Universal predictive tools for two-phase pressure drop and heat transfer in boiling and condensing mini/micro-channel flows

Experiments are performed to investigate condensation of FC-72 along parallel, square micro-channels with a hydraulic diameter of 1 mm and a length of 29.9 cm, which are formed in the top surface of a solid copper plate. The condensation is achieved by rejecting heat to a counter flow of water through channels brazed to the underside of the copper plate. Using high-speed video imaging and photomicrographic techniques, five distinct flow regimes are identified: smooth-annular, wavy-annular, transition, slug, and bubbly, with the smooth-annular and wavy-annular regimes being most prevalent. A new heat transfer coefficient correlation is proposed for annular condensation in mini/micro-channels. It shows excellent predictive capability based on both the present experimental data and the database for mini/micro-channel flows amassed from eight previous sources. A theoretical control-volume-based model for annular condensing flow is proposed where mass and momentum conservations are applied to control volumes encompassing the liquid film and the vapor core separately. The model accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of a new eddy diffusivity model specifically tailored to shear-driven turbulent films. The new model accurately captures the present pressure drop and heat transfer coefficient data in both magnitude and trend. This study also explored heat diffusion effects in micro-channel heat sinks. Detailed analytical models are constructed for heat sinks having micro-channels with rectangular, inverse trapezoidal, triangular, trapezoidal, and diamond-shaped cross sections. For a circular micro-channel, an alternative analytical power series solution technique is presented due to a singularity point in the governing heat diffusion equation. The analytical results are compared to one-dimensional and two-dimensional numerical simulations for different micro-channel diameters, aspect ratios, fin spacings, and Biot numbers. This study proves the analytical models are very effective tools for the design and thermal resistance prediction of micro-channel heat sinks. Universal approaches to predicting the two-phase frictional pressure drop and the heat transfer coefficient in condensing and boiling mini/micro-channel flows are proposed that are capable of tackling many fluids with drastically different thermophysical properties and very broad ranges of all geometrical and flow parameters of practical interest. For the frictional pressure drop in condensing and adiabatic flows, a universal approach is proposed by incorporating appropriate dimensionless relations in a separated flow model to account for both small channel size and different combinations of liquid and vapor states. For the heat transfer coefficient in mini/micro-channel condensing flows, two new correlations are proposed, one for predominantly annular flows, and the second for slug and bubbly flows. For the frictional pressure gradient in boiling mini/micro-channel flows, the separated flow technique developed for non-boiling (condensing and adiabatic) mini/micro-channel flows is modified to account for the differences in frictional pressure gradient predictions between non-boiling versus boiling mini/micro-channel flows. For the boiling heat transfer in mini/micro-channel flows, a correlation for the dryout incident quality is first proposed to exclude the partial dryout data. Two new correlations for saturated boiling mini/micro-channel flows are then proposed, one for nucleate boiling dominant regime, and the other for convective boiling dominant regime. These new approaches are shown to provide excellent predictions of the consolidated database. It is shown this accuracy is fairly even for different working fluids, and over broad ranges of hydraulic diameter, mass velocity, quality and pressure, and for both single and multiple mini/micro-channels