G. Le Palec

Numerical and experimental study of mixed and forced convection in a junction

Abstract This paper presents a theoretical and experimental investigation of the laminar steady 2D-mixing flow in a junction. The numerical method developed by Gosman et al. has been applied to derive velocities and temperatures profiles in the mixing zone. The effects of the angle between the two branches of the junction and the air-flow rate upon the structure flow are analysed, for both forced and mixed convection cases. The experimental procedure is based upon a flow visualization technique and L.D.V. velocity measurements: a reasonably good agreement between theoretical and experimental results is found.

Study of transient laminar free convection over an inclined wet flat plate

Abstract A new analysis of the transient natural convection between an inclined wet flat plate and ambient air is presented. The problem is treated by considering two separate regions—i.e. the boundary layer and the capillary-porous plate—for which a specific differential system of equations is developed. The two systems are linked with the wall heat and mass balances from which the local and average Nusselt and Sherwood numbers are deduced. For some particular cases, quantitative comparisons with previous works reported in the literature agree with each other. Moreover, the agreement between theoretical results and experimental data is satisfactory in the boundary-layer region.

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 heat and mass transfer from an inclined flat plate with wet and dry zones

Abstract This paper presents a theoretical study of momentum, heat and mass transfer in the laminar boundary layer which develops over an inclined flat plate by either free or forced convection when a constant wall heat flux is specified: this plate is made with two wet zones which are separated by a dry one. From the theoretical analysis, a set of dimensionless coupled equations is deduced and numerically solved by using an implicit finite difference method. The results show the effect of most of the main physical quantities upon velocity, temperature and concentration profiles in the boundary layer: from this study, the length of the dry separation zone appears as an essential criterion.

Study of laminar heat transfer over a sinusoidal-shaped rotating disk

Abstract A theoretical and experimetnal investigation of laminar convective heat transfer over a sinusoidal-shaped rotating disk with a constant wall heat flux condition is presented. The theory is based upon the boundary-layer approach. The results show the effects of roughness on the local and average Nusselt numbers. Theoretical wall temperatures are obtained from a simple thermal balance and compared with the experimental ones measured by means of an infra-red thermography technique for the case of air (Pr = 0.7).

Numerical study of the compressible turbulent flow in a laser cavity

Abstract This paper presents a numerical study of the structure flow and heat transfer in the discharge tube of a high power CO2 laser. A compressible turbulent model was developed in connection with simplifying assumptions. The resulting set of partial differential equations describing the flow was solved by means of the PHOENICS code. Several types of thermal boundary conditions were tested and the numerical results were found in a quite good agreement with the experimental data obtained in a previous study. The location of the reattachment point was found highly correlated with the values of the turbulent energy and the dissipation rate of turbulent energy in the entrance section. The size of the recirculation zone and the shock waves created near the exit section of the nozzle have no significant effect upon the structure flow downstream of the reattachment point.

Aerothermic characterization of a high-power fast axial flow CO2 laser

Abstract Because the structure of the flow in glow discharge tubes for high-power CO 2 lasers is one of the main physical processes to control in order to avoid arcing, an experimental procedure for studying the velocity and temperature fields in such tubes was performed. Two kinds of injection system have been studied. For the first one, the gas mixture is injected by a single sonic nozzle located on the axis of the tube. Because the power of the laser may be increased by associating several tubes in line, the other injection system consists of a series of 12 holes radially arranged in order to move injectors away from the axis. For both cases, the axial mean time velocity profile was measured (without electric discharge) using a laser Doppler anemometry technique in several sections of the discharge tube. The temperature field was investigated during the discharge by using a special thermofluorescent probe. It is found that turbulence facilitates the homogenization of the discharge.

Dispersion of a bent chimney fume around a variably oriented building

In addition to experimental investigations, a computational study was conducted in order to analyse the structure of the flow issued from a bent chimney around a parallelepiped obstacle. The particle image velocimetry (PIV) technique was applied to generate an experimental database suitable for validation of the authors numerical simulations. The numerical model is based on the resolution of the NavierStokes governing equations by means of the finite volume method associated with the second-order model. This gives satisfying results in the exit region and in the trailing zone of the jet. Comparing the obtained numerical results with the experimentally tracked data confirmed the authors opinion. The comparison concerned both the mean and the fluctuating dynamic flow features. Once validated, their model allowed the evaluation of the effect of the obstacle attack angle (0,15,30,45, and 75) on the dynamic and mass flow features.

Model of a glow discharge in a turbulent flow

In this paper, a model of the temperature field in a laser cavity, is proposed. This model includes the calculation of the electron concentration in a turbulent flow. First, a 1D study is validated with experimental data of Leys et al. Second, the model is applied to a 2D argon flow. This study shows that the knowledge of the temperature field allows us to characterize the correlation between the plasma oscillations and turbulence fluctuations of the neutral gas.

Study of turbulent compressible flow structure in a CO2 laser cylindrical discharge tube

In order to increase high power closed cycle CO2 laser performance, an experimental investigation of the velocity field in a cylindrical glow discharge tube was carried out. The axial velocity profile was measured (without electric discharge) using a laser Doppler anemometry technique at several sections of the discharge tube. The turbulent compressible flow has also been theoretically investigated by means of the PHOENICS code and comparisons of numerical results with experimental data show a reasonably good agreement.