Experimental analysis on the condenser design for a thermosyphon cooling system for next-generation telecommunications electronics

Passive two-phase cooling systems are an efficient and viable alternative to conventional air-cooling schemes for thermal management of electronics, thanks to their capability of dissipating higher power densities with lower power consumption and noise levels. The present study aims to assess the performance of a novel two-phase air-cooled thermosyphon for cooling shelf-level telecommunications equipment that is designed with 18 line cards. In particular, the main objective is to investigate the effect of the condenser technology on the thermal-hydraulic performance of the entire cooling system. The thermosyphon consists of a multi-microchannel evaporator with 18 individual micro-channel zones (one per card) connected to an air-cooled condenser via riser and downcomer tubes. The total height of the loop, from mid-evaporator to mid-condenser, is approximately 50 cm. An extensive experimental campaign is performed to evaluate the performance of two different air-cooled condensers in a front-to-back air flow configuration. The two condensers tested here are: (i) a single-pass, louvered-fin, flat-tube design, and (ii) a multiple-pass, wavy-fin, circular-tube design. Thermosyphon experiments are carried out with R134a as the working fluid, varying filling ratios from 45% to 65%, heat loads from 102 W to 1023 W under uniform and non-uniform conditions over the 18 heat sources operating in parallel. A number of air-side fan configurations are also investigated by varying the number of fans from five to seven, their speed from 7302 min-1 to 15480 min-1, and by testing different fan-tray heights from 65 mm to 103 mm. The experimental results demonstrate that the single-pass louvered-fin flat-tube condenser provides lower liquid-side frictional pressure drops, and consequently, higher refrigerant mass flow rate in the loop, as well as a higher critical heat flux value, which is ideal for increasing power densities. On the other hand, the multiple-pass wavy-fin, circular-tube condenser offers lower thermosyphon thermal resistances over a larger range of dissipated heat loads, mainly thanks to its lower air-side pressure drop.