This research aims to investigate the nonlinear dynamics of the non-reacting jets and non-premixed lifted jet flames. The goal is to understand better how the flow system dynamics change over time and identify the path toward unwanted conditions such as flashback, extinction, or blowout to limit damage to the combustor’s critical components. The existence of these undesirable conditions is bound to the fluid’s history, meaning that initiated perturbation may persist in the system for time scales comparable to large-scale flow timescales. Hence, the notion is to utilize jet and jet flames as a study test case to work out how the flow evolves dynamically with the hope of understanding how to limit occurrences of the chaotic unwanted condition.
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Lab personnel: Sina Rafati
Sponsor: National Science Foundation and Air Force Office of Scientific Research
We are studying the non-linear dynamics of lifted turbulent jet flames in a turbulent co-flow as shown in the figure below. We use time-resolved (kHz-rate) particle image velocimetry (PIV) and chemiluminescence imaging to obtain the data needed for our analysis. The figure below shows the setup for simultaneous tomographic-PIV and chemiluminescence imagining; the tomo-PIV enables us to measure three-dimensional velocity fields.
Turbulent jet flame with laser illumination for PIV
We have used different frameworks to investigate the development of chaos in this dynamic system originated from the interaction of the turbulence and flame. Chaotic systems have considerable sensitivity to the initial condition with the presence of a the so called strange attractor. In such a sensitive system, any two trajectories that start close to one another in the phase space might move exponentially apart, which is an indication of chaotic motion. Such an evaluation can be parametrized with the so-called Lyapunov exponent (LE), which quantifies the divergence rate of the two initially nearby points in the phase space as evolving in time. The figure below represents the snapshot attractor obtained by using the ensemble framework for a non-reacting turbulent jet obtained from the time-resolved PIV velocity field. The gif shows how the snapshot attractor shape changes in time.
Phase portrait of velocity components for data acquired in a turbulent jet
The particles can be also tracked in the measurement domain based on the obtained velocity fields. The figure below depicts a matrix of notional particles tracked based on the planar velocity fields. Interestingly the particles are forming some hidden flow patterns that are not readily observed using an Eulerian perspective.
Demonstration of the deformation of an initially square fluid element
Hence, in this context, we use Lagrangian Coherent Structures (LCSs), which are defined as manifolds that are locally Euclidean and invariant, to study the relationship between Lyapunov exponent changes with flow topological features. The Lagrangian framework is shown to be effective at revealing the kinematics associated with the flame-turbulence interaction. The animated gif below depicts a planar measurement of turbulence-flame interaction. The PIV velocity field and repelling and attracting LCSs are showing how the flame interacts with these structures. It’s apparent that the flame is pushed away by repelling LCSs at and attracting LCSs draw the flame toward themselves.
Sample data inferred from the velocity field. (left) velocity magnitude, (center) repulsive LCS, and (right) attractive LCS interacting with the flame.
More information on our methodology can be found in the following publication:
Rafati, S. and Clemens, N.T., “Frameworks for investigation of nonlinear dynamics: Experimental study of the turbulent jet,” Physics of Fluids, Vol. 32, Issue 8, p. 085112, 2020.