An experimental and numerical investigation of thermocapillary driven phenomena for evaporating menisci in capillary tubes related to microelectronics cooling

The present work is an experimental and numerical investigation of the hydrodynamics of a meniscus formed by a liquid in capillary tubes undergoing phase change. The non uniform evaporation process along the liquid-vapour interface leads to self-induced temperature field that in turn generates surface tension gradients along the meniscus. The interfacial stress so created drives a vigorous liquid convection that is measured by using a micro-Particle Image Velocimetry (micro-PIV) technique. Temperature measurements using advanced Infra Red (IR) and Thermochromic Liquid Crystal (TLC) techniques allowed us to measure the interfacial temperature profile (IR) and external capillary tube temperature (TLC). Measuring velocity and temperature at such scales is not trivial because conventional techniques such as thermocouples cannot be used. Therefore, efforts have been made to compare the measurements gathered from different techniques: TLC and IR external wall measurements for the temperature field and micro-PIV measurements and numerical analysis for the velocity field. The TLC temperature measurements show the important sink effect close to the meniscus near contact line region. By measuring the evaporation mass flux and performing a heat transfer analysis based on the measured wall temperature of the system, a very good agreement was found, giving confidence to the temperature measurements performed with TLC. IR measurement of the external capillary tube wall was also taken and compares well with the TLC ones. Tube sizes ranging from 200 to 1,630 mm and four volatile liquids were investigated. Different experimental studies were conducted with both vertically and horizontally oriented capillaries, with and without external heating. It is observed that symmetrical flow patterns found in horizontal diametrical sections of the tube are dramatically distorted in vertical diametrical sections, presumably due to gravity. The IR measurements of the liquid-vapour interface temperature revealed clearly the effect of the asymmetrical flow pattern on temperature field. The direction of the induced convection rolls can be reversed by applying a temperature gradient along the capillary wall. With the tubes plunged in a pool of liquid, it is interesting to note that providing external heating to the system can substantially change the convection patterns. In particular is was revealed that the flow is enhanced when the heating element lies below the meniscus; on the contrary, when the heating element is above the meniscus and sufficient power is provided, the flow pattern is reversed with respect to the unheated situation. This important result confirms the fact that the phenomena being studied are strongly dominated by the action of surface tension. The numerical code used is a FVM with coordinate transformation in order to take into account the curved meniscus. The code was used to first reproduce some of the results obtained experimentally and successively was used to investigate those cases not easily accessible to experiment. Good quantitative (when possible) and qualitative agreement was found between experimental and numerical results.;Results gained, both of velocity and flow map, meniscus temperature and finally numerical confirmation clearly lead to the conclusion that the heat and mass transfer in such small systems is strongly affected by the mechanism taking place in the meniscus contact line region.;In this work, two cases of practical interest were also examined. The first concerns a heat pipe module for micro-electronic cooling where the effect of surface coating on the fin stack unit was experimentally investigated. The second is an extension of existing heat pipe technology to cool a heavily thermally loaded nozzle of future spacecraft vehicles of high specific impulse. (Abstract shortened by UMI.).