Etude du transfert thermique local et identification des structures d'écoulement lors de la condensation dans un microcanal en silicium.

摘要

The use of micro-channels offers an advantage of contributing to a significant increase in the compactness of heat exchangers and improves systems energy performance. The study of two-phase flow patterns and local heat transfers represent a real scientific obstacle given its effect on the life and energy performance of energy systems such as fuel cells and miniature coolers. Unfortunately, the appearance of hydrodynamic flow and heat transfer (measurement of heat flux densities and local heat exchange coefficients) in a single micro-channel is still unclear. As part of this thesis, we are interested in studying the various phenomena occurring during condensation in a single micro-channel by identifying the various hydrodynamic instabilities and deducting the various physical mechanisms influencing heat transfer coefficients. To this end, we developed a test bench for testing the condensation in micro-channels and wherein the micro-channel is instrumented with micro-thermocouples 20 µm in diameter. This aspect of micro-instrumentation represents a genuine originality of this work because it allows to measure local surface temperatures along the micro-channel. A speed camera is used for visualization of flow patterns in condensation occurring in micro-channel. An image processing procedure is developed to characterize the different parameters of the two-phase flow in the micro-channel. i.e. : bubble size, range of bubbles, the meniscus shape, speed and frequency of bubble, etc... The influence of these parameters on the flow patterns and the intensification of transfer are studied. It is shown that the presence of unsteady flows and cyclical change structure during each period. The temperature variation for each period is related to the structure of the condensate flow in the micro-channel. We also identified different developed flow structures. We also demonstrated that the local thermal flux density depends not only on the mass flux and condensation rate but also on the structure of the flow condensation. Finally, our results demonstrate the influence of microstructure on the surface flow structure during condensation in a micro-channel and provide new methods for improving the heat intensification.