A CFD study of the parameters influencing heat transfer in microchannel slug flow boiling
Boiling heat transfer in microchannels has been investigated extensively in the last two decades. However, most results from independent experimental studies display a substantial disagreement on the influence of the flow conditions and mechanisms on the heat transfer performance. The objective of the present paper is to clarify by means of CFD simulations the effect of the primary flow parameters on the saturated flow boiling heat transfer performance of a slug flow within a horizontal, circular micro channel. The vapor quality, heat flux, mass flux, channel diameter, bubble frequency, and saturation temperature are the parameters under analysis. The numerical model is based on the volume of fluid method included within ANSYS Fluent v.14.5, which is here augmented by implementing ad-hoc models to calculate the surface tension force, the mass and energy transfer at the interface due to phase change, and to generate a continuous stream of bubbles with an arbitrary frequency. The high fidelity of the numerical framework provides unprecedented insight on the effect of these flow parameters on the bubble dynamics and heat transfer performance, which are tightly interrelated in microchannel slug flow boiling. In principle, the heat transfer performance is enhanced by flow conditions promoting a thin liquid film surrounding the elongated vapor bubbles and by the flow within short liquid slugs between the bubbles. Furthermore, numerous interesting secondary effects driven by the different flow conditions are observed and described according to the underlying flow dynamics: time-dependent interfacial waves on the bubble surface which significantly reduce the local film thickness; the response time of the channel wall temperature to changes of the flow boundary conditions at different channel sizes; the liquid film thickness dependence on the bubble length and on the liquid slug length; the heat transfer dependence on heat flux via the bubble generation frequency. A dimensional analysis is finally conducted in order to identify the nondimensional groups relevant to the microchannel bubble dynamics and heat transfer in the slug flow regime. (C) 2016 Elsevier Masson SAS. All rights reserved.