Instabilities and soot formation in high-pressure, rich, iso-octane-air explosion flames - 1. Dynamical structure

Simultaneous OH planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements have been performed on 2-bar rich iso-octane–air explosion flames obtained in the optically accessible Leeds combustion bomb. Separate shadowgraph high-speed video images have been obtained from explosion flames under similar mixture conditions. Shadowgraph images, quantitative Rayleigh images, and normalized OH concentration images have been presented for a selection of these explosion flames. Normalized experimental equilibrium OH concentrations behind the flame fronts have been compared with normalized computed equilibrium OH concentrations as a function of equivalence ratio. The ratio of superequilibrium OH concentration in the flame front to equilibrium OH concentration behind the flame front reveals the response of the flame to the thermal–diffusive instability and the resistance of the flame front to rich quenching. Burned gas temperatures have been determined from the Rayleigh scattering images in the range 1.4⩽ϕ⩽1.9 and are found to be in good agreement with the corresponding predicted adiabatic flame temperatures. Soot formation was observed to occur behind deep cusps associated with large-wavelength cracks occurring in the flame front for equivalence ratio ϕ⩾1.8 (C/O⩾0.576). The reaction time-scale for iso-octane pyrolysis to soot formation has been estimated to be approximately 7.5–10 ms

Planar laser induced fluorescence for temperature measurement of optical thermocavitation

Pulsed laser-induced cavitation, has been the subject of many studies describing bubble growth, collapse and ensuing shock waves. To a lesser extent, hydrodynamics of continuous wave (CW) cavitation or thermocavitation have also been reported. However, the temperature field around these bubbles has not been measured, partly because a sensor placed in the fluid would interfere with the bubble dynamics, but also because the short-lived bubble lifetimes (∼70–200 µs) demand high sampling rates which are costly to achieve via infrared (IR) imaging. Planar laser-induced fluorescence (PLIF) provides a non-intrusive alternative technique to costly IR imaging to measure the temperature around laser-induced cavitation bubbles. A 440 nm laser sheet excites rhodamine-B dye to fluoresce while thermocavitation is induced by a CW 810 nm laser. Post-calibration, the fluorescence intensity captured with a high-speed Phantom Miro camera is correlated to temperature field adjacent to the bubble. Using shadowgraphy and PLIF, a significant decrease in sensible heat is observed in the nucleation site– temperature decreases after bubble collapse and the initial heated volume of liquid shrinks. Based on irradiation time and temperature, the provided optical energy is estimated to be converted up to 50% into acoustic energy based on the bubble's size, with larger bubbles converting larger percentages

Holographic measurement of drop-on-demand drops in flight

The analysis of images of ink drops in flight can provide information about jet straightness, drop velocity and volume. However trade-offs between field of view, optical and digital resolution and other factors such as depth of field and optical distortion, limit the accuracy and amount of information available from a single image. In-line, digital holograms of drops in flight can capture information from fields of view at least as large as the area of the digital sensor. Using mathematical reconstruction techniques particularly suited to sparse, small objects of regular geometry the accuracy of measurement can potentially be submicrometer on drop position and diameter. This paper describes our experimental apparatus, hologram reconstruction techniques and the results of experiments on imaging drops. We also discuss techniques to improve the accuracy of the technique in the direction of the optical axis

Picosecond ionization dynamics in femtosecond filaments at high pressures

We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse. Our study suggests that this picosecond evolution can be attributed to collisional ionization including Penning and associative ionizations and electron-impact ionization of excited atoms generated during the pulse. The dominance of excited atoms over ionized atoms at the end of the pulse also indicates an intrapulse inhibition of avalanche ionization. This delayed ionization dynamics provides evidence for diagnosing atomic and molecular excitation and ionization in intense laser interaction with high-pressure gases

High speed shadowgraphy for the study of liquid drops

The book contains invited lectures and selected contributions presented at the Enzo Levi and XVII Annual Meeting of the Fluid Dynamic Division of the Mexican Physical Society in 2011.This was work was sponsored by EPSRC grant number RG5560

Probes and monitors for the study of solidification of molten semiconductors

The purpose is to examine solidification in the LiCl-KCl system to determine if phenomena such as solute rejection can be obseved by laser schlieren imaging. Molten salts have attributes that make them attractive as physical models in solidification studies. With optical techniques of investigation such as schlieren imaging, it is possible to study fluid flow phenomena in molten salts and to watch the trajectory of the solid-liquid interface