  High-fidelity flow boiling simulations are conducted in a vertical
minichannel with offset strip fins (OSF) using R113 as a working fluid.
Finite-element code PHASTA coupled with level set method for interface
capturing is employed to model multiple sequential bubble nucleation using
transient three-dimensional approach. The code performance is validated against
experiments for a single nucleation site in a vertical rectangular channel. To
test the code performance, the studies for a bubble departing from the wall in
a minichannel with OSF are carried out first. Due to low heat flux values
applied to the channel (1 kW/m2) contribution from the microlayer is not
considered. The influence of surface characteristics such as contact angle and
liquid superheat on bubble dynamics are analyzed. Local two-phase heat transfer
coefficient is also investigated. To achieve higher void fractions, two conic
nucleation cavities are introduced in the same channel with OSF. Observed
bubble characteristics (departure diameter, bubble departure frequency) are
evaluated and bubble trajectories are presented and analyzed. Local heat
transfer coefficient is evaluated for each simulation case. The results show
approximately 2.5 times increase in the local heat transfer coefficient when
individual bubbles approach the wall. With a smaller bubble nucleation
diameter, heat transfer coefficient increases by two times. The current work
shows the capability of modeling flow boiling phenomena in such complex
geometry as OSF as well as data processing advantages of high-resolution
simulations.
