Air cooled loop thermosyphon cooling system for high heat load CPUs: design and performance simulation

A new innovative partially passive cooling system was conceived and designed for cooling future generation CPUs of 2U servers operating in datacenter racks with a heat load target of 600W. The cooling includes passive flow of a two-phase working fluid inside of a closed loop thermosyphon (LTS), comprised of an evaporator, a condenser, and the connections between them (a riser and a downcomer). The buoyance in the riser is the driver of the flow in the loop, which increases when the heat load dissipated from the CPU is increased (self-controlled system). The newly designed LTS is entirely in aluminum, meaning it is cost effective and light in weigh. Two units can be installed side-by-side in standard 2U server racks to handle both CPUs. The new design (patent pending [1]) is characterized by a new concept of a duo-thermosyphon with headers, riser and downcomer that maximize the thermal-hydraulic performance of the entire loop and frontal face for airflow. The cooling system is partially passive since the heat transported by the LTS is removed via forced air convection, i.e. a fan. Using louvered fin coil technology adapted from automotive applications for the condenser, the energy consumption of the fan can be significantly reduced compared to conventional cold plates, if one so wishes. The total height of the LTS unit is 66mm and its footprint handles large size CPUs without any need of an oversized heat spreader. Simulations with JJ Cooling Innovation's inhouse code considering different environmentally friendly working fluids, air volumetric flow rates and heat loads were performed and are shown to demonstrate the potential of the proposed LTS cooling system.