Thermocapillary Phenomena and Performance Limitations of a Wickless Heat Pipe in Microgravity

Abstract

A counterintuitive, thermocapillary-induced limit to heat- pipe performance was observed that is not predicted by current thermal-fluid models. Heat pipes operate under a number of physical constraints including the capillary, boiling, sonic, and entrainment limits that fundamentally affect their performance. Temperature gradients near the heated end may be high enough to generate significant Marangoni forces that oppose the return flow of liquid from the cold end. These forces are believed to exacerbate dry out conditions and force the capillary limit to be reached prematurely. Using a combination of image and thermal data from experiments conducted on the International Space Station with a transparent heat pipe, we show that in the presence of significant Marangoni forces, dry out is not the initial mechanism limiting performance, but that the physical cause is exactly the opposite behavior: flooding of the hot end with liquid. The observed effect is a consequence of the competition between capillary and Marangoni-induced forces. The temperature signature of flooding is virtually identical to dry out, making diagnosis difficult without direct visual observation of the vapor-liquid interface.

Condensation on Highly Superheated Surfaces: Unstable Thin Films in a Wickless Heat Pipe

“…Here, we demonstrate that condensation of the working fluid on the superheated surface of a wickless heat pipe in microgravity first appears beyond a threshold thermal load to the device and is the source of the flooding phenomenon reported earlier [26]. As we increase the thermal load and raise the temperature at the heated end, the amount of condensed liquid increases.…”

“…The interfacial region is located between the heater end and the "central drop" and first appears at a power input between 0.6 -0.7 W. At larger power inputs, the temperature gradient was high enough to generate a significant Marangoni flow that drove the liquid away from the heater end. Instead of the heater end becoming dry, liquid accumulated there and reduced the performance of the heat pipe [26,34]. In the sharp corners of the device, the competition between the capillary return flow and the Marangoni flow formed a junction vortex leading to a thick liquid "drop" that formed on the flat surface of the cuvette.…”

“…The experimental details have been described in previous publications [26,[33][34][35][36][37] and a synopsis is given in the Supplemental Information section [38]. Figure 1 shows the temperature profiles in a 40 mm, microgravity CVB experiment carried out using a cold finger temperature of 292 K and electrical power inputs to the heater of 1.6 to 3W.…”

“…They are a measure of the average vapor temperature. The green markers are the location of the central drop obtained from the vapor-liquid interface images [26] and are the points of maximum evaporation. At positions beyond the green markers, toward the cooled end, the system operates as a classic heat pipe.…”

“…Recently, microgravity experiments demonstrated that large Marangoni forces may have the opposite effect and lead to flooding of the heater end, not dryout [26].…”