Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526391
Title: Large eddy simulation of isothermal and reacting sprays
Author: Lyra, Sgouria
ISNI:       0000 0004 2694 5782
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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Abstract:
The rapid growth of civil aviation and the pressing demand to reduce greenhouse gas emissions from the growing air transport industry has drawn attention to the development of lean combustion technologies which promise to improve combustion efficiency and minimise emissions from aero-engines. The present thesis concerns the application of Large Eddy Simulation (LES) on lean combustors burning liquid aviation kerosene. A Lagrangian formulation is adopted for the description of the fuel spray coupled with an Eulerian framework used for the carrier gas. Stochastic sub-grid models are used to address the sub-grid spray diffusion and evaporation in chemically reacting gaseous environments. The sub-grid turbulence-spray-chemistry interactions are represented through the LES-PDF methodology coupled with the eulerian stochastic field method which is devised and applied to the numerical prediction of spray fuelled flames. At first, LES is applied to a droplet laden mixing layer and a numerical investigation is performed on the effect of turbulence on the transport and preferential concentration of the spray. Heat and mass transfer phenomena are neglected and the sub-grid droplet dispersion model is initially validated in isothermal flows. The droplet diffusion is found to be a size selective process, strongly dependent on the filtered, unresolved scales and accurately represented by the sub-grid dispersion model. In a second stage, the effect of turbulence and temperature fluctuations on the structure of evaporating turbulent kerosene and acetone spray jet flows is investigated. The concentration, spreading and evaporation rate of the liquid is well described by the employed models which reproduce successfully the dispersion and low-temperature evaporation in two-phase shear flows. Subsequently, the methodology is validated against a more complex geometry and flow field. An isothermal confined annular swirling jet characterised by a high swirl number is simulated and LES is found to predict accurately the first and second moments of the velocity field. The flow features a challenging combination of recirculation zones and coherent structures. The latter are identified from the high intensities in the centreline RMS profiles and an analysis is performed using the frequency spectra of the velocity fluctuation signal for the specification of their characteristic frequency. The flow is nearly 'sub-critical', with the central recirculation zone reaching the outflow boundary. The isothermal single-phase simulations form the basis for the simulation of kerosene spray flames studied in the same configuration. The effect of the spray properties on the burning behaviour of a stable (Flame A) and an extinguishing (Flame B) flame is investigated. The evaporating droplets create a lean mixture of kerosene vapour/air and a flame front is formed which envelops the central recirculation zone. The LES-PDF methodology reproduces the temperature evolution and the flow field of both chemically reacting flows. Discrepancies are observed in the temperature RMS levels which are over-estimated in the 'middle' and far field of the flow. The reduced chemistry mechanism used allows for qualitative comparison of the species concentrations with the experimental data. The high sensitivity of the flame topology, burning and mixing regions and species concentrations to the spray characteristics is found to be a challenge for the calculation of liquid fuelled flames.
Supervisor: Jones, W. P. ; Marquis, A. J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.526391  DOI: Not available
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