Combustion Dynamics
Since the 90's, there have been increasingly stringent regulations on pollutants emitted out of gas turbines. These have led engine manufacturers to operate combustors with premixed fuel and air. This allows for temperature control of the combustion process and hence the concentration of emissions. One significant drawback of operation in the premixed mode though, is the dynamic behavior of these combustors.
Combustion dynamics includes the phenomenon of pressure oscillations in both, aircraft and land-based power generation engines. These lead to cracks and thermal hot-spots in different parts of the engine. Understanding and control of these oscillations are key to the reliable and robust operation of power-plants and propulsion systems.
The objectives of our efforts at MIT are to develop a fundamental understanding of combustion dynamics in cases when flames are stabilized by recirculating flows formed in the wake behind sudden expansions or in swirl stabilized flows. We are currently working with syngas at varying hydrogen &ndash carbon monoxide ratios, premixed with air over a range of equivalence ratios, inlet temperatures and Reynolds numbers. We are conducting experiments in both a planar combustor and a newly designed axisymmetric combustor. The insight gained from these experiments will be used to suggest different variations on the flame stabilizations environments aimed at passively stabilizing the flames.
Fuel Flexible Combustion
Syngas, a mixture of hydrogen and carbon monoxide produced by burning coal in pure oxygen and steam in high pressure gasifiers, has been proposed as the fuel of choice for modern high efficiency low emission combined cycles power plant, in which gas turbine engines are used as topping cycles that take advantage of high temperature combustion products to improve the overall efficiency of the power plant. Incorporating technologies to filter out turbine-corroding gases from the gasifier products and delivering clean syngas to the gas turbine combustor enables the use of syngas in gas turbines. Furthermore, efficient separation technologies can be used to produce hydrogen from the syngas, which can then be fed into fuel cells to further improve the energy conversion efficiency of the plant. The sequestration of carbon dioxide, following the production of steam for the bottoming cycle, makes this plant environmentally ideal.
Time-Resolved Simulation of Reaction Kinetics
Numerical Combustion with detailed chemical kinetics is a very challenging problem. The set of the species conservation equations is mathematically very stiff. The stability of the explicit projection schemes for such problems requires a very strict CFL condition, limiting the maximum timestep to a few nanoseconds. To increase this stability restriction by orders of magnitude, semi-implicit stiff integration is carried out in the projection scheme. A set of ODEs is solved implicitly using commercially available stiff ODE solvers during the process. The next limiting CFL condition, that does not allow the full potential usage of semi-implicit scheme, is the diffusion of intermediate species like H ion. Diffusion sub-stepping with Operator-Splitting is performed to get around this problem. As a result, stable time stepping increases from few nanoseconds to hundreds of nanoseconds. These techniques, add considerable function overhead to the numerical code, however, the overall speed gain in the computations is tremendous.
Such schemes are currently used to investigate two-dimensional steady and unsteady simulations of perforated-plate stabilized laminar premixed flames. Flame-wall interactions and flame-acoustic interactions in such combustion systems are being studied. The above summarized numerical technique makes the time accurate simulations of unsteady flames faster by orders of magnitude compared to purely explicit numerical schemes.
Instability Mitgation