confined bubble growth during flow boiling in a mini-/micro-channel of rectangular cross-section part ii: approximate 3-d numerical simulation

this part ii of the paper reports the three-dimensional (3-d) numerical modelling on bubbly flow in confined mini-/micro-channels using the volume of fluid (vof) method in commercial cfd code fluent. the numerical simulation aims to provide detailed information of the fields of velocity, temperature and pressure so as to further understand the effect of bubble growth on the flow field and heat transfer from the channel wall. in part i, the experiment of flow boiling in a mini-/micro-channel of rectangular cross-section was carried out and a simple one-dimensional (1-d) model for the interaction of the pressure fluctuations during the growth of a confined bubble with various kinds of upstream compressibility was developed as an aid to the rational specification of inlet resistance. in part ii, the experimental observers and the theoretical model developed in part i are tested by performing the 3-d numerical simulation of bubble growth from nucleation to full confinement. the simulation involves some approximations based on a concept of pseudo-boiling to avoid the well-known difficulties of modelling bubble generation and growth. during the simulation, the volumetric growth rate of the bubble is defined to match the experimental observations. at small times prior to bubble detachment, a vapour flow was injected through a small hole in the wall to simulate nucleation. following partial confinement, vapour injection was stopped and growth was driven by the generation of vapour at a defined rate at the contact area between the bubble and the superheated wall. the 3-d simulation reproduces the experimental observations of the distorted profile of the bubble and its trajectory during partially confined growth and provides information about flow and heat transfer in the bulk liquid outside the thin film region. the 3-d and 1-d predictions of the development of axial pressure distributions during partially and fully confined growth are in satisfactory agreement.