. For all tests with rough nanocoated surfaces and nanocoated smooth one with high nanofluid concentration, there was degradation of the HTC due to the filling effect of the cavities and the formation of an additional thermal resistance. For low heat fluxes, the HTC increased for confined cases compare to unconfined ones, as consequence of the liquid film evaporation present between the heated surface and the vapor bubble. However, with heat flux increase, the dryout phenomenon incipience is precipitated in relation to unconfined tests, which compromises the performance of components under these conditions. In the confined tests, enhancement in dryout incipience heat flux were observed for all nanocoated surfaces tested, reaching 52% for the nanocoated smooth surface in high concentration, compared to the smooth surface without nanostructure. This shows that nanostructure, while not promoting HTC enhancement, helps to delay the dryout incipience heat flux, which is the operational limit of systems that work under confinement.