144,859 research outputs found
Modelagem da transferência de calor com e sem mudança de fase no resfriamento por spray (Spray Cooling)
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2010Nos últimos vinte anos, com a progressiva miniaturização de quipamentos e sistemas de engenharia e o conseqüente aumento das taxas com que calor é dissipado por unidade de área, diferentes tecnologias vêm sendo avaliadas para o resfriamento de altos fluxos de calor em componentes eletrônicos. Sistemas de resfriamento envolvendo a mudança de fase líquido-vapor de um fluido de trabalho, como termossifões bifásicos e ebulição em convecção forçada em microcanais, são atualmente experimentados em diversas aplicações. Dentre as alternativas mais promissoras, destaca-se o resfriamento por jatos bifásicos (spray cooling), o qual consiste na atomização de um fluido de trabalho no estado líquido através de um injetor (ou um arranjo de injetores) direcionado à superfície a ser resfriada. Dependendo das condições de operação, uma fina película de líquido pode cobrir totalmente a superfície aquecida e diferentes regimes de transferência de calor por ebulição podem se fazer presentes. Tais cenários proporcionam elevados valores do coeficiente de transferência de calor por convecção, fazendo com que um alto fluxo de calor possa ser removido mediante um pequeno grau de superaquecimento da superfície do aquecedor. Este trabalho apresenta uma análise teórica do processo de resfriamento com um único spray incidindo perpendicularmente a uma superfície plana voltada para cima. O modelo matemático para o campo de velocidades, temperaturas e concentração de gotículas por unidade de volume de spray é baseado em uma formulação de dois fluidos diferencial quasi-bidimensional que considera a variação radial dos campos de velocidade no spray por meio de perfis gaussianos. Um critério para a existência de um filme contínuo sobre a superfície aquecida é apresentado com base na relação entre a frequência com que as gotas atingem a superfície e o tempo de ciclo de vida das mesmas, o qual é definido com o tempo decorrido ao longo do impacto, espalhamento e secagem total de uma gota. Quando as condições de operação são tais que um filme líquido contínuo é formado, um modelo matemático é proposto para se determinar a variação da espessura da película em função da distância radial a partir do eixo de simetria do jato. Tal modelo é baseado em um balanço de quantidade de movimento no filme, levando em conta a transferência de quantidade de movimento ao filme por parte das gotas incidentes e assumindo que o perfil de velocidades no filme obedeça à lei-log universal. A transferência de calor por ebulição no filme fino é computada com base em um modelo de superposição onde a parcela de ebulição nucleada é computada por meio da correlação de Nishikawa para filmes líquidos finos. O modelo é verificado a partir de dados experimentais obtidos por diversos autores para fluidos de trabalho como água, FC-72 e R-134a, onde se observa uma concordância da ordem de ± 30%para o coeficiente de transferência de calor médio. Um modelo matemático também é apresentado para a transferência de calor no regime de filme líquido descontínuo, o qual é baseado naquele proposto por Aoki para a evaporação de uma gotícula impactando sobre uma superfície. O modelo proposto combina o modelo de Aoki para a fração da superfície coberta pelas gotas com um modelo para a convecção forçada monofásica na fração da superfície em contato direto com o vapor.Over the last twenty years, with the progressive miniaturization trend of engineering systems, there has been a continuous increase of the rates with which heat is dissipated per unit area in electronic equipment. Among the many technologies under constant evaluation and development for cooling of high heat fluxes, such as two-phase thermosyphons, heat pipes and forced convection boiling in microchannels, spray cooling is one of the most promising due to the high heat transfer coefficients involved. The technique consists of atomizing a working fluid through a nozzle (or a nozzle array) directed to ward the surface to be cooled. Depending on the operating conditions, a thin liquid film can cover completely the heated surface and various regimes of boiling heat transfer can be present. These conditions provide high values for the heat transfer coefficient, which enables the removal of the large heat transfer rates with a small superheating of the heater wall. This work presents a theoretical analysis of the cooling process with a single spray perpendicular to an upward-facing flat plate heater. The mathematical model for determining the phase velocities, temperatures and droplet concentration per unit volume in the spray is based on a two-fluid quasi two-dimensional differential formulation which considers the variation of velocities in the radial direction via Gaussian distributions. A criterion for the existence of a continuous film on the heated surface is presented based on the relation between the droplet lifetime on the heated surface and the frequency with which it reaches the heated surface. When the operating conditions are such that a continuous liquid film is formed, a mathematical model is proposed to determine the variation of the film thickness as a function of the radial distance from the spray center line. This model is based on mass and momentum balances, taking into account the momentum transfer to the film by the drops which impact on its free surface. A universal log-law velocity profile is assumed in the liquid film. Boiling heat transfer in the thin film is computed based on a superposition model using the thin liquid film boiling correlation of Nishikawa. The model is verified against experimental data obtained by several authors for working fluids such as water, FC-72 and R-134a, with an average agreement of ± 30%for the average heat transfer coefficient. A mathematical model was also developed for heat transfer in the discontinuous liquid film regime based on the theory advanced by Aoki for the evaporation of a single droplet impacting on a surface. The proposed calculation method combines the Aoki model for the fraction of the surface covered by the drops with a model for the single-phase forced convection heat transfer in the fraction of the surface in direct contact with the vapor
Numerical Investigation of Spray Formation in Coaxial Nozzles
No full textIn this paper the results of numerical investigation of atomization process of liquid fuel (Diesel) in Laval and coaxial nozzles are presented. The calculations have been performed using CFD codes of STAR-CD. The dependences between the different cases of pressure drop and spray formation have been analysed. For that purpose a model to simulate unsteady two-phase atomization process has been employed. Results of transient flow through various 3D nozzle shapes and the resulting spray development have to be discussed. The three type of nozzle (Laval, converging and coaxial) are considered. It was found that for the nozzles under consideration the finer atomization occurs for low initial mass flow rates. The atomization process in such nozzles occurs by fragmentation between high velocity gas jet and fuel volume. Further, the pressure conditions of gas flow are of great relevance for atomization process
Experimental Investigation of Spray Characteristics in Subsonic Crossflows
No full textThe breakup of a liquid jet, in a crossflow leading to the evolution of the spray plume downstream is an important aspect that needs to be well understood for improving combustion efficiency and reduction of emissions for airbreathing propulsion engines. The drop size characteristics of a liquid jet in a non swirling crossflow of air were investigated experimentally at conditions relevant to ramjets and gas turbine afterburners. Experiments were done with a 1.0 mm diameter plain orifice nozzle which was flush mounted on the bottom plate of test section to provide normal injection. Laser diffraction using Malvern Spraytec particle analyzer was used to measure drops size and distributions in the near field of the jet. The momentum flux ratio was varied to address a reasonable range of liquid flow rates as in practical devices. The sprays were characterized using the non dimensional parameters such as the Weber number and the momentum flux ratio and drop sizes were measured at an axial distance of 50 mm from the injector. Results
indicate that as one goes from the bottom to the top of the spray plume, the drop size distribution becomes narrower. Further with increase in the momentum flux ratio the volume concentration across the plume becomes highly non uniform and depends on the measurement point in the expanding spray lume
The 2016 thermal spray roadmap
No full textS.1376-1440Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.25Nr.
Caracterização e atividade antimicrobiana de micropartículas carregadas de ácidos graxos de cadeia curta obtidas por spray drying
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Agrárias, Programa de Pós-Graduação em Ciência dos Alimentos, Florianópolis, 2014.No presente trabalho micropartículas contendo ácidos graxos de cadeia curta (AGCC) foram obtidas por atomização em Spray Dryier. Dez formulações contendo diferentes proporções de amido modificado Capsul®, maltodextrina, goma arábica, L-fosfatidilcolina e Tween foram caracterizadas através de análises físico-químicas (eficiência da microencapsulação, teor de umidade, densidade, Espectroscopia de infravermelho - FTIR (Fourier transform infrared spectroscopy), microscopia eletrônica de varredura - MEV, calorimetria exploratória diferencial - DSC (Differential Scanning Calorimetry) e análise termogravimétrica - TGA (Thermogravimetric analysis). Os AGCC foram submetidos às análises de concentração mínima inibitória (CMI) e concentração mínima bactericida (CMB) a fim de avaliar o potencial antimicrobiano, antes e após o processo de atomização. De acordo com os resultados obtidos, as micropartículas com surfactantes na composição (fosfatidilcolina e Tween 80) apresentaram melhor eficiência de microencapsulção (80%; 77,3% e 76,5% respectivamente), menor teor de umidade e menor densidade aparente. Para estas amostras também foram observadas estruturas na forma de esferas com menor tendência a agregação. Através dos espectros de FTIR foi possível observar que as partículas obtidas apresentaram diferenças na concentração de AGCC retido. Os termogramas de DSC e TGA revelaram a presença de um evento endotérmico, com temperatura onset de fusão de 137,17°C e com perda de massa ( m) de 54,92% entre 304,79°C e 413,87°C. A determinação da concentração mínima inibitória (CMI) e concentração mínima bactericida (CMB) indicaram atividade antimicrobiana para todas as cepas testadas (Escherichia coli - ATCC 25922; Salmonella enteritidis - ATCC 13076; Staphylococcus aureus - ATCC 25923; Staphylococcus aureus - ATCC 29213; Listeria monocytogenes - ATCC 19117 e, Listeria monocytogenes sorotipo 2 - ATCC 19112), antes e após a obtenção das micropartículas carregadas de AGCC. A técnica de spray drying se mostrou um método eficaz para a obtenção de um produto com maior estabilidade e pode tornar mais fácil e eficiente a utilização dos ACGC com atividade antimicrobiana na indústria.Abstract : Microcapsules containing short-chain fatty acids (SCFA) were obtained by atomization Spray dryier. Ten formulations containing different proportions of Capsul ® (modified starch) , maltodextrin, gum arabic , L- phosphatidylcholine and Tween were characterized by physico-chemical analysis (efficiency of microencapsulation , moisture content, density, infrared spectroscopy - FTIR (Fourier transform infrared spectroscopy), scanning electron microscopy - SEM, differential scanning calorimetry and thermogravimetric analysis. The SCFA were analyzed for determinate the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) to evaluate the antimicrobial activity before and after the atomization process. According to the results, it was observed that formulations containing surfactants in the composition (L-phosphatidylcholine and Tween 80) showed better efficiency microencapsulção (80%; 77,3% and 76,5% respectively), less moisture content and less bulk density. For these samples also were observed structures in the form of spheres with fewer tendencies to aggregation. Through the FTIR spectra was observed that the particles obtained showed differences in the concentration of AGCC retained. The DSC and TGA thermograms revealed the presence of an endothermic event with onset melting temperature of 137.17 ° C and weight loss ( m) 54.92 % from 304.79 °C and 413.87 °C. The minimum inhibitory concentration and minimum bactericidal concentration showed antimicrobial activity against all tested strains (Escherichia coli - ATCC 25922 , Salmonella enteritidis - ATCC 13076 , Staphylococcus aureus - ATCC 25923 , Staphylococcus aureus - ATCC 29213, Listeria monocytogenes - ATCC 19117 and Listeria monocytogenes serotype 2 - ATCC 19112) before and after the atomization process of SCFA. The Spray Drying technique proved to be an effective method for obtaining a product with greater stability and can make use of ACGC with antimicrobial activity in the industry easier and more efficient
Large-eddy simulation of kerosene spray combustion in a model scramjet chamber
No full textLarge-eddy simulation (LES) of kerosene spray combustion in a model supersonic combustor with cavity flame holder is carried out. Kerosene is injected through the ceiling of the cavity. The subgrid-scale (SGS) turbulence stress ensor is closed via the Smagorinsky’s eddyviscosity model, chemical source terms are modelled by a finite rate chemistry (FRC) model, and a four-step reduced kerosene combustion kinetic mechanism is adopted. The chamber wallpressure predicted from the LES is validated by experimental data reported in literature. The test case has a cavity length of 77mm and a depth of 8mm. After liquid kerosene is injected through the orifice, most of the droplets are loaded with recirculation fluid momentum inside the cavity. Due to lower velocity of the carrier fluid inside the cavity, sufficient atomization and evaporation take place during the process of droplet transportation, resulting in a rich fuel mixture of kerosene vapour accumulating inside the cavity. These rich fuel mixtures are mixed with fresh air by the approachmixing layer at the front of the cavity and are thus involved in burning accompanied with the approach boundary layer separation extending towards upstream. The combustion flame in the downstream impinges onto the rear wall of the cavity and is then reflected back to the front of the cavity. During the recirculation of hot flow, heat is compensated for evaporation of droplets. The circulation processes mentioned above provide an efficient flame-holdingmechanism to stabilize the flame.Comparisons with results froma shorter length of cavity (cavity length of 45mm) show that, due to insufficient atomization and evaporation of the droplets in the short distance inside the cavity, parts of the droplets are carried out of the cavity through theboundary layer fluctuation and evaporated in the hot flame layer, thus resulting in incomplete air fuel mixing and worse combustion performance. The flow structures inside the cavity play an important role in the spray istribution, thus determining the combustion performance
Spray Drying of beta- Lactoglobulin-Vitamin A and beta-Lactoglobulin-Vitamin D Complexes
No full textβ- Lactoglobulin (β-LG) is a major whey protein of ruminant species. Its amino-acid sequence and 3-dimensional structure show that it is a lipocalin, a serum retinol binding protein. β-LG has been reported to be able to bind a variety of ligands, many of which are hydrophobic compounds. These compounds include retinol (vitamin A), fatty acids, vitamin D, cholesterol, etc. The importance of the binding property is that it can be implemented to deliver these nutrients using β-LG as a carrier without the presence of the fat in which they normally associate.
In the study, two complexes (β-Lactoglobulin-Vitamin A and β-Lactoglobulin-Vitamin D) were produced through spray drying. During the drying process, heat tends to denature the protein and causes the dissociation of the complex, which can result in low retention of vitamins. Certain sugars, such as lactose, were found to stabilize whey protein during spray drying. The study tested the hypothesis that the addition of lactose into the complexes would yield higher recovery of vitamins from spray drying.
The β-Lactoglobulin-Vitamin A complex was formed by mixing retinyl palmitate with 2% β-LG solution in DI water. Cholecalciferol was mixed with 2% β-LG solution in DI water to form β-Lactoglobulin-Vitamin D complex. Both complexes were incubated at 40°C for 2 hr. Binding of vitamin to β-LG was confirmed by fluorescence quenching of the protein at wavelength 332 nm. Lactose was then added into the above complex solutions at 5:1 weight ratio to the protein. Each mixed complex was then pumped into the spray dryer and powder was collected. The content of vitamins in the powder complexes was determined by HPLC analyses.
The results showed that in the presence of lactose, the spray-dried powder had 4-5 times more retention of vitamin A than without lactose and that the content of vitamin D in the powder complex was 1-2 times greater than without lactose.
The study also used the β-lactoglobulin-vitamin A complex to fortify a lemon lime soda and studied the stability of the vitamin A in the complex under different types of light for a period of seven (7) days. The results demonstrated that both lights and acidity lead to the quick degradation of vitamin A in the beverage
Y2O3:Eu micronic particles synthesised by spray pyrolysis: Global modelling and optimisation of the evaporation stage
No full textThere are a number of some major advantages to be gained in processing micronic europium-doped yttrium oxide Y2O3 particles for phosphor applications using spray pyrolysis. In order to maximise production rates, it is tempting to use relatively dense sprays, but then coalescence occurs increasing final particle diameters, which must be prevented. Moreover, the influence of the operating conditions on the process behaviour is poorly understood. A complete one-dimensional model of the evaporation stage of micronic water/Y(NO3)3 droplets considering only the evaporation process and then both evaporation and gravity-induced coalescence phenomena has been established. Calculations of pure evaporation have shown that the amounts of evaporated water and droplet compositions depend only on the local temperature and not on the thermal history of the spray. Coupled calculations have shown that, in comparison with evaporation, coalescence plays a minor role on droplet diameter, but non-negligible as the increase of the final mean droplet diameter due to coalescence reaches up to 10% at low flow rates in the operating conditions tested. Injecting a preheated air flow directly into the nebuliser is a promising method to minimise coalescence effects: optimal operating conditions for which coalescence is completely insignificant were obtained by simulation
Numerical simulation of spray combustion using bio-mass derived liquid fuels
No full textThe main objective of this work is to create a robust model for two-phase liquid spray
combustion flow using vegetable oils, to investigate the flow structure generated by a
swirler array with different fuels, and secondly to assess and optimise the capability of
the CFD to predict accurately the results obtained experimentally and eventually
enhance CFD model development and simulation. Validation is achieved by
comparing the numerical results obtained with CFD with the experimental
measurements.
The purpose of this research is to increase the scientific understanding of the
fundamental mechanisms of the spray combustion process using a carbon neutral fuel
such as ethanol and biodiesel. In fact, very few numerical simulations of liquid
biomass fuels in gas turbine systems are available in the literature.
The flames are simulated using the commercial code FLUENT. The
combustion/turbulence interaction is modelled using the laminar flamelet approach
with detailed chemistry modelled using the OPPDIFF model from CHEMKIN.
While the experiments could be carried out only up to 3 atm, the simulations were
further extended to a maximum pressure of 10 atm. The FLUENT results were
assessed qualitatively and quantitatively between the experimental measurements and
the simulation. The cold flow features have been captured by the present simulations
with a good degree of accuracy. Effect of air preheating was investigated for the
biodiesel, and sensitivity to droplet size and spray angles variation were analysed.
Good agreement was obtained for ethanol except in the fuel lean region due to failure
of the FLUENT laminar flamelet model to capture local flame extinction while
biodiesel simulation resulted in a significant overprediction of the flame temperature
especially in the downstream region and satisfactory results further upstream. The
results show the importance of setting proper droplet initial conditions, since it will
significantly affect the structure of the flame
Numerical investigations of thermal spray coating processes: combustion, supersonic flow, droplet injection, and substrate impingement phenomena
No full textThe aim of this thesis is to apply CFD methods to investigate the system characteristics of high speed thermal spray coating processes in order facilitate technological development. Supersonic flow phenomena, combustion, discrete droplet and particle migration with heating, phase change and disintegration, and particle impingement phenomena at the substrate are studied. Each published set of results provide an individual understanding of the underlying physics which control different aspects of thermal spray systems.A wide range of parametric studies have been carried out for HVOF, warm spray, and cold spay systems in order to build a better understanding of process design requirements. These parameters include: nozzle cross-section shape, particle size, processing gas type, nozzle throat diameter, and combustion chamber size. Detailed descriptions of the gas phase characteristics through liquid fuelled HVOF, warm spray, and cold spray systems are built and the interrelations between the gas and powder particle phases are discussed. A further study looks in detail at the disintegration of discrete phase water droplets, providing a new insight to the mechanisms which control droplet disintegration, and serves as a fundamental reference for future developments of liquid feedstock devices.In parallel with these gas-particle-droplet simulations, the impingement of molten and semi-molten powder droplets at the substrate is investigated and the models applied simulate the impingement, spreading and solidification. The results obtained shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity. The results obtained also visually describe the freezing induced break-up phenomenon at the splat periphery
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