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

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

Characteristics of turbulent n-heptane jet flames in a hot and diluted coflow

Distinctive behaviour of turbulent n -heptane jet flames is revealed by conventional photography and laser-induced fluorescence of the hydroxyl radical (OH-LIF) performed in a Jet in Hot Coflow (JHC) burner. Prevaporised n -heptane, carried by air, issues into a hot vitiated coflow at two temperatures (1250 K and 1315 K) with three coflow oxygen levels (X O 2 = 3%, 6%%, and 9%), mimicking moderate or intense low oxygen dilution (MILD) combustion conditions. Results for n -heptane flames are compared to those for ethylene, ethanol, and natural gas flames. Two main discrepancies are revealed: firstly, the apparent liftoffheight of n -heptane flames decreases gradually with the increasing coflow oxygen level, while the appar- ent liftoff height of other fuels exhibit a non-monotonic trend; secondly, a transitional flame structure of n -heptane occurs in a coflow with X O 2 = 3%, while this structure occurs in a coflow with X O 2 >/-9% for other fuels. Calculations using a closed homogeneous reactor model support the interpretation of the experimental data, namely that the temporal profiles of OH in the n -heptane flames are similar for the 3% and 9% O 2 cases, differing from the behaviour of other fuels. A comparative analysis of n -heptane and ethanol chemistry, focused on the fuel pyrolysis and net negative heat release regions, suggests that it is more difficult for n -heptane than ethanol to meet one of the criteria of MILD combustion.Jingjing Ye, Paul R. Medwell, Michael J. Evans, Bassam B. Dall

A study of combustion characteristics of pulverised coal under MILD combustion conditions

In this experimental work, a laboratory-scale recuperative furnace has been used to investigate the sustainability of Moderate or Intense Low Oxygen Dilution (MILD) combustion with pulverised coal. Low-rank and high volatile Kingston brown coal and high-rank and low volatile Bowen Basin black coal with particle size in the range of 38-180 μm were injected into the furnace using either CO2 or N2 as a carrier gas. Operating conditions for stable MILD combustion of pulverised coal have been identified and evidencing MILD condition is achievable without any additional pre-heating of the air. The O2 and CO emissions were measured in parallel with NO emission. A water cooled sampling probe was used to conduct in-furnace gas sampling. Measurements of in-furnace gas concentration of CO and NO and in-furnace temperatures are presented. It was found that a significant reduction of NO emission owing to the strong NO reburning reaction inside the furnace. These findings, together with the potentiality of MILD conditions for soot depression and destruction, open the possibility of using high rank black and low rank brown coal with this technology.M. Saha, B. B. Dally, P. R. Medwell and E. M. Clearyhttp://cfe.uwa.edu.au/news/acs2013http://www.anz-combustioninstitute.org

The influence of wall temperature distribution on the mixed convective losses from a heated cavity

Available online 12 March 2019An experimental investigation is presented of the effects of wind speed (0–9 m/s), yaw angle (0° and 90°), and tilt angle (15° and −90°) on the mixed convective heat losses from a cylindrical cavity heated with different internal wall temperature distributions. The internal wall comprised 16 individually controlled heating elements to allow the distribution of the surface temperature to be well controlled, while the air flow was controlled with a wind tunnel. It is found that temperature distribution has a strong influence on the convective heat losses, with a joint dependence on the wind speed and its direction. For the no-wind and side-on wind conditions, the measured range of the heat losses varied by up to 50% with a change in the wall temperature distribution. However, for high head-on wind speeds, this variation reduced down to ∼20%. In addition, the heat losses from downward tilted were ∼3 times larger than the upward facing heated cavity for high wind speeds (typical of tower-mounted and beam-down configurations, respectively). Also, the measured heat losses were found to be only slightly dependent on wind speed and direction in contrast with the downward tilted cases.Ka Lok Lee, Alfonso Chinnici, Mehdi Jafarian, Maziar Arjomandi, Bassam Dally, Graham Natha

Electromagnetic Meson Form Factors in the Salpeter Model

We present a covariant scheme to calculate mesonic transitions in the framework of the Salpeter equation for $q\bar{q}$-states. The full Bethe Salpeter amplitudes are reconstructed from equal time amplitudes which were obtained in a previous paper\cite{Mue} by solving the Salpeter equation for a confining plus an instanton induced interaction. This method is applied to calculate electromagnetic form factors and decay widths of low lying pseudoscalar and vector mesons including predictions for CEBAF experiments. We also describe the momentum transfer dependence for the processes $\pi^0,\eta,\eta'\rightarrow\gamma\gamma^*$.Comment: 22 pages including 10 figure

Temperature and reaction zone imaging in turbulent swirling dual-fuel flames

Gaseous and liquid dual-fuel flames present both a practical approach to emissions reduction and a challenge to current state-of-the-art combustion modelling. This paper uses simultaneously imaged temperature and normalised OH signal fields to investigate flame structure and provide experimental data for model validation across a range of swirl-stabilised n-heptane spray flames. These data are obtained by non-linear excitation regime two-line atomic fluorescence (NTLAF) of indium, and planar laser-induced fluorescence (OH-PLIF), respectively. Swirling gas streams are varied by flowrate (63–88% of blow-off), premixed equivalence ratio (including air-only), and by type of gaseous fuel (natural gas and hydrogen). Results are used to describe how hot combustion products interact with the fuel spray: heating and diluting the region above the apex of the spray cone at low air flowrates but drawing fuel into outer branches of the flame with increasing air flowrates. Adding natural gas to the swirling air stream, at a concentration below the lean flammability limit, gives rise to a temperature increase in the outer branches with little effect on the hot region above the apex of the spray, along the burner centreline. The size of this region is significantly reduced; however, using hydrogen. As the concentration of gaseous fuel increases towards the lean flammability limit, peak temperatures shift towards the outer branch of the flame. Exceeding the lean flammability limit, an additional reaction zone begins to form in the premixed swirling stream, adjacent to the outer branch of the swirl flame. Stable outer branches of the swirl flame, however, become less prevalent and the peak temperatures of the spray flame return to burner centreline. This study provides insight into the complex behaviour of dual-fuel flames, a complementary dataset to related, PLIF-only studies and validation data for the development of numerical modelling tools.M.J.Evans, J.A.M.Sidey, J.Ye, P.R.Medwell, B.B.Dally, E.Mastorako