G. Bergeles

The Flow Past a Surface-Mounted Obstacle

Etude experimentale a laide dun fil chaud de la longueur de la region de recirculation autour dun obstacle bidimensionnel

2D LES OF VORTEX SHEDDING FROM A SQUARE CYLINDER

Abstract In the high Reynolds number flow past a square cylinder, turbulent fluctuations are superimposed on the periodic vortex shedding motion making numerical calculation of the flow a difficult task. In the present work a two-dimensional large eddy simulation is performed with no-slip boundary conditions at the solid walls. A filtering procedure is introduced in frequency space to separate the periodic from the turbulent fluctuations and the kinetic energy of both is calculated along the centerline behind the rod. The drag coefficient, vortex shedding frequency and spacing of the vortices in the wake are also calculated and the results are validated against experimental measurements. It is found that the two-dimensional large eddy simulation using a fine grid resolution, especially in the near wall region, gives a good representation of the quasi-two-dimensional mechanisms of the flow since they are directly simulated instead of being modeled as with statistical turbulence models.

Dispersion of particles in anisotropic turbulent flows

Abstract A Lagrangian approach is used to simulate particle dispersion in anisotropic turbulent flows. Discrete particles are tracked in three dimensions, influenced by the fluids turbulent velocity fluctuations. The fluctuations are temporally and directionally correlated through a statistical sampling method reflecting the anisotropy of the flow field. They are calculated at the discrete particles position through a spatial correlation which takes into consideration the anisotropy of the Reynolds stresses. The method is first tested as to the performance of the temporal and directional correlation features and is then combined with a Eulerian scheme and an algebraic Reynolds stress model for the prediction of the carrier phase. The final form of the model is used to predict a two-phase turbulent round jet and a two-phase inert flow from a quarl burner, with swirl. In parallel to these, the method is compared to a previous particle-tracking method based on an isotropic hypothesis.

Modelling of slurry droplet drying

Heat, mass and momentum transfer between a slurry droplet and a gas flow are investigated numerically. The developed model can be applied to assess drying and combustion properties of slurries inside spray dryers or combustors and to estimate the time needed to reach ignition of the solid component in slurry fuels. The model was applied to coal water droplet slurries the properties of which are available in the literature but can also be used for study of drying of any other slurry such as that encountered in flue gas desulfurization systems or in food industry. The parametric study revealed that the most important factor in slurry drying is the ambient temperature and that the injection velocity, the ambient pressure of the flowing medium or the particle initial temperature affect very little the drying rate.

Modeling wall impaction of diesel sprays

Abstract A model for diesel spray wall impaction is presented, which is assessed against experiments for a number of test cases, including normal or angled injection to a wall into a quiescent space or a cross-flowing gas at various gas pressures. New relationships are given for the velocities of the droplets rebounding from the wall. These relationships take into account the wall roughness and the possible break-up of the droplets during their impingement. The impingement model was incorporated in a spray model based on the stochastic particle technique (Dukowicz 1980) and accounts for the phenomena of droplet injection, break-up, collision and coalescence, turbulent dispersion, and evaporation. The spray model was incorporated in a recently developed three-dimensional (3-D) computational fluid dynamics (CFD) code that simulates the unsteady compressible flow of the gas in internal combustion engines by solving the full Navier-Stokes equations. It was found that the motion of the surrounding gas caused by the spray injection plays a minor role on the predicted results. The latter concern the wall spray radius and the wall spray height. The validity of the spray model is demonstrated through extensive comparisons with experiments over a wide range of gas conditions.

Particle-Surface Interactions in Heat Exchanger Fouling

The problem of fouling is of vital importance to heat exchanger efficiency and should be considered during the design phase of the heat exchanger. The purpose of the present paper is to introduce a novel method to aid in the evaluation of the various parameters that affect the fouling phenomenon. The method considers the particle-surface interaction from the energy balance at the point of impaction and takes into consideration the material properties of the particle and surface as well as the effect of the surrounding flow field on the particle movement and impaction. The calculated deposition flux is used to form the deposit evolution in time considering the removal mechanisms resulting from fluid shear stress and the eroding impacts of the particles. The model is validated against experimental measurements of particle deposition from a two phase flow of hot gases around a circular cylinder. The experimentally measured flow field around a staggered tube bundle is also predicted and a fouling analysis is performed regarding the validity of fouling studies in scaled down model configurations. It is concluded that the actual dimensions of the heat exchanger configuration are of vital importance to its fouling behavior and scaling laws are difficult tomorexa0» apply.«xa0less

The computation of flow in a spirally fluted tube

Abstract The paper describes the development and application of a computational procedure for calculating the velocity field in fully-developed flow through spirally fluted tubes. Topics covered include the choice of coordinate system, the resultant form of the governing equations, the validation of the accuracy of the procedure by comparison with known analytical solutions and a discussion of the results of applying the procedure to the spirally fluted tube flow itself. The computations indicate that the effectiveness of the flutes in imparting swirl to the flow increases markedly with the number of flutes. The swirling motion and the turning over of fluid within the flute region produces very little increase in drag; indeed, when friction factor and Reynolds number are based on hydraulic diameter, the drag is less than for a smooth tube.