Current Research

Combustion-based power generators in small dimensions have advantages over batteries in favor of their high power density. One of the main challenges is the difficulty in flame stabilization in the combustor with a high surface-to-volume ratio. Bluff-body flame holder is one approach to promote combustion stability. In this project, high fidelity simulations are conducted in order to understand the fundamental mechanisms of flame stabilization and blow-off behind a bluff body. Advanced analytical diagnostic tools are utilized to identify and characterize the blow-off and re-attachment processes.
The effect of turbulence on premixed flame propagation characteristics has been a long-standing fundamental question in combustion research. Many existing theoretical descriptions are based on simple chemistry and at weak to moderate levels of flame wrinkling, thus need to be carefully assess at higher levels of turbulence with consideration of detailed chemical pathways for combustion. To this end, high resolution direct numerical simulations (DNS) of three-dimensional premixed flame propagation through homogeneous isotropic turbulent flows are conducted at high Reynolds and Karlovitz numbers, and the detailed simulation data are analyzed to investigate various statistical characteristics of turbulent flame dynamics.
The present study attempts to develop an integrated multi-phase and multi-species computational capability including surface tension and evaporation effects and to demonstrate the feasibility of the mechanism of the fuel-vapor jet ejection observed experimentally from an evaporating droplet under micro-gravity conditions. The study is relevant in characterizing spray evaporation behavior in internal combustion engines.
Combustion-generated soot and particulates poses strong environmental concerns. Due to the complexities in the physical and chemical processes involved, predictive models require detailed description of the gas-phase chemistry including key soot precursor species, along with aerosol dynamics model to describe the nucleation, growth, and oxidation processes. This is a comprehensive research effort combining the expertise in the development of detailed chemical kinetic mechanisms for hydrocarbon fuels all the way up to higher polycyclic aromatic hydrocarbons (PAHs), systematic reduction of the reaction mechanisms for efficient flame simulations, description of soot aerosol model based on statistical approaches, and high fidelity simulations of laminar and turbulent flames.
Adaptive mesh refinement (AMR) can reduce the computational cost significantly, while maintaining the capability to capture flow features at different scales. We develop solvers for compressible multi-species reactive Navier-Stokes equations based on finite-volume methods, with AMR capability. 
Recent drive for cleaner and more efficient internal combustion engines has increased focus on low temperature combustion (LTC) strategies in compression ignition engines. Homogeneous charge compression ignition (HCCI), employing compression ignition of a lean premixed fuel-oxidizer mixture is one of the advanced LTC concepts that have been receiving substantial attention over the last few decades. However, it is very difficult to control the ignition timing and heat release rate in HCCI engines, resulting in very rapid pressure rise rates at high loads and high level of unburned hydrocarbon and CO emissions at low loads. In-cylinder stratification in composition and temperature can prevent simultaneous auto-ignition by changing the ignition delay of the mixture locally, thereby resulting in lower peak pressure and heat release rate.  This can be achieved by a number of ways such as variable valve timing for residual gas trap, spark-assisted compression ignition and multi-stage spray injection. These advanced strategies, however, introduce considerable complexity to the combustion system. Numerical simulations can aid in the design process of modern engines, but require high-fidelity combustion models that are valid across a wide range of combustion regimes. Towards high-fidelity modeling of in-cylinder combustion, the aim of this project is to develop improved flamelet combustion models to accurately simulate mixed-mode combustion.
Ignition quality tester (IQT) is a standard device to measure the cetane rating of various fuels. While it is widely used for ignition characterize the ignition property of many practical fuels as industry standard, the existing measurement techniques lack in details about the spray evaporation and dispersion characteristics in order to clearly distinguish the physical and chemical nature of the ignition behavior. CONVERGE is being used with detailed chemical kinetic mechanism to predict the details of the spray dynamics, dispersion, and autoignition characteristics within the IQT. The images show the n-heptane spray development within the IQT at 1ms (left) and 3ms (right) after the injection.
Steady and unsteady counterflow laminar flames have been extensively used to investigate fundamental flame characteristics. The information is valuable in turbulent combustion modeling in the framework of laminar flamelet approach. Although flame response to unsteady velocity fluctuations has been widely studied, the effects of unsteady composition fluctuations has not been fully explored. In collaboration with the experimental studies at Keio University, computational simulations of premixed counterflow flame are conducted with forced oscillation of equivalence ratio at the inlet. The responses of flame location and flame temperature, and flammability limit are studied.