TO-98-19-3 -- Heat Transfer Characteristics in the U-bend of a Microfin Tube Evaporator Using R-407C PDF

This work experimentally investigated the effects of parameters on the heat transfer coefficients in the U-bends and the straight sections of smooth and microfin tube evaporators using R-22 and R-407C refrigerants. The parameters were mass flux of 200 and 400 kg/m²·s, heat flux of 6 and 12 kW/m², and inlet quality of 0.1 and 0.2. The system pressure was 0.65 MPa. The apparatus consisted of a test section of four straight sections and three U-bends, preheater, condenser, refrigerant pump, mass flowmeter, and other equipment.

One-third of the downstream straight section was affected by the upstream U-bend due to the centrifugal force and mixing of two-phase flow. The average heat transfer coefficients in the U-bends were higher by 4% to 33% than those in the straight sections. The average heat transfer coefficients in the microfin tube were higher by 19% to 49% in the straight section and by 33% to 69% in the U-bend than those in the smooth tube.

The average heat transfer coefficients for R-407C were lower by 33% to 41% for the smooth tube and by 17% to 29% for the microfin tube than those for R-22.

Units: SI