Conditional methods in application for Lagrangian modeling

The exact unclosed equation for the phase-space density function (or corresponding Lagrangian pdf) in turbulent flows is obtained using conditional techniques. The equation has direct implications for stochastic Lagrangian models based on the assumption of similarity with a Markov process. The problem of random particle sources is examined and the appropriate correcting term is suggested. (C) 1998 American Institute of Physics

Examining the cascade hypothesis for turbulent premixed combustion

The cascade hypothesis, which was introduced in application for premixed combustion by Yakhot (1988) and Sivashinsky (1988), is examined against the correlations of the turbulent burning velocity data by Bradley, Lau and Lawes (1992) (BLL data). A new, less restrictive formulation of the cascade hypothesis is suggested. The formulation is shown to be consistent with the BLL data. Various flame characteristics- fractal dimensions, inner cutoffs, main asymptotic regimes - are determined form the BLL data on the basis of the cascade hypothesis

Matching the conditional variance as a criterion for selecting parameters in the simplest multiple mapping conditioning models

The simplest model within the multiple mapping conditioning (MMC) approach, that involves a single mixture-fraction-like reference variable, is considered in the Brief Communication. An important parameter-the minor dissipation time-remains unknown in the probabilistic version of the model. The present work demonstrates by the specially developed asymptotic analysis that the simplest MMC possesses an ability (although somewhat limited) to match the physical intensity of the conditional fluctuations and this match represents the criterion for proper selection of the minor dissipation time. (C) American Institute of Physics

A small disturbance in the strong vortex flow

A small disturbance in the axisymmetric, bathtub-like flow with strong vorticity is considered and the asymptotic representation of the solution is found. It is shown that if the disturbance is smaller than a certain critical scale, the conventional strong vortex approximation cannot describe the field generated by the disturbance not only in the vicinity of the disturbance but also at the distances much larger than the critical scale. (C) 2001 American Institute of Physics

Exergy optimisation of reverse combustion linking in underground coal gasification

Underground coal gasification (UCG) is a gasification process carried on in non-mined coal seams using injection and production wells drilled from the surface, which enables the coal to be converted into product gas. A key operation of the UCG is linking the injection and production wells. Reverse combustion linking (RCL) is a method of linking the process wells within a coal seam, which includes injection of an oxidant into one well and ignition of coal in the other so that combustion propagates towards the source of oxidant thereby establishing a low hydraulic resistance path between the two wells. The new theory of the RCL in typical UCG conditions has been recently suggested. The key parameters of the RCL process are determined using the technique of intrinsic disturbed flame equations. The present study is concerned with extending the results of the RCL theory to incorporate hydrodynamics of air injection and flow during RCL operation to derive mass flow rate of air to the combustion front as a function of the injection pressure. The results enabled an optimisation procedure maximising the exergy efficiency of RCL process

Propagation of nonstationary curved and stretched premixed flames with multistep reaction mechanisms

The propagation speed of a thin premixed flame disturbed by an unsteady fluid flow of a larger scale is considered. The flame may also have a general shape but the reaction zone is assumed to be thin compared to the flame thickness. Unlike in preceding publications, the presented asymptotic analysis is performed for a general multistep reaction mechanism and, at the same time, the flame front is curved by the fluid flow. The resulting equations define the propagation speed of disturbed flames in terms of the properties of undisturbed planar flames and the flame stretch. Special attention is paid to the near-equidiffusion limit. In this case, the flame propagation speed is shown to depend on the effective Zeldovich number Z(f) , and the flame stretch. Unlike the conventional Zeldovich number, the effective Zeldovich number is not necessarily linked directly to the activation energies of the reactions. Several examples of determining the effective Zeldovich number for reduced combustion mechanisms are given while, for realistic reactions, the effective Zeldovich number is determined from experiments. Another feature of the present approach is represented by the relatively simple asymptotic technique based on the adaptive generalized curvilinear system of coordinates attached to the flame (i.e., intrinsic disturbed flame equations [IDFE])

On premixed flames as gasdynamic discontinuities: A simple approach to derive their propagation speed

We consider the propagation speed of thin premixed flames disturbed by the fluid flow and suggest a new approach based on using the generalized curvilinear system of coordinates and tenser calculus. The suggested technique is shown to be more simple than the traditional technique used by Sivashinsky (1976), Matalon and Matkowsky (1982) and also in other publications. The result of Matalon and Matkowsky (1982) is generalized for an arbitrary temperature dependent diffusion coefficient