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 mix- ture of 94 vol%-pentane and 6 vol%-isohexane at different cooling temperatures in the microgravity envi- ronment 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 tem- perature 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 coef- ficient 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 liq- uid 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.
Research Containing: Marangoni flow
This study reports experimental evidence of gas phase micro-convection induced by support fibers used in droplet combustion experimentation. Soot aggregates formed during combustion of n-octane and n-decane droplets (initial diameters ranging from 0.5 mm to 5 mm) provide natural seeds to reveal the thermal and flow asymmetries involved. The experiments are carried out in an environment that reduces the influence of forced and buoyant convection for both free-floating (unsupported) and fiber-supported droplets. Under these conditions, the soot trapping patterns (due to a balance of thermophoretic and flow-induced drag) would be spherical. However, this situation is only observed for unsupported droplets, or for fiber-supported droplets when the fiber is small relative to the droplet diameter. For Do < 1 mm a ground based drop tower employed two 14 μm diameter SiC fibers to fix the droplet’s position during burning; unsupported droplets were also examined. For Do > 1 mm the International Space Station provided capabilities for anchoring test droplets onto a single 80 μm SiC fiber, and for deploying unsupported droplets. Results clearly indicate that a non-symmetric gas flow field exists in some cases (i.e., for 1 mm < Do < 3 mm, with an 80 μm fiber) near to where the fiber enters the droplet. This gas motion originates from the presence of the fiber that introduces asymmetries in the temperature and flow fields resulting in localized force imbalances on the soot particles, which cause vortical flow patterns near the fiber. This may in part be explained by flow asymmetries induced by droplet shape distortions coupled with heat exchanges between the fiber and surrounding gas and conduction into the droplet, resulting in a Marangoni flow near the droplet surface. For very small fibers (or for unsupported droplets) spherical soot shells are found suggesting that no thermal and flow asymmetries exist.
The Constrained Vapor Bubble (CVB) experiment concerns a transparent, simple, "wickless" heat pipe operated in the microgravity environment of the International Space Station (ISS). In a microgravity environment, the relative effect of Marangoni flow is amplified because of highly reduced buoyancy driven flows as demonstrated herein. In this work, experimental results obtained using a transparent 30 mm long CVB module, 3 mm x 3 mm in square cross-section, with power inputs of up to 3.125 W are presented and discussed. Due to the extremely low Bond number and the dielectric materials of construction, the CVB system was ideally suited to determining if dry-out as a result of Marangoni forces might contribute to limiting heat pipe performance and exactly how that limitation occurs. Using a combination of visual observations and thermal measurements, we find a more complicated phenomenon in which opposing Marangoni and capillary forces lead to flooding of the device. A simple one-dimensional, thermal-fluid flow model describes the essence of the relative importance of the two stresses. Moreover, even though the heater end of the device is flooded and the liquid is highly superheated, boiling does not occur due to high evaporation rates.