Submitted by Cátia Araújo (catia.araujo@unipampa.edu.br) on 2017-09-29T13:05:00Z No. of bitstreams: 1 Mariovane Sabino Donini - 2017.pdf: 4347435 bytes, checksum: b83edb6c2d0b7868757722dc435be9fa (MD5) / Approved for entry into archive by Marlucy Farias Medeiros (marlucy.farias@unipampa.edu.br) on 2017-09-29T16:25:43Z (GMT) No. of bitstreams: 1 Mariovane Sabino Donini - 2017.pdf: 4347435 bytes, checksum: b83edb6c2d0b7868757722dc435be9fa (MD5) / Made available in DSpace on 2017-09-29T16:25:43Z (GMT). No. of bitstreams: 1 Mariovane Sabino Donini - 2017.pdf: 4347435 bytes, checksum: b83edb6c2d0b7868757722dc435be9fa (MD5) Previous issue date: 2017 / In the present work, vaporization and combustion of an isolated fuel droplet at diferente ambient temperatures are examined numerically in order to analyze the effect of buoyancy force on the flame. Generally, fuel droplets in combustion devices are so small that the influence of buoyancy force on vaporization and combustion of droplets is negligible. On the other hand, fuel droplets in experimental devices are affected by the buoyancy force due to their diameters being around or more than 1 mm. To reduce the buoyancy effects, expensive experimental studies are performed in microgravity ambient (drop-tower or out of space). In normal-gravity conditions, the buoyancy force is induced by temperature gradient on ambient atmosphere. The buoyancy is positive in regions of hot gases and negative in regions of cold gases compared with the ambient atmosphere gas. Hot gases move upward and cold gases downward. Playing with the positive buoyancy force of hot gases around the flame and with the negative (cold) buoyancy force of cold gases around the droplet via ambient atmosphere temperature, it is possible to modify the flame shape. In the numerical simulations, incompressible Navier–Stokes equations along with mixture fraction and excess enthalpy conservation equations are solved using a finite volume technique with a uniform structured grid. An artificial compressibility method was applied to reach steady state solutions. The numerical predictions have been compared with analytical results for a zero gravity condition, showing good agreement. For normal gravity condition the numerical results showed that when the ambient temperature increases, the velocity gradient and buoyancy source term decreases. Despite that, the flame increased in all directions. The results have also shown that increasing the ambient temperature, decreases the temperature gradient in the flame, which ends up affecting the flame position. / No presente trabalho, a vaporização e a combustão de uma gota de combustível isolada a diferentes temperaturas ambiente são examinadas numericamente para analisar o efeito da força de flutuação na chama. Geralmente, as gotículas de combustível em dispositivos de combustão são tão pequenas que a influência da força de flutuação na vaporização e na combustão de gotículas é insignificante. Por outro lado, as gotículas de combustível em dispositivos experimentais são afetadas pela força de flutuabilidade devido ao seu diâmetro em torno de ou mais de 1 mm. Para reduzir os efeitos de flutuabilidade, estudos experimentais caros são realizados em ambiente de microgravidade (drop-tower ou fora do espaço). Em condições de gravidade normal, a força de flutuação é induzida por gradiente de temperatura na atmosfera ambiente. A flutuabilidade é positiva em regiões de gases quentes e negativas em regiões de gases frios em comparação com o gás atmosférico ambiente. Os gases quentes movem-se para cima e os gases frios para baixo. Jogando com a força de flutuação positiva dos gases quentes ao redor da chama e com a força de flutuação negativa (fria) dos gases frios ao redor da gota através da temperatura da atmosfera ambiente, é possível modificar a forma da chama. Nas simulações numéricas, as equações de Navier-Stokes incompressíveis juntamente com a fração de mistura e as equações de conservação de entalpia em excesso são resolvidas usando uma técnica de volume finito com uma grade estruturada uniforme. Foi aplicado um método de compressibilidade artificial para alcançar soluções de estado estacionário. As previsões numéricas foram comparadas com resultados analíticos para uma condição de gravidade zero, mostrando boa concordância. Para a condição de gravidade normal, os resultados numéricos mostraram que, quando a temperatura ambiente aumenta, o gradiente de velocidade e o termo da fonte de flutuação diminuem. Apesar disso, a chama aumentou em todas as direções. Os resultados também mostraram que aumentar a temperatura ambiente, diminui o gradiente de temperatura na chama, o que acaba afetando a posição da chama.

In spite of recent attention to renewable sources of energy, liquid hydrocarbon fuels are still the main source of energy for industrial and transportation systems. Manufactures and consumers are consistently looking for ways to optimize the efficiency of fuel combustion in terms of cost, emissions and consumer safety. In this regard, increasing burning rate of liquid fuels has been of special interest in both industrial and transportation systems. Recent studies have shown that adding combustible nano-particles could have promising effects on improving combustion performance of liquid fuels. Combustible nano-particles could enhance radiative and conductive heat transfer and also mixing within the droplet. Polymeric additive have also shown promising effect on improving fire safety by suppressing spreading behavior and splatter formation in case of crash scenario. Polymers are also known to have higher burning rate than regular hydrocarbon fuels. Therefore adding polymeric additive could have the potential to increase the burning rate. In this work, combustion dynamics of liquid fuel droplets with both polymeric and nanoparticle additives is studied in normal gravity. High speed photography is employed and the effect of additive concentration on droplet burning rate, burning time, extinction and soot morphology is investigated. Polymer added fuel was found to have a volatility controlled combustion with four distinct regimes. The first three zones are associated with combustion of base fuel while the polymer burns last and after a heating zone because of its higher boiling point. Polymer addition reduces the burning rate of the base fuel in the first zone by means of increasing viscosity and results in nucleate boiling and increased burning rates in the second and third stages. Overall, polymer addition resulted in a higher burning rate and shorter burning time in most of the scenarios. Colloidal suspensions of carbon-based nanomaterials in liquid fuels were also tested at different particle loadings. It was found that dispersing nanoparticles results in higher burning rate by means of enhanced radiative heat absorption and thermal conductivity. An optimum particle loading was found for each particle type at which the maximum burning rate was achieved. It was observed that the burning rate again starts to reduce after this optimum point most likely due to the formation of large aggregates that reduce thermal conductivity and suppress the diffusion of species.

No description available.

<p>Conventional rocket propellants such as monomethyl hydrazine (MMH) and hydrazine have been used for decades due to their high specific impulse and performance. However, interest in greener alternatives, including HAN or HAN-based propellants, has grown due to high levels of toxicity and difficulties in the handling and storage of conventional fuels. Included among potential propellants are monopropellants nitromethane (NM) and isopropyl nitrate (IPN) and their blends. Though large-scale investigations on the ignition and combustion of these fuels have been done, the ignition and combustion processes of these monopropellant fuels are still not well understood. Droplet studies have been traditionally and extensively employed to decipher the influence of ambient conditions and fuel properties on ignition and combustion of different fuels. These fundamental studies allow for the isolation of different factors such as ambient temperature and initial droplet size among others, to provide a deeper understanding of their effects in overall spray combustion.</p> <p> </p> <p>The research described here seeks to add to the knowledge on the ignition and combustion processes of NM and IPN through single droplet ignition and combustion studies. To this end, the first effort has been to establish a suitable method of studying the ignition and combustion of droplets in conditions similar to those in practical systems. Droplet ignition delay measurements for NM and IPN droplets have also been conducted, and the influence of ambient temperature and droplet size has been studied. The double flame structures of NM and IPN, representative of hybrid combustion, have also been observed. In addition, the applicability of the hybrid combustion model, developed to predict mass burning rates for hypergolic fuels exhibiting hybrid burning including MMH, UDMH and hydrazine, has been assessed. Lastly, the ability of the quasi-steady droplet ignition model to predict ignition delays of IPN and NM monopropellant droplets is also discussed.</p>

No description available.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

This thesis reports a model to predict the auto-ignition time of single droplets of n-paraffin fuel mixtures using the method of continuous thermodynamics. The model uses experimental data for pure fuels to fit rate parameters for a single-step global chemical reaction equation; from this, correlations for rate parameters as a function of species molecular mass are derived, which are integrated to produce a continuous thermodynamics expression for mixture reaction rate. Experiments were carried out using the suspended droplet-moving furnace technique. The model was then tested and compared to experimental data for three continuous mixtures with known compositions: one ranging from ¬n-octane to n-hexadecane, the second ranging from n-dodecane to n-eicosane, and the third being a combination of the first two mixtures to produce a “dumbbell” mixture. Discrete and continuous mixture models of the ASTM standard distillation test were compared to design the experimental mixtures and provide the distribution parameters of the continuous mixtures intended to simulate them. The results of calculations were found to agree very well with measured ignition times for the mixtures.

Concerns about the use of certain chemical species within the aerospace field are growing in recent years. A European regulation, REACh, now makes the use of hydrazine uncertain in – among others- attitude control thrusters. Green monopropellants, which are alternatives for this species already exist, but they all require a catalyst to react. Catalysts constitute the limiting factor for the lifespan of satellites because of the number of thermal cycles they endure. A joint project between ONERA, the French aerospace research center and CNES, the French space agency, was born to develop a high-performance green monopropellant thruster operating without any catalyst. Sizing the thruster and particularly its combustion chamber is not an easy task because of the explosive properties and the lack of knowledge regarding the monopropellant reaction process. The thesis aims at simulating the flow in a combustion chamber using CNES05, a new promising green monopropellant. This monopropellant has a very low vapor pressure and is an energetic liquid. As such, its reaction above a certain temperature -which is called decompositionis not well understood and must be observed closely. For this matter, a test bench was created, and it paved the way for the development of a specific model of decomposition. Indeed, even if the CNES05 decomposition cannot be modeled with the classical theory of isolated droplets, the setup showed us the order of magnitude of the reaction kinetics and the presence of a break up phenomenon. Using this model, the simulations of the flow inside the combustion chamber give us the heat flux profile through its walls, a sizing parameter for the thruster. Large recirculation zones are observed and the influence of the angle of injection seems to be the major injection parameter of influence. The sensitivity of the parameters used in the model is also studied.