André Lallemand

A model for calculation of steam injector performance

Abstract Steam injectors can be used in many applications, but especially for security water injection in steam generators of nuclear reactors. Using a one-dimensional model, a steam injector with a centered liquid supply has been simulated. General relationships are presented from the nozzle exit to the steam injector outlet. It is shown that the flow contains a condensation shock. To achieve modelling of the mixing zone, and empirical correlation giving an equivalent pressure with value of condensation rate is found using experimental results obtained at the CETHIL. A parametric study is then made to determine the influence of significant parameters and the functioning range of the steam injector. Calculated values are compared with experimental results and are found to be in good agreement.

Prediction of thermal protection of walls by blowing with different fluids

Abstract This work concerns the modelling of heat and mass transfer in the boundary layer and inside a plane porous plate which is below a hot fluid flow and submitted to cold fluid blowing. A preliminary study of the heat transfer rates in the boundary layer without blowing permits us to validate, comparing with experimental results, the RNG κ-ϵ model. The RNG κ-ϵ model, with kinematic and thermal laws for the wall, linked with a model of blowing, is then used to study the heat and mass transfer rates at the wall when the main flow and the injected fluids are the same species - air - but at different temperatures. The comparison between calculated friction factors, Stanton numbers and published results confirms the validity of our model. We also show the strong influence of the injection rate on the thermal convective coefficient of the wall. In the last part, results on cooling by blowing with water vapour in a main flow of air are given. Comparisons of the evolution of Stanton numbers and friction factors show that blowing with water vapour is more efficient than air injection in terms of momentum transfer and thermal protection of walls.

Étude en soufflerie thermique du refroidissement de parois poreuses par effusion de gaz

Abstract A heated air tunnel and specially its test section features, designed for studies of wall protection from high temperature gases, are described. A coolant, moving through the porous wall, flush fitted in the horizontal test section floor, achieves its thermal protection. Preliminary velocity measurements, carried out at ambient temperature and without injection, within the test section, pointed out the following aerodynamic characteristics: stationarity and turbulence intensity in the potential flow, boundary layer features over the porous wall according to the entrance test section boundary conditions, wall friction coefficient estimation. Velocity and temperature profiles, in non-isothermal conditions, over the porous wall with or without cooling air injection, are reported. They point out the injection rate influence, variable from 0 to 2 %.

Blowing models for cooling surfaces

Abstract To study the cooling of surfaces exposed to high temperature stress and heat flux, the blowing, or transpiration, technique is numerically investigated in the case of a porous circular cylinder. Two models are developed to simulate the blowing impact on the outer flow and an experimental set-up available allows for direct comparison and validation of the numerical simulations. The heat exchange occuring within the porous wall itself between the coolant and the solid part of the wall is accounted. The results show an excellent effectiveness of the blowing in terms of surface temperature reduction, even for low blowing ratii. The incident heat flux exhibits a maximum for medium blowing rates due to a decreasing heat transfer coefficient and a growing temperature difference between the surface and the main flow with the injection rate. Finally, the blowing is demonstrated to be very effective in cooling heavily thermally stressed parts in terms of homogeneity and coolant rate required.

Local measurements in the flow of a steam injector and visualisation

A steam injector is a thermocompression device where the steam drags directly a subcooled liquid and delivers a liquid with an outlet pressure higher than both upstream fluid pressures. The functioning principle of injectors involves sophisticated thermohydraulic processes whose present knowledge is essentially empirical. This work presents an experimental approach of steam injector in order to finely understand and quantify the physical laws driving the flow in the mixing chamber. Local measurements (2D) are carried out on a special device with a rectangular section, where the flow was visualised. These measurements are the void fraction, the static pressure and the static temperature. They permit the calculation of all the variables and a well understanding of the physical phenomena involved in the flow. Especially, we show, in some well-defined zones, the existence of important non-equilibrium kinetic, thermal and thermodynamic phenomena.

Near wake of a circular cylinder submitted to blowing. II: Impact on the dynamics

Abstract The near wake of a porous circular cylinder in cross-flow submitted to blowing through its whole surface is experimentally studied. The blowing impact on the Strouhal number exhibits a linear decrease of the vortex shedding frequency with the blowing ratio. A simple model representing this decrease as a function of the injection rate is developed, based on the wake static pressure profile evolution. The main flow temperature influence is also investigated in case of non-isothermal blowing and is shown to have no effect on the Strouhal number evolution. Finally, the interaction between the blowing and the shear layer is investigated through a spectral analysis of the velocity signal. A modification of the shear layer power spectrum is observed when injection occurs. The dynamics are slowed down and characteristic patterns, denoted sub-peaks, appear while the relationship between the von Karman and the shear layer frequencies without blowing remains valid.

Étude d’run fluide frigoporteur diphasique: 2: Analyse expérimentale du comportement thermique et rhéologique

Resume La premiere partie de cet article a ete consacree a la determination des caracteristiques thermophysiques intrinseques du coulis de glace forme a partir d’une solution aqueuse d’ethanol. Dans cette deuxieme partie, on donne les resultats des etudes experimentales thermiques et rheologiques realisees sur ce fluide frigoporteur. On s’interesse, dans un premier temps, a la formation des cristaux de glace dans un echangeur a surface brossee et plus particulierement aux transferts thermiques qui y ont lieu. On en deduit une correlation d’echange thermique entre le coulis et la paroi de l’echangeur. On analyse ensuite l’incidence de la presence de la suspension de glace sur les pertes de pression dans des longueurs droites de canalisation et dans diverses singularites. Enfin, une etude hydrostatique du melange dans une cuve de stockage met en evidence une tres forte tendance a la stratification avec une redristribution de la concentration en glace et en alcool.

Prediction of turbulent heat transfer with surface blowing using a non-linear algebraic heat flux model

Abstract The paper reports on the prediction of the effects of blowing on the evolution of the thermal and velocity fields in a flat-plate turbulent boundary layer developing over a porous surface. Closure of the time-averaged equations governing the transport of momentum and thermal energy is achieved using a complete Reynolds-stress transport model for the turbulent stresses and a non-linear, algebraic and explicit model for the turbulent heat fluxes. The latter model accounts explicitly for the dependence of the turbulent heat fluxes on the gradients of mean velocity. Results are reported for the case of a heated boundary layer which is first developed into equilibrium over a smooth impervious wall before encountering a porous section through which cooler fluid is continuously injected. Comparisons are made with LDA measurements for an injection rate of 1%. The reduction of the wall shear stress with increase in injection rate is obtained in the calculations, and the computed rates of heat transfer between the hot flow and the wall are found to agree well with the published data.

STUDIES OF THE TRANSPIRATION COOLING THROUGH A SINTERED STAINLESS STEEL PLATE

This study concerns the liquid transpiration cooling effect on thermal protection of a porous plate wall. The results indicate that the effectiveness reaches more than 95% for a very weak effusion rate, about 0.1%, that is to say 50 times weaker than that of gas effusion. The concentration profile in the boundary layer is calculated experimentally and the rate of liquid evaporated is then calculated numerically, using a model based on utilization of momentum equations in laminar flow for the boundary layer. The results of this numerical study confirm evaporation rates calculated by semi-empirical relations.

Comparison between two models of cooling surfaces using blowing.

Abstract: To protect surfaces against high temperatures, the blowing through a porous material is studied. The geometry is that of a circular cylinder in cross‐flow and the effectiveness of the blowing for the thermal protection is numerically investigated. Two models are developed for the blowing simulation and comparisons are made with experimental data obtained in a heated wind‐tunnel. It is shown that the blowing strongly affects the dynamical and thermal profiles over the surface, thickening the boundary layers and decreasing the external transfer coefficients. It results in a lower viscous drag and thermal stress. The wall temperature dramatically decreases with blowing and the heat flux is also affected.