فهرست مطالب

Applied Fluid Mechanics - Volume:12 Issue: 1, Jan-Feb 2019

Journal Of Applied Fluid Mechanics
Volume:12 Issue: 1, Jan-Feb 2019

  • Special Issue
  • تاریخ انتشار: 1397/11/23
  • تعداد عناوین: 8
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  • H. C. Sadouk *, S. Chikh, G. Lauriat Pages 1-8
    Turbulent forced convection heat transfer in a parallel plate channel partially or totally filled with a porous layer is studied numerically. The walls of the duct are non-permeable and heated uniformly at the same temperature. The present model adopts a single domain approach using continuity conditions at the interface. The governing equations (continuity, momentum, energy and κ-ε turbulence model) are solved by applying the finite volume method. The Darcy-Brinkman-Forchheimer model is used in momentum equations, kinetic energy and turbulent dissipation rate using the κ-ε turbulence model with adapted law of the wall. The obtained numerical results allow us to evaluate the effect of key parameters namely the Reynolds number, the thickness of the porous layer, its permeability, its porosity and the effective thermal conductivity on the flow and heat transfer. It is highlighted that the turbulent kinetic energy is strongly influenced by the permeability and the porous layer thickness. It is found that the presence of the porous medium substantially increases the heat transfer for an optimum thickness for very specific conditions.
    Keywords: Heat transfer, ?-? model, Finite volume method, Parallel plates, Porous media
  • I. B. Abdellaziz* , M. Bouterra , O. Vauquelin, A. E. Cafsi Pages 9-18
    The natural smoke extraction in a corridor with a vertical duct is analyzed numerically using the software Fire Dynamics Simulator. A parametric study is conducted to investigate the influence of the duct geometry (i.e. section, shape and height) on the smoke extraction efficiency. Results show that the section of the duct appears as the most influential geometrical parameter. There exists an optimal duct section, for which, the hot smoke adheres to the left and right walls of the duct. In this case, the mixing process and the stack effect are the dominating factors influencing the natural extraction performance. The thermal stratification is also investigated using the Newman parameter to quantify the smoke stratification stability. It was found that this parameter is predicted by the two-dimensional nature of the flow pattern in the downstream direction and that the degree of stratification decreases as the duct section and height increase.
    Keywords: Tunnel fires, Stratification, Fire dynamics simulator
  • A. Hadef *, A. Mameri, Z. Aouachria, F. Tabet Pages 19-28
    Nowadays, combustion intervenes in more than 80% of primary energy consumption, the control of the combustion process is essential. Fuel consumption, emissions reduction and energetic efficiency increasing are challenges to which researchers are faced. The most interesting invention that meets nearly all these prerequisites is the flameless combustion regime. This regime is based on the dilution and preheating of reactants by recirculated exhaust gases. This regime is well adapted for the low calorific fuels such as biofuels. In this context, this work presents a contribution to flameless combustion which differs from the conventional one by its low emissions and high efficiency. This numerical study considers the effects of air contained nitrogen substitution by water vapor, the oxygen volume concentration in the oxidizer stream and hydrogen doping of the fuel. To simplify analysis, the opposed jet configuration is adopted with a full diffusive transport approach. The MILD combustion is described by the well-known Gri 3.0 mechanism. It has been noticed that oxygen reduction within a range of 4% to 6%, which is a characteristic of flameless combustion, reduces significantly temperature and emissions whereas hydrogen addition to the fuel, which increases temperature and emissions, has a lower impact. Dilution by water vapor reduces temperature and emissions by thermal and chemical effects except OH radical.
    Keywords: Hydrogenated biogas, Dilution by H2O, Flameless combustion, Thermal, chemical effects of H2O
  • G. Laguna, M. Vilarrub, M. Ibaez, J. Rosell, F. Badia, H. Azarkish, L. Michel Collin, L. Fréchette, J. Barrau * Pages 29-39
    Previous studies have demonstrated that the performance of a cooling scheme based on a matrix of microfluidic cells with self-adaptive valves under unsteady and non-uniform heat load scenarios improves in terms of pumping power and temperature uniformity, compared to the ones from conventional microchannels and hybrid jet impingement/microchannel cooling devices. The behavior of the thermally dependent self-adaptive valves varies as a function of some design parameters. In this work, the impact of the valve’s characteristic curve on the cooling device is assessed to establish the basic rules for the valve design. The performance of a 3×3 microfluidic cell array is numerically studied under an unsteady and non-uniform heat load scenario. The results show that the valves which open at the most elevated temperature (control temperature of 90ºC) reduce by 15.5% the pumping power with respect to the valves opening at 60ºC, while improving by 25.0% the temperature uniformity and reducing both the overcooling and the fatigue.
    Keywords: Adaptive cooling, Temperature uniformity, Distributed cooling, Pumping power
  • D. Alibert*, M. Coutin, M. Mense, Y. Pizzo, B. Porterie Pages 37-47
    The purpose of this experimental study was two-fold: first, to explore and understand the effects of oxygen availability on the combustion of liquid and solid fuels; second, to provide data for comparison with CFD models. Experiments were conducted in the controlled-atmosphere calorimeter of IRSN, called CADUCEE, varying the oxygen concentration in the oxidizing stream and the size of the fire. Polymethylmethacrylate (PMMA) and heptane were used as fuels. Results are found to be in good agreement with the literature data. As the oxygen level decreases, the mass loss rate and flame heat feedback decrease, as well as the flame height and maximum flame temperature, for both fuels whatever the sample size. For heptane pool fires, temperature measurements in the liquid layer reveal a decrease in heat transfer at the fuel surface and inside the fuel with the oxygen molar fraction. For PMMA, the radiative and convective contributions to the total heat flux remain nearly constant, with about 65% and 35% respectively, regardless of sample size and oxygen concentration.
    Keywords: Fire, Pool fires, Pyrolysis, PMMA, Heptane, Calorimeter
  • M. Mense *, Y. Pizzo, H. Prétrel, B. Porterie Pages 49-58
    This study focuses on fire behavior in a mechanically-ventilated compartment, with a special emphasis on the low-frequency (LF) oscillatory behavior that has been occasionally observed. LF oscillations, typically in the order of a few mHz, can lead to large thermodynamic pressure variations, which in turn can cause fire safety issues (e.g. loss of confinement, mechanical damages). To address these issues, small-scale experiments are conducted varying the air renewal rate (ARR) in the compartment from 8 to 20 h-1 and the ventilation configuration. The fire source consists of heptane fuel loaded in a pan with a diameter of 18 cm. Depending on the ARR, LF oscillations are observed on the time evolution of the burning rate, and thus of all the other variables, with a frequency in the range of 16-26 mHz. As the ARR increases, there are three distinct regimes of burning behavior observed in this study: (1) rapid extinction due to smoke filling; (2) LF oscillating burning, followed by blow-off extinction after a few oscillations; and (3) LF oscillating burning, where extinction occurs because of the burning rate “runaway” due to an intensification of the heat transfer through the rim of the container. The oscillatory behavior and fire extinction result from the competition between oxygen supply and fuel vapor supply due to the heat feedback to the fuel tray. Both regimes (2) and (3) are accompanied by displacements of the flame out of the pan towards regions where oxygen is present. The influence of ARR and ventilation configuration (i.e. air inlet location and blowing direction) on the burning rate and LF oscillation properties (frequency, amplitude) is examined and discussed.
    Keywords: Compartment fire, Forced ventilation, Experiments, Oscillating flame
  • S. Chatti *, C. Ghabi, A. Mhimid Pages 59-70
    Smoldering phenomenon can be described as a slow, low-temperature, flameless form of combustion, sustained by heterogeneous reactions with oxygen occurring at the surface of a condensed-phase fuel. In this work a computational study on the smoldering ignition and propagation in polyurethane foam is carried out. First, we investigated numerically the heat transfer and the fluid flow in porous media using the generalized lattice Boltzmann method (LBM). Our appropriate code is validated through the study of a thermal injected flow. LBM results are compared to analytical solutions and numerical results obtained using the Finite Difference Method. Second, the numerical model is extended to account for chemical reactions. We introduce the two-dimensional, transient, governing equations for smoldering combustion in a porous fuel. The model  describes opposed and forward propagation according to appropriate assumptions. The kinetics model isbased on a three-step mechanism. The temperature and char mass fraction profiles are studied at different cross-sections. Obtained results are compared to literature solutions. At the beginning, the important quantity of char is produced near the ignited boundary. To follow the phenomenon, the isotherms are presented at different instants. The results reproduce the features of the smoldering process and represent a significant step forward in smoldering combustion modeling.
    Keywords: Chemical kinetics, Heat transfer, Lattice Boltzmann method, Porous media.
  • S. Adjmi *, S. Launay, C. Abid Pages 71-77
    An experimental study was conducted on a two-phase thermosyphon loop with a total length of 2.1 m, using Ethanol as a working fluid. The influence of the filling ratio, at 40 and 62%, on the thermo-hydraulic behavior was investigated. The applied heat fluxes on the evaporator are ranged between 10 and 60 W/cm2, which corresponds to high values compared to those published elsewhere. One can notice that the confinement conditions associated with high filling ratios contribute to the training of the liquid phase from the evaporator to the condenser, which increases the drying limit of the evaporator due to a significant increase of the flow rate of the sub-cooled liquid at the inlet of the evaporator.
    Keywords: Liquid-vapor, Heat flux, Thermal resistance, Filling ratio, Heat transfer, Gravity