Revue Générale de Thermique最新文献

The purpose of the present paper is to develop a mathematical model for predicting thermal field and heat transfer inside short length/height ratio, rectangular and nonhomogeneous material pipe connecting a combustion chamber with outdoors, while the gas combustion temperatures are in the range of 2 000 to 3 000 K and the pipe is made of heat-resistant material. The used approach is based on a finite difference simulation and makes certain idealizations and approximations in order to get to an engineering workable and at the same time adequate, solution. Main assumptions are an equivalent temperature cavity to describe radiant heat transfer due to both the combustion chamber luminous flame and outdoors.

The proposed model shows a very good agreement with experimental results coming from an experimental set-up used for materials testing, that is embodied in a refractory rectangular pipe, at high temperature.

The structure of turbulent flow within a rotor-stator enclosure is investigated using two different methodologies. The first one consists of integrating the unsteady Navier-Stokes equations to determine its statistical moments. The numerical algorithm integrates the equations in stream function-vorticity formulation using second order space centred approximations and a second order time stepping scheme. This algorithm was implemented on a parallel machine through an algebraic multi domain decomposition technique. The second consists of integrating the Reynolds averaged equations with a κ-ε model accounting for rotation effects. After a brief discussion of the space-time structure of the unsteady solutions, comparisons between the results produced by both methods are performed. We analyze the differences on the turbulence kinetic budgets whose main consequence is an underestimation of the angular velocity in the core region.

This paper is an assessment of a new discrete-ordinates algorithm recently developed by the authors for the numerical treatment of radiative participating media in both two- and three-dimensional enclosures. The algorithm is based on the utilization of general characteristic relations instead of the traditional differencing schemes for the spatial marching procedure. It is ideally suited for the treatment of complex geometries, the grid being formed from triangles (2D) or tetrahedra (3D). The method is exempt of any numerical oscillation and may be readily interfaced with the finite-element method for the solution of problems involving other modes of heat transfer. The mathematical derivation is detailed in the text and several examples are given for complex enclosures. The method proves to be very accurate and of good flexibility.

The results of an experimental investigation concerning the heat transfer from three cylindrical heaters to a water jet are reported in the form of correlating equations, which express the Nusselt number versus the Reynolds, Prandtl and Grashof numbers and some dimensionless ratios characterising the configuration. As the experienced range of the thermal flux is wide (2·10⁴ ≤ q̇ ≤ 6·10⁵ W·m⁻²), the influence of the free convection, which was shown to be negligible in previous studies, is carefully investigated in the present one. This influence appears still negligible up to the maximum value of q̇ for the heater impinged by the jet; on the contrary it is remarkable for the heaters lying in its wake. Another aspect which is carefully studied is the influence both of the ratios characterising the configuration and of the impingement direction: accordingly the values of these ratios and the kind of impingement which yield the maximum Nusselt number are clearly singled out. The investigation is completed by some visualization experiments which allow us to qualitatively clarify some aspects of the interaction between the dynamic and thermal fields.

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.

The present paper reports the results of a visualization study of the burnout in subcooled flow boiling of water, with square cross-section annular geometry (formed by a central heater rod contained in a duct characterised by a square cross-section). In order to obtain clear pictures of the flow phenomena, the coolant velocity is in the range 3–9 m·s⁻¹ and the resulting heat flux is in the range 7–13 MW·m⁻². From video images (single frames were taken with a light exposure of 1 μs) the following general behaviour of vapour bubbles was observed: when the rate of bubble generation is increasing, with bubbles growing in the superheated layer close to the heating wall, their coalescence produces a sort of elongated bubble called a vapour blanket. One of the main features of the vapour blanket is that it is rooted to the nucleation site on the heated surface. Bubble dimensions, as well as those of the hot spots, are given as a function of thermal-hydraulic tested conditions.

Assessment of such a measure has been revealed to be especially difficult due to the lake of mathematical criteria applicable to conservative systems. Existing tools are mainly based on the characterization of the ‘strangeness’ of the attractors. These asymptotic measures are thus limited to dissipative systems. We have adapted some of these tools in order to apply them to conservative systems, based on short time observations of the system instead of asymptotic observations. In this study we have used the alternating Dean flow as a benchmark and on the basis of which the mathematical tools have been constructed. Instead of observing an attractor, we suggest observing the image formed by the cross section of a tracer filament injected upstream of the flow. Such an image is simulated by using a numerical model for the flow. With the image, we evaluate the ‘information dimension’ as well as the ‘integral correlation dimension’. Since we use short time observations, the dimensions depend on the initial position of the injected filament. However, their evolution follows the apparent disorder observed on the images. As a global measure of the chaotic behavior, we suggest calculating the mean value of the dimensions for all injection positions defined as ‘mean integral correlation dimensions’.

Combined radiation and natural convection in a participating medium between concentric or vertically eccentric horizontal cylinders is investigated numerically. The annular medium is considered as a gray, emitting, absorbing, and isotropically scattering gas. The equations of steady, laminar, two-dimensional, thermal, natural convection are written by using a two-cylindrical coordinate system, the stream function, and the vorticity. The finite volumes method is used to discretize the coupled equations of momentum, energy, and radiative transfer. To solve the global nonlinear algebraic equations the successive-over-relaxation iterating scheme is applied. Numerical solutions are obtained for a Rayleigh number in the range 10³–10⁵ and radiation-conduction parameter ranging from 0 to ∞. The influences of radiation-conduction parameter, Rayleigh number and other parameters on flow and temperature distributions and heat transfer are discussed.