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Title: Turbulent inhomogeneous autoignition of liquid fuels
Author: Gupta, Ajay
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Autoignition is a multi-physics phenomenon that occurs in wide-ranging engineering applications, such as diesel engines, gas turbines and jet-engine afterburners. In these applications, autoignition occurs in the presence of flow inhomogeneities and turbulence. Further, liquid fuel autoignition presents a case of chemically reacting flow where processes such as jet break-up, atomisation, droplet evaporation (interfacial heat and mass transfer), turbulent mixing and chemical reaction occur simultaneously in the presence of important flow, mixture and phase inhomogeneities. The multi-scale nature and direct coupling between these various contributing processes, present a challenging fundamental problem which cannot be understood by extrapolation from homogeneous, inhomogeneous (gaseous) and even single droplet combustion studies. This thesis presents an experimental study on autoignition of polydispersed droplets generated by single liquid jets of pure liquid n-heptane and n-pentane injected axisymmetrically from a circular nozzle into a confined turbulent hot coflow of air at atmospheric pressure. Distinct phenomena are identified concerning the emergence of various autoignition regimes --- no autoignition, random spots and continuous flame; the occurrence of these regimes depends on the reaction conditions of air temperature, air velocity, and liquid fuel injection velocity. In the random spots regime, autoignition appears in the form of well-defined localised spots occurring randomly along the length of the reactor. Optical measurements of these random spots are made from which the autoignition lengths/locations are measured and are used to infer average delay (or residence) times from injection. At higher air temperatures and lower liquid fuel injection velocities, autoignition is observed to move closer to the injector; the corresponding delay times also decrease. With increasing air velocity and hence turbulent fluctuations, autoignition moves downstream but the delay times decrease. The shorter delay times are also associated with faster evaporation of the liquid fuel. The results from this work suggest that turbulent inhomogeneous autoignition of liquid fuels cannot be directly predicted from chemical delay times from homogeneous studies and that the effects of evaporation, and turbulent mixing, as quantified by the mixture fraction and conditional scalar dissipation rate must be accounted for. The results also provide insights into the flow conditions leading up to the various observed autoignition phenomena for liquid fuels, and constitute a valuable data-set that can contribute towards developing and validating models of advanced multiphase turbulent combustion and chemically reacting flows.
Supervisor: Markides, Christos Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral