Hatem Mhiri

Three-Dimensional Numerical Calculations of a Jet in an External Cross Flow: Application to Pollutant Dispersion

We present a three-dimensional numerical simulation of a circular turbulent jet issuing transversely into a uniform air stream. In the first part an air-helium jet is considered and the three-dimensional structure of the flow field is discussed. Then, a comparison between the numerical results of four turbulence closure models (three first-order models and a second-order one) are presented and compared with the experimental data given by Crabb et al. , and Andreopoulos et al.

Influence des conditions d'émission sur un écoulement de type jet plan laminaire isotherme ou chauffé

Abstract We propose numerical solutions for a laminar jet, accounting for emission conditions at the exit of the nozzle. Two emission cases are considered in this study: velocity and temperature profiles are uniform or parabolic, respectively. A finite difference scheme is developed for the resolution of the equations governing the isothermal and non-isothermal free jet and wall jet developing tangentially along an adiabatic flat plate. The analysed results are the centerline velocity and centerline temperature for the free jet, and wall temperature and shear stress for the wall jet. The results obtained are compared to another method that is based on two constraints of integration, i) conservation of momentum and ii) conservation of energy, to replace the emission conditions at the exit of the nozzle for the resolution of equations. Our results of the velocity and temperature profiles compare well with those obtained by the latter method solely in the plume region, where buoyancy forces are responsible for flow.

Étude numérique des conditions d'émission sur un écoulement de type jet plan turbulent isotherme ou chauffé

Abstract We intend to solve equations governing turbulent plane-vertical isotherm and non isotherm jets by taking into account inflow conditions at the exit of the nozzle. The analysis is focused on the influence of these conditions on this type of flow. Two cases are considered (uniform and parabolic velocity and temperature profiles). A finite difference scheme is developed to solve the governing equations. This numeric model allows us to show that the region of fully developed regime begins much nearer the nozzle for the turbulent case than for the laminar flow case. Indeed, the turbulence increases the mixing between the incoming gas from the nozzle and the ambient fluid, and consequently the size of the potential core zone decreases. The results are compared to other works introducing mathematical variables based on the energy conservation for the case of the mixed convection and the momentum conservation for the forced convection, which allows the validation of our results.

Numerical study of momentum and heat transfer in a pulsed plane laminar jet

Abstract In this paper, we propose numerical solutions for a two-dimensional pulsed plane jet in unsteady laminar regime. At the exit of the nozzle, the pulsating flow is imposed with a uniform temperature T 0 and a velocity u = u 0 (1+ A sin( ωt )). Two cases are considered: the free and the wall pulsed plane jet. For the wall jet case, the wall may either be considered adiabatic or subjected to a uniform temperature. Equations are treated with an appropriate finite difference method. The effect of the important governing parameters, such as the amplitude and the frequency of the pulsation, the Reynolds and Grashof numbers on the flow behavior are also investigated in detail. The results obtained show that the pulsation affects the flow in a vicinity region of the nozzle to reach the same asymptotic regime than the steady jet. The results also indicate that the initial development of the jet is considerably accelerated and the entrainment in the first diameters is enhanced.

Numerical Study and Optimization of Parabolic Trough Solar Collector Receiver Tube

In this study, the parabolic trough collectors (PTC) performance is analyzed. In order to achieve this goal, the adopted procedure comprises two main steps. In the first step, the concentrated solar heat flux densities in the solar concentrator focal zone are calculated by soltrace software. In the second step, computational fluid dynamics (CFD) simulations are carried out to analyze and to optimize the thermal performance of the tube receiver. The calculated heat flux densities by soltrace software are used as wall heat flux boundary conditions for the receiver tube. The effect of the receiver tube diameter variation on the PTC thermal performance is studied. A new type of receiver tube is tested. This latter is covered with a metallic thickness. The performance of tube receiver covered with a metallic layer for different diameters is compared to those of the same diameters without the addition of metallic thickness. It has been found that increasing tube metallic thickness enhances the performance of PTC system comparing to the tubes of the same diameter and crossed by the same flow rates.

Flow Field Measurement in a Crossflowing Elevated Jet

Structural features resulting from the interaction of a turbulent round jet issuing transversely into a uniform stream are described with the help of flow visualization and the PIV technique. The jet exits from a rigidly mounted pipe projecting at a distance from the floor of a tunnel. The aim of the present work is to investigate the flow structure in the near-field jet-pipe exit. Jet-to-crossflow velocity ratios from 0.375 to 3 were revealed at Reynolds numbers from 1660 to 6330. Flows in the vertical symmetry plane and horizontal plane across the jet-wake, jet-exit, and pipe-wake regions are investigated. The measured velocity fields present quantitative characteristics of the streamlines, vortices, and topological features of the flow structures. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies.

Numerical Analysis of Recirculation Bubble Sizes of Turbulent Co-Flowing Jet

Abstract This paper deals with the numerical prediction of an axisymmetric turbulent jet discharged into a co-flowing stream. Two computational codes have been employed, in-house code using the standard k-ɛ model and commercial CFD software including the standard k-ɛ and the RSM model. The main object of our research is to investigate the recirculation phenomena induced by the change of the inlet and boundary conditions. We observed different recirculating regions (central and corner) induced by the inclusion of the nozzle thickness δe. The size and the strength of these recirculation bubbles are intensified by increasing δe. We also observed that the co-flowing jet emitted with a different velocity profile generates a wide central recirculation bubble and a small corner vortex. However, the formation of the recirculation bubble is destroyed by decreasing Froude number. Furthermore, the analysis of the size and strength of the recirculation bubbles is necessary in the case of the jet pump and the flame in combustion chambers.

NUMERICAL STUDY OF A HEATED PULSED AXISYMMETRIC JET IN LAMINAR MODE

In this work, we have studied numerically the influence of a pulsation on the flow generated by an axisymmetric immerged jet in a laminar mode. A finite-difference method is used to solve the dimensionless equations governing the flow. The simulation enabled us to determine the space-time evolution of the flow variables, such as the velocity components, temperatures, length of the potential core, and both the dynamic and thermal half-thicknesses. The results obtained are the outcome of various factors such as the pulsation amplitude, the frequency which affects the Strouhal number value, and the Reynolds and the Grashof numbers. All of them show that a pulsed jet reaches an asymptotic mode identical to that of the steady one. On the other hand, the pulsation considerably accelerates the expansion of the jet and clearly improves the entrainment at the nozzle exit for distances of some diameters. The results reached in this work are validated with those obtained for a steady jet in its various aspects.

Influence of a Coflowing Ambient Stream on a Turbulent Axisymmetric Buoyant Jet

This paper reports numerical results on turbulent buoyant axisymmetric jets in a coflowing ambient stream. The objective of this study is to compare the performance of the Reynolds stress algebraic model (ASM) with that of the k-e turbulence model in predicting the flow field. A finite difference method has been used to solve a system of coupled partial differential equations. A comparison has been carried out between the numerical results obtained in the present work and experimental and numerical data reported in the literature. It has been found that the two investigated models reasonably predict the mean flow properties of the flow field. Nevertheless, the ASM proves to be better than the k-e method to predict the effects of buoyancy and the turbulence structure. It has been found that the increase of the coflow can slow the development of the jet to the state of similarity of mean characteristic profiles. A jet with a ratio of coflow velocity Ū ∞ to jet discharge velocity Ū 0 less than 0.05 has developed to closely approximate a free jet in a stagnant medium while a jet with higher Ū ∞ /Ū 0 ratio never reaches a similarity state. In buoyant jets, only a flow with u ∝ /u 0 ≤ 0.05 reaches a similarity state. Buoyancy ensures that the similarity region begins at a distance closer to the nozzle exit than if the medium is stagnant.

Numerical study of the thermal and aerodynamic insulation of a cavity with a vertical downstream air jet

Abstract This work is devoted to a numerical approach of the laminar mixed convection in a cavity which one of the boundaries is materialized by a laminar vertical downstream air jet. The purpose is to analyze the interaction of this flow with the natural movement that grows in the cavity under the combined action of boundary thermal gradients and external medium of the cavity in order to examine thermal insulation qualities of the jet. Calculations have been made with the help of the finite volume method.