Supersonic Combustion Modeling
Scramjets or supersonic-combustion RAM jets form the backbone of hypersonic vehicles. One of the main attractions of scramjets is their simpler design compared to conventional jet engines. Compression of incoming air is achieved solely through a system of compression shocks. Depending on the flight Mach number, hydrogen or hydrocarbon based fuel is used. Fuel-air mixing needs to be fast since the residence time of the fluid inside the engine is very short. Scramjet stability critically depends on the fuel-air mixing and the subsequent combustion processes. The focus of our work is to develop detailed large-eddy simulation (LES) based combustion models for such scramjets. In particular, we are interested in modeling the interaction of shocks with the turbulent combustion process.
Graduate students: Heeseok Koo, Pratik Donde
Funding source: NASA
Ablation of Surfaces Exposed to High-speed Flows
Re-entry vehicles are exposed to extremely hostile flow environments with very high heat loads and large aerodynamic forces. Typically, a thermal-protection shield (TPS) is used to protect the vehicle. TPS ablation during re-entry, while a necessary aspect of the protection system, also alters the turbulent flow in the boundary layer close to the vehicle. In particular, material dislodging from the surface can lead to blowing effects or particle transport in the boundary layer. This, in turn, will affect the aerodynamic load the vehicle dynamics. This project aims at understanding the modification of the turbulent boundary layer flow under these ablation conditions. This is a joint experimental/computational program that will use uniquely designed experiments (carried out by Prof. Clemens’s group) to develop insight into the modeling process. A new paradigm for model development based on an optimal estimation strategy is being formulated.
Graduate students: Kalen Braman
Funding source: NASA, DoD
Modeling Turbulent Spray Combustion
Liquid fuels are commonly used in aircraft engines as well as automobile engines. The liquid fuel is typically passed through an atomizer that breaks down the liquid jet into small droplets. The performance of these engines is determined, in large part, by the dispersion of the atomized liquid fuel drops inside the engine combustor. Droplets possess inertia and many not follow the fluid elements. In addition, non-uniform dispersion will lead to very complex evaporation process. The combustion chamber itself is a highly chaotic environment that will affect and be affected by the droplet dispersion process. Successful modeling of this complex multiphase system will help minimize soot and NOX emissions, thereby reducing the environmental footprint of these devices. In this work, we focus on developing a spray combustion model that can account for some of the unique properties of such flows. For instance, fuel-air mixing in spray combustion is considerably different from a single-phase system. While the boundary conditions determine the combustion regime in single-phase systems, the droplet dispersion process controls the flame propagation mechanism in spray combustion. A probabilistic framework is being developed to incorporate these features.
Graduate students: Colin Heye, Heeseok Koo
Funding source: NSF
Sub-filter Modeling for LES of Turbulent Combustion
Large-eddy simulation (LES) has emerged as a viable tool for modeling turbulence. A conventional description is that LES resolves the large-scale motions of the flow on a computational grid while requiring modeling of small-scale motions. The underlying requirement is that small-scale motions should carry an insignificant amount of energy, and statistical models used to describe their evolution should not alter the dynamics of the large-scale motions. Two industrially relevant processes - wall-bounded flows and turbulent combustion - violate this requirement since in these flows the physical processes occurring at the small-scales affect the large-scale dynamics. While both flows are still actively being researched, turbulent combustion involves certain benign features that slightly simplify the modeling process for a sub-set of problems. When chemical reactions are sufficiently strong and far away from extinction, the small-scale combustion process can be described through low-dimensional models. The evolution of the parameters used in these models is dependent on the large-scale mixing process, which is very well captured by LES. Consequently, combustion LES has become one of the biggest success stories in the application of the LES approach. Still, rigorous analyses of this modeling process and the associated errors are largely absent. The focus of this work is to understand the errors incurred by current LES models for turbulent combustion and to provide a new modeling approach based on error minimization strategies.
Graduate students: Colleen Kaul, Shaun Kim
Funding source: NASA,ACS PRF
Kinetic Modeling of Soot Formation in Aircraft Engines
Soot is high carbon-content solid particle formed as a by-product in high-temperature combustion process. Soot release is a health hazard, implying that future engines need to reduce the amount of soot formed. The science of soot formation in these engines is very complex. Particulate formation is affected by a host of conditions, including the gas-phase composition and temperature, the spatial distribution of the fuel and the combustion products, the turbulent gas-phase flow. The objective of this project is to develop a comprehensive modeling framework that incorporates detailed description of the gas-phase chemistry, turbulence-chemistry-soot interactions, and the background turbulent flow.
Grad students: James Sung
Funding Source: DoD SERDP