Boiling of multicomponent mixtures in confined geometries

Flow boiling is an efficient heat transfer mechanism and such it is the main choice when it comes to the high heat fluxes encountered in miniaturized process equipment such as micro- and mini-heat exchangers, micro-reactors and fuel cells. The development of two-phase micro-channel technology requires a thorough understanding of the hydrodynamic and thermal behavior of two-phase flow in such confined geometries. In particular, with the introduction of mixture refrigerants as an alternative for the non-environmental friendly refrigerants previously used, the problem of predicting the heat transfer coefficient and heat transfer mechanisms with mixture refrigerants has become more complex. Better predictive models for flow boiling in confined spaces are needed. From the modelling point of view, researchers around the world have not yet reached a consensus on the main physical mechanism dominating the heat transfer phenomena in these systems. Numerical simulations are widely used in order to complement experimental and modelling studies and help to better understand the underlying physics during flow boiling in small diameter channels. However, available numerical techniques have not yet succeeded in overcoming all the challenges present in the simulation of two-phase flows systems, especially with phase change involved. This project focuses on the study of bubble dynamics inside small diameter channels during flow boiling. The main goal of the project is to improve the fundamental knowledge on the hydrodynamics of bubble flow in micro-channels and their effect on heat transfer. A spectral numerical solver based on the least-squares method and a diffuse interface description approach has been developed. Here, the Navier-Stokes equations have been coupled to the Cahn-Hilliard equation in order to be able to describe interfacial phenomena such as coalescence and breakup. In order to incorporate evaporation effects, a model based on the Navier-Stokes and Korteweg equations has been chosen instead. The two numerical models have been validated against corresponding reference cases and used for the simulation of a rising bubble and a falling droplet, two-droplet coalesce, growing isolated bubbles during evaporation and Taylor bubbles travelling in a microchannel with and without evaporation.

** Achieved and potential outcomes and impacts ** A main outcome of the project has been the development of a numerical model based on a phase field method to simulate phase change phenomena. In particular, the new numerical framework based on Navier-Stokes and Korteweg will allow further studies of the physics involved in the evaporation and condensation of bubbles and droplets.

Flow boiling is an efficient heat transfer mechanism and such it is the main choice when it comes to the high heat fluxes encountered in miniaturized process equipment such as micro- and mini-heat exchangers, micro-reactors and fuel cells. The development of two-phase micro-channel technology requires a thorough understanding of the hydrodynamic and thermal behaviour of two-phase flow in such confined geometries. With the introduction of mixture refrigerants as an alternative for the non-environmental fri endly refrigerants previously used, the problem of predicting the heat transfer coefficient and heat transfer mechanisms has become more complex. In particular, the heat transfer coefficient has been reported to be lower than the one predicted by correlat ions from single-component studies. This project will focus on the study of bubble dynamics inside small diameter channels during boiling of multicomponent refrigerants. The focus will be both on a single-bubble growth process and the Taylor bubble flow regime. The main goal of the project is to improve the fundamental knowledge on the hydrodynamics of slug flow in micro-channels and the effect of components diffusion in the heat transfer phenomena. The project will be carried out over a period of 3 years (Jul. 2012 - Jun. 2015), with one PhD and one Postdoc projects included in the proposed research activities. The problem will be investigated in a two-dimensional setup, with bubble growth and/or flow of Taylor bubbles occurring bet ween two parallel heated plates. Channel diameters of 0.5-2mm (typical sizes of compact heat exchangers) will be considered, with constant heat flux as the boundary condition at the solid walls. The numerical models will be implemented using diffuse interface methods together with the least squares method for solving the resulting system of equations.