Direct-to-Chip Two-Phase Cooling for High Heat Flux Processors

Abstract Due to the surge in electronics power density, single-phase liquid cooling technologies are emerging to replace legacy air-cooling technologies. However, this surge in electronics power densities is accelerating abruptly, which will cause a single-phase liquid cooling operational lifetime to be much shorter than air cooling. Accordingly, it is essential to look for alternative cooling technologies such as two-phase cooling to replace liquid cooling when its time is up. This work presents comprehensive analyses of two-phase rack-level cooling systems deployment. These analyses can be divided into three main categories which are benchtop testing, rack-level deployment, and choosing a green refrigerant replacement. On the benchtop part of the project, five different cold plates that have different internal geometry are considered. These cold plates are used to build cooling loops with various configurations namely parallel, serial, and hybrid (parallel and serial). An EES code is used to design and evaluate the cold plates and cooling loops based on the existing correlations and modeling techniques. To evaluate this code, a benchtop two-phase experimental setup is built. This setup is designed to test single cold plates and full cooling loops while maintaining system stability. In this setup, high-power-density TTVs with 2.5 kW rated heaters are used to test these cold plates and cooling loops. The work on the benchtop level is just a preparation stage for the rack level deployment, where a custom-built CDU distributes refrigerant to the cooling loops through rack and row manifolds. These cooling loops are placed in multiple racks and attached to TTVs to simulate the thermal load of high-power density servers. In this part of the study, some design perspectives are introduced, and the impact of different operational parameters on CDU performance is explored. The last part of this study discusses the criteria for choosing a green refrigerant to replace existing high GWP ones. Most commonly used refrigerants such as R134a are expected to be phased out very soon due to their high GWP. Therefore, it is necessary to look for an alternative green refrigerant that can be adopted in the system without significantly impacting its performance. Preliminary results showed that R1234yf is the most appropriate replacement for R134a in two-phase rack-level cooling systems.