Experiments on the simultaneous two-phase liquid cooling of multiple simulated servers at differing vertical rack positions in steady state

An experimental study was conducted to examine the behavior of a refrigerant two-phase system for cooling multiple servers at differing vertical locations within a standard data center rack. In such racks, the vertically stacked servers may operate at different utilization levels and hence may have differing power dissipations. Furthermore, these distinct power dissipations may occur at differing vertical levels in the rack and may be time-dependent as a result of IT workload scheduling. A reliable two phase cooling system must operate in a stable and controllable fashion under these conditions and the design and characterization of such a system is the topic of this study. An experimental rig was developed for evaluating both pumped and non-pumped (thermosyphon) refrigerant two-phase systems for cooling simulated CPU's in both steady and transient scenarios, and with multiple simulated CPU's operating at distinct vertical positions. Each server flow branch was supplied by a common supply manifold, absorbing the heat at the CPU's using a mini-channel evaporator and returning the two-phase flow to a chilled water cooled plate condenser. Precise measurements were made of the mass flow rate to each branch as well as temperatures and pressures at all key system locations, allowing the identification of thermodynamic state at all relevant system positions. This paper presents preliminary experimental results for two simultaneously operating servers at different vertical positions and with different heat loads operating in steady state, in both pumped and non-pumped modes. It is shown that the system operation is stable in both modes for the two-server case. The flow rate branches evenly in the pumped case, with little effect of vertical position. In the non-pumped thermosyphon operation, flow rate to each server location is not affected by is power dissipation and vertical position.