Niklas Zettervall

Combustion LES of a Multi-Burner Annular Aeroengine Combustor using a Skeletal Reaction Mechanism for Jet-A Air Mixtures

In this study we describe combustion simulations of a single sector and a fully annular generic multi-burner aero-engine combustor. The objectives are to facilitate the understanding of the flow, mixing and combustion processes to help improve the combustor design and the design process, as well as to show that it is now feasible to perform high-fidelity reacting flow simulations of full annular gas turbine combustors with realistic combustion chemistry. For this purpose we use a carefully validated finite rate chemistry Large Eddy Simulation (LES) model together with a range of reaction mechanisms for kerosene-air combustion. The influence of the chemical reaction mechanism on the predictive capability of the LES model, and on the resulting understanding of the combustion dynamics has recently been proved very important and here we extend this for kerosene-air combustion. As part of this work a separate study of different kerosene-air reaction mechanism is comprised, and based on this evaluation the most appropriate reaction mechanisms are used in the subsequent LES computations. A generic small aircraft or helicopter aero-engine combustor is used, and modeled both as a conventional single sector configuration and more appropriately as a fully annular multi-burner configuration. The single-sector and fully annular multi-burner LES predictions are similar but with the fully annular multi-burner configuration showing different combustion dynamics and mean temperature and velocity profiles. For the fully annular multi-burner combustor azimuthal pressure fluctuations are clearly observed, resulting in successive reattachment-detachment of the flames in the azimuthal direction. (Less)

Simulating jet exhaust plumes for optical propagation calculations

Laser beam propagation in severe environments such as a jet engine exhaust may influence performance of Airborne Optical Systems such as Missile Early Warning Systems or Directed Infrared Countermeasures. Laser beam propagation in close vicinity of the engine plume causes performance degradation due to beam wander, and beam broadening effects. In this study aero-optical effects are studied based on four CFD simulations of the same engine geometry. The CFD simulations encompasses ReynoldsAveraged Navier-Stokes (RANS), Large Eddy Simulation (LES) and hybrid RANS-LES methods, as well as different spatial resolution for the LES simulations. The CFD simulation results are used to compute estimates on laser beam wander and laser beam profile after passage of the jet.

A Combined Experimental and Computational Study of the LAPCAT II Supersonic Combustor

Dual-mode ramjet propulsion systems are suggested for the next generation high-speed flight vehicles. Here, we combine experimental measurements of high-speed (subsonic and supersonic) combustion at different operating conditions in the LAPCAT-II dual-mode ramjet combustor with Large Eddy Simulations (LES) using finite rate chemistry models and new skeletal H2-air combustion chemistry. The LAPCAT II combustor consists of four sections, and experiments have been performed for wall injection of H2 in a Ma 2.0 vitiated air-flow for total pressures and temperatures of p0=0.40 MPa, 1414 K<T0<1707 K, and a fixed equivalence ratio of φ=0.15. For this p0 the combustor is over-expanded, and the transition from supersonic to subsonic flow occurs at the start of the fourth combustor section. The flow and combustion diagnostics include measurements of p0 and T0 upstream of the combustor, wall-pressure profiles and Schlieren and OH* chemiluminescence imaging. The computational set-up consists of the full combustor, from the nozzle to the dump-tank. The computational model is composed of a compressible finite rate chemistry LES model, using the mixed subgrid flow model and the Partially Stirred Reactor (PaSR) combustion model, together with a new 22 step H2-air reaction mechanism. Qualitative as well as quantitative comparisons between experiments and simulations show reasonable agreement, but also reveal a high sensitivity of both the LES predictions and the experiments to T0. The LES results are further used to describe the underlying mechanisms of flow, wall-injection, mixing, self-ignition and turbulent combustion, and how these interrelated processes are modified by increasing the total temperature under otherwise identical conditions.

Investigations of microwave stimulation of turbulent flames with implications to gas turbine combustors

Efficient and clean production of electrical energy and mechanical (shaft) energy for use in industrial and domestic applications, surface- and ground transportation and aero-propulsion is currently of significant general concern. Fossil fuels are mainly used for transportation and aero-propulsion, but also for power generation. Combustion of fossil fuels typically give rise to undesired emissions such as unburned hydrocarbons, carbon dioxide, carbon monoxide, soot and nitrogen oxides. The most widespread approach to minimize these is to apply various lean-burn technologies, and sometimes also dilute the fuel with hydrogen. Although efficient in reducing emissions, lean-burn often results in combustion instabilities and igniteon issues, and thus become challenging itself. Another desired aspect of today’s engines is to increase the fuel flexibility. One possible technique that may be useful for circumventing these issues is plasma-assisted combustion, i.e. to supply a small amount of electric energy to the flame to stimulate the chemical kinetics. Although not new, this approach has not yet been fully explored, partly because of it’s complexity, and partly because of apparent sufficiency. Recently, however, several research studies of this area have emerged. This paper attempts to provide a brief summary of microwave-assisted combustion, in which microwaves are utilized to supply the electrical energy to the flame, and to demonstrate that this method is useful to enhance flame stabilization, delay lean blow-off, and to increase combustion efficiency. The main effect of microwaves (or electrical energy) is to enhance the chemical kinetics, resulting in increased reactivity and laminar and turbulent flame speeds. Here we will demonstrate that this will improve the performance of gas turbine combustors. (Less)