Experimental evaluation and simulation validation of an air-cooled loop thermosyphon designed for high heat load CPUs

A newly designed passive loop thermosyphon cooling system (whose design and simulation are described in a companion ITHERM2021 paper) was prototyped and tested at different levels of CPU heat load, air temperature and volumetric flow for two environmentally friendly working fluids: R1233zd(E) and R1234ze(E). The light and compact all aluminum cooling unit (patent-pending [1]) has a total height of 66mm and it is dimensioned to allow two units to fit side-by-side on two CPUs in 2U datacenter servers. The prototype was bench tested to operate safely for R1233zd(E) at heat loads from 100W up to a very high heat load of 700W, air inlet temperatures from 25°C to 30°C and air flow rates from 40CFM to 178CFM, yielding measured junction temperatures from 35°C and 90°C and thermal resistance as low as 0.085 °C /W. For R1234ze(E), the tests covered heat loads from 50W to 500W, air inlet temperatures from 26°C to 30°C and air flow rates from 40CFM to 118CFM with observed junction temperatures between 32°C and 92°C and thermal resistance as low as 0.081 °C /W. The pseudo-CPU footprint area was 16cm
²
, corresponding to heat fluxes from 3.125W/cm
²
to 43.75W/cm
²
. Cold start-up tests were performed and are discussed. Finally, a validation of JJ Cooling Innovation’s inhouse simulator used for the thermal-fluidic design of the unit is presented, showing the capability of the inhouse solver to predict the performance of this LTS cooling system with a high level of confidence (with mean error of 2.1°C and maximum error of 5.8°C in junction temperature).