OPTIMIZATION OF THE THERMAL DESIGN OF A CRYOGENICALLY COOLED COMPUTER

The recent design of large computers having heat dissipation rates in excess of 1 kW suggests research in the area of cryogenic cooling systems. Direct immersion liquid cooling of the CPU boards can be. implemented in order to maintain low board temperatures and avoid damage to the circuit components. In this paper, the performance and cost of three types of cryogenic cooling systems are analyzed; a ''once through'' system in which liquid nitrogen is allowed to boil on the CPU boards and is then vented to the surroundings; a saturated pool boiling refrigerated system in which gaseous nitrogen generated from boiling on the CPU boards is condensed in the vapor space above the liquid nitrogen pool; and a subcooled pool boiling refrigerated system in which the ''condenser'' is completely submerged in a subcooled liquid nitrogen pool surrounding the CPU boards. Thermal/fluid numerical models used in conjunction with an optimization code, GRG2, were used to predict the performance and cost of the cryogenic cooling systems. The uniform annual cost of the ''once through'' system was calculated to be $28.8K. This compares with $34.6K for the optimal saturated pool boiling system and $34.7K for the optimal subcooled pool boiling system. The critical heat flux for the subcooled system can be increased by as much as 30% over the value for the saturated system. This increase in critical heat flux may become significant in preventing thermal failure or degradation of the electronic components if local values of the heat flux exceed approximately 15 W/cm2. However, due to the extremely high initial cost of the refrigerated systems, the ''once-through'' cryogenic cooling system is recommended if the local heat flux on the CPU board remains below 15 W/cm2.