Experimental Investigation of Soot Particle Size Evolution in Microflow Tube Reactor

The acute health and environment effects of soot emissions are well recognized. Most importantly, soot particulate matters have been strongly associated with pulmonary and cardiovascular diseases due to their small size (< 2.5 μm). Wide spread experimental and computational efforts are ongo- ing in an attempt to gain deeper understanding of soot formation processes. However, information about soot formation mechanisms in practical combustion conditions remains limited, especially under conditions of elevated pressures and relatively low temperatures (around 1200 K). Moreover, previously proposed principles of soot nucleation and growth need experimental evidence for their firm establishment. In fact, validation of predictive models of soot formation requires reliable ex- perimental data that the model prediction can be tested against.
In light of these requirements in combustion and soot research, the present study undertakes an experimental investigation of soot formation from fuel pyrolysis in a micro-flow tube reactor. Soot formation is monitored by tracking the evolution of particle size distributions under different ex- perimental conditions using a high resolution Scanning Mobility Particle Sizer comprising of TSI 3080 nano-DMA and TSI 3788 nano-Water based CPC. An analysis of soot particle size distribu- tions so obtained provides critical information about the various processes involved in soot formation.
The objectives of this study are mainly two fold. The first objective of this study to to obtain experimental training data set for soot formation under elevated pressure conditions. The second objective of this study is to experimentally verify the role of acetylene in soot formation, particu- larly the importance of hydrogen abstraction/carbon addition (HACA) mechanism in soot surface growths. Accordingly, the first part studies soot formation during the pyrolysis of 2% ethylene in 98% inert N2 bath at 1200 K under elevated pressure conditions (2.5 atm - 2.8 atm) in a novel micro-flow tube reactor with 231 μm orifice (corresponding to 240 ms residence time) at the reactor exit. The orifice allows choked flow conditions, and hence allows us to realize constant elevated pressure conditions. Analysis of soot formation over this range of pressures showed that increasing pressures resulted in an apparent increase in the density of soot precursor, which is evident from the increase in total soot number density (this indicates the formation of more soot nuclei). As a conse- quence of strengthened soot nucleation rates, soot mean particle size undergoes a noticeable increase.
The second part of this study examines the effect of acetylene on soot formation during ethylene py- rolysis. Hence this study aims to identify the role of acetylene as a soot precursor, as well as validate the well-known hydrogen-abstraction/carbon-addition (HACA) mechanism for soot growth. These experiments involve the pyrolysis of ethylene doped with small amount of acetylene in a micro-flow straight tube reactor at different temperatures (1210 K - 1230 K) and residence times (215 ms -315 ms). The molar ratio of ethylene/acetylene was maintained at 6. The reactant composition was 4% fuel mix in 96% N2 bath. Soot particle size distributions were compared with those obtained from baseline ethylene pyrolysis (4% pure ethylene in 96% N2 bath). Soot number density, mean diameter and total volume fraction showed a significant increase as a result of acetylene doping. The results clearly demonstrate the crucial role of acetylene in soot inception as well as soot growth resulting in an increase in soot sizes. This correlation between acetylene doping and formation of more soot nuclei (evinced by the higher number density), and larger soot particles (as proved by larger mean diameter and higher soot volume fraction) lends credence to the assessment that acetylene is an important soot precursor, and HACA mechanism is the dominant soot growth pathway.
This study is a valuable preliminary work towards the establishment of a consistent and experimen- tally verified soot formation theory.