Convective vaporization in micro-fin tubes of different geometries

An experimental investigation was performed for convective vaporization of R22 and R410A inside one smooth tube and five micro-fin tubes with the same outer diameter of 5 mm. Data are for mass fluxes ranging from 100 to 620 kg/m2 s at 279 K saturation temperature. The results suggest that the tube with fin height of 0.15 mm, apex angle of 25° and 38° starts has the best thermal performance for convective vaporization when mass velocity is less than 400 kg/m2 s, while the tube with fin height of 0.12 mm, apex angle of 25° and 58° starts has the best heat transfer performance at larger mass velocities, which is probably due to the relative size between fin height and liquid film thickness. Considering the effects of micro-fin on flow boiling, a new general semi-empirical model has been developed based on the present data and recent data from literature. The new model is applicable for intermittent and annular flow patterns, covering different fluids, nominal diameters from 2.1 to 14.8 mm, mass fluxes from 100 to 650 kg/m2 s, heat fluxes based on the total inner surface area from 0 to 30 kW/m2, and reduced pressure from 0.08 to 0.69. The model predicts the parametric trends correctly and the average and local heat transfer coefficients accurately. The heat transfer mechanism can also be observed clearly by the new model.

► Flow boiling in micro-fin tubes with outer diameter of 5 mm is experimentally studied.
► Effects of geometric parameters on heat transfer are analyzed.
► A new general semi-empirical model is developed.
► Heat transfer mechanism is qualitatively discussed.