Effects of Translational Nonequilibrium on Performance of Flowing Chemical Oxygen-Iodine Laser

The effect of the translational nonequilibrium on performance modeling of flowing chemical oxygen-iodine lasers (COIL) is emphasized in this paper. The spectral line broadening (SLB) model is a basic factor for predicting the performances of flowing COIL. The Voigt profile function is a well-known SLB model and is usually utilized. In the case of gas pressure in laser cavity less than 5 torr, a low pressure limit expression of the Voigt profile function is used. These two SLB models imply that ail lasing particles can interact with monochromatic laser radiation. Basically, the inhomogeneous broadening effects are not considered in these two SLB models and they cannot predict the spectral content. The latter requires consideration of finite translational relaxation rate. Unfortunately, it is rather difficult to solve simultaneously the Navier-Stokes (NS) equations and the conservation equations of the number of lasing particles per unit volume and per unit frequency interval. In the operating condition of flowing COIL, it is possible to obtain a perturbational solution of the conservational equations for lasing particles and deduce a new relation between the gain and the optical intensity, i.e., a new gain-saturation relation. By coupling the gain-saturation relation with other governing equations (such as the NS equations, chemical reaction equations and the optical model of gain-equal-loss), We have numerically calculated the performances of flowing COIL. The present results are compared with those obtained by the common rate-equation (RE) model, in which the Voigt profile function and its low pressure limit expression are used. The difference of different model's results is great. For instance, in the case of lasing frequency coinciding with the central frequency of line profile and very low gas pressure, the gain-saturation relation of the present model is quite different with that of the RE model.

Semi-gas kinetics model for performance modeling of flowing chemical oxygen-iodine lasers (COIL)

A semi-gas kinetics (SGK) model for performance analyses of flowing chemical oxygen-iodine laser (COIL) is presented. In this model, the oxygen-iodine reaction gas flow is treated as a continuous medium, and the effect of thermal motions of particles of different laser energy levels on the performances of the COIL is included and the velocity distribution function equations are solved by using the double-parameter perturbational method. For a premixed flow, effects of different chemical reaction systems, different gain saturation models and temperature, pressure, yield of excited oxygen, iodine concentration and frequency-shift on the performances of the COIL are computed, and the calculated output power agrees well with the experimental data. The results indicate that the power extraction of the SGK model considering 21 reactions is close to those when only the reversible pumping reaction is considered, while different gain saturation models and adjustable parameters greatly affect the output power, the optimal threshold gain range, and the length of power extraction.

Gain saturation effects of a flowing chemical oxygen-iodine laser

A new oxygen-iodine medium gain model is developed to include pumping and deactivation of the upper laser levels of the iodine atoms, hyperfine and translation relaxation, as well as the flowing effect. The rate equations for gain of a supersonic flowing cw oxygen-iodine laser (COIL) are described when the medium is stimulated by a single-mode field. The general solution of the self-consistency integral equation is obtained. The result shows that the saturation behaviour in low pressure of the COIL differs from both the inhomogeneous and homogeneous broadening, and exhibits an 'anomalous' saturation phenomenon.

Numerical analysis of spatial evolution of the small signal gain in a chemical oxygen-iodine laser operating without primary buffer gas

A chemical oxygen iodine laser (COIL) that operates without primary buffer gas has become a new way of facilitating the compact integration of laser systems. To clarify the properties of spatial gain distribution, three-dimensional (3-D) computational fluid dynamics (CFD) technology was used to study the mixing and reactive flow in a COIL nozzle with an interleaving jet configuration in the supersonic section. The results show that the molecular iodine fraction in the secondary flow has a notable effect on the spatial distribution of the small signal gain. The rich iodine condition produces some negative gain regions along the jet trajectory, while the lean iodine condition slows down the development of the gain in the streamwise direction. It is also found that the new configuration of an interleaving jet helps form a reasonable gain field under appropriate operation conditions. (c) 2007 Elsevier Ltd. All rights reserved.

Modified Simplified Rate Equation Model of Flowing Chemical Oxygen-Iodine Laser and its Application

A modified simplified rate equation (RE) model of flowing chemical oxygen-iodine laser (COIL), which is adapted to both the condition of homogeneous broadening and inhomogeneous broadening being of importance and the condition of inhomogeneous broadening being predominant, is presented for performance analyses of a COIL. By using the Voigt profile function and the gain-equal-loss approximation, a gain expression has been deduced from the rate equations of upper and lower level laser species. This gain expression is adapted to the conditions of very low gas pressure up to quite high pressure and can deal with the condition of lasing frequency being not equal to the central one of spectral profile. The expressions of output power and extraction efficiency in a flowing COIL can be obtained by solving the coupling equations of the deduced gain expression and the energy equation which expresses the complete transformation of the energy stored in singlet delta state oxygen into laser energy. By using these expressions, the RotoCOIL experiment is simulated, and obtained results agree well with experiment data. Effects of various adjustable parameters on the performances of COIL are also presented.

Microstructure control and iodine doping of bismuth telluride

On the materials scale, thermoelectric efficiency is defined by the dimensionless figure of merit zT. This value is made up of three material components in the form zT = Tα²/ρκ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the total thermal conductivity. Therefore, in order to improve zT would require the reduction of κ and ρ while increasing α. However due to the inter-relation of the electrical and thermal properties of materials, typical routes to thermoelectric enhancement come in one of two forms. The first is to isolate the electronic properties and increase α without negatively affecting ρ. Techniques like electron filtering, quantum confinement, and density of states distortions have been proposed to enhance the Seebeck coefficient in thermoelectric materials. However, it has been difficult to prove the efficacy of these techniques. More recently efforts to manipulate the band degeneracy in semiconductors has been explored as a means to enhance α.

The other route to thermoelectric enhancement is through minimizing the thermal conductivity, κ. More specifically, thermal conductivity can be broken into two parts, an electronic and lattice term, κ_e and κ_l respectively. From a functional materials standpoint, the reduction in lattice thermal conductivity should have a minimal effect on the electronic properties. Most routes incorporate techniques that focus on the reduction of the lattice thermal conductivity. The components that make up κ_l (κ_l = 1/3Cνl) are the heat capacity (C), phonon group velocity (ν), and phonon mean free path (l). Since the difficulty is extreme in altering the heat capacity and group velocity, the phonon mean free path is most often the source of reduction.

Past routes to decreasing the phonon mean free path has been by alloying and grain size reduction. However, in these techniques the electron mobility is often negatively affected because in alloying any perturbation to the periodic potential can cause additional adverse carrier scattering. Grain size reduction has been another successful route to enhancing zT because of the significant difference in electron and phonon mean free paths. However, grain size reduction is erratic in anisotropic materials due to the orientation dependent transport properties. However, microstructure formation in both equilibrium and nonequilibrium processing routines can be used to effectively reduce the phonon mean free path as a route to enhance the figure of merit.

This work starts with a discussion of several different deliberate microstructure varieties. Control of the morphology and finally structure size and spacing is discussed at length. Since the material example used throughout this thesis is anisotropic a short primer on zone melting is presented as an effective route to growing homogeneous and oriented polycrystalline material. The resulting microstructure formation and control is presented specifically in the case of In₂Te₃-Bi₂Te₃ composites and the transport properties pertinent to thermoelectric materials is presented. Finally, the transport and discussion of iodine doped Bi₂Te₃ is presented as a re-evaluation of the literature data and what is known today.

Habitats regulations for stage 3 assessments radioactive substance authorisations

This is the Habitats regulations for stage 3 assessments: radioactive substances authorisations report from the Environment Agency, published on October 2003. The report focuses on the stage 3 assessments of radioactive substances authorisations in UK (to take place over the next five years, starting in 2003), which may have a potential impact on European designated Natura 2000 sites such Special Protection Areas (SPA), Special Areas of Conservation (SAC); and thus require further detailed assessment. This Environment Agency R&D project was commissioned to ERC, University o f Liverpool, in conjunction with Westlakes Scientific Consulting and the Centre for Ecology and Hydrology, as part o f the agency's preparation for the Stage 3 Assessments o f radioactive substances authorisations. The aim was to prepare site information sheets containing all data relevant for individual Natura 2000 sites needing Stage 3 Assessment and to stylise and represent species that require protection under the Habitats Regulations by the reference organism geometries listed in R&D Publication 128 (Copplestone et al., 2001).

Development of low-pH cements for immobilisation of intermediate level radioactive waste: Achievements and challenges

Although cementation is a widely recognized solidification/ stabilization process for immobilisation of Intermediate Level Radioactive Waste (ILRW), the low resistance to hyperalkaline pore waters compromises the effectiveness of the process when Portland Cement (PC) is employed. Moreover the manufacture of PC is responsible for significant CO2 emissions. In this context, low pH cements are environmentally more suitable and have emerged as a potential alternative for obtaining secure waste forms. This paper summarises the achievements on development of low-pH cements and the challenges of using these new materials for the ILRW immobilisation. The performance of waste forms is also discussed in terms of radionuclides release. Reactive magnesium oxide and magnesium phosphate cements are emphasised as they feature important advantages such as consumption of available constituents for controlling acid-base reactions, reduced permeability and higher density. Additionally, in order to identify new opportunities for study, the long-term modelling approach is also briefly discussed. Copyright © 2013 by ASME.