Experimental And Numerical Study Of Laminar Premixed Flat Flame Structure In Low Pressure

In purpose of solving combustion instability and low efficiency in Ramjet which work in low pressure at high altitude, it's important to work on basic research of combustion in low pressure. Laminar premixed flame is the natural basis of complicated combustion; it has high academic meaning and applied value to study the laminar premixed flame structure. By means of numerical simulation and experimental studies, this dissertation studies the characteristic and mechanism of laminar premixed flat flame structure systematically and thoroughly.An experimental system has been established which includes a modularized burner, an accommodation system, a measurement and control system, three different optical diagnostic system based on spontaneous emission, PLIF and TDLAS technology.Choosing experimental system as the object, characteristic and variation of flat flame structure with four different gas combinations (CH4/Air, C2H4/Air, H2/Air, CH4/O2) are studied experimentally. One dimensional laminar premixed flame structure is simulated by using PREMIX with GRI Mech 3.0 detailedly. The parameter influence of low pressure and equivalence ratio on flame structure is analyzed, flame speed diversification of different gas combinations is investigated when pressure and equivalence ratio change.Basing on the numerical simulation results, the method which describe flame structure as three parts: preheat zone, flame surface, product zone is proposed. The temperature near flame surface is mearsured by TDLAS, the flame shape and radical chemiluminescence images are screened by ICCD. Original chemiluminescence images are decorated by MATLAB code based on adaptive-filtering method, distribution information of OH and CH radicals is extracted from these images. The rule of flame surface movement becomes different when fuel component changes in low pressure by experimental data. Combustion stability and the variation of flame structure infected by pressure are influenced by oxygen content. Also, the diffusion effect of thermal and mass influenced on flame instability is discovered.In order to study flame structure more accurately, by technology of OH-PLIF and CH, C2 intensified spontaneous emission, the chemiluminescence images of elaborate flat flame structure with different premixed gas are collected. The method which describe flame structure as four parts: unburnt zone, preheat zone, reaction zone, product zone is proposed, and use OH half width to define the boundary and thickness of the reaction zone. The variation of flat flame structure with different premixed gas is studied in fuel-lean, stoichiometric and fuel-rich flames. Stoichiometry and pressure dependence of elaborate flat flame structure is discussed detailedly; reaction zone thickness rises when pressure descends. The existence of pressure lower limit and turning point is testified and also found these two parameters depend on stoichiometry by experiment. We also found the combustion efficiency will be improved by organising combustion with fuel-lean premixed gas in 4.5kpa CH4/O2 experiment. The variation of laminar non-premixed flame structure is studied in ways of changing equivalence ratio and inner-outer liquid velocity difference at last.