Air-Cooled Loop Thermosyphon Cooling System for High Heat Load CPUs—Part II: Experimental Results and Validation

A two-phase passive loop thermosyphon cooling system (LTS, whose design and simulation are described in a companion Part I) was prototyped and tested with R1233zd(E) and R1234ze(E) as environmentally friendly working fluids. The lightweight and compact aluminum LTS cooling system (patent Error! Reference source not found.) has a height of 66 mm and can be fit as two units side-by-side to cool two central processing units (CPUs) in 2U data center servers. The prototype operates safely for R1233zd(E) under heat loads between 100 and 700 W, air inlet temperatures from 25 °C to 30 °C, and air volumetric flow rates from 40 to 178 CFM. For this working fluid and test conditions, the junction temperature remained in the range between 35 °C and 90 °C with a minimum thermal resistance of 0.085 K/W. When operating with R1234ze(E), heat loads between 50 and 500 W, air inlet temperatures from 26 °C to 30 °C, and air volumetric flow rates from 40 to 118 CFM were tested. For this case, the junction temperature stayed between 32 °C and 92 °C with a minimum thermal resistance of 0.081 K/W. The pseudo-CPU footprint area (copper block with cartridge heaters) was 16 cm
²
, and thus, heat fluxes from 3.125 to 43.75 W/cm
²
. Cold startup of the pseudo-CPU/LTS assembly was done and no critical instabilities were observed. Finally, a validation of JJ Cooling Innovation’s solver (considered in the companion Part 1) is presented, which showed a high accuracy to predict the performance of the LTS cooling system (mean error of 2.1 K and maximum error of 5.8 K for junction temperature).