International Journal of Heat and Mass Transfer - Characterization of evaporation and boiling from sintered powder wicks fed by capillary action
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 1200 μm and particle sizes from 45 to 355 μm, 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 500 W cm−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.
The team says they expect commercial coolers utilizing the tech to hit the market within a few years
In 2008, Purdue University developed a plate containing holes for "microjets," and a device containing "microchannels," both used in a new design that has been shown to dramatically increase cooling for microchips in computers and electronics. The technology circulates a liquid coolant supplied through the microjets into the microchannels and is capable of cooling chips that produce 1,000 watts of heat per square centimeter, about a five-fold increase over the capacity of today’s high performance cooling systems in computers and electronics.
The Purdue-developed technique circulates a cooling liquid called a hydrofluorocarbon, which is a "dielectric," meaning it will not conduct electricity or cause short circuits.
The cooling system is made of grooves narrower than a millimeter, or thousandth of a meter, wide. These channels are formed on top of a chip and covered with a metal plate containing tiny holes. The coolant is pumped through the holes in microjets, and the liquid then flows along channels to cool the chip. As the liquid is heated by the hot chip inside the channels, it bubbles and momentarily becomes a vapor, facilitating the cooling process, Mudawar said.
IBM is using a cooling liquid to increase the power density of concentrated photovoltaic (CPV) power and reduce its cost
Improved cooling will help enable higher power computer chips and increased power density for CPV.
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