Hazem El-Rabii

Mode coupling and output beam quality of 100-400 μm core silica fibers.

Propagation and mode coupling within relatively short (∼1-10 m) large core, nominally multimode, fibers are of interest in a number of applications. In this research, we have studied the output beam quality and mode coupling in various fibers with core diameters of 100-400 μm and lengths of 2 m. Output beam quality (M2) and mode-coupling coefficients (D) have been studied for different clad dimensions, numerical apertures, and wavelengths. The mode-coupling coefficients have been determined based on modal power diffusion considerations. The results show that D scales approximately as the inverse square of the clad dimension and inverse square root of the wavelength. Output from a 2 m length fiber of 100 μm core and 660 μm clad fiber is close to single mode (M2=1.6), while output from a 200 μm core and 745 μm clad fiber also has high beam quality.

Properties of an air plasma generated by ultraviolet nanosecond laser pulses

Temperatures and compositions of plasma environments created by laser pulses are wavelength-dependent, and they have not been determined as yet for air plasmas produced by ultraviolet radiation. This paper fills this gap and reports time-resolved spectral measurements of laser-induced sparks created in air using a Q-switched Nd : YAG operating at 355 nm with a pulse width of 7 ns. Temperatures are determined from Boltzmann analysis of emission from atomic oxygen lines at 715 and 777 nm. Electron number densities are inferred from Stark broadening of the H-alpha line (656 nm). The measurements have been carried out over a time interval of 90 ns–3 µs after the plasma formation that corresponds to the variation of the temperature and the electron number density from 27 000 K to 12 000 K and from 1.2 × 10 18 cm −3 to 1.6 × 10 17 cm −3 , respectively. Simple rate estimates show that the plasma is likely to be in local thermodynamic equilibrium (LTE). Using a new approach to calculate time-dependent pressures, which employs blast-wave theory, we perform thermochemical computations of the plasma thermodynamic properties and compositions. The results strongly confirm the validity of the LTE assumption for the plasma investigated and yield electron number densities in very good agreement with the measured values. (Some figures in this article are in colour only in the electronic version)

LASER IGNITION IN A LEAN PREMIXED PREVAPORIZED INJECTOR

Laser-induced spark ignition has proved to be an alternative way of achieving ignition. In this paper, we demonstrate the feasibility of laser spark ignition at the outlet of a lean premixed prevaporized injector, similar to those used in low-NOx air jet engines. This injector uses liquid n-heptane as fuel in preheated air with a high level of turbulence. After a description of the combustion facility and of the experimental setup, experimental results are presented and discussed. Special attention is paid to ignition energy thresholds for various positions of the focusing point in the combustion chamber. The influence of energy level on ignition probability is also presented. Finally, shadowgraphy visualization with a high-speed camera of one ignition event is presented and discussed.

Temperature measurement of laser heated metals in highly oxidizing environment using 2D single-band and spectral pyrometry

Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000–4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3% and 6% of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperature, even with unexpected variations of the emissivity during the observed process, and to give a relevant value of this emissivity. A validation of a comsol numerical model of the heating cycle of W, Ta, Nb, V rods has been obtained by comparison with the measurement.Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000–4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3% and 6% of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperatu...

Fuel Mass-Loss Rate Determination in a Confined and Mechanically Ventilated Compartment Fire Using a Global Approach

Fire development is generally characterized in terms of evolution of heat release with time, and its determination thus represents an essential aspect of a fire hazard analysis. Since it is not a fundamental property of a fuel, heat release cannot be calculated from the basic material properties, and one generally resorts to experiments to determine it. In this article, we present a theoretical formulation that allows the determination of the burning rate of fuels for pool fires in a closed compartment. It is based on an energy balance at the pool fire surface and includes radiative and convective heat components from the flame to the pool surface by relating them to the ambient oxygen mass fraction at the flame base. Fuel response to vitiated air as well as burning enhancement due to hot gases and confinement are taken into account. The formulation was first compared with the empirical correlation determined by Peatross and Beyler before being implemented in a computational fluid dynamics (CFD) code and validated against two experiments involving a hydrogenated tetra-propylene pool fire test enclosed in a confined and mechanically ventilated compartment. These experiments were conducted in conditions for which external heat fluxes were either negligible or significant. It is shown that this approach is able to correctly predict the fuel mass loss rate and provides a reasonable assessment of the heat flux from the flame to the pool surface.

On-shell description of unsteady flames

The problem of a non-perturbative description of unsteady premixed flames with arbitrary gas expansion is addressed in the two-dimensional case. Considering the flame as a surface of discontinuity with arbitrary local burning rate and gas velocity jumps, we show that the flame-front dynamics can be determined without having to solve the flow equations in the bulk. On the basis of the Thomson circulation theorem, an implicit integral representation of the downstream gas velocity is constructed. It is then simplified by a successive stripping of the potential contributions to obtain an explicit expression for the rotational component near the flame front. We prove that the unknown potential component is left bounded and divergence-free by this procedure, and hence can be eliminated using the dispersion relation for its on-shell value (i.e. the value along the flame front). The resulting system of integro-differential equations relates the on-shell fresh-gas velocity and the front position. As limiting cases, these equations contain all the theoretical results on flame dynamics established so far, including the linear equation describing the Darrieus-Landau instability of planar flames, and the nonlinear Sivashinsky-Clavin equation for flames with weak gas expansion.

An Accurate Equation-of-State Model for Thermodynamic Calculations of Chemically Reactive Carbon-Containing Systems

We present an equation-of-state (EOS) model that allows for reliably computing the thermodynamics and chemical compositions of reactive carbon-containing systems over a wide range of temperatures and densities covering both high pressures up to tens of gigapascals and moderate pressures. The model includes a theoretical EOS of a multicomponent fluid phase and a thermodynamically consistent multiphase EOS of carbon nanoparticles. The results of thermochemical computations based on this model are shown to be in good agreement with a variety of shock wave and static measurements, as well as with detonation experiments that were not used for calibrating the intermolecular potential parameters.

Laser Ignition of Bulk Iron, Mild Steel, and Stainless Steel in Oxygen Atmospheres

The ignition of pure iron, mild steel S355J, and stainless steel 316L has been investigated. The whole ignition and combustion processes have been monitored using a high-speed video camera and adapted pyrometry. Our results show that the absorptivity of the iron and mild steel to laser radiation increases rapidly at 850 K, from 0.45 to 0.7, and that of stainless steel increases more gradually during the heating process from 0.45 to 0.7. The ignition of iron, mild steel, and stainless steel is controlled by a transition temperature, at which the diffusivity of the metal increases sharply. The transition temperature of pure iron and mild steel is around 1750 K, when molten material appears, and that of stainless steel is around 1900 K, when the solid oxide layer loses its protective properties. These temperatures are independent of the oxygen pressure (from 2 to 20 bar) and of the laser intensity (from 1.6 to 34 kW·cm). During ignition, the temperature increases very strongly at first, and after that a change in the heating rate of the surface is observed. A diffusive-reactive model, provided with equations describing the diffusion of oxygen in the metal and the transfer of heat released by the oxidation reactions has been solved. The model correctly reproduces the sharp rise of temperature as well as the decrease in the heating rate that follows. Comparison between calculated and experimental data shows that, without liquid convection flow in the melt, combustion would extinguish as soon as the metal surface is fully oxidized and that the combustion front moves into the metal.

Temperature and Electron Density Measurements of Laser-Induced Plasmas in Air at Elevated Pressures

ABSTRACT We present time-resolved spectroscopic measurements of 1064-nm Nd:YAG laser-produced plasmas in air at pressures from 0.85 to 48.3 bar. We report temperatures and electron number densities of the plasmas at times between 200 ns and 3 µs after the plasma onset. Neutral atomic oxygen lines at 715 nm and 777 nm are used for temperature measurement through a Boltzmann analysis. Electron number density is measured using Stark broadened atomic hydrogen (Hα) line at 656 nm. We compare experimental results with modeling results obtained from using Taylor-Sedov blast-wave theory coupled with local thermal equilibrium composition calculations.

Premixed Flame Propagation in Channels of Varying Width

A theory of flame propagation in two-dimensional channels with adiabatic and impermeable curved walls is developed within the framework of the on-shell description of premixed flames. Employing the Green function appropriate to the given channel geometry, an implicit integral representation for the burnt gas velocity is constructed. It is next used to derive an explicit expression for the rotational component of the gas velocity near the flame front by successive separation of irrotational contributions. We prove that this separation can be performed in a way consistent with boundary conditions at the channel walls. As a result, the unknown irrotational component can be projected out by applying a dispersion relation stemming from its analyticity, thus leading to a closed system of equations for the on-shell fresh gas velocity and the flame front position. These equations show that, in addition to the usual nonlocality associated with potential flows, vorticity produced by a curved flame leads to specific nonlocal spatial and temporal influence of the channel geometry on the flame evolution. To elucidate this influence, three special cases are considered in more detail: a steady flame stabilized by incoming flow in a bottle-shaped channel, quasi-steady flames, and unsteady flames with small gas expansion propagating in channels of slowly varying width. In the last case, analytical solutions of the equations derived for the front shape are obtained in the first post-Sivashinsky approximation on using the method of pole decomposition.