Passive scalar interface in a spatially evolving mixing layer (A. Attili and D. Denker)

Quartz nozzle sampling (D. Felsmann)

Dissipation element analysis of a planar diffusion flame (D. Denker)

Turbulent/non-turbulent interface in a temporally evolving jet (D. Denker)

Dissipation elements crossing a flame front (D. Denker and B. Hentschel)

Particle laden flow (E. Varea)

Turbulent flame surface in non-premixed methane jet flame (D. Denker)

DNS of primary break up (M. Bode)

Diffusion flame in a slot Bunsen burner (S. Kruse)

Various quantities in spatially evolving jet diffusion flame (D. Denker)

OH layer in a turbulent wall bounded flame (K. Niemietz)

Particle Image Velocimetry for the determination of laminar flame speeds

Laminar flame speed is one of the most important fundamental properties of a combustible mixture, regarding its reactivity, diffusivity, and exothermicity. It is defined as the propagation speed relative to the unburned mixture of a steady, laminar, one-dimensional, planar, stretch-free, and adiabatic flame. The accurate knowledge of laminar flame speed is essential for validating kinetic mechanisms. It also serves as a key scaling parameter in turbulent premixed combustion models.

The spherically expanding flame configuration is the most established and widely used technique to derive flame speeds at high-pressure and high-temperature, close to engine-relevant conditions. Several optical diagnostics are available to track the flame front during its propagation, e.g., Schlieren, shadowgraphy, or laser tomography. The instantaneous velocity field within the unburned mixture can be estimated by classical 2D-2C Particle Imaging Velocimetry (PIV). This approach eliminates any assumptions related to the burned gas. Therefore, it provides a more direct derivation of the flame speed. In reality, radiation affects the flame propagation, and the assumption of equilibrium in the burned gas is questionable since the density varies in time and space. Based on the images recorded, a specific post-processing routine has been developed to measure the velocity field within the unburned mixture.

Description of the setup

The ITV setup consists of a spherical vessel with an inner radius of 59.9 mm. Four optically accessible sapphire crystal windows are positioned on opposite sides (two cylindrical with a radius of 25 mm, and two rectangular with a height of 45 mm and a width of 10 mm). Gaseous mixtures, which can be burned in this experimental device, are flow controlled by mass flow controllers (MFC). The chamber is continuously flow fed by the fuel/oxidizer mixture at a given temperature, pressure, and composition. The inlet and outlet valves of the chamber are closed simultaneously once the desired pressure is obtained. Before sparking, the mixture is allowed to settle for 5 min. Subsequently, the combustible mixture is ignited at the center of the chamber using a spark ignition system, which can deliver energies up to 7 J. In order to avoid ignition disturbances in the propagating flame, a minimum spark ignition energy is applied.

Flame front positions and velocity fields are simultaneously measured by means of Mie scattering laser tomography. The flow is seeded with silicone oil droplets (Rhodorsil) supplied by an atomizer are used. The light source is a double cavity Nd:YLF laser (Litron LDY303HE), which can deliver up to 35.1 W at 527 nm. The light scattered by the droplets is captured by a high-speed camera (LaVision HighSpeedStar 6) working at an acquisition rate of 5,000 Hz. The camera magnification is more than 20 px/mm. The spatial location of the flame is close to the position at which the droplets evaporate. In the present case, the evaporation temperature of the droplets is 580 K.

An additional feature of this chamber is that the ignition can be initiated in the center of the vessel by laser ignition. A Lambda Physik EMG150 MSC excimer laser at a wavelength of 248 nm or a Spectra Physics GCR-150 at wavelengths of 355 and 532 nm can be used. The laser pulse is focused with a spherical lens in the middle of the vessel serving as a point ignition source.

An absorber tube has been employed to allow for neutralization of harmful combustion by-products, such as hydrofluoric acid, which is produced in the fire safety assessment of fluorinated low global-warming-potential refrigerants.