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Here, we demonstrate that heat removed in pool boiling from a heater mimicking high-power microelectronics could be used to facilitate a swing-like motion of the heater before being finally dissipated. This swing-like motion could be beneficial for shedding a large vapor bubble that encapsulates high-power heaters in microgravity where buoyancy force is unavailable for vapor bubble removal. The swing-like motion is propelled by vapor bubble recoil, the force which exists irrespective of gravity and buoyancy. We also demonstrate that this force could be significantly enhanced by depositing on the heater surface supersonically blown polymer nanofibers with cross-sectional diameters below 100 nm. These nanofibers provide additional nucleation sites, resulting in much more frequent bubble nucleation and departure, and thus a higher overall vapor recoil force propelling the heater motion. Such nanofibers strongly adhere to the heater surface and withstand prolonged harsh pool boiling. The measured velocity of the model swing-like heater in Novec 7300 fluid is about 1 cm/s.

Pool boiling of ethanol and self-rewetting fluids on bare copper surface and copper surface with polymer nanofibers were studied experimentally. No significant effect of the depths of ethanol layer on the heat removal rate was found. That indicates that only the heat transfer in the liquid microlayer near the heater surface is a dominant factor. As a result, one can expect that selfrewetting fluids can significantly affect boiling performance. Accordingly, several alcohol solutions including the self-rewetting ones were investigated as working fluids in the boiling chamber. It was found that at the 0.1% (v/v) concentration, only the high carbon-alcohol, nheptanol in aqueous solution, improved boiling heat transfer considerably. Furthermore, the experimental study of the effect of surface nano-texture on boiling characteristics was undertaken. For that aim, polyacrylonitrile (PAN) nanofibers were deposited onto the copper heater surface. Measurements of the boiling curve revealed a detrimental effect of such nano-texture in the case of such working fluids as ethanol and self-rewetting n-heptanol solutions. On the other hand, when polystyrene (PS) nanofibers were deposited onto the copper heater surface instead of PAN nanofibers, a significant improvement in boiling heat transfer was observed. The more hydrophobic nature of PS compared to copper is responsible for this effect, i.e. is the reason of the heat transfer enhancement on such a nano-textured surface compared to the pure copper one. In addition, the critical heat flux in the case of n-heptanol solution was found to be reduced considerably on the PS nano-textured surface compared to the pure copper one. This stems from the increased propensity of the heater surface to be covered by vapor, while the rewetting is insufficiently effective at high heat fluxes in presence of PS nanofibers.

No abstract

The present work aims at the experimental investigation of the effect of an increased thermal bonding pressure on the melting point (the so‐called Clapeyron effect) of three polymers employed in nonwovens. Namely, this work quantifies the dependence of melting temperature on pressure in these polymers. The following three polymers were used in the present experiments: polybutylene terephthalate (PBT), polyethylene terephthalate (PET), and polypropylene (PP) (all three already received in the form of nonwovens). A simple novel method of measurements of melting points of such polymers under different pressures was proposed and developed. The results revealed: (i) the melting point of PBT nonwovens increased by about 12°C when the applied pressure was increased up to 277.79 atm; (ii) the melting point of the PET nonwovens increased by about 7°C when the applied pressure increased up to 104.86 atm; (iii) the melting point of PP nonwovens increased by about 6°C when the applied pressure was increased up to 104.86 atm. The melting temperature measurements by the present method were also validated through differential scanning calorimetry measurements with the above‐mentioned three polymers without applied pressure.