| Some practical combustion processes,such as flame synthesis of nanomaterials,coal combustion,flame-wall interactions,electric-field and plasmas assisted combustion,etc.,usually involve complex co-existing phases of gas,condensed and plasma,which are termed as complex combustion.For the interactions and transitions among different phases,many basic scientific issues urgently needs to be addressed.In this thesis,focusing on the flame synthesis of nanomaterials,the author develops in-situ laser/optical diagnostics for complex multiphase combustion systems,studies the phase transformation and transportation in flame synthesis,and reveals the controlling mechanisms of complex flame dynamics by solid walls or flame plasmas.The first part of the current thesis concentrates on the development of the novel laser/optical diagnostic methods for complex systems involving gas,condensed and plasma phases.Firstly,the underlying absorption-ablation-excitation mechanism of a novel phase-selective laser induced breakdown spectroscopy(PS-LIBS)is elaborately uncoverred.This new PS-LIBS is further successfully developed into a multidimensional diagnostic method for demonstrating the gas-to-particle conversion with atomic information and for quantifying the particle volume fractions.Then,for near-wall flame measurements,a wavelength modulation spectroscopy(WMS)in absence of non-absorption transmission losses is developed and utilized in beam-cutting diagnostics.Thirdly,for the flame plasma measurements,the chemiluminescence from CH radicals,closely related to the chemi-ionization process,is detected and tomographic reconstructed in both axisymmetric and non-axisymmetric cases.Detailed applications of these novel diagnostic methods are given below.In the flame aerosol synthesis involving both gases and particles,the current thesis studies the turbulent flame synthesis and doping synthesis,both of which play important roles in practical industrial processes.The single-shot two-dimensional measurement of nanoparticle distributions in turbulent combustions is achieved for the first time by the PS-LIBS.The proper orthogonal decomposition(POD)analyses of the PS-LIBS snapshots further reveal that the particle volume fraction fluctuations originate from the unsteady flame surface at the upstream and large-scale mixing at the downstream.In the doping synthesis system,the atomic emissions of V and Ti from PS-LIBS suggest a bandgap shift process because of their rapid collisions and doping at the nucleation stage.The PS-LIBS signal ratios between different elements can quantitatively reflect in-situ element ratios in the condensed phase.In the stagnation flame synthesis involving gases and solid walls,the present work investigates the effect of substrate on the flame structure and the extinction limit,as well as on the formation and deposition of the nanoparticles.The near-unity local Karlovitz number(Ka_L)can be regarded as the extinction criterion at various heat fluxes.The deposition of flame-synthesized nanoparticles onto the substrate is imaged by two-dimensional PS-LIBS.The low-temperature substrate can inhibit the nanoparticle coalescence,induce the gathering of nanoparticles at the boundary layers,and promote their thermophoretic deposition.In the electric-field assisted flame synthesis involving plasmas and gases,the present work focuses on the manipulation of electric fields on flame dynamics.A low-frequency AC electric field can actively manipulate a stagnation flame and cause thermoacoustic oscillations,which provides a promising active control method of suppressing original flames fluctuations.The complex flame dynamics under electric fields is caused by a two-way interaction between gases and plasmas.The interaction can form a positive feedback loop and leads to a novel electro-hydrodynamic flame instability. |
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