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Current thermal fluid models, according to research observation did not predict thermocapillary-induced (counterintuitive) to heat-pipes performance. Heat pipes operate under various physical constraints that include boiling, capillary, entrainment, and sonic limits that significantly affect their performances. Heated end of the heat pipe usually exhibits high-temperature gradient that leads to the generation of Marangoni forces opposing the cold end return-flow of fluids. The Marangoni forces act by forcing premature attainment of capillary limits besides exacerbating dry-out conditions. By using thermal and image data from international Space Station experiments, this work shows the presence of Marangoni forces, mechanisms that limit performance are not limited to dry out but the physical cause that is the opposite behavior.
The empirical effects are the implications of competition between the Marangoni induced and capillary forces. In addition, the dry out and flooding have a virtually identical temperature signature that makes the examination complex without observing the vapor-liquid interface.
Heat pipes, also known as the passive heat transfer apparatus are used in the applications entailing a high level of heat flux, a process with no forces of the convention processes.
The desirability of Heat pipes are majorly in microgravity that entails the reliability and robustness of devices. Consequently, these devices are attractive where the gravity to the forces of surface tension, and low Bond (Bo) that allows the significant transfer of heat. Heat pipes, mostly found in laptops, primarily used as cooling devices for the microprocessor. Heat pipes work through a capillary action where evaporated liquid at heated regions condenses after flowing to cooled ends.
With the use of a wick through the capillary action, the condensed heat returns to the heated ends. Due to the environmental conditions in the International Space Station (ISS), the low bond was relatively small, reinforcing the interfacial forces, small transparent devices that allowed for the use of a wickless design (simple) applicable for low visualization(Peterson, 1994).
Heat-pipes have a complex, and well-developed equations that govern the performance and operations of limits. For instance, considering a vapor-liquid distribution in a confined geometry or three dimensional and isothermal have been calculated theoretically. At the heat end of a heat pipe, a large temperature gradient generates significant Marangoni force used in driving the liquid from a hot to a cold region due to the presence of low surface tension towards the hot region of the pipe compared to the cold that exhibits high surface tension. According to the contemporary studies of the heat pipes in fluid mechanics, the Marangoni forces induces wetting of fluids that act against the gravitational force by climbing the walls, exhibiting teardrop formations, and fingering instabilities useful in coating flows and enhancement to boiling(Pal, Kochat, & Ghosh, 2012). The detrimental possibilities of the Marangoni flow affecting the performance of capillary limits of heat pipes were theoretically studied. This led to an experimental fluid self-re-wetting that exhibits an increasing surface tension with an increase in temperaturethat restore the liquid's movement to heater ends of the device. However, no direct, visual, and detailed observation to verify the interfacial existence has ever been reported despite performance enhancement reports of using self-re-wetting fluid.
In an empirical investigation on interfacial phenomenon in a heat-pipe, a device (transparent) fused with Silica spectrophotometer cells of 20-30 mm length, 9mm2 cross section, and a simple wicklesswas developed. Series of tests and experiments were conductedusing heat-pipes in ISS microgravity environments. The ISS environment eliminated problems associated with liquid pooling or pumping the liquid against the forces of gravity, and negating internal and external flows attributed by Mother Nature convections. The US Destiny Module conducted the CVB (Constrained Vapor Bubble) housed within the FIR (Fluids Integrated Racks) that is a facility of fluid physics used for multiple purposes(Chatterjee, Plawsky, Wayner, Chao, Sicker, Lorik & Zoldak, 2013).
The Fluid Integrated rack consists of the LMM (Light Microscopy Module), an Automated Optical Microscope providing interferometry facility applied in obtaining in-depth images. The Constrained Vapor Bubble apparatus was placed on the LMM stage with the surrounding temperatures controlled by the cold plate that keeps it uniform. The experiment's orientation was along the ISS's y-axis, having2systems sensing acceleration that measures the transient acceleration(Liu, Guo, Xie, Liu & Luo, 2012). The MAMS (Microgravity Measurement System) and SAMS (Space Acceleration Measurement System) responds to the 0.001-400 Hz frequencyranges and a 0-1.01 Hz. The CVB is insensitive to jitter (g) at SAMS but averaging MAMS over the duration the experiment took exhibited low frequency in the y-axis acceleration to be 0.19.
Pentane was the heat pipe's working fluid, a simple van-der-Waals fluid wetting the silica fused surfaces of the silica(Pal, Kochat, & Ghosh, 2012). High resolutions of vapor-liquid mappingswere taken inthe device through interferometry. The Interferometry method measured the temperature levels along the x-axis using thermocouples, drilled in one glass wall, and measured the general internal pressure in figure 1. The Low Bow represented the ration of gravitational forces to the Surface tension. with p as the fluid's density and acceleration (g) due to the force of gravity. hrepresented the dimension(linear) such as the cell's half-width,surface tension of the liquid. Low and Earth ranged from 0.8 to 27 formed the basis where the experiments requirements were designed. The experiment's design was to drive and enhance the device with the hope of attaining the capillary limits. Consequently, this was also to attain, at the pint of dry out release, the vapor-interface image. According to Chatterjee (2013), the power inputs required driving the temperature of the heated wall below auto-ignition pentane temperatures of 533K or raising the pressure to 345Pa were the safe limits. Figure 2with temperature profiles attained from the highest power input level with the temperature gradient saturating indicated the device had dried pit and attained the capillary level (limit).
Through operating the transparent wickless heat-pipe under the ISS microgravity environments, it is evident that the previous limitations to the microgravity operated heat pipe are not among classically predicted limits. Instead of boiling or drying out at the heated points, the Capillary and Marangoni forces flooded the heated points of the pipe thus inducing opposite traits degrading the experiment's performance. According to Plawsky & Wayner (2012), Dry outs and the temperature signatures of the floodinghad close resemblance, perhaps why it led to opaque heat pipes' misdiagnosis. Driving it harder will break down the flooding phenomenon. However, continuous ISS's experiments will be needed to report what happens at high pressure, temperature, and heat inputs. The experimented model did not discuss the supplemental material substantively, thus unable to produce the observed phenomena in space(Plawsky & Wayner, 2012)
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