Numerical modelling of flow boiling inside microchannels: A critical review of methods and applications

Boiling heat transfer in microchannels has been a very hot topic in heat transfer research over the past two decades, fuelled by the dramatic need for high heat flux cooling of miniaturised electronics and a number of high energy density applications. Two-phase numerical simulations have emerged as a very powerful tool to investigate fundamental fluid mechanics structures and heat transfer mechanisms, and thus complement experimental observations. Boiling flows in microchannels possess distinctive fluid dynamics features such as clear separation of liquid and gas phases, dominance of surface tension forces, very thin liquid films, that require tailored numerical models to achieve high-fidelity results. Hence, there has been growing interest towards computational developments and numerical studies, which has resulted in an extensive publication output. This article presents a comprehensive review of the vast literature of scientific papers dedicated to numerical simulations of boiling in microchannels. First, the most recent advances in traditional and emerging computational techniques for interface-resolved simulations of microchannel flows are reviewed, covering from macroscale models based on the solution of the continuum Navier–Stokes equations, to mesoscale and molecular dynamics models. The review then focuses on numerical studies that investigated the prevailing fluid dynamics features in microchannel flow boiling, such as the confined bubble dynamics, flow pattern development, conjugate heat transfer and flow instabilities deriving from multi-channel configurations. Last, the results of computational studies dedicated to practical applications in heat transfer enhancement through engineered surfaces and novel geometrical arrangements are illustrated. The review is then completed by providing recommendations for future two-phase computational research and by proposing a wishlist for experimental analyses. The main challenges for numerical simulations of flow boiling in microchannels remain the accurate estimation of surface tension forces which is paramount due to the dominance of capillarity, the availability of sub-grid thin film models applicable to flow boiling conditions made necessary by the disparity of scales between sub-micron thin films and channel sizes, and the physics-based modelling of nucleation which is currently missing in all continuum-scale models. A closer integration of simulation and experimental activities is recommended to design fundamental microchannel flow boiling experiments, where the initial and boundary conditions of the flow can be represented faithfully by simulations for validation of the numerical methods.