A Numerical Study of NOx and Soot Emissions in Counterflow Methane/ n-Heptane Triple Flames

There is significant interest in developing alternative and environmentally-friendly fuels with the objective of reducing global emissions and our dependence on conventional fuels. Both engine manufacturers and researchers have focused their efforts in using cleaner fuels, such as syngas and natural gas (NG), in transportation and power generation systems. In this work we report a computational study to investigate NOx and soot emissions in n-heptane/methane triple flames in an opposed-jet configuration. The objective is to assess various fuel injection scenarios in a dual-fuel diesel engine using a simple configuration that can imitate partially premixed combustion in a diesel engine. Numerical simulations are performed by combining a detailed fuel and NOx chemistry model with a soot model. Three fuel blending strategies are examined. In strategy 1, both fuels are introduced through the fuel nozzle, while in strategy 2, n-heptane is introduced from the fuel nozzle and methane from the fuel and oxidizer side nozzles, and in strategy 3, n-heptane and methane are introduced from the fuel and oxidizer nozzles, respectively. These three strategies represent different injection methods in a dual fuel diesel engine. The first strategy is similar to the condition when both methane and n-heptane are injected from the same nozzle or the nozzles are very close to each other. The second strategy represents the case where methane is injected into the cylinder first, mixed with air and then near the end of compression stroke, diesel is injected. The third strategy emulates the case where there are two separate nozzles for methane and n-heptane and they are away from each other. These are three of several possible injection strategies in a dual-fuel diesel engine which can be investigated in further detail in future for better performance of the diesel engine. SUMMARY (Continued) For each strategy, NOx, PAH and soot emissions, and thermal efficiency are characterized by varying the energy content between the two fuels, while keeping the total energy input rate fixed. The most important difference between strategy 1 and other two strategies is that the flame formed with the first strategy is a double flame, whereas strategies 2 and 3 result in a triple flame. As n-heptane in the blend is reduced, the benzene emission index (EIBENZ) and soot emission decrease with all three strategies. However, the reduction is much more pronounced with strategies 2 and 3 compared to strategy 1. This shows that the flame structure plays an important role in the emissions. In addition, reducing n-heptane in the blend leads to higher efficiency with strategies 2 and 3, but lower efficiency with strategy 1. With regards to NOx emission, results indicate an optimum fuel blending ratio corresponding to a minimum EINO for strategies 2 and 3. In contrast, for strategy 1, EINO increases monotonically as n-heptane in the blend is reduced. Thus strategy #1 yields higher NOx, PAH and soot emissions, and lower efficiency compared to other two strategies, and is not recommended.