Modelling and Parametric Studies of Multi-Phase Heat Transfer in Micro-Scale Sinks

In this study, the RPI (Rensselaer Polytechnic Institute model) boiling model is used to predict the cooling of a heat sink submerged in a pool of Novec 7100. The pool boiling of Engineered Fluid– Novec 7100 using ANSYS Fluent is observed. The study is modelled after saturated and subcooled conditions over a horizontal aluminum surface measuring 25.4mm x 25.4mm with trapezoidal fins. The element sizes used for meshing for the sink was 1mm, 3mm for epoxy and 5mm for the sink. Within the RPI model as presented by ANSYS Fluent, there are several boiling model parameters including bubble departure diameter, frequency of bubble departure, nucleation site density and area influence coefficient. The number of mass transfer is changed to 1 from liquid to vapor to enable boiling. The boiling was modelled after Tolubinski-Kostanchuk for bubble departure diameter, Cole for frequency of bubble departure, kocamustafaogullari – ishii for nucleation site density and Delvalle-Kenning for area influence coefficient. Drag was modelled after ishii, lift after moraga and heat after ranz-marshall. Vapor formation is captured by the vapor volume fraction x-y plot along the line surface with a z-coordinate value of 0.82579 and a contour plot along the y-mid iso surface. The Eulerian model in ANSYS Fluid is enabled to solve the multiphase problem. The Eulerian model in ANSYS Fluent allows for the modeling of multiple, separate, yet interacting phases. Due to the  physical nature of vapor bubbles and the dynamics related to them, k-ε model is selected. Troshko Hassan is chosen for turbulence interaction. The parametric studies are performed on the bases of the trapezoidal fins located on the sink. Numerical values were gathered for the wall superheat for heat fluxes ranging between 2.32W/cm2 to 15.57w/cm2. Results are validated with previous works performed by Kojo Osafo Asiamah validated with experimental data from Arden et al. It is observed that when the wall temperature exceeds the saturation temperature, heat transfer increases which increases bubble generating frequency and hence influences bubble departure from the wall surface. Decreasing the fin spacing of the sink increases the wall superheat temperature. The total heat flux also decreases with decreasing fin spacing. The relatively lower prediction of vapor volume fraction in the decreased bases simulation indicates that the system is dominated by convective heat loss. The temperature rise predicted with the computer model showed a good correlation with the model validated by the experimental results.

Keywords: Pool boiling, RPI boiling model, Eulerian Model.