Microscale Temperature Measurements at the Triple Line of an Evaporating Thin Film

Thin-film evaporation from a meniscus in a confined space, which is the basis for many two-phase cooling devices, is experimentally investigated. The meniscus formed by heptane, a highly wetting liquid, on a heated, fused quartz substrate is studied. Microscale infrared temperature measurements performed near the thin-film region of the evaporating meniscus reveal the temperature suppression caused by the intensive evaporation in this region. The high spatial resolution (~6.3 μm) and high temperature sensitivity (~20 mK) of the infrared camera allowed for accurate measurements. The effects of meniscus thickness and applied heat flux on the thin-film heat transfer distribution and rate are also explored. INTRODUCTION Evaporation from a meniscus serves as the basic mechanism in many two-phase cooling devices such as heat pipes, thermosyphons, vapor chambers, two-phase cold plates and capillary pumped loops. Thin-film evaporation, which takes place near a solid-liquid-vapor junction, has long been believed to be the dominant mode of heat transfer in such systems [1]. The efficacy of heat transfer in thin films is attributed to a high disjoining pressure gradient [2] which results in liquid being pulled into the thin-film region, as well as the very low thermal resistance resulting from the small film thickness. The intensive evaporation near the triple line creates a temperature gradient along the meniscus. This results in a surface tension gradient that gives rise to thermocapillary convection. Both the evaporation from the thin film as well as the thermocapillary convection induced have been reported in the literature [3,4] to play a major role in the total heat transferred. However, the exact role and nature of these processes is still not completely understood. Wayner and co-workers [ 5 ] carried out extensive theoretical and experimental studies in this field and delineated several important factors influencing thin-film evaporation. Ma and Peterson [6] developed experimentally verified models for evaporation from V-grooves to predict their maximum heat transport limit. Holm and Goplen [7] were the first to develop an approximate method for predicting heat transfer from capillary grooves. They found that nearly 80% of the total heat transfer takes place from the thin-film transition region. Stephan and Busse [8] developed a model to describe the heat and mass transport in the microregion of the meniscus in a Vgroove. Their numerical model agreed well with the measured heat transfer data and showed that up to 50% of the entire evaporation can take place in the microregion despite its small geometrical dimensions. Other researchers [ 9 , 10 ] have attributed as much as 90% of heat transfer to the thin-film region. However, Park and Lee [11] suggested that the thinfilm contributed less than 5% of total heat transfer due to its small geometrical extent. A recent model developed by Wang et al. [12] showed that 20% of the heat transfer takes place from the thin-film region but as much as 60% of the overall heat transfer takes place from a 1 μm thick micro-region. In terms of experimental investigations, interferometry and ellipsometry have been widely employed in the study of thin liquid films. Mirzamoghadam and Catton [13] focused on a 2D meniscus generated by an inclined, partially submerged, heated flat plate in a pool of liquid. They studied the temperature in the liquid using laser holographic interferometry. The general characteristics of the meniscus region, where the combined influence of natural convection, conduction, and surface tension gradients are important, were deduced. Xu and Carey [9] assumed that heat transfer occurs only through the thin-film region and developed a model whose results were in reasonable agreement with their experiment. Dasgupta et al. [ 14 ] studied a 2D extended meniscus of heptane using Proceedings of IMECE2007 2007 ASME International Mechanical Engineering Congress and Exposition November 11-15, 2007, Seattle, Washington, USA