The stability of a quantum superposition of two different stationary mass distributions is examined, where the perturbing effect of each distribution on the space-time structure is taken into account, in accordance with the principles of general relativity. It is argued that the definition of the time-translation operator for the superposed space-times involves an inherent ill-definedness, leading to an essential uncertainty in the energy of the superposed state which, in the Newtonian limit, is proportional to the gravitational self-energyEΔ of the difference between the two mass distributions. This is consistent with a suggested finite lifetime of the order of ħ/EΔ for the superposed state, in agreement with a certain proposal made by the author for a gravitationally induced spontaneous quantum state reduction, and with closely related earlier suggestions by Diosi and by Ghirardiet al.

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On Gravity's Role in Quantum State Reduction

A new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modelling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Here, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent of reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast-chemistry and steady-flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fuelled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data, while the fast-chemistry and steady-flamelet models both predict an attached flame.

Progress-variable approach for large-eddy simulation of non-premixed turbulent combustion

Turbulent combustion modeling

On gravity's role in quantum state reduction

A subgrid scale model for large eddy simulations of turbulent premixed combustion is developed and validated. The approach is based on the concept of artificially thickened flames, keeping constant the laminar flame speed sl0. This thickening is simply achieved by decreasing the pre-exponential factor of the chemical Arrhenius law whereas the molecular diffusion is enhanced. When the flame is thickened, the combustion–turbulence interaction is affected and must be modeled. This point is investigated here using direct numerical simulations of flame–vortex interactions and an efficiency function E is introduced to incorporate thickening effects in the subgrid scale model. The input parameters in E are related to the subgrid scale turbulence (velocity and length scales). An efficient approach, based on similarity assumptions, is developed to extract these quantities from the resolved velocity field. A specific operator is developed to exclude the dilatational part of the velocity field from the estimation of...

A thickened flame model for large eddy simulations of turbulent premixed combustion

Partially premixed flames are observed in nonpremixed turbulent combustion when fuel and oxidizer have mixed before burning. This combustion regime combines the properties of both premixed and diffusion flames. A procedure based on the resolved fields is proposed to associate premixed and diffusion flame descriptions in large eddy simulation. Using basic and well known subgrid modelling of premixed and diffusion flames, the proposed methodology is tested for flames lifted in a two-dimensional turbulent wake. Very recent experimental observations concerning the dynamics of the flow field at the turbulent flame base are reproduced.

Partially premixed flamelets in LES of nonpremixed turbulent combustion

To characterize the dynamics and the physical properties of isoconcentration surfaces of a random reactive scalar field, an instantaneous isosurface quantity and its transport equation are introduced. For turbulent flows, the mean area of isoconcentration surface per unit volume is studied through its transport equation derived by using the probability density function (pdf) formalism. This approach allows the value of the reactive scalar used to define the level surface to be treated as an independent variable. It also leads to the definition of a surface density function. The developed formalism is applied to premixed turbulent combustion and a bridge is built between modeling approaches based on pdf, and others based on the flame surface concept.

Surface density function in premixed turbulent combustion modeling, similarities between probability density function and flame surface approaches

Role of the progress variable in models for partially premixed turbulent combustion

Surface ignition in internal combustion engines – where hot sport in the combustion chamber can cause fuel to ignite either before or after the normal spark ignition – is both inefficient and harmful to the engine. Detection usually involves the engine being modified in some way, but a promising noninvasive technique is to use the spark plug to detect the passage of flame 'front'. This novel technique has been demonstrated recently in several research laboratories including that of M swain and colleagues at the University of Miami 1989 J. Phys. E: Sci. Instrum. 22 838).

Putting a stop to surface ignition

Premixed turbulent combustion of propane and air is studied experimentally using counterflowing axisymmetric jets. Attributes of this type of burner arrangement for studying turbulent combustion are discussed in terms of flame geometry, bulk and turbulent stretch rates, solid boundaries and measurement access. Results are preliminary, but a turbulent extinction is observed and it appears to be related to the total stretch rate applied to the flames. Extinction occurs at Karlovitz numbers of the order of unity. Laser tomography is used to show the shape and location of the wrinkled flame surfaces.

Ken Bray

Papers

Partially premixed flamelets in LES of nonpremixed turbulent combustion

Surface density function in premixed turbulent combustion modeling, similarities between probability density function and flame surface approaches

Role of the progress variable in models for partially premixed turbulent combustion

Putting a stop to surface ignition

Premixed turbulent combustion in counterflowing streams