Soot Particle Size Distribution of a Microflow Tube Reactor: Experimental and Sectional Modeling Investigations

The adverse health, environmental, and climatic effects of soot particulate emissions are well known. Extensive research has been conducted to understand the fundamental mechanisms of soot particle formation, namely nucleation, growth, and oxidation, but these processes under realistic conditions are still not completely understood due to insufficient experimental data. In this work, a Scanning Mobility Particle Sizer (SMPS) with a nano-differential mobility analyzer (n-DMA) is utilized for real time sampling and measurement of nascent soot particles in the size range of 3-100 nm over a range of temperatures and residence times.

The flow reactor employed consists of a novel micro-flow tube reactor (MFTR) developed at UVa with provision for feeding any fuel mixed with a carrier inert gas, typically nitrogen heated to a target temperature (< 1200 K). For soot oxidation investigations, the fuel stream can be mixed with oxygen. In the present initial investigation, the fuel considered was ethylene as it is the primary product of fast thermal pyrolysis of large hydrocarbon molecules used in propulsion systems. With the SMPS system and the probing approach developed, soot particle size distributions (PSD’s) were obtained for a range of temperatures and residence times during pyrolysis and oxidation of ethylene in MFTR. Unlike other laboratory flames investigated with the SMPS showing bimodal distributions of nascent soot, the present PSDs for ethylene pyrolysis and oxidation shows a normal distribution with a single mode. As expected, the soot particles mass and number density is shown to increase with increasing in flow residence time at a constant temperature. The residence times were varied from 150 to 600 ms over a range of temperature from 1100 to 1200 K to define the temperature and residence time limits that lead to onset of soot formation under ethylene pyrolysis conditions. Ethylene oxidation was also studied for two equivalence ratios and it was found that the particles were concentrated towards smaller sizes indicating substantial decrease in soot growth rates.

The experiment results were also compared to the results obtained from a zero dimensional numerical model with soot particle nucleation, growth, and oxidation described by discrete sectional approach, for both oxidation and pyrolysis cases. For soot nucleation, pyrene was treated as the monomer that leads to formation of soot nuclei. There was a general agreement between the experiment and the model results with respect to trends in total soot particle mass and number concentrations over the range of residence times in the case of pyrolysis of ethylene. In the oxidation case, the mode and the mean diameter of the size distribution agreed well in experimental and model results.