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|  Pumped liquid cooling has been used in numerous applications including automotive engine cooling, avionics thermal control and nuclear reactor cooling. A typical pumped liquid cooling loop consists of a pump, a cold plate, a heat exchanger/sink and liquid lines. In many cases, reservoirs and valves are used to control the fluid volume and flow rate. The pump circulates the fluid in the loop, which picks up the heat in the cold plate and dissipates the heat through the heat exchanger. Compared to capillary driven (passive) two-phase devices such as heat pipes, loop heat pipes and capillary pumped loops, pumped liquid loops provide more robust operation. Compared to pumped two-phase loops, pumped liquid loops are inherently simpler and more reliable. Traditional channel flow cold plates have limited heat flux capability. Porous media and micro channels have been used to improve the cold plates' heat flux capability up to thousands watts per cm2. However, these cold plates require large pumping power, and the system pressures are typically on the order of 50psi. This presents significant difficulties in designing a pump that can operate reliably for many years in such a high pressure and high lifting environment. For microchannel cold plates, flow imbalance among the channels is often a problem that can potentially cause temperature non uniformity. ACT has been developing advanced liquid cooling technologies that are capable of cooling very high heat fluxes with minimal pressure drop penalty. In particular, ACT's R&D efforts in this area have been on two different technologies: oscillating liquid heat transfer and liquid jet impingement. ACT's oscillating liquid heat transfer technology incorporates a mechanical actuator to generate an oscillatory motion of the liquid. The oscillating liquid absorbs heat with very high efficiency, thanks to the disrupted liquid-wall boundary layers. The fluid oscillation effectively "relays" the heat from the heat source to the heat sink. The heat transfer performance can be controlled by adjusting the amplitude and frequency of the actuator output. The next figure (top, opposite column) shows the test results of oscillating water in a 1/8" OD copper tube. The heat source and sink were attached to the two ends of this 12" long tube. The oscillation was generated by a mechanical assembly capable of producing fluid oscillations at frequencies of 0 to 20Hz and amplitudes of 0 to 12". As shown, the oscillating flow heat transfer, with a proper combination of oscillating frequency and stroke, can remove heat fluxes in excess of 1,300W/cm2 at an equivalent thermal conductivity close to 250,000W/m-K. For comparison, a 1/8" OD and 12" long copper/water heat pipe can only handle heat fluxes up to 40W/cm2. The thermal conductivities of copper and diamond materials are on the order of 380 and 1,200W/m-K, respectively.
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) Oscillating Liquid Heat Transfer Test Results (click graph to enlarge) ACT's liquid jet impingement technology is developed for applications that require unconventional form factors and very high heat flux capabilities. ACT has developed and validated correlations to accurately predict the heat transfer coefficients and critical heat fluxes (CHFs) of jet impingement of various fluids. The two figures below show the measured heat transfer coefficients of water and fluorinert jet impingement, respectively.
) Water Jet Impingement Test Results (click graph to enlarge)
) Fluorinert Jet Impingement Test Results (click graph to enlarge) ACT is currently developing these advanced liquid cooling technologies for a diverse range of applications, including cooling of power electronics and computer microprocessors. Additional information on ACT's pumped liquid cooling technologies: |
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  Pumped Two-Phase Cooling Technologies Contact and Visit ACT Information ACT Brochures and Product Datasheets
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