Computational fluid dynamics simulation of diesel spray combustion

Abstract

CFD simulations must be validated by comparison to experimental results before being used as part of the design cycle. This thesis presents CFD simulations for a specific spray combustion experimental configuration, Sandia spray A, defined by the Engine Combustion Network and comparison of CFD predictions with experimental results. Two parameter variations are examined within the CFD model. Different fuel injection rate profiles and chemical mechanisms are used to show changes in combustion behavior. Three different fuel injection rate profiles and two chemistry reaction mechanisms are tested, creating six possible simulation cases, with all other parameters held constant (temperature, pressure, O2 concentration, injection duration, and injection mass as well as all other subgrid models, including turbulence and spray physics). The injection rate profiles considered are a constant injection rate, a logarithmic profile for injection rate, and an injection profile with a ramp up to constant injection then a ramp down. The chemical kinetic reaction mechanisms considered are a simplified empirical model that applies generically to a variety of fuels, developed by Gowdadiri and Oehlschlaeger (Gowdadiri & Oehlschlaeger 2014) and a relatively complex mechanism created by Luo et al. (Luo et al. 2014). The simulation outputs examined and compared to experiment are the spray liquid length, vapor penetration, chamber pressure profile and ignition delay (timing of chamber pressure rise due to the onset of combustion). Visualization of the simulated temperature field during spray penetration and combustion are also presented.

Diesel combustion is widely employed in many aspects of industry, especially shipping and transportation. There are many factors that motivate continued research and development of diesel engines and fuels, including potential cost reductions and efforts to increase efficiency and power density and reduce emissions. While physical testing of new fuels and engines is important, computational fluid dynamics (CFD) simulation play a role in the design cycle for new engines as simulations greatly reduce the cost of their development. CFD simulations, with submodels for turbulence, spray physics, and combustion, can be used to predict a number of details of the spray combustion phenomena that takes place in diesel engines including spray phenomena, such as liquid length or vapor penetration, and combustion phenomena such as the pressure profile or ignition delay.