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Understanding the fluid dynamics and the phase change heat transfer process within a thin liquid film is important to improve the performance of many industrial processes like coating or distillation. Studies by our group and other research teams showed that thin liquid films begin to oscillate spontaneously as the heat flux increases. We also found that the oscillation amplitude and frequency increase with increasing heat input. This implies that there is a heat transfer advantage to an oscillating thin film. We developed a numerical model to try and understand if there is an advantage to oscillation and under what conditions that advantage occurs. We found that oscillation can enhance net evaporative heat transfer but only if a short period of condensation exists within each oscillation cycle. Such condensation can be driven by intermolecular forces, capillary forces, Marangoni forces, or combinations of all three as we concluded from recent heat pipe experiments. Condensation increases the liquid film thickness at the contact line, and therefore decreases the disjoining pressure impediment to evaporation. These short condensation periods followed by fast evaporation appear as “spikes” in the liquid film thickness over time. These “spikes” were observed experimentally and mimicked by the simulations. Our calculations also show that the heat transfer efficiency increases with increasing oscillation frequency and amplitude in qualitative agreement with experiments.

The Constrained Vapor Bubble (CVB), a simple, wickless, heat pipe design that depends on interfacial forces to drive corner flow in a square cuvette, was studied in the microgravity environment aboard the International Space Station (ISS). In this paper, we consider the effects of different condenser temperatures on the heat transfer and fluid flow behavior using pentane as the working fluid. As the condenser temperature was decreased, the performance of the system decreased. This performance decrease using the pure working fluid was opposite to the behavior observed when using a mixture of 94 vol% pentane and 6 vol% isohexane. The mechanism for the decline in performance as the condenser temperature was lowered was a stronger than expected increase in the apparent strength of Marangoni flows at the heater end of the system. A simple mathematical model was fit to the experimental data and used to extract an evaporator heat transfer coefficient for experiments where we held the condenser temperature constant while increasing the heater power and where we held the heater power constant while decreasing the condenser temperature. All the results could be collapsed onto a single Nusselt number vs. Marangoni number curve. In this formulation, the Nusselt number was found to decrease with increasing Marangoni number to the 1/3 power.

Understanding the dynamics of phase change heat and mass transfer in the three-phase contact line region is a critical step toward improving the efficiency of phase change processes. Phase change becomes especially complicated when a fluid mixture is used. In this paper, a wickless heat pipe was operated on the International Space Station (ISS) to study the contact line dynamics of a pentane/isohexane mixture. Different interfacial regions were identified, compared, and studied. Using high resolution (50×), interference images, we calculated the curvature gradient of the liquid-vapor interface at the contact line region along the edges of the heat pipe. We found that the curvature gradient in the evaporation region increases with increasing heat flux magnitude and decreasing pentane concentration. The curvature gradient for the mixture case is larger than for the pure pentane case. The difference between the two cases increases as pentane concentration decreases. Our data showed that the curvature gradient profile within the evaporation section is separated into two regions with the boundary between the two corresponding to the location of a thick, liquid, “central drop” region at the point of maximum internal local heat flux. We found that the curvature gradients at the central drop and on the flat surfaces where condensation begins are one order of magnitude smaller than the gradients in the corner meniscus indicating the driving forces for fluid flow are much larger in the corners.

The effect of cooling temperature on heat pipe performance has generally received little consideration. In this paper, we studied the performance of a Constrained Vapor Bubble (CVB) heat pipe using a liquid mixture of 94 vol%-pentane and 6 vol%-isohexane at different cooling temperatures in the microgravity environment of the International Space Station (ISS). Using a one-dimensional (1-D) heat transfer model developed in our laboratory, the heat transfer coefficient of the evaporator section was calculated and shown to decrease with increasing cooler temperature. Interestingly, the decreasing trend was not the same across the cooler settings studied in the paper. This trend corresponded with the change in the temperature profile along the cuvette. When the cooling temperature went from 0 to 20 °C, the temperature of the cuvette decreased monotonically from the heater end to the cooler end and the heat transfer coefficient decreased slowly from 456 to 401 (W m −2 K −1 ) (at a rate of 2.75 W m −2 K −2 ). However, when the cooling temperature increased from 25 to 35 °C, a minimum point formed in the temperature profile, and the heat transfer coefficient dramatically decreased from 355 to 236 (W m −2 K −1 ) (at a rate of 11.9 W m −2 K −2 ). A similar change in decreasing trend was observed in the pressure gradient and liquid velocity profile. The reduced heat pipe performance at high cooling temperatures was consistent with the reduced evaporation which was indicated by the decreasing internal heat transfer and the increasing liquid film thickness along the cuvette as seen in the surveillance images. The result obtained is important for future heat pipe design because we now have a better understanding of the working temperature ranges of these devices.

The Constrained Vapor Bubble (CVB) is a transparent, wickless heat pipe experiment carried out in the US Labs of the International Space Station (ISS). Experiments were carried out using the 40 mm CVB, 3 mm× 3 mm in cross-section, pentane as the working fluid, with the power inputs of up to 3 W. Due to the low Bond number (Bo) in microgravity and materials of construction, the CVB system was ideally suited to determine the contribution of the Marangoni forces toward the limiting heat pipe performance, and the transparent quartz shows exactly how that limitation occurs. Previous literature models and experimental temperature and pressure measurements suggested that at high enough temperature gradients, the working fluid should be subjected to enough Marangoni force to force it away from the heater and ultimately, dry out the hot end. The CVB experiment shows that high temperature gradients lead to a totally opposite behavior, i.e., ‘flooding’ of the heated end. Flooding of the heater end is attributed to a competition between Marangoni-induced flow due to high temperature gradients at the heater end and capillary return flow from the cooler. This creates a thick liquid layer in the corner of the cuvette at the heater end. At the point of flow balance, a thick layer of liquid is observed on the flat surface of the quartz cuvette. This is defined as the central drop. The region from the top of the heater end to the central drop is referred to as the interfacial flow region. The interfacial flow region develops at a power input of around 0.7 W, and increases in length to the power input of 2 W. At 2 W, the strength of the Marangoni forces saturate. As a result, the forces in the flooded interfacial region are not able to push the liquid further into the capillary region and a further penetration of liquid down the axis of the heat pipe is arrested. As the power input is increased to nearly 3 W, an increase in the vapor space is observed near the heater end at 3 W. This behavior suggests that the flooding might just be an intermediate stage in reaching the dry-out limitation. The flat quartz surface at the hot end is covered by a wavy thin liquid film due to the interfacial forces. The hot end region closest to the heater is a superheated vapor region that leads to the condensation. This additional observation is discussed in Appendix.

Previous studies on wickless heat pipes showed that a temperature induced “Marangoni flow” prevents liquid from recirculating to the heater end, and therefore reduces the effectiveness of the heat pipe. Recently, several research groups used a water and alcohol mixture, with a low concentration of alcohol, resulting in better performance of the heat pipe. The alcohol/water combinations were peculiar in that for a certain composition range, the surface tension increases with increasing temperature thereby driving liquid toward the hotter end. It was believed that changing the direction of the Marangoni stress or reducing its magnitude by differential evaporation of an ideal binary mixture would also improve the performance of the heat pipe. For the first time, an ideal fluid mixture of 94 vol%-pentane and 6 vol%-isohexane was used as the working fluid in the Constrained Vapor Bubble (CVB) heat pipe experiment on the International Space Station (ISS). Using a simple heat transfer model developed in our laboratory, an internal heat transfer coefficient in the evaporator section was determined and shown to be almost twice that of the case where pure pentane was used under the same conditions. The Marangoni stress in the mixture was five times lower. Interestingly, reducing the Marangoni stress led to less liquid accumulation near the heater end and surveillance images of the device, taken at the steady state, showed that the bubble gets much closer to the heater end in the mixture case instead of being isolated from the heater by a thick liquid pool as in the pure pentane case. The proximity of the bubble to the heater wall led to more evaporation at the heater end in the mixture case, and therefore a higher heat transfer coefficient. The pressure profile calculated from the Young–Laplace equation supports the observations made from the surveillance images.

The liquid-vapor distribution and its effects on the fluid dynamics and heat transfer occurring within a wickless heat pipe are little understood, especially in a microgravity environment. Such information is vital to the design of thermal management systems for deep space robotic and manned exploration missions, especially if unexpected behaviors arise. We observed an unusual analog of a terrestrial rip current during the operation of a wickless heat pipe on the International Space Station. The current arose spontaneously as the heat input increased, flowing along the flat surfaces of the device toward the heater end, and was driven by a pair of counterrotating vortices that formed from the interaction of opposing surface tension and capillary driven corner flows. The current served as a natural way of increasing the total contact line length within the device and this enabled higher evaporation rates than would have been possible based on the engineered geometry of the device alone.

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.