<div>Gas-turbine engines use film cooling to actively cool turbine components and keep thermal loads on the materials at acceptable levels for structural integrity and service life. The turbulent mixing between the film-cooling jet and the crossflow decreases the coolant temperature, which reduces the cooling performance. This turbulent mixing is sensitive to parameters such as density ratio (DR), blowing ratio (BR), velocity ratio (VR), and momentum-flux ratio (MR) and understanding the effects of these parameters on the turbulent mixing is critical for improving film cooling. </div><div><br></div><div>This research seeks to improve understanding by using large-eddy simulation (LES) as a tool to analyze the turbulence of film cooling. With this knowledge it is possible evaluate more fundamental turbulence modeling assumptions utilized by Reynolds-Averaged Navier-Stokes (RANS) approaches as they apply to film cooling. This analysis can provide insight regarding how to improve turbulence models.</div><div><br></div><div>The film-cooling problem studied involves the cooling of a flat plate, where the cooling jets issued from a plenum through one row of circular holes of diameter $D$ and length 4.7$D$ that are inclined at 35$^\circ$ relative to the plate. Parameters studied include BR = 0.5 - 1.3, DR = 1.1 - 2.1, VR = 0.3 - 0.9, and MR = 0.16 - 0.9. For LES, two different boundary layers upstream of the film-cooling hole were investigated - one in which a laminar boundary layer was tripped to become turbulent from near the leading edge of the flat plate, and another in which a mean turbulent BL is prescribed directly without any superimposed turbulent fluctuations. For RANS, two different turbulence models were investigated - realizable $k$-$\epsilon$ and $k$-$\omega$ shear-stress-transport (SST). The wall-resolved LES solutions generated are extensively verified and validated using analytical, DNS, and experimental measurements to ensure high quality. </div><div><br></div><div>LES results obtained show that having an upstream boundary layer that does not have turbulent fluctuations enhances the cooling effectiveness significantly at low VRs when compared to an upstream boundary layer that resolved the turbulent fluctuations. However, these differences diminish at higher VRs. Instantaneous flow reveals a bifurcation in the jet vorticity as it exits the hole at low VRs, one branch forming the shear-layer vortex, while the other forms the counter-rotating vortex pair. At higher VRs, the shear layer vorticity is found to reverse direction, changing the nature of the turbulence and the heat transfer. Results obtained also show the strength and structure of the turbulence in the film-cooling jet to be strongly correlated to VR. </div><div><br></div><div>RANS results obtained show the turbulent and thermal structure of the jets predicted by the two RANS models to differ considerably. However, both models are consistent in underpredicting the spread of the film-cooling jet. The counter-rotating vortex pair dominates the interaction of the jet and crossflow in the near-wall region, and neither RANS model could predict the strength and structure of this interaction. The gradient-diffusion and Boussinesq hypotheses were evaluated by using the LES data. Comparing LES and RANS results shows that $k$-$\epsilon$ tends to overpredict eddy viscosity, while SST tends to underpredict the eddy viscosity. Additionally, both models predict very low values of eddy viscosity near the wall which leads to incorrect Reynolds stresses. While regions of counter-gradient diffusion and stress-strain misalignment were identified in the near-wall region, further above the wall, the jet behaved according to the hypotheses.</div><div><br></div><div>The turbulence scaling when VR is fixed at 0.46 and 0.63 was investigated. The LES results show that separation and spreading of the film-cooling jet increase as BR, DR, and MR increase while VR remains constant. For a given VR, the LES predicts an absolute difference between the minimum adiabatic effectiveness of the lowest and highest MRs to be 2 to 5 times greater than those predicted by RANS. This is because RANS with either model cannot respond appropriately to changes in MR. However, RANS can correctly predict that adiabatic effectiveness decreases as VR increases. The LES results show the turbulent kinetic energy and Reynolds stresses near the film-cooling hole to change considerably with MRs at a constant VR, while turbulent heat flux changes negligibly. This suggests that while improved turbulence models for heat flux can improve RANS’ prediction of spreading, capturing trends, however, requires improved modeling of the Reynolds stresses.</div>

Thesis (M.Sc.(Engineering))--University of the Witwatersrand, Faculty of Engineering, 1999

Orientador: Paulo César Razuk / Banca: Augusto Ronchi Junior / Banca: Waldemar Gastoni Venturini Filho / Resumo: O produtor de leite de cabra, com o apoio e incentivo de entidades governamentais, privadas e ONGs; agrega tecnologia ao leite garantindo uma melhor distribuição de renda e qualidade de vida, fixando-se cada vez mais no campo. O resfriamento do leite após a ordenha é a melhor medida para retardar a proliferação natural das bactérias e sua temperatura deverá ser reduzida de 36 ºC para 4 ºC no período máximo de 2 horas em cada ordenha. O tempo transcorrido entre a primeira ordenha e seu recebimento na usina de beneficiamento deverá ser, no máximo, de 48 horas. Neste trabalho propõe-se projetar e analisar tanques de resfriamento de leite com capacidades úteis de 320, 500 e 2000 litros em aço inoxidável austenítico AISI 304, utilizando-se do coeficiente global médio (U) e das temperaturas médias de saturação do fluido de refrigeração (X) obtidos em ensaios experimentais. Um objetivo secundário foi verificar a possibilidade da utilização de caixas d'água de polietileno com a mesma capacidade dos tanques, por ser um produto acessível, de baixo custo e que atende as exigências sanitárias, visando obter um produto que possa substituir o aço inoxidável, mas descartou-se esta possibilidade, pois o fundo plano destas caixas dificultaria a retirada do produto e a limpeza. Nos cálculos envolvidos na geometria para a construção dos tanques, utilizou-se da relação entre a altura cilíndrica e o diâmetro que contemplasse respectivamente, o número... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The goat milk producer, with support and encouragement of governmental and private agencies as well as NGOs, aggregate technology to milk production ensuring a better distribution of income and life quality for all his family, making it possible the maintenance of the country life. The milk cooling ater the milking process is the best alternative to delay the natural proliferation of bacteria. The milk temperature should be reduced from 36 ºC to 4 ºC in a maximum period of time of two hours for each milking. The elapsed time between the first milking and the milk delivery in the processing plant shall be at most 48 hours. In this work, we propose to design and analyze milk cooling AISI 304 austenitic stainless ateel 320, 500 and 2000 liters tanks, using the mean global coefficient (U) and mean saturation temperatures of the refrigeration fluid (X), obtained experimentally. Besides stainless steel built tanks, accessible low cost polyethylene watertanks, with same dimensions of the steel tanks, attending the necessary sanitary requirements were used, aiming for an alternative product that can replace stainless steel. However, this possibility was rejected, because its flat basis would make it hard the product removal and its cleaning. During the geometric calculations in the manufacturing of the tanks, the relationship betwwen the cylindrical height and the diameter was defined in order to provide the golden ratio and batch square... (Complete abstract click electronic access below) / Mestre

Neste trabalho foi realizada a modelagem fenomenológica do desempenho de uma torre de resfriamento de água e de um sistema de duas torres de resfriamento em série, com temperatura de água de entrada de até 65 ºC. Verificou-se a validade do modelo através de comparação dos resultados previstos com os obtidos em ensaios em uma unidade piloto. Em seguida, através de simulação matemática, a partir do modelo desenvolvido, foi feito o estudo de diversos casos de aplicação. Estudou-se a influência das principais variáveis de operação (vazão de ar, vazão de água e temperatura de bulbo úmido) no desempenho de torres de resfriamento acopladas (em série e em paralelo) e no custo operacional. Verifica-se que a variável de maior influência é a vazão de água que circula pela torre. / This is a study about the phenomenological modeling of the performance of a water cooling tower and a system of two cooling towers in series, with water temperature input up to 65°C. The validity of model was verified by comparing the expected results with those obtained in tests on a pilot plant. After this, through a mathematical simulation, based on the model developed, several cases of application were analyzed. It was studied the influence of main operating variables (air flow, water flow and wet bulb temperature) on the performance of couples cooling towers (in series and parallel) and on operational cost. It was verified the most influential variable is the water flow rate through the tower.

Programa computacional em Matlab-Simulink para simular o comportamento do sistema de arrefecimento de veículos comerciais (vans, ônibus e caminhões) equipados com motores diesel. O programa está embasado em uma modelagem matemática que visa caracterizar o funcionamento dos componentes principais do sistema: motor, radiador, termostato, by-pass e bomba d\'água. Tendo como entrada dados característicos do veículo em estudo, o programa computacional fornece a distribuição de temperatura ao longo do tempo para o líquido de arrefecimento - parâmetro preponderante em uma análise do sistema. Os resultados da simulação permitem aos projetistas prever a atuação do sistema de arrefecimento em diversas condições, o que possibilita a realização de um número menor de testes de pista. Os benefícios de um sistema de arrefecimento projetado adequadamente podem ser notados em relação à economia de combustível, ao aumento de desempenho e à redução do desgaste de determinadas peças do motor e da emissão de poluentes. / A software was developed having Matlab-Simulink as basis and it simulates the behavior of the cooling system in commercial vehicles (vans, bus and trucks) equipped with diesel engines. The program is based on mathematical models to describe the main components of the system: engine, radiator, thermostat, by-pass and water pump. Having as input data the features of the vehicle in studied, the software supplies the cooling fluid temperature distribution during certain time - preponderant parameter in the analysis of the system. The results of the simulation allow the designers to foresee the cooling system performance in several conditions, decreasing the number of track tests. The benefits of an adequate designed cooling system project can be noticed considering fuel economy, performance improvement and decrease of wearing of specific parts in the engine as well as pollutant emissions.

Resumo: Em alguns tipos de indústria, como por exemplo a de laticínios, bebidas e frigoríficos, o sistema de refrigeração é responsável por uma parcela significativa no consumo de energia elétrica. Tendo como objetivo a redução deste consumo, foi realizado um estudo de caso em uma indústria produtora de bebidas, sendo analisados todos os componentes do sistema de refrigeração. O sistema de refrigeração analisado utiliza amônia anidra (NH3) como fluido refrigerante no circuito principal e solução de etanol no circuito secundário, sendo composto por compressores do tipo parafuso, condensadores evaporativos, evaporadores de placas, conjunto de bombas e válvulas expansoras com controle eletrônico. Usualmente, os sistemas de refrigeração atendem diversos processos e etapas de produção, que diferem nos requisitos de temperatura de produto. Além disso, por serem projetados para atender a maior demanda esperada, constituem uma grande oportunidade de otimização nos períodos fora de pico de consumo. Foram descritos e detalhados os componentes do sistema de refrigeração, bem como todas as ações e requisitos necessários para melhoria na eficiência energética da instalação. A implementação destas ações de melhoria de eficiência na planta industrial estudada contou com um investimento total de R$ 302.000,00, e foram responsáveis por um ganho anual de R$ 241.000,00, correspondentes a uma redução no consumo elétrico de 1,7 GWh/ano. O payback simples, portanto, é de 1,25 anos / Abstract: In some types of industry where cooling systems are required, for example, dairy products, beverages and frozen foods, the cooling system is responsible for a significant portion of electricity consumption. Aiming to reduce this consumption, a case study was performed in a beverage industry, and all the refrigeration systems components were analyzed. The cooling system operates using anhydrous ammonia (NH3) as the refrigerant in the main circuit and ethanol solution in the secondary circuit, it consists of screw compressors, evaporative condensers, plate and frame coolers, pumps and expansion valves with electronic control. Usually, refrigeration systems have several processes and production stages to meet for different products, depending on the products temperature requirements. Moreover, they are designed to attain the highest demand expected, which gives a great opportunity to optimize the off-peak periods of consumption. We have described and itemized whole refrigeration systems components, as well as all actions and requirements to improve the energy efficiency of the installation. The energy efficiency improvements actions that were implemented in this industrial plant study had a total investment of R$ 302,000.00. These improvementes resulted in an annual saving of R$ 241,000.00, as well a relative a reduction in electricity consumption of 1,7 GWh/year. Therefore, the simple payback is 1.25 years / Orientador: Pedro Magalhães Sobrinho / Coorientador: José Luz Silveira / Banca: José Alexandre Matelli / Banca: Christian Jeremi Coronado Rodriguez / Mestre

No description available.

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. / Includes bibliographical references. / M.S.

Ultracold molecules promise a revolutionary test bed for quantum science with applications ranging from experiments that probe the nature of our universe, to hosting new platforms for quantum computing. Cooling and trapping molecules in the ultracold regime is the first step to unlocking the wide array of proposed applications, and developing these techniques to control molecules is a key but challenging research field. In this thesis, we describe progress towards a new apparatus designed to cool and trap barium monohydride (BaH), a molecule that is amenable to laser cooling and has prospects as a precursor for ultracold atomic hydrogen. The same complexity that makes molecules interesting objects of study creates challenges for optical control. To mitigate some of these challenges, we first cool the molecules using cryogenic techniques and technologies. Our apparatus uses a cryogenic buffer gas to thermalize BaH within a contained cell. The molecules are extracted into a beam with millikelvin transverse temperature, and forward velocities <100 m/s. The BaH beam in this work is the brightest hydride beam to date, with molecule density and kinetic characteristics well suited for laser cooling and trapping.

The role of radiative cooling plays an important role in the formation of structures in collapsing gas. In this dissertation I examine the impact of cooling in two formation scenarios: first, the role of H2 cooling in collapsing gas in primordial dark matter halos in the possible formation of supermassive black holes; second, low metallicity cooling in collapsing clouds and its possible role in explaining low-metallicity globular clusters. Further, I introduce a new hydrodynamics code, with a design guided by current software principles. In chapter 2, I examined the proposed mechanism to explain the formation of super-massive black holes through direct collapse. The presence of quasars at redshifts z > 6 indicates the existence of supermassive black holes (SMBHs) as massive as a few times 10^9 mass of the sun, challenging models for SMBH formation. One pathway is through the direct collapse of gas in T_vir ≳ 10^4 K halos; however, this requires the suppression of H2 cooling to prevent fragmentation. In this dissertation, I examine a proposed mechanism for this suppression which relies on cold-mode accretion flows leading to shocks at high densities (n > 10^4 cm^−3 ) and temperatures (T > 10^4 K). In such gas, H2 is efficiently collisionally dissociated. I use high-resolution numerical simulations to test this idea, demonstrating that such halos typically have lower temperature progenitors, in which cooling is efficient. Those halos do show filamentary flows; however, the gas shocks at or near the virial radius (at low densities), thus preventing the proposed collisional mechanism from operating. I do find that, if we artificially suppress H2 formation with a high UV background, so as to allow gas in the halo center to enter the high-temperature, high-density “zone of no return”, it will remain there even if the UV flux is turned off, collapsing to high density at high temperature. Due to computational limitations, we simulated only three halos. However, we demonstrate, using Monte Carlo calculations of 10^6 halo merger histories, that a few rare halos could assemble rapidly enough to avoid efficient H2 cooling in all of their progenitor halos, provided that the UV background exceeds J_21 ∼ few at redshifts as high as z ∼ 20. In chapter 3, I explore the relative role of small-scale fragmentation and global collapse in low-metallicity clouds, pointing out that in such clouds the cooling time may be longer than the dynamical time, allowing the cloud to collapse globally before it can fragment. This, I suggest, may help to explain the formation of the low-metallicity globular cluster population, since such dense stellar systems need a large amount of gas to be collected in a small region (without significant feedback during the collapse). To explore this further, I carried out numerical simulations of low-metallicity Bonner-Ebert stable gas clouds, demonstrating that there exists a critical metallicity (between 0.001 and 0.01 metallicity of the sun ) below which the cloud collapses globally without fragmentation. I also run simulations including a background radiative heating source, showing that this can also produce clouds that do not fragment, and that the critical metallicity – which can exceed the no-radiation case – increases with the heating rate. Lastly in chapter 4, I describe the structure and implementation of the new open-source parallel moving-mesh hydrodynamic code, Python Hydro-Dynamics (phd). The code has been written from the ground up to be easy to use and facilitate future modifications. The code is written in a mixture of Python and Cython and makes extensive use of object-oriented programming. I outline the algorithms used and describe the design philosophy and the reasoning of my choices during the code development. I end by validating the code through a series of test problems.