The effect of thermal expansion on diffusion flame instabilities

In this paper we examine the effect of thermal expansion on the stability of a planar unstrained diffusion flame and provide a comprehensive characterization of diffusive-thermal instabilities while realistically accounting for density variations. The possible patterns that are likely to be observed as a result of differential and preferential diffusion are identified for a whole range of parameters including the distinct Lewis numbers associated with the fuel and oxidizer, the initial mixture strength and the flow conditions. Although we find that thermal expansion has a marked influence on flame instability, it does not play a crucial role as it does in premixed combustion. It primarily affects the parameter regime associated with the onset of the instabilities and the growth rate of the unstable modes. Perhaps the most surprising result is that its has a different influence on the various modes of instability - a destabilizing influence on the formation of cellular structures and a stabilizing influence on the onset of oscillation

The effect of thermal expansion on diffusion flame instabilities

In this paper we examine the effect of thermal expansion on the stability of a planar unstrained diffusion flame and provide a comprehensive characterization of diffusive thermal instabilities while realistically accounting for density variations. The possible patterns that are likely to be observed as a result of differential and preferential diffusion are identified for a whole range of parameters including the distinct Lewis numbers associated with the fuel and oxidizer, the initial mixture strength and the flow conditions. Although we find that thermal expansion has a marked influence on flame instability, it does not play a crucial role as it does in premixed combustion. It primarily affects the parameter regime associated with the onset of the instabilities and the growth rate of the unstable modes. Perhaps the most surprising result is that its has a different influence on the various modes of instability a destabilizing influence on the formation of cellular structures and a stabilizing influence on the onset of oscillations

Hydrodynamic and thermodiffusive instability effects on the evolution of laminar planar lean premixed hydrogen flames

Numerical simulations with single-step chemistry and detailed transport are used to study premixed hydrogen/air flames in two-dimensional channel-like domains with periodic boundary conditions along the horizontal boundaries as a function of the domain height. Both unity Lewis number, where only hydrodynamic instability appears, and subunity Lewis number, where the flame propagation is strongly affected by the combined effect of hydrodynamic and thermodiffusive instabilities are considered. The simulations aim at studying the initial linear growth of perturbations superimposed on the planar flame front as well as the long-term nonlinear evolution. The dispersion relation between the growth rate and the wavelength of the perturbation characterizing the linear regime is extracted from the simulations and compared with linear stability theory. The dynamics observed during the nonlinear evolution depend strongly on the domain size and on the Lewis number. As predicted by the theory, unity Lewis number flames are found to form a single cusp structure which propagates unchanged with constant speed. The long-term dynamics of the subunity Lewis number flames include steady cell propagation, lateral flame movement, oscillations and regular as well as chaotic cell splitting and mergin

Detection of ultra-high energy cosmic ray showers with a single-pixel fluorescence telescope

We present a concept for large-area, low-cost detection of ultra-high energy cosmic rays (UHECRs) with a Fluorescence detector Array of Single-pixel Telescopes (FAST), addressing the requirements for the next generation of UHECR experiments. In the FAST design, a large field of view is covered by a few pixels at the focal plane of a mirror or Fresnel lens. We report first results of a FAST prototype installed at the Telescope Array site, consisting of a single 200 mm photomultiplier tube at the focal plane of a 1 m$^2$ Fresnel lens system taken from the prototype of the JEM-EUSO experiment. The FAST prototype took data for 19 nights, demonstrating remarkable operational stability. We detected laser shots at distances of several kilometres as well as 16 highly significant UHECR shower candidates.Comment: Accepted for publication in Astroparticle Physic

Premixed edge-flames under transverse enthalpy gradients

We describe flame propagation between two opposed reactive streams which may differ in their composition and temperature. A two-dimensional counterflow configuration and an irreversible Arrhenius reaction are adopted, along with the constant density approximation. Attention is focused on the influence of two nondimensional parameters. The first one, denoted by γ, represents the difference in the enthalpy of the feed streams. The second one, ε, quantifies the ratio between the characteristic chemical time and the strain time. After a general formulation of the problem, we begin by an analysis of the one-dimensional case consisting of two parallel planar flames of unequal strength. The flames behavior is described analytically and numerically. In particular, two extinction regimes are identified: for values of γ smaller than a critical value γ*, the flames extinguish by quenching against each other at the stagnation plane; for γ > γ* they extinguish while at a finite distance from each other which increases with γ. These behaviors are similar to those, known in the literature, associated with the influence of Lewis numbers on the extinction of twin-flames. We then describe the propagation of two-dimensional flame fronts along the stagnation line, in a direction perpendicular to the plane of strain. The flame front is thus curved under the combined effects of the flow field and the transverse enthalpy gradient in the frozen mixture ahead of it; far behind the state of the gas is that of the pair of flat flames introduced above. The problem is studied numerically and complemented by an analytical description of the fast-chemistry situations corresponding to small values of ε. In particular we describe, for different fixed values of γ, the evolution of ignition fronts, characterized by a positive propagation speed, to extinction fronts, characterized by negative speeds, as ε is increased. In addition to the marked change in the flame shape, the most noticeable effect of an increase in γ is the decrease in the propagation speed of the flame front. These effects are associated with the increased front curvature for higher values of γ, along with a shift of the front leading edge towards the stream with higher enthalpy

Combustion theory and modeling

In honor of the fiftieth anniversary of the Combustion Institute, we are asked to assess accomplishments of theory in combustion over the past fifty years and prospects for the future. The title of our article is chosen to emphasize that development of theory necessarily goes hand-in-hand with specification of a model. Good conceptual models underlie successful mathematical theories. Models and theories are discussed here for deflagrations, detonations, diffusion flames, ignition, propellant combustion, and turbulent combustion. In many of these areas, the genesis of mathematical theories occurred during the past fifty years, and in all of them significant advances are anticipated in the future. Increasing interaction between theory and computation will aid this progress. We hope that, although certainly not complete in topical coverage or reference citation, the presentation may suggest useful directions for future research in combustion theory

Liesegang patterns: Effect of dissociation of the invading electrolyte

The effect of dissociation of the invading electrolyte on the formation of Liesegang bands is investigated. We find, using organic compounds with known dissociation constants, that the spacing coefficient, 1+p, that characterizes the position of the n-th band as x_n ~ (1+p)^n, decreases with increasing dissociation constant, K_d. Theoretical arguments are developed to explain these experimental findings and to calculate explicitly the K_d dependence of 1+p.Comment: RevTex, 8 pages, 3 eps figure

A simple and robust method for connecting small-molecule drugs using gene-expression signatures

Interaction of a drug or chemical with a biological system can result in a gene-expression profile or signature characteristic of the event. Using a suitably robust algorithm these signatures can potentially be used to connect molecules with similar pharmacological or toxicological properties. The Connectivity Map was a novel concept and innovative tool first introduced by Lamb et al to connect small molecules, genes, and diseases using genomic signatures [Lamb et al (2006), Science 313, 1929-1935]. However, the Connectivity Map had some limitations, particularly there was no effective safeguard against false connections if the observed connections were considered on an individual-by-individual basis. Further when several connections to the same small-molecule compound were viewed as a set, the implicit null hypothesis tested was not the most relevant one for the discovery of real connections. Here we propose a simple and robust method for constructing the reference gene-expression profiles and a new connection scoring scheme, which importantly allows the valuation of statistical significance of all the connections observed. We tested the new method with the two example gene-signatures (HDAC inhibitors and Estrogens) used by Lamb et al and also a new gene signature of immunosuppressive drugs. Our testing with this new method shows that it achieves a higher level of specificity and sensitivity than the original method. For example, our method successfully identified raloxifene and tamoxifen as having significant anti-estrogen effects, while Lamb et al's Connectivity Map failed to identify these. With these properties our new method has potential use in drug development for the recognition of pharmacological and toxicological properties in new drug candidates.Comment: 8 pages, 2 figures, and 2 tables; supplementary data supplied as a ZIP fil