Two Phase Immersion Cooling Study For OCP Accelerator Module OAM

Author: Jiu Xu, Thermal Engineer, Meta; Jaylen Cheng, Assistant Technical Manager, Wiwynn Collection: 2023 Global Summit - Cooling Environments

Summary

Meta and Wiwynn jointly conducted a systematic two-phase (2P) immersion cooling study targeting the OCP Accelerator Module (OAM) form factor, motivated by growing power density in AI/ML data center infrastructure. The core premise is that two-phase immersion outperforms single-phase immersion by enabling direct chip contact and exploiting phase-change heat transfer (evaporation and condensation) for a substantially higher heat transfer coefficient. This study is part of Meta's broader immersion cooling program—a companion to a high-level Meta overview presented by colleague Jay Chen earlier at the same summit—and focuses specifically on OAM as the thermal target to characterize the performance limits of 2P immersion at representative AI accelerator power levels.

The experimental work covered four areas: (1) boiling plate design optimization across four sample variants differing in base material (vapor chamber vs. copper) and boiling enhancement coating (3D pin-fin vs. 2D center-layer), targeting R_th < 0.02 °C/W at 1 kW; (2) tank-level testing examining the effects of TTV row position (preheating effect) and submersion depth on thermal resistance; (3) whole-system PUE analysis demonstrating 1.02 PUE at tank level versus 1.09 for Meta's best-in-class air-cooled data centers and ~1.3 for a typical data center; and (4) an apple-to-apple fluid comparison across three dielectric fluids—Opteon SF33, Opteon 2P50, and FMD-50. The test rig used a liquid-to-liquid CDU with >8 kW cooling capacity, an OCP UBB 1.0 compliant 1U tray, and 8x OAM Thermal Test Vehicles (TTVs) each capable of up to 1 kW, for a total test power of 8 kW matching the CDU capability.

The study concluded that two-phase immersion delivers superior thermal and energy performance but faces a gating constraint in dielectric fluid availability under PFAS regulation. Opteon 2P50 showed the lowest thermal resistance of the three fluids tested; all three were compatible with the TTV test hardware. Vapor chamber base boiling plates outperformed copper-base plates at all power levels above ~300 W, with both vapor chamber samples meeting the 0.02 °C/W goal at 1 kW. The team acknowledged CoolerMaster and Chemours as collaborators and issued a call to action for the OCP Cooling Environment community to navigate fluid regulatory uncertainty and expand chemical compatibility work.