Travelling waves in the cool flame regime

Hydrocarbon oxidation develops through a complex network of elementary steps. Depending on the initial thermodynamic conditions, different behaviours are observed ranging from slow combustion to hot ignition [1]. Chain reactions involving radicals, govern all the combustion processes. Most of the time, the operating kinetic mechanism can be approximated by a reduced kinetic scheme which is depending on the initial conditions. In an intermediate range of temperature, cool flames appear as a transition between slow combustion and hot ignition. The existence of cool flames is often associated with knocking is engines

Premixed flames with nonbranching chain-reactions (structure and dynamics)

A description of the flame structure, and its response to wrinkling and unsteady perturbations, is given for a simplified multiple step reaction mechanism (representative for example of halogen-hydrogen reactions), involving a dissociation reaction, a nonbranching chain reaction, both with high activation energy, and a recombination reaction. We describe the changes in flame structure that take place with increasing values of the flame temperature. We encounter first a "merged" flame regime, for which the chemical reactions are confined to a thin zone, on the hot side of the flame, and the quasi-steady state approximation, applicable at lower flame temperatures can no longer be used. Then, we reach a slow recombination regime for which the radical is produced in a thin zone and diffuses out to recombine in the outer transport zones. New effects, due to the radical diffusivity on the flame dynamics, are found in this regime. Finally, in describing the flame structure for high enough flame temperatures, the effects of radical recombination can be neglected, but no longer the endothermic effect of dissociation

Reap the rewards from reseeding

Permanent grass clover leys are a vital component of productive organic grassland farms. In all, there are 3 main ways to re-seed - full reseeding of permanent pastures, under-sowing with a cereal crop and thirdly over-sowing onto an existing sward. In this article aspects of full reseeding of permanent pastures will be discussed

Asymptotic behaviour of the Rayleigh--Taylor instability

We investigate long time numerical simulations of the inviscid Rayleigh-Taylor instability at Atwood number one using a boundary integral method. We are able to attain the asymptotic behavior for the spikes predicted by Clavin & Williams\cite{clavin} for which we give a simplified demonstration. In particular we observe that the spike's curvature evolves like $t^3$ while the overshoot in acceleration shows a good agreement with the suggested $1/t^5$ law. Moreover, we obtain consistent results for the prefactor coefficients of the asymptotic laws. Eventually we exhibit the self-similar behavior of the interface profile near the spike.Comment: 4 pages, 6 figure

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

Video meeting signals: experimental evidence for a technique to improve the experience of video conferencing

We found evidence from two experiments that a simple set of gestural techniques can improve the experience of online meetings. Video conferencing technology has practical benefits, but psychological costs. It has allowed industry, education and social interactions to continue in some form during the covid-19 lockdowns. But it has left many users feeling fatigued and socially isolated, perhaps because the limitations of video conferencing disrupt users' ability to coordinate interactions and foster social affiliation. Video Meeting Signals (VMS™) is a simple technique that uses gestures to overcome some of these limitations. First, we carried out a randomised controlled trial with over 100 students, in which half underwent a short training session in VMS. All participants rated their subjective experience of two weekly seminars, and transcripts were objectively coded for the valence of language used. Compared to controls, students with VMS training rated their personal experience, their feelings toward their seminar group, and their perceived learning outcomes as significantly higher. Also, they were more likely to use positive language and less likely to use negative language. A second, larger experiment replicated the first, and added a condition where groups were given a version of the VMS training but taught to use emoji response buttons rather than gestures to signal the same information. The emoji-trained groups did not experience the same improvement as the VMS groups. By exploiting the specific benefits of gestural communication, VMS has great potential to overcome the psychological problems of group video meetings

Effects of equivalence ratio variations on turbulent flame speed in lean methane/air mixtures under lean-burn natural gas engine operating conditions

Direct numerical simulations of turbulent premixed CH4/air flames were carried out using an inflow-outflow configuration to study the effects of equivalence ratio on the turbulent flame speed in lean mixtures. The inflow velocity was dynamically adjusted at a run-time to stabilize the flame brush location within the computational domain. Linear forcing was applied to maintain the turbulent intensities at desired levels. Numerous equivalence ratios near the lean limit were selected for the same turbulence properties and the normalized turbulent flame speed was shown to be a function of the equivalence ratio. Simulations were performed for over 80 eddy turnover times with the turbulent flame speed obtained by averaging the inflow velocity. Results revealed that the equivalence ratio does not have an explicit effect on the normalized turbulent flame speed over the lean limit. Analysis of flame surface area showed that the surface wrinkling produced by eddies of varying scales was not influenced by the change in equivalence ratios when the Karlovitz and Damkohler numbers are fixed. Finally based on the flame surface statistics flame surface normal is preferentially parallel to the most compressive strain rate direction for all equivalence ratios