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Determination of the Effect of Water on Soot Formation, 03-9073

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Principal Investigator
Charles E. Roberts

Inclusive Dates: 04/01/98 - 10/01/99

Background - Meeting new and increasingly stringent emission regulations for diesel engines requires a method for reducing NOx and soot emissions. Most emission-reduction strategies reduce one pollutant while increasing the other. Water injection is one of the few promising emission reduction techniques with the potential to reduce soot and NOx simultaneously in diesel engines. While it is widely accepted that water reduces NOx via a thermal effect, the mechanisms behind the soot reduction are not well understood. The water could reduce the soot via physical, thermal, or chemical effects.

Approach - To aid in developing water injection strategies, this project attempts to determine the mechanism by which water enters the soot formation chemistry. Linked experiments and modeling in a rich premixed flame will determine the magnitude of the chemical effect of water on soot formation and potentially identify a kinetic mechanism to explain it. In a diesel engine, the early soot inception results from rich premixed combustion; thus, the rich premixed flame provides an appropriate venue in which to isolate the influence of water on the kinetics. Experiments will quantify the soot inception point and the relative amounts of soot formation in premixed flames with and without water addition. Subsequent modeling will predict the soot inception and amount of soot formation using currently accepted kinetic soot mechanisms. Comparison of the experimental and modeling data will assay the accuracy of the soot-formation mechanisms and ultimately yield an understanding of the soot-formation chemistry and the role of additional water.

Accomplishments - The experimental, premixed burner and test assembly have been completed. The burner has been used for experimental measurements of the critical soot-inception point for various fuel and air mixtures with and without water addition. Further tests have been conducted to determine the effects of mixture temperatures on the soot-formation process. Hence, the project has extended the available data on soot formation to include the combined effects of temperature and water on the soot-formation process. The experimental work has been complemented by numerical modeling of the premixed flame, including a full simulation of the chemical kinetics associated with soot-precursor formation. The numerical results have identified discrepancies within the current chemical kinetic mechanisms available and are being used to identify potential kinetic pathways to explain the discrepancies between the experimental and numerical results. Summaries of the work performed have been presented to members of a large industrial-client consortium, resulting in requests for further work in this area and potential client-sponsored project funding.

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The experimental premixed burner is used to measure the critical soot-inception point for various fuel and air mixtures with and without water.

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