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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Modelling of premixed turbulent propagating flames

Patel, Samir N. D. H. January 2001 (has links)
Combustion has an active role in our modern lives as we continue to exploit its potential for many of our requirements. For example, its use to produce electricity and to power land, air and space transport vehicles. Increasing competition from the onset of the Industrial Revolution has led to a greater emphasis on improving technology. Furthermore, the ongoing issue of global warming has led to government legislation on emissions. These problems have led to increasing interest in gaining fundamental critical details on flow and combustion in simple and complex engineering geometries. Over the past twenty to thirty years numerical methods have demonstrated their success at obtaining information on flow and combustion. However, there is a continuing need to develop many of the components comprising a numerical method. The work reported here stems from the modelling of turbulent premixed flames. Turbulent premixed flames is a mode of combustion where the fuel and air mix before reacting. Such a combustion mode is present in spark ignition (SI) and gas turbine (GT) engines, and in explosions. Modelling of the combustion process within these practical applications can provide useful information. For example, in aiding the design of the piston bowl and the combustion chamber of SI and GT engines, respectively. Furthermore, the simulation of explosions can result in safer designs for fuel storage and supply facilities. A central parameter to be modelled in turbulent premixed flame propagation is the rate of chemical reaction. This is a crucial parameter since it controls the rate of flame propagation, flame structure, and resulting pressure history. However, to date the challenge of accurately modelling the rate of chemical reaction over a range of turbulence conditions remains. Therefore, in this thesis, mathematical models for the mean rate of reaction are examined, developed, and validated against time-resolved experimental data. The aim of the work is to improve the modelling of the mean rate of reaction in order to achieve closer agreement with available experimental results on rates of flame propagation, flame structure, and pressure history. Recent, practical and numerical experiments have provided support for algebraic and transport equation models for the flame surface area to volume ratio to model the mean rate of reaction. Here, these models are examined and developed with one-, two-, and three-dimensional computational fluid dynamics (CFD) calculations. The simulations were carried out using both an in-house code (Turbulent Reacting Flows, TRF) and a commercially available CFD code (FIRE). The TRF code was used to investigate the ability of existing and developed models to accurately predict turbulent burning velocity. The models were then validated further by simulating turbulent flame propagation in two combustion chamber configurations with built-in solid obstacles. Hence verifying the models for different turbulence and geometry conditions. A nonlinear eddy-viscosity model was implemented into the TRF code to assess the significance of turbulence modelling in turbulent premixed flames. Finally, the developed models were implemented in the FIRE code to carry out three-dimensional calculations to verify reproducibility of the TRF code results and to investigate secondary flow effects. Two reaction rate models were developed namely the algebraic (BML) and transport flame surface density (FSD) models. Both BML and FSD models yield plausible results for flame propagation in turbulent premixed combustion. However, modifications to the BML model were required for low turbulence conditions, and superior results were obtained with the FSD model. Both models struggled in capturing the interaction between flame and turbulent wakes behind obstacles when the standard linear eddy-viscosity turbulence was used. However, the application of a non-linear version of the eddy-viscosity model yields improved results for flame structure and speed around the obstacle, highlighting the importance of the turbulence model. The 3D calculations using the developed combustion model show good reproducibility of the 2D findings. Furthermore, the flame propagation, pressure history, and flame speed results are found to be in plausible agreement with the experimental data. It is shown that secondary flow mainly has the effect of increasing the rate of flame propagation in the single obstacle combustion chamber, and that the influence of secondary flow is dominant in the turbulent wake behind the obstacles.
2

Development of a micro-retarding potential analyzer for high-density flowing plasmas

Partridge, James M. January 2005 (has links)
Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: Ion Energy Distribution; Current Collection Theory; Energy Diagnostic; Retarding Potential Analyzer; Electric Propulsion. Includes bibliographical references. (p.91-95)
3

Development and Implementation of diagnostics for unsteady small-scale plasma plumes

Partridge, James Michael. January 2009 (has links)
Dissertation (Ph.D.)--Worcester Polytechnic Institute. / Keywords: TLP; RPA; retarding potential analyzer; triple Langmuir Probe; plume; probe; diagnostic; thruster; Plasma. Includes bibliographical references (leaves 183-190).
4

The potential of fuel cells to reduce energy demands and pollution from the UK transport sector

Adams, Victor W. January 1998 (has links)
Atmospheric carbon dioxide and pollution due to the burning of fossil fuels is increasing. Many scientists attribute global warming to the rising levels of carbon dioxide and other pollutants, some of which also pose risks to health. These can be reduced by the more efficient use of conventional fuels and the development of non-polluting energy resources. Fuel cells offer a highly efficient and low polluting method of generating electricity, and are under development for both the power generation and transport sectors. There is a need to assess (a) emissions from fuel cells using various fuels and (b) ways of introducing such technology to transportation in the near future. Fuel consumption, energy and emissions from the production and use of fuels (hydrogen, methane, propane, petrol, diesel, alcohols and rape methyl ester) are calculated per kilowatt hour of fuel cell output over a range of efficiency. These are compared with those for internal combustion engines with advanced exhaust control and for the recharging of battery driven vehicles. The results, which are applicable to both transport and power generation, enable the best low pollution fuels to be selected and are used to calculate through life emissions for public transport buses. Fuel cells are an ideal solution to reduce pollution from transport, but their commercial development in this field is further away than that for stationary applications. Thus, a transition stage is recommended where fuel cell electrical power stations, based on existing demonstrators, are used to recharge fleets of battery driven vehicles during the development of mobile fuel cell systems. These fleets include public transport and commercial vehicles. Also, fuel cell power stations could provide energy for electric trains. A combined system is proposed where electric trains recharge battery driven commercial vehicles during long journeys. The above proposals would enhance fuel cell development, introducing them alongside current transport systems, possibly using the same fuel.
5

Discharge characteristics and instabilities in the UK-25 ion thruster operating on inert gas propellants

Edwards, Clive Henderson January 1997 (has links)
No description available.
6

Thermal-fluid analysis of a lithium vaporizer for a high power magnetoplasmadynamic thruster

St. Rock, Brian Eric. January 2007 (has links)
Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: vaporizer; two-phase flow; electric propulsion. Includes bibliographical references (leaves 152-156).
7

Numerical analysis of transient Teflon ablation in pulsed plasma thrusters

Stechmann, David Paul. January 2007 (has links)
Thesis (M.S.) -- Worcester Polytechnic Institute. / Keywords: Teflon ablation; pulsed plasma thrusters; numerical analysis; PPT; depolymerization. Includes bibliographical references (p.102-105).
8

Development of a high power density motor for aircraft propulsion

Dibua, Imoukhuede Tim Odion 25 April 2007 (has links)
Electric propulsion has been studied for a long time. Most of the electrically propelled vehicles that have been developed however have been ground vehicles. Recent research by NASA has promoted the development of electric aircraft. Most aircraft are currently powered by heavy gas turbine engines that require fueling. The development of electric motors to replace gas turbines would be a big step towards accomplishing more efficient aircraft propulsion. The primary objective of this research extends previous work by developing a high power density motor for aircraft propulsion. This design is novel because it does not require a dynamometer to provide the torque to drive the vehicle. Equally important for successful testing of the motor was the design and development of a spin pit interface that was used as a containment vessel during testing. The research led to a designed, fabricated, assembled, modeled, and tested motor. Voltages, currents and power outputs of the motor were measured and used to determine the motor’s efficiency. The gaps between the motor’s magnets were related to the current and power it produced, and modifications were made based on this relation. The vibrations of the motor were also studied and MATLAB codes were written and used to reduce these vibrations. Significant among the objectives was monitoring the temperatures of the motor’s stators due to their close association with the rotating parts. The windage and friction losses between the stators and the magnets provided a challenging hurdle in the research. These windage and friction losses were predicted, analyzed and measured, and modifications were made to reduce them. Finally, results were compiled, tabulated, and analyzed. Results obtained before and after the modifications were compared, and these comparisons were used to assess the necessity and effectiveness of the modifications. The efficiency of the motor was found to be 82.9% and the power density was evaluated as 33.1 W/lb based on a rotor weight of 497 lb. It was concluded that the litz wire used in the motor has high, frequency related impedances that could be reduced but not eliminated.
9

Anode fall as relevant to plasma thrusters

Horner, Brigitte 06 1900 (has links)
The behavior of the electric field together with the electron and ion densities in the vicinity of a nonemitting, plane anode is investigated. The selected approach involves non-linear analysis techniques on the continuum equations for steady-state, isothermal conditions where both ionization and two-body recombination are included. Ions, created through electron bombardment of neutral atoms, are repelled toward two stagnation regions: within or near the sheath boundary and near the plasma interface. These equilibria form as a result of the chemistry present: recombination establishes the latter while ionization stipulates the former. As presented, the sheath is fundamentally unstable - ions are driven toward the negative electrode. Using nitrogen data for a numeric example, the following observations are made: a sufficiently strong applied electric field pushes the ion density toward that ofthe electrons through a well - a constrictive phenomenon. Both a transition region, dominated by density gradients, and a diffusion-driven zone are found to move the system toward the plasma interface. The characteristics of this process are influenced by the applied electric field, but the instability of the chemistry-induced stagnation regions precludes numeric convergence. Insufficient dissipation may prevent the stability of the anode fall model as presented. Suggested improvements to the model descriptions include considering the effects of temperature gradients, magnetic fields, three-body recombination, diffusion written in terms of the electric field, multi-dimensionality and/or timedependencies^
10

Very low earth orbit propellant collection feasibility assessment

Singh, Lake Austin 12 January 2015 (has links)
This work focuses on the concept of sustainable propellant collection. The concept consists of gathering ambient gas while on-orbit and using it as propellant. Propellant collection could potentially enable operation in very-low Earth orbits without compromising spacecraft lifetime. This work conducts a detailed analysis of propellant collection from a physics perspective in order to test the assertions of previous researchers that propellant collection can dramatically reduce the cost of propellant on-orbit. Major design factors for propellant collection are identified from the fundamental propellant collection equations, which are derived in this work from first principles. A sensitivity analysis on the parameters in these equations determines the relative importance of each parameter to the overall performance of a propellant-collecting vehicle. The propellant collection equations enable the study of where propellant collection is technically feasible as a function of orbit and vehicle performance parameters. Two case studies conducted for a very-low Earth orbit science mission and a propellant depot-type mission serve to demonstrate the application of the propellant collection equations derived in this work. The results of this work show where propellant collection is technically feasible for a wide range of orbit and vehicle performance parameters. Propellant collection can support very-low Earth operation with presently available technology, and a number of research developments can further extend propellant-collecting concepts' ability to operate at low altitudes. However, propellant collection is not presently suitable for propellant depot applications due to limitations in power.

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