Research
Funded research opportunities available!
Dr. Lignell's research group studies turbulent reacting flows using computer simulation. Primary areas of interest include modeling turbulent nonpremixed combustion, soot formation and transport, flame extinction and reignition processes, and multi-phase flows. Simulation techniques include direct numerical simulation, one dimensional turbulence modeling (ODT), large-eddy simulation (LES), and a new technique under development, autonomous microstructure evolution (AME). Samples of previous and ongoing research results are highlighted below.
Soot formation
Soot formation in flames is an important physical process as it accounts for the majority of flame radiation and is a pollutant. Soot is a particulate species with a low diffusivity resulting in strong differential transport between soot and gaseous species comprising the flame which affects soot temperature, formation and destruction, and emission. Modeling these processes in turbulent flames is aided by direct numerical simulation. The image at right shows a two-dimensional nonpremixed ethylene flame decaying turbulence. Soot mass fraction is shown along with the stoichiometric isocontour colored by the relative velocity between the soot and the flame.
Flame Extinction and Reignition
Nonpremixed flames occur at the stoichiometric interface between fuel and oxidizer streams. The rate of combustion in these flames is limited by the rate of diffusive mixing of the streams. Turbulence acts to increase flame surface area and scalar gradients, hence increasing mixing. As mixing rates increase, finite rate chemical kinetic effects become important, and flame extinction may occur if rates of diffusive heat loss exceed the heat release rates of combustion. Local flame extinction results in flame holes through which unburned fuel may escape, reducing combustion efficiency and allowing fuel emission. High rates of flame extinction may result in unstable combustion, flame liftoff, and if excessive, global flame blowout, posing operational and safety hazards.
Flame extinction and reignition are notoriously difficult processes to model due to the finite rate chemical kinetic effects, and complex reignition mechanisms that depend on the turbulent environment and flame structure. The degree of mixing in extinguished regions prior to reignition may affect the mode of reignition. At right are a series of three parametric simulations of planar nonpremixed jets with increasing levels of extinction. The simulations allow detailed investigation into reignition modes, and flame structures, providing high fidelity data for model validation, and fundamental insight into turbulent flame structure.
One Dimensional Turbulence Modelling (ODT)
Full resolution of three-dimensional turbulence fields is very costly from
a computational viewpoint. The ODT model fully resolves a flow field along a notional
line-of-sight. Turbulent advection is simulated through stochastic
eddy events that are implemented through grid rearrangements via triplet
maps. These eddy events occur with a given size, location, and frequency
that depends on the evolution of the velocity field itself, in a way that
mimimics that turbulent energy cascade. This figure at right shows
a snapshot of a turbulent channel flow at Re=395. The flow is vertical,
with walls on the right and left. A new, adaptive mesh ODT code is
under development that is being used to build an AME code in which coupled
sets of ODT lines evolve to provide detailed, affordable three-dimensional
flows.