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Captação de vapor de água em sílica gel para sistema de refrigeração a vácuo

Dinis, Cláudio dos Santos
Fonte: Universidade de Coimbra Publicador: Universidade de Coimbra
Tipo: Dissertação de Mestrado
Português
Relevância na Pesquisa
46.48%
Resumo A presente dissertação está inserida num trabalho de investigação, a decorrer no Departamento de Engenharia Mecânica, onde é estudada a refrigeração evaporativa a vácuo, sendo que a produção de vapor de água é o principal inconveniente do processo. Assim, a captação de vapor de água pela sílica gel é o principal objectivo deste trabalho, que se baseia na realização de ensaios experimentais, para avaliar o comportamento da sílica gel neste processo. No presente trabalho, foi elaborado um programa de aquisição de dados, que permite visualizar a variação de todos os parâmetros medidos durante a actividade experimental. Este programa permite também a gravação desses mesmos dados, para tratamento futuro. Para estudar e perceber o fenómeno de adsorção, e como seria possível a sua utilização neste processo, foram analisados todos os aspectos com ele relacionados, com especial incidência na adsorção de vapor de água em sílica gel. Analisaram-se diversos estudos nesta área, que permitiram obter um plano de análise da capacidade de adsorção da sílica gel. Durante a realização dos ensaios experimentais, parâmetros como a quantidade de sílica gel e o tipo de suporte utilizado foram analisados detalhadamente. Verifica-se que quanto maior é a quantidade de sílica gel utilizada no arrefecimento de uma determinada quantidade de água...

Desenvolvimento de uma metodologia de calibração independente para um LIDAR Raman na obtenção e estudo de perfis de vapor d'água atmosférico; DEVELOPMENT OF A METHODOLOGY FOR AN INDEPENDENT WATER VAPOR RAMAN LIDAR CALIBRATION TO STUDY WATER VAPOR ATMOSPHERIC PROFILES.

Torres, Ani Sobral
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 17/12/2008 Português
Relevância na Pesquisa
56.43%
Neste trabalho são apresentados resultados relativos ao desenvolvimento de uma metodologia de calibração independente de um sistema LIDAR Raman para medidas de razão de mistura do vapor dágua, avaliando-se a eficiência total de transmissão óptica do sistema LIDAR Raman. Uma calibração independente ou absoluta de um sistema LIDAR foi possível por meio de medidas e cálculos dos parâmetros relevantes. A metodologia de calibração independente apresentada é baseada na decomposição das equações da razão de mistura do vapor dágua, especialmente na avaliação do fator de calibração e as incertezas relativas à transmissão óptica e secções diferenciais de espalhamento. A eficiência de transmissão incluiu fatores como a refletividade do telescópio, transmissão dos componentes ópticos e filtros. A avaliação da eficiência do sistema foi realizada com do uso de uma lâmpada calibrada de tungstênio em duas configurações distintas de filtros de interferência, (banda larga e banda estreita) instalados no sistema LIDAR Raman da Howard University (HURL). A avaliação da secção diferencial de espalhamento Raman para o vapor dágua foi realizada pelo uso de um modelo téorico descrito na literatura e por uma primeira aproximação experimental. Os resultados mostraram consistência e que essa metodologia de calibração pode ser usada em uma base rotineira para medidas de vapor dágua atmosférico.; For this study we present the results of the development of an absolute Raman LIDAR calibration methodology for measuring a water vapor mixing ratio. An absolute calibration of the system is possible by measuring or calculating the relevant system parameters based on the decomposition of the calibration instrumental function. The efficiency of the LIDAR system transmission at the laser and Raman produced wavelengths including factors such as reflectivity of the telescope and the transmission of conditioning optics and filters. We have evaluated the efficiency of the system with a calibrated tungsten lamp using two different sets of interference filters (wideband and narrow-band) that were installed at the Howard University Raman LIDAR system. The methodology is based on the decomposition of the water vapor mixing ratio equations...

Evapotranspiração máxima da cultura de pimentão em estufa plástica em função da radiação solar, da temperatura, da umidade relativa e do déficit de saturação do ar; Maximum evapotranspiration of sweet pepper in plastic greenhouse as a function of solar radiation, temperature, relative humidity and water vapor pressure deficit of the air

Dalmago, Genei Antonio; Heldwein, Arno Bernardo; Nied, Astor Henrique; Grimm, Edenir Luis; Pivetta, Carina Rejane
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Artigo de Revista Científica Formato: application/pdf
Português
Relevância na Pesquisa
56.11%
Um experimento foi conduzido para avaliar a relação da evapotranspiração máxima (ETm) da cultura de pimentão por unidade de índice de área foliar (ETmf) com a radiação solar global incidente (Rge) e saldo de radiação (Rne), externos à estufa, e com a temperatura (tam), umidade relativa (URm) e déficit de saturação (D) do ar no interior da estufa. A ETm foi determinada por lisimetria. Apenas a temperatura e a umidade relativa do ar foram registradas e as demais variáveis foram estimadas. O Rne apresentou o maior efeito isolado, seguido pelo D e pela URm. A temperatura do ar às 15 horas e a temperatura máxima diária foram as variáveis pontuais de maior associação com a ETmf. A Rge melhorou sua relação quando analisada em diferentes faixas de tam. Os resultados confirmaram o Rne e o D como as variáveis meteorológicas de maior efeito preditivo da evapotranspiração das culturas em estufas plásticas no outono, porém com graus diferenciados de ajuste para o pimentão em relação às outras culturas.; An experiment was carried out to study the relationship between sweet pepper maximum evapotranspiration (ETm) per unit leaf area index (ETmf) and incident solar radiation (Rge) and net radiation (Rne) outside greenhouse...

GPS performance in the quantification of integrated water vapor in Amazonian regions

Sapucci, Luiz Fernando; Monico, João Francisco Galera; Plana-Fattori, Artemio; Machado, Luiz Augusto Toledo; Lima, Wagner Flauber Araujo
Fonte: Universidade Estadual Paulista Publicador: Universidade Estadual Paulista
Tipo: Conferência ou Objeto de Conferência Formato: 2362-2369
Português
Relevância na Pesquisa
56.31%
The Amazonian regions are characterized by large space-time variability in the humidity fields due to the intense convective process in those areas associated with the great humidity potential generated by high temperatures. An experiment denominated RACCI/DRY-TO-WET (RAdiation, Cloud, and Climate Interactions in the Amazonia during the DRY-TO-WET Transition Season) was carried out in the Brazilian Amazonian Region in 2002. The IWV values from GPS and other techniques, such as radiosondes, radiometer and humidity sounding satellites were used in this experiment to supply subsidies to evaluate the aerosols influence in the associated processes modifications to seasonality of atmospheric water vapor. Those regions are one of the most humid of the planet, where IWV (Integrated Water Vapor) average values are in the order of 50 kg/m2. As according the literature the IWV quantification using GPS has not been explored in those circumstances, the objective this paper is to present the preliminary results obtained in the evaluation of the GPS performance in Amazonian Regions when comparing with other techniques. The tendency measurement values indicated that the IWV values from GPS tend to be larger than those from radiosondes and smaller than those from radiometer. On the other hand...

Absorption of solar radiation by water vapor in the atmosphere. Part I: a comparison between selected parameterizations and reference results

Plana-Fattori,A.; Souza,E. P. de; Chagas,J. C. S.
Fonte: Sociedade Brasileira de Geofísica Publicador: Sociedade Brasileira de Geofísica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/11/1997 Português
Relevância na Pesquisa
66.29%
Selected schemes conceived for providing reliable and computationally efficient estimates of the absorption of solar radiation by the atmospheric water vapor are discussed. Special attention is devoted to the scheme proposed by Lacis & Hansen, which is included in radiative codes of atmospheric models running at Brazilian centers of research and numerical weather forecasting. Additional schemes, proposed from relatively modern estimates of the effects due to the water vapor absorption, are also included in the analysis. In the goal of assessing systematic errors associated to these schemes, comparisons are performed with benchmark results available in the literature which have been obtained through careful line-by-line integration throughout the solar spectrum. Modern schemes allow more atmospheric absorption, and therefore less downward shortwave irradiance at the ground, when compared with schemes implicitly adopted at present in Brazilian centers.

Effect of Water Vapor on Lyophilized Serratia marcescens and Escherichia coli

Dewald, R. R.; Browall, K. W.; Schaefer, L. M.; Messer, A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Publicado em /11/1967 Português
Relevância na Pesquisa
46.48%
Dried Serratia marcescens ATTC 14014 and Escherichia coli ATTC 4157 cells were exposed to various partial pressures of purified water vapor. The colony-forming ability of the S. marcescens was unimpaired when the dried organisms were stored in water-vapor atmosphere such that P/P0 < 0.55 or P/P0 = 1.0 (where P is the pressure of the water vapor in contact with the organisms, and P0 is vapor pressure of pure water at 25 C). During storage under water-vapor atmospheres with P/P0 between 0.6 and 1.0, the colony-forming ability of the dried S. marcescens was destroyed. The inactivation by water vapor followed the expression — ln N/N0 = Kt1/2, where N0 and N are the number of viable organisms before and after exposure, respectively, t is time, and K is a pseudo constant which is dependent upon the partial pressure of the water vapor at 25 C. Similar results were obtained with dried E. coli. The addition of solutes to the suspending media before freeze-drying was found to influence the stability of the organisms during exposure to water vapor.

The Response of Stratospheric Water Vapor to a Changing Climate: Insights from In Situ Water Vapor Measurements

Sargent, Maryann Racine
Fonte: Harvard University Publicador: Harvard University
Tipo: Thesis or Dissertation
Português
Relevância na Pesquisa
66.53%
Stratospheric water vapor plays an important role in the Earth system, both through its role in stratospheric ozone destruction and as a greenhouse gas contributing to radiative forcing of the climate. Highly accurate water vapor measurements are critical to understanding how stratospheric water vapor concentrations will respond to a changing climate. However, the past disagreement among water vapor instruments on the order of 1 – 2 ppmv hinders understanding of the mechanisms which control stratospheric humidity, and the reliable detection of water vapor trends. In response to these issues, we present a new dual axis water vapor instrument that combines the heritage Harvard Lyman-(alpha) hygrometer with the newly developed Harvard Herriott Hygrometer (HHH). The Lyman-(alpha) instrument utilizes ultraviolet photo-fragment fluorescence detection, and its accuracy has been demonstrated though rigorous laboratory calibrations and in situ diagnostic procedures. HHH employs a tunable diode near-IR laser to measure water vapor via direct absorption in a Herriott cell; it demonstrated in-flight precision of 0.1 ppmv (1-sec) with accuracy of 5%±0.5 ppmv. We describe these two measurement techniques in detail along with our methodology for calibration and details of the measurement uncertainties. We also examine the recent flight comparison of the two instruments with several other in situ hygrometers during the 2011 MACPEX campaign...

The Sources and Significance of Stratospheric Water Vapor: Mechanistic Studies from Equator to Pole

Smith, Jessica Birte
Fonte: Harvard University Publicador: Harvard University
Tipo: Thesis or Dissertation
Português
Relevância na Pesquisa
66.4%
It is the future of the stratospheric ozone layer, which protects life at Earth’s surface from harmful ultraviolet (UV) radiation, that is the focus of the present work. Fundamental changes in the composition and structure of the stratosphere in response to anthropogenic climate forcing may lead to catastrophic ozone loss under current, and even reduced, stratospheric halogen loading. In particular, the evolution toward a colder, wetter stratosphere, threatens to enhance the heterogeneous conversion of inorganic halogen from its reservoir species to its catalytically active forms, and thus promote in situ ozone loss. Water vapor concentrations control the availability of reactive surface area, which facilitates heterogeneous chemistry. Furthermore, the rates of the key heterogeneous processes are tightly controlled by the ambient humidity. Thus, credible predictions of UV dosage require a quantitative understanding of both the sensitivity of these chemical mechanisms to water vapor concentrations, and an elucidation of the processes controlling stratospheric water vapor concentrations. Toward this end, we present a set of four case studies utilizing high resolution in situ data acquired aboard NASA aircraft during upper atmospheric research missions over the past two decades. 1) We examine the broad scale humidity structure of the upper troposphere and lower stratosphere from the midlatitudes to the tropics...

Eggshell Permeability: A Standard Technique for Determining Interspecific Rates of Water Vapor Conductance

Portugal, S.; Maurer, G.; Cassey, P.
Fonte: Univ Chicago Press Publicador: Univ Chicago Press
Tipo: Artigo de Revista Científica
Publicado em //2010 Português
Relevância na Pesquisa
46.48%
Typically, eggshell water vapor conductance is measured on whole eggs, freshly collected at the commencement of a study. At times, however, it may not be possible to obtain whole fresh eggs but rather egg fragments or previously blown eggs. Here we evaluate and describe in detail a technique for modern laboratory analysis of eggshell conductance that uses fragments from fresh and museum eggs to determine eggshell water vapor conductance. We used fresh unincubated eggs of domesticated chickens (Gallus gallus domesticus), ducks (Anas platyrhynchos domesticus), and guinea fowl (Numida meleagris) to investigate the reliability, validity, and repeatability of the technique. To assess the suitability of museum samples, museum and freshly collected black-headed gull eggs (Larus ridibundus) were used. Fragments were cut out of the eggshell from the blunt end (B), equator (E), and pointy end (P). Eggshell fragments were glued to the top of a 0.25-mL micro test tube (Eppendorf) filled with 200 μL of distilled water and placed in a desiccator at 25°C. Eppendorfs were weighed three times at 24-h intervals, and mass loss was assumed to be a result of water evaporation. We report the following results: (1) mass loss between weighing sessions was highly repeatable and consistent in all species; (2) the majority of intraspecific variability in eggshell water vapor conductance between different eggs of the same species was explained through the differences in water vapor conductance between the three eggshell parts of the same egg (B...

Multispectral analysis of maritime clouds at night in the presence of atmospheric water vapor

Brooks, Christopher K.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
46.48%
Approved for public release; distribution is unlimited.; Multispectral analysis methods are exercised using AVHRR channels 3, 4, and 5 to improve upon single-wavelength thermal imagery at night. An algorithm was developed yielding cloud location and water vapor distribution from channel 3-4 and 4-5 differences, respectively. Water vapor effects on pixel registration for cloud were examined using two candidate subscenes, one cloudy and dry, the other, cloudy and moist. A positive water vapor/cloud-free correlation was found using statistical techniques on the candidate subscene scatter plots. TOVS water vapor channels verified the analyzed water vapor pattern in the 4 - 5 difference image. V-shaped cloud/clear thresholds were applied to various subscene scatter plots to account for the positive correlation of water vapor-to-cloud registration. Results showed that in regions of higher water vapor concentrations, pixels nearest cloud boundaries were likeliest to be misclassified as clear. Images containing significant water vapor gradients and cloud variations required threshold refinement for best results.

The effect of relative humidity on tensile strength and water vapor permeability in chitosan, fish gelatin and transglutaminase edible films

ALVARADO,Sofia; SANDOVAL,Glória; PALOS,Isidro; TELLEZ,Simón; AGUIRRE-LOREDO,Yaneli; VELAZQUEZ,Gonzalo
Fonte: Sociedade Brasileira de Ciência e Tecnologia de Alimentos Publicador: Sociedade Brasileira de Ciência e Tecnologia de Alimentos
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/01/2015 Português
Relevância na Pesquisa
66.35%
AbstractComposite films of chitosan, fish gelatin and microbial transglutaminase (MTgase) were developed. Films were produced by the casting method and dried at room temperature for 30 h, conditioned for 7 days at 30 °C at a relative humidity (RH) from 11 to 90%, and characterized. Chitosan:fish gelatin films in different proportions (100:0, 75:25, 50:50) with MTgase, were subjected to tensile properties and water vapor transmission (WVT) testing. The results showed that tensile strength decreased with an increase in RH and with an increase in gelatin content. Percent of elongation also increased with increasing RH and gelatin concentration. Water vapor transmission showed an increase proportional to an increase in RH with the presence of gelatin being unfavorable for reducing WVT. Results in this work allowed studying the effect of relative humidity on tensile and water vapor properties of chitosan and fish gelatin films.

Dinâmica e variabilidade do vapor de água na Península Ibérica

Costa, Pedro Daniel Melo da
Fonte: Universidade de Aveiro Publicador: Universidade de Aveiro
Tipo: Dissertação de Mestrado
Português
Relevância na Pesquisa
46.51%
O papel da atmosfera no Ciclo Hidrológico é fundamental. É na atmosfera que ocorrem os movimentos verticais que, com o arrefecimento adiabático, levam à condensação e à formação de nuvens. E as nuvens são as “células” geradoras da precipitação que inicia o ramo terrestre do ciclo da água. É, também, na atmosfera que se realiza o transporte de água, sobretudo na fase vapor, que suscita o importante ramo aéreo do Ciclo Hidrológico. Os propósitos principais desta dissertação foram: descrever a dinâmica do vapor de água na atmosfera, indagar as causas físicas que estão por detrás da variabilidade da divergência do vapor de água e sua ligação com os padrões de circulação atmosférica. Para descrever a dinâmica, considerando uma escala global e regional da Península Ibérica (PI), foi necessário proceder a um estudo do comportamento latitudinal, meridional e vertical de alguns campos meteorológicos, como por exemplo, humidade específica e água precipitável. O campo da divergência é seguramente o mais importante para a conexão dos ramos aéreo e terrestre, pois o valor numérico é equivalente à taxa da diferença entre a evaporação e a precipitação. A variabilidade foi estudada pela análise de Componentes Principais...

Parallelization and implementation of methods for image reconstruction: applications in GNSS water vapor tomography

Bento, Fábio André de Oliveira
Fonte: Universidade da Beira Interior Publicador: Universidade da Beira Interior
Tipo: Dissertação de Mestrado
Publicado em //2013 Português
Relevância na Pesquisa
66.33%
Algebraic reconstruction algorithms are iterative algorithms that are used in many area including medicine, seismology or meteorology. These algorithms are known to be highly computational intensive. This may be especially troublesome for real-time applications or when processed by conventional low-cost personnel computers. One of these real time applications is the reconstruction of water vapor images from Global Navigation Satellite System (GNSS) observations. The parallelization of algebraic reconstruction algorithms has the potential to diminish signi cantly the required resources permitting to obtain valid solutions in time to be used for nowcasting and forecasting weather models. The main objective of this dissertation was to present and analyse diverse shared memory libraries and techniques in CPU and GPU for algebraic reconstruction algorithms. It was concluded that the parallelization compensates over sequential implementations. Overall the GPU implementations were found to be only slightly faster than the CPU implementations, depending on the size of the problem being studied. A secondary objective was to develop a software to perform the GNSS water vapor reconstruction using the implemented parallel algorithms. This software has been developed with success and diverse tests were made namely with synthetic and real data...

Absorption of solar radiation by water vapor in the atmosphere. Part II: sensitivity tests with a general circulation model

Souza,E. P. de; Dias,P. L. da S.; Plana-Fattori,A.; Chagas,J. C. S.
Fonte: Sociedade Brasileira de Geofísica Publicador: Sociedade Brasileira de Geofísica
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/11/1997 Português
Relevância na Pesquisa
66.2%
The impact of implementing the Chou & Lee scheme for absorption of solar radiation by water vapor in the CPTEC/COLA's General Circulation Model is analyzed in this paper. Comparison with results of the present operational version, which uses the Davies scheme, shows that Chou & Lee scheme leads to more shortwave absorption, thus providing an extra warming of the atmosphere, mainly in the upper troposphere. The changes in the meridional distribution of temperature causes weakening of the subtropical jet in the Southern Hemisphere and strengthening of the Northern Hemisphere jet, which is slightly shifted poleward. The increase of the static stability weakens the meridional circulation cells. The increasing shortwave absorption in the atmosphere decreases the downward shortwave irradiance at the surface. As a consequence, the fluxes of enthalpy and latent heat from the surface are reduced. As a result of all these features, zonal mean precipitation and evaporation are also significantly reduced. Comparison with atmospheric fields reported by the Climate Prediction Center (NCEP/NOAA) shows that the configuration of the large-scale fields is, in a general manner, improved with the use of the Chou & Lee scheme in a monthly integration for January 1997.

Efeitos da interação de vapor d’água, de nitrogênio e de hidrogênio com estruturas dielétrico/SiC; Effects of the interaction of water vapor, of nitrogen and of hydrogen with dielectric/SiC structures

Corrêa, Silma Alberton
Fonte: Universidade Federal do Rio Grande do Sul Publicador: Universidade Federal do Rio Grande do Sul
Tipo: Tese de Doutorado Formato: application/pdf
Português
Relevância na Pesquisa
66.54%
No presente trabalho, foram investigados os efeitos de tratamentos térmicos em vapor d’água, em óxido nítrico e em hidrogênio nas propriedades físico-químicas e elétricas de filmes dielétricos crescidos termicamente e/ou depositadas por sputtering sobre lâminas de carbeto de silício. A caracterização foi realizada antes e após tratamentos térmicos nesses ambientes através de técnicas que utilizam feixes de íons. Em alguns casos, a caracterização elétrica também foi realizada. A investigação da incorporação e distribuição em profundidade de hidrogênio e oxigênio após tratamentos de SiO2/SiC e SiO2/Si em vapor d’água mostrou que há diferenças marcantes na interação da água com as duas estruturas. Observou-se maior incorporação de oxigênio no filme pré-existente de SiO2 sobre o SiC do que em SiO2/Si, evidenciando uma maior concentração de defeitos nos filmes sobre SiC. A incorporação de hidrogênio também foi maior nas estruturas SiO2/SiC, sendo observada em todas as regiões do filme de SiO2 crescido sobre SiC. Nos filmes crescidos sobre Si, no entanto, a incorporação deuse, principalmente, na região da superfície do filme de óxido. A interação do vapor d’água com estruturas SiO2/SiC e SiO2/Si com filmes depositados por sputtering também foi investigada. Foi constatada uma incorporação distinta da observada para essas estruturas quando seus óxidos foram crescidos termicamente. A incorporação de hidrogênio do vapor d’água em estruturas com filmes de SiO2 depositados por sputtering sobre SiC e sobre Si ocorre...

Human modification of Global Water Vapor flows from the Land Surface

Gordon, Line J; Steffen, Will; Jonsson, Bror F; Folke, Carl; Falkenmark, Malin; Johannessen, Ase
Fonte: National Academy of Sciences (USA) Publicador: National Academy of Sciences (USA)
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
56.14%
It is well documented that human modification of the hydrological cycle has profoundly affected the flow of liquid water across the Earth's land surface. Alteration of water vapor flows through land-use changes has received comparatively less attention, d

Water vapor distribution in protoplanetary disks

Du, Fujun; Bergin, Edwin A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 09/08/2014 Português
Relevância na Pesquisa
46.51%
Water vapor has been detected in protoplanetary disks. In this work we model the distribution of water vapor in protoplanetary disks with a thermo-chemical code. For a set of parameterized disk models, we calculate the distribution of dust temperature and radiation field of the disk with a Monte Carlo method, and then solve the gas temperature distribution and chemical composition. The radiative transfer includes detailed treatment of scattering by atomic hydrogen and absorption by water of Lyman alpha photons, since the Lyman alpha line dominates the UV spectrum of accreting young stars. In a fiducial model, we find that warm water vapor with temperature around 300 K is mainly distributed in a small and well-confined region in the inner disk. The inner boundary of the warm water region is where the shielding of UV field due to dust and water itself become significant. The outer boundary is where the dust temperature drops below the water condensation temperature. A more luminous central star leads to a more extended distribution of warm water vapor, while dust growth and settling tends to reduce the amount of warm water vapor. Based on typical assumptions regarding the elemental oxygen abundance and the water chemistry, the column density of warm water vapor can be as high as 1E22 cm^{-2}. A small amount of hot water vapor with temperature higher than ~300 K exists in a more extended region in the upper atmosphere of the disk. Cold water vapor with temperature lower than 100 K is distributed over the entire disk...

Determination of effective water vapor diffusion coefficient in PEMFC gas diffusion layers

LaManna, Jacob
Fonte: Rochester Instituto de Tecnologia Publicador: Rochester Instituto de Tecnologia
Tipo: Tese de Doutorado
Português
Relevância na Pesquisa
56.38%
Proton exchange membrane fuel cells have emerged as one of the leaders for the replacement of fossil fuel powered internal combustion engines. Water removal from the cell is one of the top concerns regarding fuel cell performance for transportation applications. During lower power output or high temperature operation, water removal in the vapor phase can dominate. The rate of water vapor diffusion through the porous cathode gas diffusion layer (GDL) of the fuel cell is limited by the porosity and tortuosity formed by the solid fiber matrix. In this work an experimental apparatus is designed to measure the rate of water vapor diffusion across the GDL to determine an effective diffusion coefficient. The effects of microporous layer (MPL) coating, GDL thickness, and polytetrafluoroethylene (PTFE) loading on the diffusion coefficient is demonstrated. Commercially available diffusion media are tested and include Mitsubishi Rayon Corp. Grafil U-105 series, SGL Sigracet® 25, 35, and 10 series, and Toray TGP-H-120 series. Standard corrections, such as the Bruggeman correction, used in fuel cell literature are found to overpredict the effective diffusion coefficient for the GDL. The MPL was found to produce a significant resistance to water vapor diffusion due to its smaller pore diameters...

Vapor de Água na Atmosfera: do efeito estufa às mudanças climáticas

Barbosa, Henrique M. J.
Fonte: Universidade de São Paulo. Superintendência de Comunicação Social Publicador: Universidade de São Paulo. Superintendência de Comunicação Social
Tipo: info:eu-repo/semantics/article; info:eu-repo/semantics/publishedVersion; Formato: application/pdf
Publicado em 22/11/2014 Português
Relevância na Pesquisa
56.4%
A water molecule interacts with both solar and longwave radiation emitted by Earth’s surface. Were it not for such absorption, which accounts for 2/3 of the natural greenhouse effect and keeps the atmosphere and surface warm, climate would not be as we know it today and possible life would not be possible on Earth. The presence of water vapor in the climate system also allows for an efficient vertical transport of energy from the surface to the troposphere, modifying the vertical profile of temperature through the heat exchanges involved in the phase changes of water to form a cloud. On their turn, clouds cover 2/3 of the sky – and their interaction with solar radiation increases the planetary albedo from 15 to 30% − whereas they also trap infrared radiation that would otherwise be lost to space. In this article we will show that if we are to keep advancing our understanding of climate and climate change, we have to keep advancing our understanding of the roles water vapor and clouds play in our atmosphere.; Uma molecula de agua interage tanto com a radiacao solar quanto com a radiacao de onda longa emitida pela superficie do nosso planeta. Se nao fosse por essa absorcao, que responde por 2/3 do efeito estufa natural e mantem a atmosfera e a superficie aquecidas...

A simple 2d pore-scale network model for the transport of water vapor and oxygen in polymeric films

Segura,L. A.; Mayorga,P. A.; Gonzalez,J. M.; Paz,J. E.
Fonte: Latin American applied research Publicador: Latin American applied research
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2012 Português
Relevância na Pesquisa
66.32%
A mathematical model was developed to describe mass transport in polymeric films based on modifications of Fick's law in a continuous macroscopic approach. The purpose of this work was specifically to model the transport of water vapor and oxygen in polymeric films at the pore level, based on physical aspects of the condensation process and the morphology and connectivity of the porous medium, with two-dimensional pore networks representing pore spaces. Using a Pore scale discrete model, we found that the pore-level distributions of liquid (condensate) and vapor as transport phenomena occurred. The vapor and oxygen diffusivities ranged from 4.22×10-6 to 3.16×10-7 cm²/s and from 3.66×10-6 to 2.86×10-7 cm²/s, respectively. Also, the vapor and oxygen permeabilities ranged from 2.61×10-3 to 3.83×10-4 nD (1nD=10-21m²) and from 6.77×10-3 to 1.35×10-3 nD, respectively. The transport properties obtained by the model were compared with the corresponding results for chitosan films obtained in previous experimental studies, showing a partial agreement.