Pumped Two-Phase R2L Direct-to-Chip Cooling in High-Density Data Center Racks: System Design and Experimental Evaluation

Abstract Data centers are experiencing rapidly escalating thermal loads driven by AI/HPC workloads with chip TDPs now exceeding 1–2 kW. This study presents the design and testing of a pumped two-phase, direct-to-chip refrigerant-to-liquid (R2L) cooling system for high-density racks. The test setup features an in-row R2L coolant distribution unit (CDU) rated at 160 kW (scalable toward 180–200 kW) integrated with row- and rack-level manifolds feeding server cooling loops, each loop containing four micro-channel cold plates.

Thermal Test Vehicles (TTVs) with 2.5 kW cartridge heaters (2,500 mm² area) emulated up to 10 kW per server. The system was evaluated under varied hydraulic conditions and heat loads, focusing on pressure drops in supply manifolds, loop regulators, and returns. Two-phase heat transfer to chilled water (using R-134a) was highly effective: at ~ 0.50 LPM/kW, the maximum CPU case temperature (T_case) was ~ 55–56°C, well below typical 80°C reliability limits.

Pressure drops remained modest (≈ 0.23 psi across row manifolds, up to ~ 7.6 psi in rack loops at full load). These results confirm that two-phase R2L cooling can safely dissipate > 160 kW per rack at high efficiency and suggest that even higher-capacity systems (e.g., 200 kW CDUs in production) are viable. The use of low-GWP refrigerants (e.g., R-515B, GWP 299) is recommended for future systems to improve sustainability.

Overall, pumped two-phase direct-to-chip cooling emerges as a promising, energy-efficient solution for next-generation high-density data centers.