Revue Générale de Thermique最新文献

New amorpheous hydrogenated carbon films have been applied successfully to promote dropwise condensation (DWC) of steam on metallic surfaces at atmospheric pressure. The highest heat transfer coefficients have been measured for completely coated surfaces, maximum contact angle, largest thermal conductivity of the base material, vertically oriented wall and minimum wall height. The effect of these parameters on the DWC performance is evaluated quantitatively. The investigation of partly coated surfaces shows, that even for a small portion of coated surface (approx. 20 % DWC and 80 % filmwise condensation (FWC)) still nearly maximum heat transfer is achieved. This phenomenon is explained qualitatively by the application of numerical simulation of the local condensation process using the finite element method (FEM). Furthermore, this analysis technique also explains the dependence of DWC heat transfer on the thermal conductivity of the base material being coated.

The temperatures which are present in a heat transformation device play a very important part: at first, the temperatures determine the maximum performance or efficiency of the cycle via the first and second laws of thermodynamics. Secondly, the temperatures determine the heat transfer area which is required to put a given heat flux through the system. Consequently, they relate power to investment cost. In order to elaborate further on these interdependencies, in this paper basic relationships between technically and thermodynamically relevant temperatures, as they are present in the heat exchangers, are being derived. To this end, we will define several temperature differences as usual: the temperature glide, the driving mean temperature difference and the thermodynamic or entropic mean temperature difference. The logarithmic temperature mean is significant for determining the heat transfer. It will be shown that, as long as the temperature gradient between external and internal fluids is larger than the difference in glide of both fluids, the log-mean can be substituted by the difference of the arithmetic mean temperatures. Consequently, it is almost identical to the entropic temperature difference. The entropic temperature difference is a measure of efficiency whereas the logarithmic temperature difference is a measure of first cost. As both temperature differences deviate only marginally from each other in most technical applications it will easily be possible to establish a relationship between performance and investment.

In the first part, the theoretical and experimental framework used to present the thermomechanical behaviour of solid materials is briefly recalled. The main feature of the experimental approach relies on the use of thermographical techniques allowing us to deduce, from the thermal data, the distribution of heat sources arising during the mechanical transformation. In the particular case of homogeneous thermomechanical tests, an energy balance can be performed and used to derive the behavioural constitutive equations. When heterogeneities occur, the infrared images facilitate the analysis of localization mechanisms. In the second part, basic aspects of homogenization techniques are reiterated. Related to thermomechanical couplings, homogenization improves the description of the behaviour of materials and structures in which microstructural phenomena have a significant influence at the macroscopic scale. Several finite element simulations are shown concerning the thermoviscoelasticity of polymers, the thermoelasticity coupled with damage in composites, and the pseudoelastic behaviour related to the solid-solid phase change of shape memory alloys.

Recent developments of new instruments for investigation or analysis originates in the efforts to miniaturize industrial products (microelectronics, mass storage, sensors…). First near-field microscopes (STM, AFM) have enabled an accurate surface observation. Emerging scanning microscopes based on photothermal or thermoelastic 3-D processes introduce specific contributions (thermal diffusivity, e.g.) and, especially, information about the close subsurface. The aim of this paper is, by using an heuristic approach, to propose an interpretation of super-resolved images and to predict the resolution and the investigation depth of these new near-field microscopes. The proposed approach extends the concept of near-field: 3-D dispersion of the waves limits the interaction distance and fixes the values of the investigation parameters. In a first part, some basic analogies between the theories associated with thermal, elastic and thermoelastic fields are placed in evidence. As predicted, the corresponding resolution is mainly related to the size of the excitation source but the thermoelastic images are less resolved. In the second part, super-resolution is experimentally demonstrated and some presently available images are discussed.

A numerical study has been conducted for natural convection dominated melting inside uniformly and discretely heated rectangular cavities. A computational methodology based on the enthalpy method for the phase change is first presented and validated with experimental data. The model is next employed to determine the effect of the source dimensions β and span η, of the aspect ratio of the cavity A and of the wall-PCM thermal diffusivity ratio $\text{α}$ on the melting process. Results show the benefits of using discrete heat sources instead of a uniformly heated wall. For high aspect ratio enclosures (A ≳ 4), configurations leading to well controlled source temperatures and relatively long melting times have been obtained. For cavities of aspect ratio A ≲ 4.0, the source span η is the most influential parameter. If η ≲ 0.45, the melting times are shorter and the temperatures of the sources remain equal and moderate during the melting process. Threshold values $\text{α}{}_{\mathrm{<mtext>min</mtext>}}$ above which melting becomes independent of the source distribution were determined for cavities of various aspect ratios.