(47b) Computational Study of 3-Phase Contact Line: Effect of Oscillations on Heat Transfer

Micro-scale transport phenomena are critical to understanding the performance of high heat flux dissipation devices in microelectronics, where the hotspots created require that each chip be cooled individually. Heat pipes have been an attractive choice for high heat flux operations because of their compact, low-maintenance design. Heat pipes are passive heat transfer devices that transport energy from one end to the other based on phase change and capillary flows. Evaporation from extended menisci at the heated end of a heat pipe drives the first phase change process. Experimental measurements have shown that as the heat input to the meniscus increases, the film becomes unstable, and the meniscus oscillates. Figure 1 shows a strip-chart representation of a slice through an extended meniscus. The x-axis represents time and the patterns shown are interferograms outlining the film thickness profile.

Simulating the spontaneous oscillation of a meniscus is a difficult task due to the large number of variables that may be responsible and the mathematical acumen required to follow the multiple solution paths that may exist. However, we were interested in one particular question, does an oscillating film increase or decrease the rate of heat dissipation? We used the evolution model developed originally by Ruckenstein and refined by Davis and Bankhoff to describe the film thickness profile. The previous authors already showed that the adsorbed film ahead of the meniscus would be unstable if Marangoni and gravity forces overwhelm the intermolecular forces holding the liquid onto the surface. We developed a finite element formulation to solve the evolution equation in the form of a pair of partial differential equations, one describing the film thickness and the other the interface curvature. Two methods of externally forcing oscillation were used. In the first case, we manipulated the surface temperature and in the second case we explicitly manipulated the film thickness at discrete points in the domain. After reaching a steady-state, the outside solid wall temperature or film thickness was oscillated at different frequencies. In either case the overall film thickness responded to the changes with oscillations being larger at lower frequencies and damping out as the frequency increased and the film could no longer respond to the imposed changes. The heat flux profiles at the solid-liquid boundary show that there is an increase, followed by a sharp drop in the heat flux at certain frequencies of oscillation with time. We will report on both methods of oscillation, which frequencies result in enhanced versus decreased overall heat transfer, and where perturbations in the film thickness profile lead to the largest increases or decreases in the net heat transfer rate due to evaporation.

Figure 1 Strip chart representation of the film thickness profile as a function of time