The current investigation reports the design, fabrication, and characterization of ultra-compact air-cooled condenser heat exchangers with exceptional power density performance in excess of 100 W/cm3. This exceptional heat exchanger performance is attributed to the implementation of high-speed compressible air flow in circular microchannels and two-phase condensation flow of refrigerant in rectangular minichannels to achieve high heat transfer performance in a compact design. These advancements are enabled by recent developments in micro-electric discharge machining to produce very high surface area compact heat exchanger devices.Air-cooled heat sinks have been identified as pivotal to the future of thermal management of microelectronics in the 21st century. Air-cooling systems provide clear advantages in overall ease of integration due to its availability and abundance over forced liquid cooling systems. The enhancement of forced air flow is especially important because it generally represents the dominant thermal resistance of the condenser. The implementation of high-speed compressible air flow in microchannels permits investigation of flow conditions with exceptionally high heat transfer coefficients. This investigation explores turbulent flow regime conditions for air-side Reynolds numbers 8,000 < Re < 20,000, in high-density parallel arrangements of circular copper microchannels, 355 and 520 μm in diameter.Two-phase heat transfer technology is one of the most efficient methods of waste heat removal in high power electronics cooling. It is especially advantageous in applications where size, weight, and efficiency are important factors. Two-phase active cooling systems consist of an evaporator and a condenser. Heat transfer performance in the evaporator is typically much higher than in the condenser; consequently the condenser is the limiting component of the entire cooling system. Improvements in condenser technology enable electronics systems to operate at a higher power while reducing the overall cooling system size and weight. The condensation phase change process in microchannels with high aspect ratios yields the formation of a thin film condensation layer on the heat transfer wall, resulting in high heat transfer coefficients with little pressure drop penalty. It is therefore of utmost importance that condensation phenomena, especially in high aspect-ratio microchannels be experimentally investigated. This investigation employs condensation in a parallel array of high-aspect ratio, 0.5 mm x 2 mm, rectangular minichannels.Compact cross flow heat exchangers are manufactured using novel micro-electro-discharge machining to produce high-density, high aspect ratio microchannels in a copper alloy. The heat exchanger performance is characterized for single-phase liquid and phase-change condensation of a refrigerant. The heat exchangers are operated using single-phase liquid flow of dielectric refrigerant R245fa at 80 °C and high-speed flow of air at ~25 °C to demonstrate a power density performance of nearly 70 W/cm3. The heat exchangers are operated using two-phase condensation of R245fa at 80 °C and high-speed flow of air at ~25 °C to achieve power density performance > 175 W/cm3 and overall thermal resistance < 0.27 K/W.Test methodologies were implemented for determination of the thermal-hydraulic performance of these novel devices. Modeling and characterization of this system were implemented using well-known methods and the results are compared with the corresponding literature for microchannel fluid flow and heat transfer. The study of this system demonstrates an advancement in the state-of-the-art in power density performance of compact air-cooled heat sinks. Power dissipation rates > 1 kW and an overall thermal resistance of < 0.05 K/W are projected with scaling of these methodologies in a 10 cm3 device.
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Design and performance characterization of high power density air-cooled compact condenser heat exchangers
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