Denis Veynante

Influence of the Repetition Rate of a Nanosecond Pulsed Discharge on the Stabilization of a Turbulent Lean Premixed Flame

A Nanosecond Repetitively Pulsed Plasma (NRPP) generator capable of delivering an electric pulse of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25 kW lean premixed propane/air flame at atmospheric pressure. We had previously shown that, when placed in the recirculation zone of the flame, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. In the present work we study the influence of the discharge’s repetition rate on the operating regimes of the burner and on both thermal and chemical effects. We present temperature and electron number density measurements based on the Stark-broadening effect of the Hβ Balmer line in the region where the discharge is created.

Influence of air flow parameters on nanosecond repetitively pulsed discharges in a pin-annular electrode configuration

The effect of various air flow parameters on the plasma regimes of nanosecond repetitively pulsed (NRP) discharges is investigated at atmospheric pressure. The two electrodes are in a pin-annular configuration, transverse to the mean flow. The voltage pulses have amplitudes up to 15 kV, a duration of 10 ns and a repetition frequency ranging from 15 to 30 kHz. The NRP corona to NRP spark (C-S) regime transition and the NRP spark to NRP corona (S-C) regime transition are investigated for different steady and harmonically oscillating flows. First, the strong effect of a transverse flow on the C-S and S-C transitions, as reported in previous studies, is verified. Second, it is shown that the azimuthal flow imparted by a swirler does not affect the regime transition voltages. Finally, the influence of low frequency harmonic oscillations of the air flow, generated by a loudspeaker, is studied. A strong effect of frequency and amplitude of the incoming flow modulation on the NRP plasma regime is observed. Results are interpreted based on the cumulative effect of the NRP discharges and an analysis of the residence times of fluid particles in the inter-electrode region.

Transformational fluctuation electrodynamics: application to thermal radiation illusion

Thermal radiation is a universal property for all objects with temperatures above 0K. Every object with a specific shape and emissivity has its own thermal radiation signature; such signature allows the object to be detected and recognized which can be an undesirable situation. In this paper, we apply transformation optics theory to a thermal radiation problem to develop an electromagnetic illusion by controlling the thermal radiation signature of a given object. Starting from the fluctuation dissipation theorem where thermally fluctuating sources are related to the radiative losses, we demonstrate that it is possible for objects residing in two spaces, virtual and physical, to have the same thermal radiation signature if the complex permittivities and permeabilities satisfy the standard space transformations. We emphasize the invariance of the fluctuation electrodynamics physics under transformation, and show how this result allows the mimicking in thermal radiation. We illustrate the concept using the illusion paradigm in the two-dimensional space and a numerical calculation validates all predictions. Finally, we discuss limitations and extensions of the proposed technique.

A Filtered Tabulated Chemistry Model for Large Eddy Simulation of Reactive Flows

A new model called F-TACLES (Filtered Tabulated Chemistry for LES) is developed to introduce tabulated chemistry methods in LES of turbulent premixed combustion. The main objective is to recover the correct propagation speed of the flltered flame front when the sub-grid scale wrinkling vanishes. The filtered flame structure is mapped by 1-D filtered laminar premixed flames. The methodology is first applied to 1-D filtered laminar flames. Computations show the capability of the model to recover the laminar flame speed and the correct chemical structure when flame wrinkling is fullyresolved on the LES lter scale. The model is then extended to turbulent regimes by introducing sub-grid scale wrinkling effects on the flame propagation. LESs of a 3-D turbulent premixed flame are performed on dierent grids, with different flame filter sizes. Objectives are to analyze the influence of the grid size, the flame filter size and the sub-grid flame wrinkling on the model performances. All these computations are compared to experimental data.