Andrea Cioncolini

Microscale Adiabatic Gas–Liquid Annular Two-Phase Flow: Analytical Model Description, Void Fraction, and Pressure Gradient Predictions

The study is devoted to the modeling of microscale adiabatic gas–liquid annular two-phase flow. The turbulent diffusion of momentum in the annular liquid film is assumed to be governed by the conditions near the channel wall, in analogy with single-phase turbulent bounded flow. This allows the universal velocity profile for single-phase turbulent flow to be extrapolated to the annular liquid film for the prediction of the local velocity. Conservation of mass applied to the liquid film allows the calculation of the average liquid film thickness, which in turn yields the void fraction. Once the void fraction is known, conventional one-dimensional, two-fluid modeling can be applied to predict all the relevant hydrodynamic parameters, an approach applied previously to macrochannel two-phase flow that in the present article is extended to microchannels. In the article, the analytical model is described and applied to an experimental database containing about 1100 data points for refrigerants R-134a and R245fa flowing through three horizontal circular glass microchannels of inner diameters 0.52 mm, 0.80 mm, and 1.0 mm, respectively. The database includes the pressure drop, mass flow rate, and vapor quality and covers operating pressures from 155 to 877 kPa, mass fluxes from 277 to 2026 kg m−2 s−1 and vapor qualities from 0.07 to 0.92. In particular, the analytical results regarding the void fraction are shown to compare favorably with macroscale empirical correlations extrapolated to microchannels, while the two-phase friction factor is successfully correlated using just one dimensionless flow parameter (defined as the ratio of a liquid film Reynolds number to a gas core Weber number), allowing a satisfactory prediction of the measured pressure gradients.

Recent Developments in Flow Boiling and Two-Phase Flow in Small Channels and Microchannels

Microscale two-phase flow is at present one of the hottest topics of heat transfer research, both in academia and in the industry. The miniaturization of two-phase flow systems, which has led to numerous experimental and theoretical challenges not yet completely resolved, is primarily related to the dissipation of high heat duties typical of compact systems such as CPU (central processing unit) chips, electronic devices, micro chemical reactors, and micro fuel cell combustors.