2PhaseEx – Two-Phase Thermosiphon Loop for High-Performance Chip Cooling in Data Centers

Increasing power densities in data centers require cooling concepts that dissipate high heat fluxes while maintaining stable and uniform chip temperatures. Within the 2PhaseEx project, a closed two-phase thermosiphon loop is investigated as a passive cooling solution for high-performance processors using environmentally friendly working fluids. The test setup consists of an evaporator, riser, condenser and downcomer.

A Thermal Design Power simulator with 13 individually controlled heating elements reproduces both homogeneous chip loads and local hot-spot scenarios based on the AMD EPYC 9005 chiplet layout. Initial measurements were conducted using a hollow baseline evaporator without internal fins under controlled coolant temperature, refrigerant charge and heat-load conditions. Temperatures, pressure, mass flow rate and heating power were recorded to assess heat transfer behavior, circulation stability and the onset of dry-out.

Stable operation was achieved up to 1.2 kW. At 1.3 kW, a sharp temperature drift indicated local dry-out. The results show that stable circulation improves temperature uniformity across the evaporator.

Future work will focus on optimizing the internal fin geometry based on the expected bubble departure diameter to enhance vapor removal, liquid rewetting and boiling stability at high heat fluxes.