| Micro and mesoscale combustion is a new research field of combustion community with the history of just more than ten years.Because the energy produced by combustion is much larger than that of traditional chemical battery,the combustion-based micro power generator is considered as an appropriate power source of the micro-electromechanical system(MEMS).Therefore,the combustion process occurred in a small confined space has attracted extensive attentions in recent years,At present,the research on micro and mesoscale combustion is mainly focused on flammable limits,combustion characteristics,stabilizations method and new combustor designs.However,due to the incomplete combustion,a large amount of soot particles will be generated in the process of actual combustion.The formation of soot will not only reduce the conversion efficiency of the practical combustion energy system,but also have serious harm to human health and the environment.Therefore,understanding the formation characteristics and physicochemical properties of soot particles from micro scale combustion is of great significance for developing clean MEMS with high efficiency and improving the basic theory of soot formation.Based on our self-designed micro and mesoscale combustion experimental platform,the effects of basic parameters(excess air ratio,flow rate),combustion atmosphere,oxygenated fuel additives and scale effect of combustor on soot formation in ethylene diffusion flame were studied.In this work,the optical diagnosis and sampling analysis,including two-color method,ordinary transmission electron microscope(TEM),high-resolution transmission electron microscope(HRTEM),thermogravimetric analysis(TGA)and gas chromatography(GC),were combined to investigate the concentration information,morphology characteristics,nanostructure,oxidation properties of soot and the components of exhaust gas.The relationship between soot formation and combustion chemistry was also established.In addition,the relevant HRTEM image processing program was applied to extract and quantify the characteristic parameters of the obtained soot HRTEM images,which helps to determine the relationship between the oxidation properties and nanostructure of soot.Firstly,the effects of basic combustion parameters on soot formation at two combustor scales(diameter of 4 and 6 mm)were investigated,and the effects of excess air ratio and flow rate on combustion characteristics and physicochemical properties of soot particles were studied.When the combustor diameter increases from 4 to 6 mm,the flame temperature and mass of generated soot will increase significantly,and the range of collectible soot will also increase.In addition,the notable distinctions in soot nanostructure could be observed between the two combustors.The soot from the combustor of d=4 mm exhibited the partial disorded structure with much short and disordered lamellae while the soot from the combustor of d=6 mm showed a typical fullerene-like structure composed of much curved lamellae with a small radii,suggesting the simultaneous existence of graphitic parts and PAHs.Furthermore,different effects on soot characteristics between both types of combustors with the same variation of flow rate were found.For the same variation in ethylene flow rate of 60-100 ml/min at a=0.5,the oxidation reactvity of soot from the combustor of d=6 mm decreased all the time,while it showed a downward trend first and then increased as for the soot from the combustor of d=4 mm.This is because the residence time and temperature variation for the same increment of flow rate between the two combustors are different.Secondly,the thermophoretic sampling method and deposition sampling method was combined to collect soot particles and exhaust gas from mesoscale combustors at different combustion atmospheres(O2/N2 and O2/CO2).The correlation between soot formation and combustion chemistry was also established.With the increase of O2 concentration,the fuel conversion and flame temperature in mesoscale combustor increase significantly,and the soot formation characteristics also exhibited significant distinctions.When the O2 concentration was 21%,the morphology and nanostructure of soot showed film-like material and amorphous structure respectively,but more chain-like aggregates and fullerenic-like structures could be observed at 30%and 40%O2 concentration.Furthermore,the thermal and chemical effects of CO2 can be both examined in the present work.Due to the larger specific heat capacity of CO2 than N2,the fuel conversion and flame temperature in CO2 diluted flame are always lower than that of N2 diluted flame.Moreover,because CO2 directly participates in the reaction CO2+H(?)CO+OH during combustion and soot formation,thus accelerating the forward process of the reversible reaction,so the CO concentration in the exhaust gas from CO2 diluted flame is higher than that of flame with N2 dilution.Then,the control mechanism of DME addition on soot formation in micro and mesoscale combustion was analyzed.The soot formation characteristics and combustion chemistry of ethylene diffusion flames with various DME additions(0%-100%)were studied by the combination of optical and sampling method.According to the optical result by two-color method,the overall temperature and soot concentration increased slightly when DME was added from 0%to 10%but they decreased rapidly with much DME addition from 20%to 100%.Such a synergistic effect was more pronounced in the larger combustor diameter.For the sampling result of transmissions electron microscopy images,soot morphology and nanostructure transitions could be clearly observed with various DME additions.The generated soot showed typical chain-like aggregates and fullerenic-like structures with 0%-10%DME additions but more film-like materials and amorphous structure were presented as DME was added beyond 20%.Through further lattice fringe analysis,the synergistic effect of DME addition could also be reflected on the soot nanostructure.Furthermore,a high correlation between soot formation and exhaust gas composition was detected.The concentrations of CO and H2 in exhaust gases decreased firstly and then increased with continuous DME additions to the fuel,peaking at 10%addition,which was exact opposite to the synergistic effect of DME addition soot formation.Finally,the detailed scale effect on soot formation and combustion chemistry during the transition process from micro scale to conventional scale is investigated by both optical and sampling method.In the case of same flow rate of fuel and oxidant,the scale effect exert a non monotonic effect on the flame temperature.To be specific,with the increase of combustor size,the flame temperature increases firstly and then decreases sharply,but the soot concentration increases at first and then maintains in a nearly constant range and the carbonization degree of soot always exhibits an increasing trend.This may be due to the different impact mechanism between flame temperature and soot formation.With the increase of combustion size,the concentration gradient of fuel and oxidant decreases so that the mixing process becomes worse and the local equivalence ratio increases.The above factors reduce the heat release of combustion but contributes the formation of soot particles. |
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