| dc.description.abstract | Turbulence interacts with
flames over a multitude of lengthscales and timescales. These interactions are
nonlinear and are ubiquitous, for example, in propulsion engines operating in space, air, land, and sea. Such
interactions also constitute processes involved in astrophysical explosions. The technological development of
the engineering devices and understanding of fascinating natural processes thus behoove deeper, fundamental
understanding of these complex interactions. Such interactions between turbulence and premixed
flames are
expressed through a change in the
flame structure and propagation rate, characterized by
flame thickness,
flame surface topology, and the
flame speed, respectively. In this thesis, we have identi fied and investigated
these crucial interactions and their effects on the
flame structure and propagation characteristics for multiple
realizations of turbulent premixed
flames using Direct Numerical Simulations (DNS).
In the first part, we have studied
flame thickness and conditional scalar dissipation rate (CSDR) for a
temporally evolving turbulent slot-jet
flame. Most practical
flames encountered either in the swirling
flow of
a gas turbine combustor or in the wake of a blu body separated
flow of an afterburner are stabilized and
stretched by the straining action of a turbulent shear layer. Contrary to the general belief, the DNS cases
investigated show that the mean
flame thickness in turbulence is lesser than that of the corresponding laminar
premixed
flame. We have explored the alignment of eigenvectors of strain-rate tensor Sij with local normal n
to the surface and found that it others an incomplete explanation to the observed phenomena. As such, the
flame thickness reduces (or CSDR increases) due to an increase in normal strain-rate n rSd due to
flame
displacement speed Sd. The strain-rate n rSd dominates over normal strain-rate due to
uid
ow nn : ru
because of the positive stretch-rate response of the local
flame structure with sub-unity Lewis number Le.
The average propagation rate of a turbulent premixed
flame is much higher than that of its laminar counter-
part due to the faster consumption of the reactant
flow through the enhanced
flame surface area. This increased
flame surface area results from the multiscale stretching, folding and continuous surface generation-annihilation
processes inherent in turbulence-
flame interaction. In the second part of the thesis, we have investigated from
where and how do the complex topology and physico-chemical state of a fully developed turbulent premixed
flame generates and evolves in time. To that end, we have developed a back in time version of Flame Particle
Tracking (FPT). Flame particles are a class of surface points that move with reactive isoscalar surfaces and
represent local states of a premixed
flame. To identify the source locations of a turbulent
flame surface, we have
developed a new tracking method called the Backward Flame Particle Tracking (BFPT). Using BFPT on DNS
data, we have found that new
flame surface elements in a turbulent
flame generate from multiple, positively-
curved, leading locations of a
flame surface. This observation is in partial consonance with Zeldovich's conjecture
of a singular leading point valid for a laminar premixed
flame. We have elucidated the generation mechanisms
and also developed a relationship between the turbulent
flame speed ST of the
flame and Sd at the leading points
with the help of Finite Strain Theory (FST). We have also investigated the pair-dispersion characteristics of
the leading points and found that they obey a modi ed Batchelor's dispersion law, which is used to shed light
on why the new surface elements generate from the leading points.
In the third and fi nal part, we have investigated models of Sd for turbulent premixed
flames. We have
again used the concept of tracking
flame particles. Using BFPT-FFPT on the DNS data, we have found that
we can broadly divide
flame particles' lifetime into two phases: non-interacting and interacting. The Phase-I
or non-interacting phase forms a signifi cant part of the
flame particle lifetime, and during this phase, Sd at
flame particles varies due to stretch and curvature effects. Since the local
flame structure is still comparable
to a standard premixed
flame, the two-parameter Markstein length model gives reasonably accurate results.
In the Phase-II or interacting phase, local
flame surface elements interact with other portions of the
flame.
Consequently, the local
flame structure is no longer comparable to a standard premixed
flame even qualitatively
and
flame particles observe steep acceleration. We have derived a new interaction model to predict Sd during
this phase. It is shown that a combined use of the Markstein length model and the interaction model can
reasonably explain the dominant features of the local propagation characteristics for an entire turbulent
flame
surface, for the fi rst time, to our knowledge. | en_US |
