Large eddy simulation of unsteady combustion

The present study concerns the application of a large eddy simulation (LES) model, capable of dealing with chemical reactions described by multistep reaction mechanisms and thermal radiation. The LES model, based on prefiltering of the balance equations of mass, momentum, and energy contains a variety of submodels far representing the residual stress tensor and flux vectors and the filtered reaction rates. Here, we have focused on the influence of modeling of the filtered reaction rates. Three different reaction rate formulations have been investigated, the eddy dissipation kinetic model, a model based on the presumed PDF approach, and finally a monotonically integrated LES model that does not explicitly take subgrid scale effects into account. The predictive capabilities of the LES model have been investigated by numerical simulations of the flow past a triangular-shaped flame holder in a rectilinear channel at various operating conditions parameterized by the equivalence ratio, inlet velocity, and temperature. In simulations of reacting flow situations, the fuel was propane and premixed conditions were enforced. Detailed experimental measurements of all operating conditions including temperature probability density functions are available. Comparison of simulated and measured quantities indicates that the LES model is capable of predicting the flow accurately under all three operating conditions and that the movement of the flame front can be captured. Moreover, a discussion describing the dissimilar modes of operation found in the test rig is presented

The enhancement of heat transfer in the tail pipe of a pulse combustor

For pulse combustors of the Helmholtz type, a heat transfer of the order two to five times higher than expected has been reported. Experiments, where the temperature profile in the tail pipe of a pulse combustor has been measured, give Ilo indication why the heat transfer should be enhanced. The interaction between the oscillating velocity field and the oscillating temperature field might explain the observed enhanced heat conduction. In order to examine this interaction, a thermomechanical pulsating flow between two parallel plates has been considered. The governing equations were coupled with the classical constitutive assumptions of linearly viscous fluid and Fourier's law for heat conduction. The pressure and temperature gradients in the axial direction were approximated as harmonically oscillating functions in time, where the coefficients were estimated from experimental data. By neglecting the viscous net power term in the energy balance equation, the postulated general equations of motion were solved analytically in the transversal direction. The solutions obtained for the velocity and temperature fields were found to be in good agreement with experimental results. The analytical expression for the temperature field was then used to determine the heal transfer at the plates. The heat transfer at the plates was found to be dependent on the phase difference between the pressure and temperature gradients

On the enhancement of heat transfer in pulsating combustion flows

It has been observed and reported that in pulse combustors of Helmholtz type the heat transfer is two to five times higher than expected. From experiments, where the temperature profile in the tail pipe of a pulse combustor has been measured, we have no indication why the heat transfer should be enhanced. In the tail pipe of a pulse combustor the radial component of the temperature gradient vanishes in the main part of the of the cross-section of the tail pipe except close to the boundary of the pipe. Evidently, a temperature drop along the tail pipe of up to 500 degrees C/m, indicates that the classical linear constitutive assumption of heat conduction, i.e. Fourier's law, is incapable of describing the phenomenon observed. A powerful coupling between the oscillating velocity field and the oscillating temperature field might be able to explain the observed enhanced heat conduction. In Fourier's law neither a direct dependence of the heat conduction on the velocity and the velocity gradient, nor an interaction between the velocity and temperature field is given. A first extension would be to introduce a more general constitutive relation for the heat conduction vector. For that reason, in order to describe the observed phenomenon, a new non-linear constitutive relation for the heat conduction vector has been suggested. An additional term, dependent on the velocity gradient operating on the temperature gradient, will effect the heat transfer. To be able to examine the consequences of the new nonlinear constitutive relation suggested, a thermo-mechanical pulsating flow between two parallel plates is considered. By approximating the general constitutive equations in the postulated general equations of motion, analytical solutions of the velocity and temperature fields can be found to be in good agreement with experimental results. The analytical expressions for the velocity and temperature profiles can then be used in the estimation of the heat transfer, which can be compared with experimental observations

Structural assessment of concrete bridges

The paper summarizes the work on concrete bridges performed in the EU project Sustainable Bridges. The work provides enhanced assessment methods that are able to provide higher load-carrying capacities and longer fatigue lives for exixixting concrete railway bridges. The work is also presented in a Guideleine available at www.sustainablebridges.netThe paper summarizes the work on concrete bridges performed in the EU project Sustainable Bridges. The work provides enhanced assessment methods that are able to provide higher load-carrying capacities and longer fatigue lives for exixixting concrete railway bridges. The work is also presented in a Guideleine available at http://www.sustainablebridges.net/Validerad; 2008; Bibliografisk uppgift: Publication no 38. ISBN 978-82-8208-011-8; 20090517 (elfgren

Structural Assessment of concrete railway bridges

The aim of the work presented here was to provide enhanced assessment methods that are able to prove higher load carrying capacities and longer service lives for existing concrete railway bridges. One main objective was to develop methods for non-linear analysis since this provides the greatest potential to discover any additional sources for load carrying capacity, and gives an improved understanding of the structural response. Another main objective was to provide methods for assessing deteriorated concrete bridge. Recommendations are given regarding the effect of reinforcement corrosion, particularly on anchorage capacity. Furthermore, a methodology for improved assessment of the remaining fatigue life of short-span concrete bridges and secondary elements is presented. Other topics treated are evaluation of material properties, simplified methods for structural analysis and the bending-shear-torsion interaction

Structural assessment of concrete railway bridges

The work presented provides enhanced assessment methods that are able to prove higher load-carrying capacities and longer fatigue lives for existing concrete railway bridges. It was performed as a part of the EU-project Sustainable Bridges. The results are implemented in the Guideline for Load and Resistance Assessment of existing European Railway Bridges, see SB-LRA (2007) and are reported in detail in a background document, see SB4.5 (2007). The work presents improved methods for the determination of in-situ material properties, for deterministic as well as probabilistic assessments. Advanced methods for structural analysis are presented, e.g. regarding combined shear, torsion and bending interaction. Recommendations are given regarding redistribution of sectional moments and forces obtained from linear finite element analysis. One main objective was to facilitate the use of non-linear analysis for structural assessment. This provides the greatest potential to discover any additional sources for load-carrying capacity, and gives a better understanding of the structural response. Another main objective was to provide methods to assess the remaining structural resistance of deteriorated concrete bridges. Recommendations are given on the effect of corrosion and a methodology is presented for assessment of remaining fatigue life, with emphasis on short-span bridges and secondary elements