טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDagan Yuval
SubjectNumerical Investigation of Flame Structure and
Stability in Gaseous and Liquid Fuel Turbulent
Combustion
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus Yoram Tambour
Dr. Eran Arad


Abstract

Unsteady turbulent flame evolution in non-premixed combustion is computationally investigated using large eddy simulations (LES). A simple coaxial combustion chamber, subjected to highly unsteady, turbulent recirculating flow is considered, following an experimental study. Large-scale flame fluctuations, previously reported in the experiment such as intermittent flame lift-off and reattachment and pulsating flames in swirling and non-swirling conditions, were identified in our computation.

New criteria for flame three-dimensional inhomogeneity are suggested and implemented in the present study, providing the ability to quantify the flame unsteadiness. Using this technique, it is shown that local, large quenched regions develop in the flame’s mixing area and rotate continuously, even when swirl is not imposed on the inlet. However, this rotation appears to be disordered, abruptly changing its direction. On the other hand, the current study shows that when swirl is imposed on the inlet, a larger quenched region is identified, rotating in steady ordered rotation in the direction of the imposed swirl. In addition, large-scale radial flame fluctuations are increased downstream with the increase of swirl number. Consequently, significant correlations between radial and circumferential flame fluctuation frequencies were retrieved. Proper orthogonal decomposition analysis reveals coherent flame structures of five dominant modes that contain most of the energy in the fluctuating flame. A simplified analytical stability model is derived and implemented here to assess the hydrodynamic contribution to the flame instability; it is shown that radial fluctuations are excited by circumferential perturbations in the mixing region, providing new insight into the mechanism responsible for the onset of radial fluctuations. The computed radial flame fluctuation spectrum is predicted well using the linear stability analysis.

In addition to the gaseous flame study, the dynamics of unsteady turbulent spray-flame is computationally investigated, employing the same geometric configuration used in the gaseous test case. It is shown that the turbulent spray-flames exhibit unique stability characteristics that were not found for the gaseous flames. The presence of a recirculation zone, which is common in jet engine combustion chambers, has a significant role in spray and flame dynamics, diverting the flame in a cyclic motion. Two repetitive developmental stages of flame structures were identified and analyzed.

Droplet clustering was found in the vicinity of large vortical structures, and flames surrounding groups of droplets were identified. Backflow of droplets was found to have ligament structures, similar to those found in turbulent shear flow. The local statistics of fuel droplet dispersion are analyzed and discussed. A new approach is used to investigate spray flame propagation by reducing the dimensionality of the problem, revealing low frequency flame repetitive motions, while retaining its turbulent characteristics.