Showing papers by "Bart Merci published in 2008"

A stable pressure‐correction scheme for variable density flows involving non‐premixed combustion

Abstract: An efficient time-accurate algorithm is presented for numerical simulations of low-Mach number variable density flows in the context of non-premixed flames. The algorithm is based on a segregated solution formalism in the class of pressure-correction methods. It shows good conservation properties and returns stable results, regardless of the difference in density between neighboring cells. A simplified Burke–Schumann flame model is used to describe the heat release due to mixing of fuel and oxidizer. The convergence rate of the method is discussed for a 1D channel flow, containing fuel and oxidizer. Copyright © 2008 John Wiley & Sons, Ltd.

Relation Between Horizontal Ventilation Velocity and Backlayering Distance in Large Closed Car Parks

Abstract: Due to the ceiling jet phenomenon, smoke above a fire source has a natural tendency to spread under the ceiling in all directions, until a barrier is reached. The present study focuses on smoke control, rather than smoke clearance, in large closed car parks. A particular situation is that the ceiling height is much lower than the horizontal car park dimensions. Also, flame heights can be in the same order of magnitude as the ceiling height, so that flames can penetrate into the smoke layer under the ceiling near the fire source. Smoke control in case of fire in large car parks can be established by horizontal mechanical ventilation. A ‘critical ventilation velocity’ exists, for which no smoke backlayering occurs, i.e. the car park is maintained smoke-free at one side of the fire source. In many cases, however, backlayering can be allowed to a certain distance. We analyse the results from a large series of CFD-simulations, used as numerical experiments, and illustrate that there is a relation between the horizontal ventilation velocity and the backlayering distance. The backlayering distance varies linearly with the difference between the critical ventilation velocity and the actual ventilation velocity of the incoming fresh air. We perform a parameter study with variation of heat release rate per unit area, fire source area, car park width and car park height. We show that the coefficient in the mentioned linear relationship is independent of the fire source area, the car park height and the car park width, but increases with decreasing heat release rate per unit area. We compare the results for the critical ventilation velocity in car parks to results obtained in tunnel fires. We confirm the observations that the critical ventilation velocity increases with fire source area and heat release rate per unit area, as well as a small influence of the car park width. We observe an increase of the critical ventilation velocity with increasing car park height. Finally, we point out that care must be taken when a smoke control system design is based on volume flow rates, calculated from cold inlet flow velocities, as differences between extraction velocities and incoming air velocities can be substantial.

Large-eddy simulation of non-premixed flames, using a novel pressure-correction approach

Abstract: An efficient time accurate algorithm with great potential in LES simulations of non-premixed flames is presented for numerical simulations of low-Mach number variable density flows. The algorithm is based on a segregated solution formalism in the class of pressure-correction methods. It shows good conservation properties and returns stable results, regardless of the difference in density between neighboring cells. In the illustrative example, a flamesheet model is used to describe the combustion of fuel and oxidizer. The convergence rate of the method is discussed for a 1D channel flow, containing both fuel and oxidizer. Furthermore, results for a computationally more demanding 2D testcase of a reacting mixing layer are discussed. Eventually, it is shown that the method can be applied to a real-life situation, of a turbulent non premixed swirling methane-air jet-flame.

Applications of an efficient low-mach algorithm to basic variable density flows

Abstract: Time-accurate simulations are more and more required (e.g. LES of reacting flows). The generally applied pressure-correction schemes can no longer be used, due to lack of stability. Instabilities arise when the density variations are too large. In many combustion applications, high density ratios appear (e.g. for methane-air combustion at atmospheric pressure, density ratios in the order of 10 near the flamefront are normal). Some tried to circumvent the stability problem by applying (unphysical) rescaling of the time derivative of the density [1]. In [2], we showed that, for a non-reacting flow, a good constraint for the velocity field can be formulated, such that the solution is stable. In [3] the propositions of [2] were extended towards non-premixed combustion simulations, making use of the mixture fraction as a conserved variable. A pressure-correction algorithm was obtained which (1) conserves mass, (2) conserves fuel mass and (3) is stable and robust, without the need for (unphysical) underrelaxation. These three properties are obtained by introduction of a chemical operator H C as ρ = HC(ρξ) (ρ: density, ξ: mixture fraction). In case of pure mixing, the operator is linear. In case of combustion, however, HC is highly nonlinear. A discrete equation for the pressure, follows from a constraint on the velocity field by combining the discrete equations of continuity and mixture fraction. Due to the nonlinearity of HC, a non-linear equation for the pressure is obtained, which can be linearized around the predicted solution for the velocity field (u ∗ ):