Experimental Investigation of Flow Boiling Characteristics of an Immersion Cooled Server Model

Abstract

The meteoric growth of global communication and networking infrastructure has resulted in demand for high-performance data centers. With the increase in power capacity of server electronics, thermal management of data centers has become a significant challenge, as the traditional air cooling technique is approaching its heat dissipation limit. Liquid immersion cooling is a liquid cooling technique that offers orders of magnitude higher heat transfer rates than air cooling, and also has the potential to reduce the energy needed for cooling data centers. While the liquid immersion solutions that are currently used in commercial applications provide some of these benefits, most of these solutions employ a large fluid inventory of expensive dielectric fluid to cool the server packages. A modular cooling solution that isolates each server blade will result in a smaller fluid inventory, which in turn would reduce the fluid costs and also improve the ease of maintenance. This study explores the thermal performance of a line replaceable, small form factor server cooling module with fluid inventory less than 2L. Flow boiling tests were performed on test boards with four high-performance dies in a square array. Three different test boards – boards with bare silicon, and silicon surfaces enhanced with microporous and microfinned heat sinks were tested. Novec 649 was used as the primary coolant in this study. Heat flux improvement of up to 50% was achieved by increasing the flow rate and decreasing the facility water temperature of the system. The use of enhanced surfaces provided significantly lower operating temperatures than the bare silicon surface.

In addition to the flow distribution system that delivers a parallel flow over to the heated surfaces, a flow distribution system that delivers impinging flow to the bottom row of heaters was also tested. For the microfinned surface, the highest heat flux recorded was 28.6 W/cm² which is roughly 185 W per die. The nucleation suppression phenomena in flow boiling was also investigated for this server model, and it was observed that the microfinned surface with impinging flow distribution system suppressed boiling activity for heat flux values of up to 18 W/cm². High-speed imaging was performed to assess the boiling activity, and to understand the change in nucleation characteristics brought about by the change in parameters of this system.