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Bubble nucleation was investigated in a 20-mm-long, wickless heat pipe on the International Space Station. Over 20 h of running experiments using pentane as the working fluid, more than 100 nucleation events were observed. Bubble nucleation at the heater end temporarily boosted peak pressures and vapor temperatures in the device. At the moment of nucleation, the heater wall temperature significantly decreased due to increased evaporation and the original vapor bubble collapsed due to increased pressure. A thermal model was developed and using the measured temperatures and pressures, heat transfer coefficients near the heater end of the system were extracted. Peak heat transfer coefficients during the nucleation event were over a factor of three higher than at steady-state. The heat transfer coefficient data were all collapsed in the form of a single, linear correlation relating the Nusselt number to the Ohnesorge number.
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On an Averaged Energy-balance Method for the Analysis of Wavy Microchannels
This study presents numerical simulations of the convective heat transfer on wavy microchannels to investigate heat transfer enhancement in these systems. The objective is to propose a methodology based on local and global energy balances in the device, instead of the commonly used Nusselt number, as an alternative for the thermal analysis. This investigation is carried out on a single-wave microchannel model of size 0.5 mm by 0.5 mm by 20 mm length, with water flowing inside the channel, exposed to a heat influx of 47 W/cm2 at the bottom. The governing equations for an incompressible laminar flow and conjugate heat transfer are first built, and then solved, for representative models, with copper as the solid-block material under a number of operating conditions (cold-water flowrates of đť‘…đť‘’=50, 100, and 150), by the finite element technique. From computed velocity, pressure and temperature fields, local and global energy balances based on cross-section-averaged velocities and temperatures enable calculating the heat rate at each section of the corresponding device. Results from this study for two different designs, namely, serpentine and divergent-convergent layouts, show that this so-called averaged energy-balance methodology enables higher accuracy than that based on Nusselt numbers since neither transfer coefficients nor characteristic temperatures are needed.
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- PAR ID:
- 10422754
- Date Published:
- Journal Name:
- Heat and Mass Transfer
- ISSN:
- 0947-7411
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1007/s00231-022-03329-5
