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Yang X.,University of Nottingham | Yan Y.Y.,University of Nottingham | Yan Y.Y.,Jilin University | Mullen D.,Thermacore
Applied Thermal Engineering | Year: 2012

Heat pipes, known as "super thermal conductors" have been widely used in many areas for more than 50 years. Currently, due to the various requirements put on cooling systems, such as lightweight, better heat transfer performance, and optimised appearance, heat pipes have been improved significantly in the past decades. This paper summarises the recent developments of lightweight, high performance heat pipes. Various methods or approaches to achieve the requirements of lightweight and high performance are introduced. The applications of lightweight materials can help reduce by up to 80% the weight of conventional copper heat pipes; however the lightweight material often has problems of corrosion. Although improving the design of wick structures and changing the size of conventional heat pipe assemblies can help to reduce weight and achieve high heat flux, there are still some limitations to the applications of lightweight materials such as magnesium due to its incompatibility with some working fluids. © 2011 Elsevier Ltd. All rights reserved. Source

Mehl D.,Thermacore
Power Electronics Technology | Year: 2010

Vapor chamber heat sinks spread heat and eliminate hot spots in power semiconductors. A vapor chamber is a vacuum vessel with a wick structure lining the inside walls that is saturated with a working fluid. As heat is applied, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it condenses, releasing its latent heat of vaporization. The capillary action of the wick enables the vapor chamber to work in any orientation with respect to gravity. Due to the way the vapor chamber operates, the heat source can be placed anywhere on the base without affecting its thermal resistance. Vapor chamber heat sinks can use a variety of working fluids, depending on the operating temperature. The thermodynamic properties of water make it an order of magnitude better than any other fluid for the majority of electronics cooling applications. If a unit were ever punctured, air would leak into the vacuum space but no water would leak out, the water being held in by the wick's capillary force. Source

Weibel J.A.,Purdue University | Garimella S.V.,Purdue University | North M.T.,Thermacore
International Journal of Heat and Mass Transfer | Year: 2010

The thermal resistance to heat transfer into the evaporator section of heat pipes and vapor chambers plays a dominant role in governing their overall performance. It is therefore critical to quantify this resistance for commonly used sintered copper powder wick surfaces, both under evaporation and boiling conditions. The objective of the current study is to measure the dependence of thermal resistance on the thickness and particle size of such surfaces. A novel test facility is developed which feeds the test fluid, water, to the wick by capillary action. This simulates the feeding mechanism within an actual heat pipe, referred to as wicked evaporation or boiling. Experiments with multiple samples, with thicknesses ranging from 600 to 1200μm and particle sizes from 45 to 355μm, demonstrate that for a given wick thickness, an optimum particle size exists which maximizes the boiling heat transfer coefficient. The tests also show that monoporous sintered wicks are able to support local heat fluxes of greater than 500Wcm-2 without the occurrence of dryout. Additionally, in situ visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. It is seen that nucleate boiling from the wick substrate leads to substantially increased performance as compared to evaporation from the liquid free surface at the top of the wick layer. The sharp reduction in overall thermal resistance upon transition to a boiling regime is primarily attributable to the conductive resistance through the saturated wick material being bypassed. © 2010 Elsevier Ltd. Source

Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 149.99K | Year: 2012

The proposed effort will seek to design and develop vacuum brazed high density folded-fin modular heat exchangers. The scalability of these heat exchangers would provide a means for fabricating large area high performance cold plates by joining via friction stir welding. Friction stir welding is a solid-state joining process that results in minimal distortion of metal characteristics. The advantages of this approach would be an increase in system performance, reliability and efficiency. From a manufacturing stand point the technology would establish the capability to facilitate future product improvement across a broad range of development programs.

Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 79.88K | Year: 2012

To assure satisfactory cooling for future JSF upgrades, the proposed solution is a vapor compression refrigeration system packaged with both a pumped PAO cooling loop and a thermal energy storage (TES) system in a standard avionics rack for the JSF. The vapor compression system, VCS, will provide the sub-ambient cooling required as avionics power is increased. A VCS gives the highest efficiency for sub-ambient cooling. The pumped PAO loop will interface to the avionics racks. PAO is already an approved coolant for aircraft. The TES will store additional thermal capacity for the cooling system for high power transient excursions. Including this feature will increase mission capability during extreme operating conditions. The Phase 1 work effort involves a system design and subscale prototype technology emonstration.

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