Immersion cooling fluid comparison using a single-phase immersion/liquid technique for data centers

• The cooling performance of 4 dielectric fluids are tested in hybrid cooling system. • The Figure of Merit (FOM1), developed by OCP, was used as an evaluation indicator. • An approximate doubling of FOM1 reduces the Prandtl number by a factor of ∼ 10. • RAM, NVME, SSD temperatures drop by ∼ 6.7, 6.4 and 5.9 °C when FOM1 is doubled. • CFD model agrees with tests with a maximum temperature deviation of 10.7 % This paper analyses the cooling performance of different immersion fluids for a single-phase immersion/liquid-cooling technique, which integrates a direct-to-chip water-cooling system with passive single-immersion cooling technology. The choice of dielectric fluid is a key factor in determining heat dissipation performance, as its thermophysical properties directly affect cooling efficiency. An experimental setup tested the impact of four dielectric fluids — SL 3568 (Shell), FES 822–6542 (Fuchs), EGEN 100R8 (Motul) and BioLife 4 (TotalEnergies Fluids) — on the thermal behavior of the system. The selection of these dielectric fluids offers insights into different development focuses such as diverse viscosity grades, flash point and biodegradability. The impact of the Figure of Merit for Natural Convection (FOM1), developed by the Open Compute Project (OCP) Immersion Cooling Sub-Project to assess the combined influence of fluid properties, was discussed and used as an evaluation indicator to investigate the impact of the dielectric fluids on the IT equipment. The results indicate that approximately doubling the FOM1 value (when comparing BioLife 4 to SL 3568) reduces the Prandtl number by around a factor of ten. This enhancement translates into notable thermal improvements for components directly immersed in the dielectric fluids, with RAMs, NVMEs, and SSDs exhibiting temperature reductions of approximately 6.7 °C, 6.4 °C, and 5.9 °C, respectively. Conversely, CPU temperatures were found to be highest when using fluids with elevated FOM1 values. Since the CPUs are cooled via cold plates and not in direct contact with the immersion fluid, this trend is attributed to the possible infiltration of the dielectric fluid into the thermal interface material applied between the CPUs and the cold plates. Such infiltration appears to significantly reduce the viscosity of the TIM, by at least 94.4 %, thereby degrading its thermal performance. To enable a more extensive evaluation of immersion fluids available in the industry, a numerical simulation of the fluid flow and thermal behavior of the IT equipment within the hybrid system was conducted and validated against experimental results.