Direct Numerical Simulation of Cool Flames for Diesel Combustion

The team works in front of a simulation plot of a cool flame. (Image courtesy of Sandia.)

Cool flames have been known about since the early 19^th century, but their utility in improving diesel combustion only recently caught researchers’ attention.

One team of researchers at Sandia National Laboratories in Livermoore, Calif., have been making use of supercomputing to perform Direct Numerical Simulations of these cool flames to determine how a flame kernel propagates inside a flame body. Knowlegdge of this can aid combustion efficiency in diesel engines and reduce emissions.

A cool flame occurs at lower temperatures than conventional flames and produces less heat, light and carbon dioxide. In the case of the research being carried out by the team, the cool flame burns at less than 1,150 degrees Kelvin, which is half the temperature of convetional flames at 2,200 degrees Kelvin.

Diesel internal combustion engines run a little differently than gas engines. In a gasoline engine, a spark is required from the spark plug in order to ignite the fuel in the cylinder, whereas diesel fuel requires heat or compression. In a diesel engine, the initial ignition is started from a glow plug before the combustion cycle becomes self-sustaining in a process known as autoignition.

Autoignition occurs in a typical four-stroke diesel engine in the following stages:

Intake stroke: During the intake stroke, the piston is moved down from top dead center (TDC) to bottom dead center (BDC) and draws in air from the atmosphere.
Compression stroke: During the next stroke, the piston moves back up (from BDC to TDC) and the air in the cylinder is compressed and undergoes adibiatic heating. At the end of the compression stroke, the injector squirts atomized diesel fuel into the top of the cylinder and into the volume of heated air.
Power stroke: Now that the air is heated up via compression, it is at the autoignition temperature of the fuel, and the fuel explodes, forcing the piston from TDC to BDC. This explosive force is converted to mechanical force and sent to the wheels of the vehicle, generating motion.
Exhaust stroke: The final stroke in the cycle expels the hot gases generated via combustion as the piston returns back to TDC. If the combustion stage is not efficient, the fuel will not burn completely and can result in unburned emissions (pollutants).

By increasing the efficiency of combustion, the fuel can be burned more completely, which means better fuel economy and fewer harmful emissions. And this is what the team at Sandia are aiming for with their research.

Kernel propagation through a flame. (Image courtesy of Sandia.)

“We're trying to quantify the influence of cool flames in stratified turbulent jets during the ignition and flame stabilization processes. The insights gleaned will contribute to more efficient, cleaner burning engines,” said Jackie Chen, Sandia mechanical engineer. “Our holy grail is to understand the physics of turbulent mixing coupled with high-pressure ignition chemistry to aid in developing predictive computational fluid dynamics models that can be used to optimize engine design.”

By using the Department of Energy’s 27-petaflop supercomputer, Titan, located at Oak Ridge Leadership Computing Facility, the team was able to perform Direct Numerical Simulations to model the flame development during the compression/power stroke. Direct Numerical Simulation is a powerful numerical experiment that resolves turbulence scales. From this experiment, the team has demonstrated that during autoignition, the cool flames accelerate the formation of ignition kernels. These kernels are localized regions of high temperature that seed a fully burning flame in fuel-lean areas—regions with too much air.

Understanding how these kernels propagate the fuel/air mix can give a better understanding of the combustion process and allow engineers to design solutions that allow more complete combustion of fuel. Because the injection phase lasts just a fraction of the second, understanding how the flame begins is crucial to ensuring complete combustion.

“Combustion processes are challenging to study because the fuel itself is quite complicated,” said Giulio Borghesi, Sandia researcher. “Fuel oxidation chemistry consists of hundreds of species and thousands of chemical reactions. A realistic simulation of diesel combustion needs to capture this complex chemistry accurately in an overall model that includes turbulent mixing and heat transfer.”

In future experiments, the team plans to research the speed and structure of flames at diesel engine conditions, and study the relationship between spray evaporation, ignition, mixing and soot processes associated with multicomponent fuels.

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