Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency

A minimal model is constructed for two-dimensional fracture propagation. The heterogeneous process zone is presumed to suppress stress relaxation rate, leading to non-quasistatic behavior. Using the Yoffe solution, I construct and solve a dynamical equation for the tip stress. I discuss a generic tip velocity response to local stress and find that noise-free propagation is either at steady state or oscillatory, depending only on one material parameter. Noise gives rise to intermittency and quasi-periodicity. The theory explains the velocity oscillations and the complicated behavior seen in polymeric and amorphous brittle materials. I suggest experimental verifications and new connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3 postscript figures upon request from author at [email protected] or [email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm

High order magnon bound states in the quasi-one-dimensional antiferromagnet α\alpha-NaMnO2_2

Here we report on the formation of two and three magnon bound states in the quasi-one-dimensional antiferromagnet $\alpha$-NaMnO$_2$, where the single-ion, uniaxial anisotropy inherent to the Mn$^{3+}$ ions in this material provides a binding mechanism capable of stabilizing higher order magnon bound states. While such states have long remained elusive in studies of antiferromagnetic chains, neutron scattering data presented here demonstrate that higher order $n>2$ composite magnons exist, and, specifically, that a weak three-magnon bound state is detected below the antiferromagnetic ordering transition of NaMnO$_2$. We corroborate our findings with exact numerical simulations of a one-dimensional Heisenberg chain with easy-axis anisotropy using matrix-product state techniques, finding a good quantitative agreement with the experiment. These results establish $\alpha$-NaMnO$_2$ as a unique platform for exploring the dynamics of composite magnon states inherent to a classical antiferromagnetic spin chain with Ising-like single ion anisotropy.Comment: 5 pages, 4 figure

The effect of jet preheating on turbulence in a premixed jet in hot coflow

Moderate or Intense Low oxygen Dilution (MILD) combustion is an important combustion regime, which utilises the combustion of gas in hot and often vitiated environment. MILD combustors are characterised by the use of exhaust gases recirculated into the combustor via dilution or mixing with the incoming fuel or alternately by rapidly mixing an incoming fuel stream into a hot environment of combustion products. However, the influence of preheating the fuel jet is not well understood. Current approaches rely heavily on assumptions about the role of heat on laminarisation of turbulence, due to viscosity. However, the density ratio between jet and environment is known to have a dominant effect on mixing. Velocity measurements are presented for a pair of turbulent premixed flames issuing into a premixed coflow. Two cases are considered, both with and without preheating of the central fuel jet. The resulting near-field velocity is measured using digital Particle Image Velocimetry (PIV). The resulting velocity maps are processed to find the turbulence intensity, u’/U. A two-point correlation technique is used to determine the turbulence integral length scale, lf. The influence of preheating the central jet on the resulting near-field turbulence characteristics are presented and discussed.P.A.M. Kalt and B.B. Dallyhttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org

Heat loss-induced oscillation of methane and ethylene in a perfectly stirred reactor

Temperature oscillations for Methane and Ethylene fuel mixtures oxidation at reduced temperatures were calculated using the transient perfectly stirred reactor model. It was found that the oscillations are controlled by the heat loss to the surroundings and are independent of inlet temperatures. Also found that the recombination channel route is not prevalent in lean and stoichiometric mixtures and that there is plenty of evidence that a simple radical scavenging by the fuel is the controlling mechanism.B. Dally and N. Petershttp://www.combustionsociety.jp/aspacc07/index.htm

Pion and Kaon Vector Form Factors

We develop a unitarity approach to consider the final state interaction corrections to the tree level graphs calculated from Chiral Perturbation Theory ($\chi PT$) allowing the inclusion of explicit resonance fields. The method is discussed considering the coupled channel pion and kaon vector form factors. These form factors are then matched with the one loop $\chi PT$ results. A very good description of experimental data is accomplished for the vector form factors and for the $\pi\pi$ P-wave phase shifts up to $\sqrt{s}\lesssim 1.2$ GeV, beyond which multiparticle states play a non negligible role. In particular the low and resonance energy regions are discussed in detail and for the former a comparison with one and two loop $\chi PT$ is made showing a remarkable coincidence with the two loop $\chi PT$ results.Comment: 20 pages, 7 figs, to appear in Phys. Rev.

Tau mesonic decays and strong anomaly of PCAC

Strong anomaly of the PCAC is found in $\tau\rightarrow\omega \pi\pi\nu$ and $\omega\rho\nu$ in the chiral limit. It originates in WZW anomaly. Theoretical result of $\tau\rightarrow\omega\pi\pi\nu$ agrees with data well and the measurement of $\tau\rightarrow\omega\rho\nu$ will confirm the strong anomaly of PCAC. The strong anomaly of PCAC is studied.Comment: 27 page

The effect of oxygen concentration on the structure of turbulent nonpremixed flames

MILD combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel saving while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at reduced temperature, with a flat thermal field and low turbulence fluctuations. An experimental burner is used in this study. Temporally, and spatially resolved measurements of reactive scalars are conducted on three different flames of H2/CH4 fuel mixture at fixed jet Reynolds number and different oxygen mass fractions in the hot oxidant stream. The results show substantial variation of the flame structure with the decrease of the oxygen level. The results also point towards a different chemical pathway for the reaction in this combustion regime, where the formation of the OH radical and NO is less dependent on temperature.B.B. Dally, A.N. Karpetis & R.S. Barlo

Performance characteristics of a hybrid solar receiver combustor utilising hydrogen or syngas

The use of hybrid solar thermal devices, which harness the energy from both concentrated solar radiation and combustion, is receiving growing attention due to their potential to provide a firm and dispatchable thermal energy supply while lowering the costs of energy systems and assisting the penetration of renewable energy. The Hybrid Solar Receiver Combustor (HSRC), which directly integrates the function of a solar receiver and a combustor into a single device, is a particularly promising hybrid technology. Its design allows the receiver to operate in three modes: solar-only, combustion-only and a mixed-mode (a combination of both solar and combustion). Compared with the present state-of-the-art in hybrid solar-combustion systems (which collect the thermal energy from the solar and combustion sources in separate devices and then combine them subsequently), the HSRC offers a reduction in total infrastructure (and hence capital costs), heat-exchange surface area, start-up/shut-down losses and pollutant emissions, due to a reduced need to start-up the back-up combustion plant. Also, the use of the HSRC with renewable fuels (e.g. hydrogen, syngas) offers the additional potential for low-cost carbon neutral or carbon-negative energy, although no data on performance for systems fed with alternative fuels are presently available. To this aim, the present work provides the first direct measurement of the performance of a Hybrid Solar Receiver Combustor (HSRC) fed with hydrogen-based fuels. A laboratory-scale HSRC prototype operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime was tested at a nominal capacity of 12-kWth for both the combustion-only and mixed mode, using hydrogen and syngas (H2/CO=2/1 v/v) as fuels. A 5-kWel xenon-arc lamp was used to simulate solar radiation into the device. The influence of the mode of operation on the thermal efficiency, heat losses, heat flux distribution within the cavity and pollutant emissions are reported for a range of values of the heat extraction. It was found that the combustion process can be successfully stabilised within the HSRC over a wide range of operating conditions, and in the mixed-mode of operation, providing ultra-low NOx emissions. The thermal performance was found to be similar for all the modes of operation, despite the different nature of the two energy sources and fuel composition. Overall, this study highlights that, if renewable H2 or syngas are used as fuels, the device can efficiently operate in all the modes of operation employing 100% renewable energy.Alfonso Chinnici, Graham J. Nathan, and Bassam B. Dall

Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow

This paper reports measurements in turbulent nonpremixed CH4/H2 jet flames issuing into a heated and highly diluted coflow. These conditions emulate those of moderate or intense low-oxygen-dilution (MILD) combustion. The spatial distribution of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature, imaged using planar laser-induced fluorescence and Rayleigh scattering laser diagnostic techniques, are measured and presented. Comparisons are made between three jet Reynolds numbers and two coflow O2 levels. Measurements are taken at two downstream locations. The burner used in this work facilitates the additional study on the effects of the entrainment of surrounding air on the flame structure at downstream locations. Reducing the coflow oxygen level is shown to lead to a suppression of OH as a result of the reduced temperatures in the reaction zone. Decreasing the oxygen level of the coflow also results in a broadening of the OH distribution. At downstream locations, the surrounding air mixes with the jet and coflow. The subsequent drop in the temperature of the oxidant stream can lead to a rupture of the OH layer. Localized extinction allows premixing of the fuel with the surrounding air. The result is an increase in the reaction rate, highlighting the need for homogeneous mixing to maintain MILD combustion conditions.Paul R. Medwell, Peter A.M. Kalt and Bassam B. Dall