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.

A constrained vapor bubble heat pipe experiment was run in the microgravity environment of the International Space Station. Here we present the initial results that demonstrate significant differences in the operation of the constrained vapor bubble heat pipe in the microgravity environment as compared to the Earth's gravity. The temperature profile data along the heat pipe indicate that the heat pipe behavior is affected favorably by increased capillary flow and adversely by the absence of outside convective heat transfer as a heat loss mechanism. The reflectivity pattern viewed through the transparent quartz wall documented complex microflow patterns. Image data of the liquid profile in the grooves of the heat pipe indicate that the curvature gradient giving capillary flow is considerably different from that on Earth. Using experimental data for the temperature and meniscus profiles, a one-dimensional model gives the inside heat transfer coefficient, which was significantly higher in microgravity. An initial discussion of some of the data collected is presented.

The Constrained Vapor Bubble (CVB) is a wickless, grooved heat pipe and is the first, full-scale fluids experiment flown on the U.S. module of the International Space Station. The CVB promises to provide new insight into the operation of a heat pipe in space. It is a relatively simple device constructed from a spectrophotometer cuvette and uses pentane as the working fluid. The pentane flows within the corners of the cuvette due to a curvature gradient in the liquid menisci associated with the cuvette corners. The curvature of the liquid interface can be determined by viewing the meniscus through the transparent quartz walls. Extremely accurate temperature and pressure measurements were obtained in addition to the images. In the article, the results from the first two CVB modules-a dry calibration module and a wet heat pipe module-are presented. We show that the axial temperature profiles are significantly different in space. The heat pipes were seen to operate at a higher pressure and higher temperature in space primarily because radiation was the only heat loss mechanism. A fin model was developed to model the data, and Churchill's correlations for natural convection were used to determine the external heat transfer coefficient. Inside evaporation and condensation heat transfer coefficients were regressed from the temperature data. We show that the heat transfer coefficient in microgravity was higher.

The constrained vapor bubble (CVB) experiment is an experiment in thermal fluid science currently operating on the International Space Station. Flown as the first experiment on the Fluids Integrated Rack on the Destiny module of the US part of the space station, the experiment promises to provide new and exciting insights into the working of a wickless micro heat pipe in the micro-gravity environment. The CVB consists of a relatively simple setup - a quartz cuvette with sharp corners partially filled with pentane as the working fluid. Along with temperature and pressure measurements, the curvature of the pentane menisci formed at the corners of the cuvette can be determined using optical measurements. This is the first time the data collected in space environment is being presented to the public. The data shows that, while the performance of the CVB heat pipe is enhanced due to increased fluid flow, the loss of convection as a heat loss mechanism in the space environment, leads to some interesting consequences. We present some significant differences in the operating characteristics of the heat pipe between the space and Earth's gravity environments and show that this has important ramifications in designing effective radiators for the space environment.

The Constrained Vapor Bubble (CVB) experiment was run in the microgravity environment of the International Space Station as part of the Increment 23-24 which ended in September 2010. Here we present preliminary results which indicate significant differences in the operation of the CVB heat pipe in the micro-gravity environment as compared to the Earth's gravity. The temperature profile data along the heat pipe indicate that the heat pipe behavior is affected favorably by increased capillary flow and adversely by the absence of convective heat transfer as a heat loss mechanism. Image data of the liquid profile in the grooves of the heat pipe indicate that the curvature gradient is considerably different from that on Earth. An initial discussion of the data collected is presented.

The constrained vapor bubble experiment scheduled to fly aboard the International Space Station in the near future promises to give us new insight into the fundamental science of interfacial thermophysics. The evaporating meniscus formed at the corner of the vapor bubble is expected to behave in a significantly different manner in the microgravity environment as compared with the Earth's gravity environment. Since the constrained vapor bubble can also behave as a micro heat pipe, it will additionally help in gaining a technical understanding of the performance of a micro heat pipe in a space environment. Earth-based experiments have been conducted for the past two decades to gain a better knowledge of the rich phenomenon observed in the relatively simple constrained vapor bubble setup. Here, some recent Earth's-gravity-environment-based data obtained on a 30-mm-long constrained vapor bubble have been presented. The data were fitted to a model, and a self-consistent value of the inside heat transfer coefficient was obtained. The external convective and radiative heat transfer coefficients were also determined. These ground. based experiments form a calibration against which the future data from space-based experiments will be compared.

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