Brahim Sarh

Turbulence Effects on the Vaporization of Monocomponent Single Droplets

Abstract An experimental facility has been developed to study the effects of turbulence on droplet vaporization. The facility allows lo generate a zero mean velocity, isotropic and homogeneous turbulence and to vary systematically the turbulence kinetic energy. The influence of turbulence on suspended single droplets of five n-alkane hydrocarbons is investigated, by determining the average vaporization rates by image analysis techniques. The experiments have been conducted under normal pressure and temperature conditions; for all cases, the length scales of energetic turbulence eddies are larger than the initial droplet diameter. For all the investigated cases, it is found that the presence of turbulent velocity fluctuations increases the average vaporization rates compared to the stagnant case. The linear regression rate of the projected droplet surface area versus time is observed under all turbulence conditions. It is observed that droplets of the five investigated fuels respond differently to the same...

Etude numérique des jets turbulents à température élevée

Numerical prediction of the structure of high temperature axisymmetric turbulent jets. Turbulent axisymmetric jets at high temperature are studied numerically by using first and second order turbulence models. Regarding the temperature fields, on which we concentrate in this work, predictions with both types of models do not show large differences. In general, predictions agree well with the measurements; the existing differences are usually favorable for the second order model. The effect of solving a transport equation for the scalar dissipation rate on the prediction of the mechanical to scalar time scale ratio and on the prediction of the scalar fluctuations is studied. The influence of varying the density ratio on parameters such as the axial decay rates of the temperature and velocity and the turbulence intensity are studied. Two definitions of the mixing efficiency are introduced. According to both definitions, the mixing efficiency decreases with increasing effects of buoyancy.

A COMPARATIVE STUDY OF TURBULENCE MODELLING IN HYDROGEN-AIR NONPREMIXED TURBULENT FLAMES

Hydrogen-hydrocarbon blend flames have recently received increased attention as alternative fuels for terrestrial and aerospace power generation applications. The combustion modelling of these composite fuels flames is complex. Turbulence modelling as well is difficult because in the near field region of the jet exit, high density gradients at high inlet velocity cause difficulties. In order to correctly predict turbulent flames if blend fuels, it is necessary to validate the models capability for hydrogen flames and hydrocarbon flames separately. In this study, pure hydrogen-air turbulent nonpremixed flames are numerically investigated. The configuration used is a co-flowing axisymetric turbulent non-premixed hydrogen flame, which is experimentally investigated by Barlow and Carter (1994) and Flury and Schlatter (1997). The model uses two turbulence closures that are the k-ε model and the Reynolds Stress Model (RSM) coupled with the steady strained laminar flamelet model. The performance of the k-ε and the RSM models are particularly discussed in the locations close to the nozzle exit. The results obtained demonstrate that the strained steady laminar flamelet approach based on the k-ε and the Reynolds stress turbulence models is, in general, capable of predicting this hydrogen-air flame at atmospheric pressure. Comparisons with measurements indicate that the predictions are sensitive to turbulence modelling and differential diffusion in the near-field region whereas the far-field region is influenced only by turbulence modelling. The RSM model gives better results than the k-ε model predictions close to the nozzle exit due to its corresponding turbulence parameters that are modelled more accurately. However, because of unity Lewis number assumption in the flamelet library generation, air entrainment is found to be not well predicted by the two turbulence models. Downstream, the k-ε model performs better and the predictions are very close to experimental data.

Electric field influence on the stability and the soot particles emission of a laminar diffusion flame

ABSTRACT Influence of electric fields on flames has been studied for many years and the ionic wind constitutes the main explanation of the observed effects on the flame structure and pollutant emissions. However, previous works have been limited to small flames. The interaction mechanisms of an electric field with longer flames, involving both ionic wind and buoyancy are not fully identified. In the present paper, the effects of a D.C. electric field on a laminar 88-mm-long ethylene diffusion flame burning in ambient air are investigated. Based on the calculated electric field configuration, the influence of both downward and upward electric field is compared via imaging, electrical diagnostic and soot measurements. The application of a negative (directed downstream) electric field triggers a flickering instability and an electric instability at higher field strength, in which self-sustained flame oscillations of flame length directly affect ion current. Conversely, the flame is stabilized by a positive electric field. In-situ soot volume fraction measurements show that the electric field decreases the average soot volume fraction measured on a stable flame axis, whereas flame oscillations lead to a sooting flame.

Characterisation of confinement and impingement effects on the near field of axisymmetric jets

Laser Doppler Anemometry and Particle Image Velocimetry are applied to a domestic burner configuration to determine the effect of geometrical parameters and gas density on the flow field and on the entrainment process. The flow field is that of a confined jet impinging onto a plate. The main design parameters of the experimental devices may be adjusted to modify the confinement ratio and the distance from the outer section of the injector to the stagnation plate. The impinging distance is known to influence the structure of the flow field, mostly when the stagnation plate is in the near field region of the jet. Two density ratios were studied: an isodensity (R ρ = 1) and a light jet (R ρ = 0.55). The two nominal flow rates were investigated, associated with low Reynolds turbulent jets and laminar jets. A totally transparent new device was manufactured in quartz. This new device presents greater optical accesses for PIV measurements inside the burner. Several configurations were investigated for the different gases and impinging distances: free jets, confined jets, impinging jets, and confined impinging jets (whole burner configurations). The effect of burner geometry and gas density on the flow field and on the entrainment process is determined.