Numerical Simulations of Boiling in Dielectric Fluid Immersion Cooling Scenarios of High Power Electronics

Abstract

As server technologies have progressed, the traditional methods of blade level cooling have become very costly. Up to this point, most server units have been cooled through forced air convection; a process that requires large air handling systems that consume a great deal of power. In this study, free convective single phase, pool boiling, and flow boiling immersion cooling of high power electronics is modeled numerically using the commercially available computational fluid dynamics (CFD) codes ANSYS Fluent. Single phase simulations of free convective, dielectric fluid immersion cooling in FC-72 were performed using Icepak, and validated against experimental results. Five 1.8 cm x 1.8 cm equally spaced die were mounted on a PCB in a cross layout. Two die were vertically aligned and two die were horizontally aligned with the center die and spaced 2.5 cm apart. Excellent agreement was found between the numerical results and experimental data. Using a five die model and a similar four die model, a numerical study was then performed to see the effect die size and spacing had on die temperature. After determining the effects of the geometry, the same setup was also modeled with using Novec 649 and HFE 7100 as a working fluid. Two-phase simulations of pool and flow boiling in subcooled Novec 649 were performed using the Eulerian two-fluid boiling model available in ANSYS Fluent known as the RPI Boiling Model. Modeled using ANSYS Icepak and imported into Fluent, four 2.4 cm x 2.4 cm die were mounted on a PCB measuring 10 cm x 10 cm in a square layout to simulate an experimental apparatus. Using various combinations of the built-in boiling model parameters in ANSYS Fluent, simulations were performed and the results were compared to experimental results. In two different cases, the combinations of boiling model parameters used showed good agreement between simulations and experimental results. After establishing the creation of a model in Icepak, importing it into Fluent, and modifying the boiling model through its boiling parameters as the appropriate methodologies to model boiling in Fluent, flow boiling was investigated. Using a model of a high performance small form factor, liquid immersion cooled server, flow boiling was simulated. Using subcooled Novec 649 as a working fluid, the effect of the inlet flow rate was evaluated. Results from these simulations were compared to experimental data and good agreement was observed.