Oscillating heat pipes : study of their oscillating motion using neutron imaging and application

Abstract The oscillating heat pipe is considered a promising candidate for high-efficiency and compact thermal control for next-generation electronics. In this paper, the visualized flow and heat transfer characteristics of two silicon-based micro oscillating heat pipes (micro-OHPs) with expanding and straight channels, respectively, were experimentally investigated. The overall size of these two micro-OHPs are both 28 mm × 23 mm × 1.025 mm and have thirty rectangular cross-section channels. The hydraulic diameter of parallel direct channel is 332.4 μm, while they are about 364.4 and 287.0 μm at the two ends of expanding channel, respectively. R141b was used as the working fluid with the volumetric filling ratio of 50%. Inside these two micro-devices, the fluid oscillating motion, including unidirectional movement and intermittent stopovers, was observed at the quasi-steady oscillation state, accompanied by bubbly flow, slug flow and annular/semi-annular flow in microchannels. The micro-OHP with expanding channels possessed better thermal performance and could achieve ephemeral circulation flow, while poorer heat transfer performance occurred for the micro-OHP with straight channels due to more localized slug/plug oscillations and intermittent stopovers. The oscillating amplitudes of liquid slugs are presented to estimate the flow behavior of working fluid inside micro-OHPs. The introduction of expanding channels in a micro-OHP is beneficial for realizing the more robust oscillating motion of liquid slugs with larger oscillating amplitudes for heat transfer enhancement.

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In situ Imaging at the NIST Neutron Imaging Facility

MRS Proceedings ◽

10.1557/opl.2011.917 ◽

2011 ◽

Vol 1318 ◽

Author(s):

David L. Jacobson ◽

Daniel S. Hussey ◽

Eli Baltic

Keyword(s):

Fuel Cells ◽

Hydrogen Storage ◽

Spatial Resolution ◽

Experimental Studies ◽

Heat Pipes ◽

Neutron Imaging ◽

Light Elements ◽

Storage Devices ◽

Tremendous Progress

ABSTRACTNeutron imaging as a method to perform in situ studies of hydrogen fuel cells, hydrogen storage devices, heat pipes, and batteries has made tremendous progress in recent years. Neutrons are useful to study light elements mixed with heavy Z elements where penetration by other forms of radiation is either impossible or incapable of contrasting the light elements. Useful spatial resolution available at neutron imaging facilities is now approaching 10 micrometers. Complimentary time resolution of 30 fps or greater is also possible with a spatial resolution approaching 300 micrometers. Here we will provide an overview of the technique of neutron imaging and experimental studies with neutrons at the National Institute of Standards and Technology. Examples of in situ studies of fuel cells, hydrogen storage devices, heat pipes and batteries will be discussed.

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An investigation into gravity independent oscillating heat pipes

10.32469/10355/43168 ◽

2012 ◽

Author(s):

◽

Aaron A. Hathaway

Keyword(s):

Heat Transfer ◽

Contact Angle ◽

Heat Pipe ◽

Heat Transfer Performance ◽

Heat Pipes ◽

Experimental Results ◽

Transfer Performance ◽

Channel Diameter ◽

Inverted Position ◽

Oscillating Motion

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] A number of oscillating heat pipes (OHPs) with unbalanced structures were investigated in an effort to develop methods enabling OHPs to be gravity independent. The unbalanced structures investigated herein include the effects of uneven turn and check valves. Two 2-D and two 3-D tubular uneven-turn OHPs, four miniature uneven-turn OHPs, and one OHP with check-valves were investigated. At the same time, the theoretical analysis of the maximum channel diameter was conducted in order to find the primary factor affecting the channel size in an OHP. A model was developed that attempts to determine the maximum channel diameter by considering the contact angle effect. It is found that the contact angle significantly affects the maximum channel diameter of an OHP. In order to verify that the uneven-turn structure can generate the oscillating motion in an OHP, a heat pipe with 3 turns in the condenser and 6 turns in the evaporator was first tested. The heat pipe with uneven turns can generate and maintain oscillating motion. When the turn number increases with 16 turns in the condenser and 20 turns in the evaporator, the heat transfer performance can be further increased. An experimental investigation of a new 3-D OHP with uneven turn design was conducted in order to further develop a gravity independent OHP. Experimental results show that the uneven turn OHP developed herein can start the oscillating motion in the negative vertical position (the evaporator being above the condenser) and demonstrate that the uneven turn OHPs can significantly reduce the effect of gravity on the heat transport capability in an OHP. Three miniature OHPs (18-turn acetone OHP, 18-turn water OHP, and 20-turn acetone OHP) were developed and tested to determine whether the uneven-turn OHPs can function in a high-g environment. Experimental results demonstrate that these miniaturized uneven turn designs are extremely capable in high gravity environments and will operate effectively in any orientation. An OHP with check valves has been successfully developed and tested to determine the check valve effect on the oscillating motion and heat transfer performance in an OHP. Experimental results show that the OHP with check valves can function well in both the inverted and vertical positions with little variation between the two positions in performance once startup occurred, while the control OHP without check valves which had the same channel layout was not able to achieve startup in the inverted position. This shows that the check valves allowed the OHP to operate in the inverted position first achieving startup and then maintain oscillating motion.

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