Numerical methods for micro heat exchangers

This chapter presents a state-of-the-art overview of numerical methods for single-phase flow and two-phase flow in microchannel heat exchangers, as well as the relevant single-phase and two-phase computational fluid dynamics (CFD) applications. Governing equations are given for both single-phase flow and two-phase flow with and without phase change. For single- phase flow, scaling effects, such as conjugate heat transfer and viscous heat dissipation, are considered for their significance on fluid flow and heat transfer in micro heat exchangers. For two-phase flow, the challenges of numerical modeling and the relative magnitudes of the dominant forces are discussed. Characteristics of the multiphase flow modeling approaches (i.e., the Eulerian-Eulerian method, the Eulerian-Lagrangian method, and direct numerical simulation [DNS]) are compared. The advantages and disadvantages of several continuum DNS methods for interface evolution (e.g., volume of fluid [VOF], level set, phase field, front-tracking, and moving mesh methods) and the mesoscopic lattice Boltzmann method (LBM) are discussed. Methods to address the mass nonconservation in the level set method are briefly provided. Because microchannels are the basic elements of micro heat exchangers, recent CFD applications of two-phase flow in microchannels, mostly limited to the scale of a few bubbles or droplets, are briefly summarized. Future research needs for numerical modeling of micro heat exchangers are suggested.