Pressurization of a Flightweight, Liquid Hydrogen Tank: Evaporation & Condensation at a Liquid/Vapor Interface

This paper presents an analysis and simulation of evaporation and condensation at a motionless liquid/vapor interface. A 1-D model equation, emphasizing heat and mass transfer at the interface, is solved in two ways, and incorporated into a subgrid interface model within a CFD simulation. Simulation predictions are compared with experimental data from the CPST Engineering Design Unit tank, a cryogenic fluid management test tank in 1-g. The numerical challenge here is the physics of the liquid/vapor interface; pressurizing the ullage heats it by several degrees, and sets up an interfacial temperature gradient that transfers heat to the liquid phase-the rate limiting step of condensation is heat conducted through the liquid and vapor. This physics occurs in thin thermal layers O(1 mm) on either side of the interface which is resolved by the subgrid interface model. An accommodation coefficient of 1.0 is used in the simulations which is consistent with theory and measurements. This model is predictive of evaporation/condensation rates, that is, there is no parameter tuning

Numerical Modeling of the Performance of a Paraffin Hybrid Rocket Motor

In this work were performed several CFD simulations of a hybrid rocket with paraffin fuel and nitrous oxide was oxidizer. A study of the regression rate of solid grain during combustion is performed to understand the influence on the performance of the motor. The numerical mesh is created using ANSYS ICEM Meshing and the commercial software ANSYS Fluent 20.2 is used to perform the numerical study. For making this, an unsteady numerical model is established with a dynamic mesh to simulate the regression process of the solid fuel surface. The viscous model used is the Reynolds Reynolds Stress model (RSM). All of the air­inlets are defined as mass­flow inlets, and the exit is defined as a pressure­outlet. In this thesis no multi­phase flow simulation will be done, all the chemical elements are considered to be in a gaseous state. First, it will be performed analyse with the swirl injection where the performance points will be compared with the experimental results. Then, a new injection configuration (axial injection) is studied to analyse the performance improvement that can bring to the hybrid rocket engine. The final results show reasonable agreement with the experimental values, with one of the biggest achievements was to visualize the flow inside the chamber and understand what is happening and where the design can be improved. Overall, it can be concluded that this CFD simulation can help the designer during the development of a new hybrid rocket motor.Neste trabalho foram realizadas várias simulações em CFD de um foguete híbrido com combustível de parafina e para o oxidante óxido nitroso. Foi realizado um estudo da taxa de regressão do combustivel sólido durante a combustão, com o intuito de compreender a influência no desempenho do motor. A malha é criada utilizando o ANSYS ICEM Meshing e o software comercial ANSYS Fluent 20.2 é utilizado para realizar o estudo numérico. Para realizar este trabalho, um modelo numérico unsteady é escolhido com uma malha dinâmica para simular o processo de regressão da superfície do combustível sólido. O modelo viscoso utilizado é o Reynolds Stress model (RSM). Todas as entradas de ar são definidos como entradas de caudal mássico, e a saída é definida como uma saída de pressão. Nesta tese não será feita uma simulação com várias fases, todos os elementos químicos são considerados como estando em estado gasoso. Em primeiro lugar, será realizada uma análise com a injecção com swirl onde os pontos de desempenho serão comparados com os resultados experimentais. Depois, é estudada uma nova configuração de injecção (injecção axial) para analisar a melhoria do desempenho que pode trazer ao motor do foguete híbrido. Os resultados finais mostram uma concordância razoável com os valores experimentais, sendo uma das maiores realizações a visualização do fluxo dentro da câmara o que permite compreender o escoamento dentro do motor e que por sua vez leva a compreender os pontos onde se pode melhorar a nível do design do mesmo. Globalmente, pode concluir­se que esta simulação CFD pode ajudar o projectista durante o desenvolvimento de um novo motor de foguetão híbrido

Low Gravity Natural Convection and Pool Boiling Predictions

Cryogenic fluids are used in various fields, such as biomedical technology, food transportation, and aerospace. Liquid hydrogen and oxygen are cryogenic fluids that can act as energy-dense fuels for long-term, large payload space flight. Much of the resulting research in long-duration cryogenic fluid storage has focused on zero-boil-off tank pressure control in low gravity environments. The high cost of testing these systems in low gravity has created a need to develop accurate models of the behavior of fluids in these environments. Simulaitons of tank pressure control require accurate models of evaporation and condensation phase change. Recent advancements in commercial fluid dynamics (CFD) simulation software offer potential to evaluate evaporation and condensation phase change models in low gravity two phase flows. The goal of this research is to use ANSYS FLUENT, a commercial CFD software, to simulate pool boiling in low gravity. The volume of fluid approach with the Lee phase change model, which predicts heat and mass transfer at the liquid vapor interface, has been recently added to FLUENT. Predictions of low gravity natural convection and nucleate boiling using FLUENT are presented and are compared to experimental data and previously published numerical simulations

Simulation of Bulk Evaporation and Condensation in Cryogenic Propellant Tanks using the Energy of Fluid Method

The design of cryogenic propellant storage systems for long duration space missions relies on accurate prediction of tank self-pressurization. Incident solar radiation heats the cryogenic liquids in the tank over time, vaporizing the cryogenic liquid. As the liquid vaporizes, the tank pressure increases. The objective of the current research is to develop a finite volume based Compuational Fluid Dynamic (CFD) model of tank pressurization in reduced gravity using an Energy of Fluid (EOF) method. A commercially available CFD model is significantly enhanced to include the EOF method, which will solve the energy equation in terms of internal energy. Model validation results are presented which include a comparison of temperature and pressure predictions to the data collected during the terrestrial experiments performed by Aydelott and the low gravity experiment conducted onboard the Saturn IB AS203 tank

Radiation Effects on Flow Characteristics in Combustion Chambers

A JANNAF sponsored workshop was held to discuss the importance and role of radiative heat transfer in rocket combustion chambers. The potential impact of radiative transfer on hardware design, reliability, and performance was discussed. The current state of radiative transfer prediction capability in CFD modeling was reviewed and concluded to be substantially lacking in both the physical models used and the radiative property data available. There is a clear need to begin to establish a data base for making radiation calculations in rocket combustion chambers. A natural starting point for this effort would be the NASA thermochemical equilibrium code (CEC)

Study and development of a holdingchamber for inhalation

Tese de Doutoramento em Engenharia MecânicaAsthma is a respiratory disease that causes chronic airway inflammation. Affecting more than 300 million individuals worldwide, it is a growing public health hazard. Inhalation therapy is the preferred strategy for medication delivery. This therapy is executed through specific delivery devices, whereas the pressurized Metered-Dose Inhaler (pMDI) is one of the most preferred. However, the pMDI efficiency is highly dependent on a correctly executed inhalation procedure. For children under 5 years old (or elderly individuals), it is advisable to use the pMDI coupled with an add-on device (i.e. spacer). Within the spacers, the Valved Holding Chamber (VHC) is the mostly used, due to its good capacity to reduce the pMDI spray coarse fraction and the oral-pharyngeal deposition ( 80%). Additionally, the VHC’s one-way valve allows the patient to maintain his tidal breathing during treatment. The VHC typically delivers a Fine Particle Mass (FPM) that is 20% of the labelled dose. Several design characteristics dictate the VHC performance, such as, the dimensions and the materials. The study herein focus on the assessment of eight commercial VHCs, through experimental and numerical methods. An experimental setup was developed, allowing the evaluation of the devices at constant flow rate (30 L/min and 60 L/min) and at variable flow (sine breath pattern). The waveform was obtained through a breathing simulator specially developed for this purpose, based in a cam-follower mechanism. The salbutamol sulphate (i.e. Ventolin) was collected using a cascade impactor (i.e. MSLI), and assessed by UV-Vis spectrophotometry analysis. Several metrics, regarding VHC performance, were calculated. Results have shown that the VHC capacity reduce the oral-pharyngeal deposition (64% - 94%), which is deeply related with the VHC valve design. It was observed that the VHC reduces the plume coarse fraction ( 70%), keeping the FPM bioequivalent to the pMDI solo. A correlation between the fine particle fraction and the volume of air passing through the VHC was proposed. Patient relevant metrics were suggested to classify the VHC devices upon quantitative and qualitative characteristics. A Computational Fluid Dynamics (CFD) model was developed where the air flow (i.e. 60 L/min)) was calculated along with the pMDI spray modelling as a discrete phase. The spray particle-wall interaction was modelled using different approaches and compared against literature and experimental data. This study, shed some light upon the spray evaporation process inside the VHC, showing that the efficiency of evaporation process is related with the VHC volume. A new VHC design, based in CFD dimensional optimisation of the VHC body is proposed, which shows an improvement of the FPM delivered.A asma é uma doença respiratória que causa a inflamação crónica das vias aéreas. Mundialmente, afeta mais de 300 milhões de indivíduos e é um problema crescente de saúde publica. A terapia de inalação é a estratégia preferida para administrar a medicação de controlo ou de alívio. Esta terapia é executada através de dispositivos específicos, entre os quais o Inalador Pressurizado com Válvula Doseadora (IPVD) é o mais usual. Contudo, a eficiência do IPVD é dependente de uma técnica de inalação correta. Para crianças com menos de 5 anos (ou idosos), é recomendável o uso do IPVD acoplado a um espaçador. Entre os espaçadores, a Câmara Expansora (CE) é a mais utilizada, devido à sua boa capacidade de redução das partículas grandes do aerossol do IPVD, e da redução da deposição orofaringeal ( 80%). Adicionalmente, a válvula de sentido único da CE, permite que o paciente mantenha a sua respiração normal durante o tratamento. A CE emite, tipicamente, uma massa de partículas finas (MPF) que é 20% da dose calibrada do IPVD. Este estudo foca-se na avaliação de oito CEs, através de uma metodologia experimental e numérica. Uma instalação experimental foi projetada para a avaliação dos dispositivos a fluxo constante (30 L/min e 60 L/min) e variável (um padrão respiratório sinusoidal). A onda foi obtida através de um simulador respiratório especialmente desenvolvido para este propósito, o qual foi baseado num mecanismo cam-seguidor. O sulfato de salbutamol (Ventilan HFA) foi recolhido utilizando um impactor em cascata em vários estágios (Aparelho C da Farmacopeia Portuguesa), e quantificado por espetrofotometria UV-Visivel. Foram calculadas várias métricas sobre o desempenho das CEs. Os resultados demonstram a capacidade da CE para reduzir a deposição orofaringeal (64% - 94%), a qual está intrinsecamente relacionada com o design da válvula do dispositivo. Foi observado que a CE reduz a fração de partículas grandes na pluma ( 70%), mantendo a MPF bioequivalente à emitida pelo IPVD. Foi proposta uma correlação entre a fração de partículas finas e o volume de ar que atravessa a CE. Foram também sugeridas métricas com relevância para o paciente, que classificam as CEs de forma quantitativa e qualitativa. Foi desenvolvido um modelo de Dinâmica Computacional de Fluidos (DCF), onde o fluxo de ar (a 60 L/min) foi calculado juntamente com o aerossol do IPVD, tendo sido este modelado como uma fase discreta. A interação entre partícula e parede foi modelada utilizando diferentes aproximações matemáticas, sendo posteriormente comparadas com a literatura e dados experimentais. Este estudo contribui com um melhor conhecimento do processo de evaporação das gotas do aerossol dentro da CE, onde se verificou que este processo está relacionado com o volume da CE. Foi proposto em novo design para CE, baseado numa otimização das dimensões do corpo da CE, que demonstra melhoria da MPF emitida

Investigation of Flow and Heat Transfer Characteristics in Two-Phase Flow Through Evaporator of Diffusion Absorption Refrigerator (DAR)

The application of the cooling system becomes very important for a comfortable and almost all public buildings in Indonesia using the cooling system. The cooling system is commonly requires considerable electrical power to drive the compressor used for the vapor compression cooling system in operation. Different with the diffusion absorption refrigeration system, all cycles in the system are occurred by natural circulation where the refrigerant is pressurized (partial) lower in the evaporator is obtained with the aid of an inert gas that can be either hydrogen or helium, which is mixed with the refrigerant gas, but not also react. The study was conducted by flowing the refrigerant at the evaporator copper tube passes that are part of a diffusion absorption refrigeration machine. Long dimension of passes evaporator is 2.38875 m. The method used in this study is the experimental and numerical approach, which is using Multiphase Flow Fluent CFD 6.3. simulation to validate the experimental observations of flow.Through this study, the fenonena happened to be validated by experimental observations numerically. Also can be seen flow characteristics of refrigerant flowing along the evaporator and heat transfer phenomena that occur

CFD simulation of pressurised metered-does inhaler (pMDI)

Pressurised metered dose inhalers (pMDI) are the most widely used aerosol delivery devices to treat asthma and COPD due to its unique ability to produce numerous inhalable droplets. However the mechanism leading to droplet generation is elusive, mainly due to small length scales and short time scale, causing experimental difficulties to obtain flow information. Such lack of insight has - to date - limited predictive capability of theoretical approaches and impeded device optimisation. The main aim of this research is to improve understanding of the thermo-fluid dynamic processes leading to droplet generation by constructing validated numerical models to predict pMDI aerosol characteristics as a function of device geometry and formulation composition. The thesis presents a systematic study of existing two-phase flow models to predict the flow conditions and the rate of propellant flow through a pMDI actuator: the homogeneous equilibrium model (HEM), the slip equilibrium model (SEM) and the homogeneous frozen model (HFM). [Continues.

AN ASSESSMENT OF THE VALIDITY OF THE KINETIC MODEL FOR LIQUID-VAPOR PHASE CHANGE BY EXAMINING CRYOGENIC PROPELLANTS

Evaporation is ubiquitous in nature and occurs even in a microgravity space envi- ronment. Long term space missions require storage of cryogenic propellents and an accurate prediction of phase change rates. Kinetic theory has been used to model and predict evaporation rates for over a century but the reported values of accommodation coefficients are highly inconsistent and no accurate data is available for cryogens. The proposed study involves a combined experimental and computational approach to ex- tract the accommodation coefficients. Neutron imaging is used as the visualization technique due to the difference in attenuation between the cryogen and the metallic container. Phase change tests are conducted using liquid hydrogen and methane at a range of saturation points between 15 psia and 30 psia. In order to account for the thermal gradient in the wall at the interface, a CFD thermal model is employed. Results from neutron imaging and the thermal model serve as boundary conditions to a transition film kinetic model. Using a combination of neutron imaging, CFD thermal model and kinetic model, there is a possibility to extract the accommodation coefficient while accounting for the curvature, disjoining pressure, nanoscale interac- tions and a variable wall temperature at the interface. An accommodation coefficient of 0.5705 ± 0.0001 is obtained for liquid hydrogen evaporating from a 10mm Al6061 cylinder at 21K using a constant wall temperature of 21.00005