Controlling Flow Instabilities in Direct-to-Chip Two-Phase Cooling for High Heat Flux Processors

As thermal dissipation from high-power density chips is anticipated to exceed the limits of single-phase cooling, two-phase cooling is being investigated as an alternative solution in the thermal management of data centers. However, it is necessary to recognize two-phase cooling issues and provide solutions to them before they can be widely implemented in data centers. The nonuniform distribution of flow in parallel pathways under the nonuniform heat load is one of the main problems two-phase systems encounters. This is explained by the fact that the pressure drop in the path increases with increasing power owing to evaporation, forcing the flow through parallel paths with less resistance. In this work, two effective solutions to eliminate flow nonuniformity namely constant flow regulators and flow restrictors were introduced. These two solutions were investigated experimentally using proof-of-concept cooling loops that have four parallel cold plates. At the entrance of each cold plate, an ultrasonic flow sensor was installed to measure the flowrate in this branch. These solutions were tested under extremely nonuniform heat loads using high-power-density Thermal Testing Vehicles (TTVs) with 2.5 kW rated heaters. Results proved that flow nonuniformity can be eliminated by using either one of these two flow stability solutions with maximum variation as low as 0.15 LPM. R134a refrigerant was used in this experiment to establish a baseline. Testing these solutions while considering a green refrigerant substitute for R134a would be the next step.