Experimental study of the effect of cycle pressure on lean combustion emissions

Experiments were conducted in which a stream of premixed propane and air was burned under conditions representative of gas turbine operation. Emissions of NOx, CO, and unburned hydrocarbons (UHC) were measured over a range of combustor inlet temperature, pressure, and residence time at equivalence ratios from 0.7 down to the lean stability limit. At an inlet temperature of 600 K, observed NOx levels dropped markedly with decreasing pressure for pressures below 20 atm. The NOx levels are proportional to combustor residence time and formation rates were principally a function of adiabatic flame temperature. For adiabatic flame temperatures of 2050 K and higher, CO reached chemical equilibrium within 2 msec. Unburned hydrocarbon species dropped to a negligible level within 2 msec regardless of inlet temperature, pressure, or equivalence ratio. For a combustor residence time of 2.5 msec, combustion inefficiency became less than 0.01% at an adiabatic flame temperature of 2050 K. The maximum combustion inefficiency observed was on the order of 1% and corresponded to conditions near the lean stability limit. Using a perforated plate flameholder, this limit is well represented by the condition of 1800 K adiabatic flame temperature

Experimental study of the effects of flameholder geometry on emissions and performance of lean premixed combustors

Emissions of NOx, CO, and unburned hydrocarbons (UHC) are reported for a lean premixed propane-air system at inlet conditions of 800K and 1MPa using twelve flameholder designs. The flameholders tested represent six design concepts with two values of blockage for each concept. Data were obtained at reference velocities of 35 m/s, 25 m/s and 20 m/s at combustor stations 10 cm and 30 cm downstream of the flameholders. Flameholder pressure drop was found to be a principal determinant of emissions performance. Designs producing larger pressure drops also produced less NOx, CO, and UHC emissions. The lean stability limit equivalence ratio was found to be approximately 0.35 for all designs. Flashback velocities (axial components in the flameholder passages) varied between 30 m/s and 40 m/s. A perforated plate flameholder was operated with a velocity as low as 23 m/s through the perforations at equivalence ratio 0.7 without producing flashback

Early life adversity, biological adaptation, and human capital: evidence from an interrupted malaria control program in Zambia

Growing evidence from evolutionary biology demonstrates how early life shocks trigger physiological changes designed to be adaptive in challenging environments. We examine the implications of one type of physiological adaptation - immunity formation - for human capital accumulation. Using variation in early life malaria risk generated by an interrupted disease control program in Zambia, we show that exposure to infectious diseases during the first two years of life can reduce the harmful effects of malaria exposure on cognitive development during the preschool years. These findings suggest a non-linear and trajectory-dependent relationship between early life adversity and human capital formation

Defects and boundary layers in non-Euclidean plates

We investigate the behavior of non-Euclidean plates with constant negative Gaussian curvature using the F\"oppl-von K\'arm\'an reduced theory of elasticity. Motivated by recent experimental results, we focus on annuli with a periodic profile. We prove rigorous upper and lower bounds for the elastic energy that scales like the thickness squared. In particular we show that are only two types of global minimizers -- deformations that remain flat and saddle shaped deformations with isolated regions of stretching near the edge of the annulus. We also show that there exist local minimizers with a periodic profile that have additional boundary layers near their lines of inflection. These additional boundary layers are a new phenomenon in thin elastic sheets and are necessary to regularize jump discontinuities in the azimuthal curvature across lines of inflection. We rigorously derive scaling laws for the width of these boundary layers as a function of the thickness of the sheet

Multiscale characterization of damage tolerance in barium titanate thin films

Barium titanate is a brittle, lead free ferroelectric and piezoelectric ceramic used in patterned and thin film forms in micro- and nano-scale electronic devices. Both during deposition and eventually during service, this material system develops stresses due to different loads acting on the system, which can lead to its failure due to cracking in the films and/or interface delamination. In situ microcantilever bending based fracture experiments and tensile tests based on shear lag tests in combination with digital image correlation were used to understand the cracking behavior of barium titanate films when deposited on flexible substrates. For the first time, the fracture behavior of these nanocrystalline barium titanate films has been quantified in terms of fracture toughness, fracture strength, and interface shear stresses for different film thicknesses. Critical defect size is estimated using the above information as a function of film thickness. It is found that damage tolerance in terms of fracture strength depends on film thickness. Furthermore, compared to a bulk single crystal, barium titanate fracture resistance of the nanocrystalline thin films is reduced. Both effects need to be considered in engineering design of reliable devices employing micro- and nano-scale barium titanate thin film structures

Topology of Event Horizons and Topological Censorship

We prove that, under certain conditions, the topology of the event horizon of a four dimensional asymptotically flat black hole spacetime must be a 2-sphere. No stationarity assumption is made. However, in order for the theorem to apply, the horizon topology must be unchanging for long enough to admit a certain kind of cross section. We expect this condition is generically satisfied if the topology is unchanging for much longer than the light-crossing time of the black hole. More precisely, let $M$ be a four dimensional asymptotically flat spacetime satisfying the averaged null energy condition, and suppose that the domain of outer communication \C_K to the future of a cut $K$ of \Sm is globally hyperbolic. Suppose further that a Cauchy surface $\Sigma$ for \C_K is a topological 3-manifold with compact boundary $\partial\S$ in $M$, and $\S'$ is a compact submanifold of \bS with spherical boundary in $\S$ (and possibly other boundary components in $M/\S$). Then we prove that the homology group $H_1(\Sigma',Z)$ must be finite. This implies that either $\partial\S'$ consists of a disjoint union of 2-spheres, or $\S'$ is nonorientable and $\partial\S'$ contains a projective plane. Further, \partial\S=\partial\Ip[K]\cap\partial\Im[\Sp], and $\partial \Sigma$ will be a cross section of the horizon as long as no generator of \partial\Ip[K] becomes a generator of \partial\Im[\Sp]. In this case, if $\S$ is orientable, the horizon cross section must consist of a disjoint union of 2-spheres.}Comment: 11 pages, plain latex (minor revision: $\Sigma$ replaced by its closure in various places.

Fluctuation Dissipation Relation for a Langevin Model with Multiplicative Noise

A random multiplicative process with additive noise is described by a Langevin equation. We show that the fluctuation-dissipation relation is satisfied in the Langevin model, if the noise strength is not so strong.Comment: 11 pages, 6 figures, other comment

General Stability Analysis of Synchronized Dynamics in Coupled Systems

We consider the stability of synchronized states (including equilibrium point, periodic orbit or chaotic attractor) in arbitrarily coupled dynamical systems (maps or ordinary differential equations). We develop a general approach, based on the master stability function and Gershgorin disc theory, to yield constraints on the coupling strengths to ensure the stability of synchronized dynamics. Systems with specific coupling schemes are used as examples to illustrate our general method.Comment: 8 pages, 1 figur