Contrôle thermique de parois de chambre de combustion. Banc d'essai du laboratoire aquitain de recherche en aérothermique

Thermal control of combustion chamber walls. LARA's test bench. Convective heat transfer along the wall of a combustion chamber is a major source of concern for a gas turbine engine manufacturer like Turboméca. This topic led to a joint research project with the University of Pau : to this end, a specific test bench with up-to-date means of measurement (IR thermography, laser doppler velocimetry) was created. An initial literature study had shown that multi-hole cooling had not been previously studied with a geometry design close to the real case. The present test facility provides and controls the dynamic and thermal parameters of a combustion chamber and the multi-hole cooling was set up on a flat plate simulating the wall chamber. This plate is submitted on one side to the hot gases resulting from combustion and on the other side to the cooling air flow. Dynamic and thermal entry conditions of the hot and cold flows were carefully controlled and are now presented. The wide possibilities of the testing facility have been explored offering scopes for other experimental studies.     

Effet de second ordre pour des protons de méthylène en ENDOR

An indirect second-order effect occurs between the methylene-group protons of a radical created by gamma irradiation in a 1, 2, 4-triazol single crystal; this gives a non-crossing phenomenon which is observed on the ENDOR transitions. It is necessary to take this into account in the expression for the transitions, which then allows us to determine the hyperfine tensors of the two protons. Moreover, the presence of this effect helps in the identification of the transitions and the signs of the couplings.     

Termes de signe positif et négatif en analyses exergétiques des flux de chaleur

Positive and negative terms in the exergetic analysis of heat fluxes. In various technical plants heat transfer rates at temperatures above that of the environment and heat transfer rates at temperatures below the temperature of the environment can be simultaneously observed. This is the case in cryogenic industrial processes where a cooling effect is produced from a heat source. This is also the case for all absorption refrigeration plants or absorption plants that provide refrigeration and heat pumping, in particular for air-conditioning of buildings. In these cases exergetic analysis leads to equations in which some terms are positive and others are negative. This can generate difficulties in the definition of exergetic, or rational, efficiencies. In this paper ways of resolving these difficulties are discussed.     

Constante de force du vibrateur CH en relation avec la structure électronique de la molécule

Une expression analytique est recherchée pour le calcul des constantes de force k _CH dans le cadre de la méthode CNDO/2. Les dérivées des intégrales de résonance qui apparaissent dans cette expression sont évaluées empiriquement. Un ajustement statistique est effectué sur les valeurs expérimentales, la paramétrisation originale de la méthode CNDO/2 étant conservée. Dans cette optique, deux évaluations des dérivées β_{μ, ν}′ et β_{μ, ν}″ des intégrales de résonance par rapport à la distance CH sont proposées:

1. β_{μ, ν}′ et β_{μ, ν}″ sont supposées constantes.

2. β_{μ, ν}′ et β_{μ, ν}″ sont posées proportionnelles aux dérivées S _{μ, ν}′ et S _{μ, ν}″ des intégrales de recouvrement.

L'une ou l'autre de ces évaluations traduit l'évolution des constantes de force grâce à l'effet conjugué d'une contribution ‘statique’ (fonction des éléments de la matrice densité) et d'une contribution ‘dynamique’ (dépendant de la variation pendant la vibration de ces mêmes éléments).

An analytical expression is sought for k _CH bond force constants, within the frame of the CNDO/2 method. Some integrals appearing in this expression are empirically valued. A statistical adjustment is effected on the experimental data, the original parameters of the CNDO/2 method having been preserved. Two appraisals of β_{μ, ν}′ and β_{μ, ν}″ derivatives of the resonance integrals are compared with each other.

1. β_{μ, ν}′ and β_{μ, ν}″ are constants.

2. β_{μ, ν}′ and β_{μ, ν}″ are in direct ratio to S _{μ, ν}′ and S _{μ, ν}″ derivatives of the overlap integrals.

Either of these valuations explains the evolution of the bond force constants, thanks to the aggregate effect of a ‘static’ contribution (function of elements of the density matrix) and thanks to a ‘dynamic’ contribution (depending on the variation during the vibration of these same elements).     

Sur le pouvoir thermoélectrique de l’alliage Cu3Au en fonction de l’ordre

The thermoelectric power of Cu₃Au alloy is calculated as a function of the order with the hypothesis of free electrons, assuming that the Brillouin zones do not change at the critical point. By assuming a square-well scattering potential it is found that the thermoelectric power decreases with increasing temperature and exhibits a discontinuity at the critical point.     

Etude du comportement de radicaux libres à la transition de phase et en phase antiferroélectrique de NH4H2AsO4

Cet article décrit le comportement de radicaux libres dans le cristal antiferroélectrique de NH₄H₂AsO₄ et le compare à celui de ces mêmes radicaux dans le ferroélectrique KH₂AsO₄. A la transition de phase, les modifications que subissent les espèces radicalaires sont interprétées en liaison avec l'abaissement de symétrie et l'établissement de l'ordre antiferro-électrique. Des transformations interradicalaires thermiquement réversibles sont mises en évidence ; elles se caractérisent par une variation du nombre de protons en interaction avec l'électron non apparié.     

Étude expérimentale de la résistance thermique de contact entre l'étain en cours de fusion et un creuset refroidi

This paper deals with the analysis of the thermal contact conditions during the melting of tin on different cooled walls, for different heat situations and for different cooling flows. Two experimental set ups are studied. A polished nickel substrate covers the melting-pot of the first one. Semi intrinsic thermocouples are implemented to measure the temperature of the substrate. This will allow a better study of the thermal contact resistance distribution on the wall. The second melting-pot is made of copper. Its surface roughness is variable. The heat system is stronger. The results obtained with the first experimental set-up show that the thermal contact resistance is time-dependent and non-uniformly spread. However, the heat evacuation is relatively uniformly spread while the heat power increases. Tests carried out with the second experimental set-up point out huge temperature oscillations which are attributed to unstable thermal contact conditions. This thermal behaviour can be explained by the buckling of the tin crust. The apparition of buckling seems to be favoured by a higher dissipated power and by a higher thermal conductivity of the melting pot.     

Etude numérique des effets de densité dans un jet de mélange turbulent en microgravité

Numerical study of density effects on a turbulent mixing jet in microgravity. The aim of this paper is to present density effects on a turbulent heterogeneous mixing free jet in microgravity. The influence of the inlet density ratio S between the jet and the surrounding gas is pointed out from numerical simulations based on second order single point closure schemes. The mean quantities, entrainment, expansion and second order moments are presented. The density ratio varies from 0,14 to 5,11. Results are discussed in comparison with both experimental data and asymptotical considerations. It is shown that the density ratio effects on the jet development vanish far from the exit leading to self-similar profiles (full self-preservation according to George [1]).     

Modélisation de la décharge luminescente dans un écoulement de gaz en régime turbulent

In this paper we propose a model of a glow discharge in a turbulent flow. The electron density is calculated using a conservation equation. We assume that the gas glow acts on the electron density and the Shwartz model is used to model the change of diffusivity due to turbulence. In order to show the effects of the turbulence on the electron density, we use a 1D model of a stable electric discharge in to a turbulent flow. The model shows that the increase in turbulent diffusivity at high Reynolds numbers tends to flatten the electron density profiles. Theoretical results are in good agreement with the reported measures. Next, the model was applied to a 2D argon axisymmetric turbulent compressible steady flow. This study shows that when plasma oscillations and turbulence fluctuations of the neutral gas are correlated the temperature profile flattens. Finally, we study electronic distribution into a 3D plasma column in a dissymmetrical flow.     

Simulation des transferts de masse et de chaleur par modélisation à bas nombres de Reynolds dans un écoulement avec effusion locale en canalisation

The turbulence model developed by Lam and Bremhorst with low values of Reynolds numbers is used to simulate a flow between two impermeable plate walls. This model, corrected by Yap, is able to simulate flow inside the boundary layer on the part of porous walls submitted to the blowing. This model of flow inside a channel with local blowing is coupled with a model of transfers through the impermeable walls and the porous wall, and with the environment. The global model is validated by comparison between experimental results from the literature and those obtained by the authors from experiences carried out with the test facilities of a subsonic thermal wind tunnel.A simulation of non-isothermal flow inside a channel with local blowing shows the attractive way of wall cooling by blowing. The optimal injection ratio is found equal to 0.01.     

Les études Séismologiques en Tchécoslovaquie de 1951 à 1953

Ohne ZusammenfassungRapport national présenté devant l'Association Internationale de Séismologie et de Physique de l'Intérieur de la Terre pendant la 10ème Assemblée Générale de l'Union Géodésique et Géophysique Internationale (UGGI) à Rome en Septembre 1954.     

Optimisation thermodynamique en temps fini du moteur de Stirling endo- et exo-irréversible

Finite-time thermodynamics optimisation of an endo- exo-irreversible Stirling Motor. This work deals with the finite time thermodynamic optimization of a Stirling engine. An endo- and exo-irreversible cycle is considered. The internal irreversibilities are due to the internal conductance of the plant and to the non-adiabatic regenerator. Also, the general irreversibilities of the non-quasistatic cycle are taken into account. The external irreversibilities are due to the linear interactions between the heat sources and the working fluids at finite temperature gaps.A sensitivity analysis was performed in order to optimize the operation conditions leading to maximum power output. The results obtained are in good agreement with the experimental data.     

Inversion angulaire de l'équation de transfert radiatif pour un milieu semi-transparent purement absorbant en présence d'une contrainte physique

We describe two methods to inverse the radiative transfer equation for a non-scattering medium in the presence of a physical constraint by means of angular data. Both techniques are based on a preliminary structural knowledge of the solution. This qualitative information is performed by analysing the direct problem. Two classes of inversion methods are considered with respect to the topological behavior of the set of physically admissible functions. An example using numerically simulated data is presented to demonstrate the practicability of our approach.     

Sur les possibiliti es de l'analyse par activation appliquée en médecine

An fur et à mesure que l'analyse par activation se développe dans toutes les branches de l,industric et de la science, en tant que méthode analytique actuelle, son application devient det plus en plus importante dans les domaines de la médecine, de la biologie et de la biochimie, ainsi que dans les domaines des recherches et des contrôles des radioéléments destinés à usage médical.     

Fluctuations de temperature et flux de chaleur en aval d'une source linéaire placée dans une couche limite turbulente

The purpose of the present communication is to present and discuss some recent experimental results concerning the dispersion downstream of a heated line source located, in a turbulent boundary layer, successively at two distances from an adiabatic wall. Information on the mean and fluctuating temperature fields and associated heat fluxes are presented and analysed by testing closure assumptions for model dispersion at first and second orders. This model, which only requires the use of temporal velocity scales of the flow, leads to a rather good agreement with measurements.     

Optimisation d'un cycle de Brayton moteur en contact avec des capacités thermiques finies

Optimization of a Brayton cycle engine in contact with fluid thermal capacities. Finite time thermodynamics studies have recently been devoted to all kind of cycles. Some works have been done on Brayton engine cycles.The proposed model is a synthesis of the preceding ones; this model completes the preceding papers, particularly regarding the two following aspects:

• 1.it takes into account the thermal losses occurring between heat source and sink, that are finite time reservoirs;
• 2.it influences the two limiting conditions for fluids at source and sink (four cases). The study is performed with partial and total regeneration, and without regeneration of heat. Influence of global internal irreversibility of the engine is also included in the sensitivity analysis of the proposed model.

Results are presented in a nondimensionnal form useful for design project. Limiting cases are enlightened by the proposed method, and allow us to disuss the whole set of previously obtained results.     

Auto-inflammation de substances explosives. Comparaison entre calcul analytique et numérique en vue d'une optimisation dans le domaine de la pyrotechnie

Self-ignition of energetic material was investigated in order to optimize safety in the field of pyrotechnic applications. Two approaches were used; the first one is relative to Frank-Kamenetskii stationary thermal explosion theory. The second approach consists of a choice of some numerical solutions of heat conduction equations in a non-stationary state. Comparison between these results was carried out in order to find the numerical scheme which is the most compatible with Frank-Kamenetskii stationary thermal explosion theory. Numerical data were used for three explosive substances. One of them was studied by the author. In all cases, the numerical stationary state is in agreement with the Frank-Kamenetskii stationary thermal explosion theory, more or less accurately. From this comparison, it may be concluded that it is preferable, for this kind of problem, to use an implicit scheme with linearization of the heat source term. Explicit numerical methods, with or without the addition of the heat term with the Zinn and Mader scheme are revealed to be less accurate and to need a greater optimization of spatial and temporal meshing.     

Étude phénoménologique de l'ébullition en espace confinéà partir d'un site de nucléation isolé

The aim of the present work is to identify the mechanisms of heat transfer enhancement during natural convective boiling in confined spaces. The influences of gap-size (0.3 to 2 mm), of pressure (1 to 3 bar), of heating surface orientation (vertical or horizontal) and that of the number of nucleation sites (no site, single site or several sites) for boiling R-113 in a narrow channel are investigated. Results for unconfined boiling are also presented as a reference. It is shown that latent heat transfer is improved when the bubbles are squeezed against the heated wall. The heat transfer enhancement is greater for a vertical than for a horizontal orientation because the two-phase fluid velocity is increased when the gap-size decreases, which enhances the sensible heat transfer. Finally, the confinement reduces the effect of an increase in pressure, which limits the influence of confinement. Indeed, both parameters lead to contrary effects on the bubble diameter.