|Multiphase numerical simulations have typically been restricted to a few standard functions for key boiling parameters. In this study, ANSYS Fluent was used to model dielectric fluid immersion cooling of multichip modules using a variety of functions for nucleation site density and bubble departure diameter under pool and flow boiling scenarios. Numerical results were validated against experimental data for the same geometry and working fluid.
The pool boiling model consisted of a single die and PCB vertically suspended in a large, quiescent pool of Novec 649. Heat was uniformly dissipated from the die at fluxes ranging from 3 to 12 W/cm2. Combinations of the built-in functions available in Fluent were simulated with results exhibiting varying degrees of agreement with experimental data and overall demonstrating relatively unpredictable behavior. Parametric studies were performed with user-defined functions for nucleation site density and bubble departure diameter. The best agreement was seen when experimental data for bubble departure diameter was implemented as a user-defined function; these results matched experimentally measured wall superheat to within 1.3°C. Other pool boiling simulations included alternate functional forms for nucleation site density, variations to the degree of subcooling and variations to the turbulence model.
The flow boiling model was based on an experimental design for a small form factor, four die cartridge module that circulated subcooled Novec 649. The user-defined functions that performed best in pool boiling simulations were applied to flow boiling. It was found that although the nucleation site density and bubble departure diameter functions resulted in good agreement under pool boiling scenarios, modifications would be needed to adapt the functions for flow boiling.