Advanced micro-fin tubes for evaporation

R-22 evaporation data are presented for new micro-fin geometries applied to the inner surface of 15.88 mm outside diameter tubes. The purpose of the work was to develop internal geometries having higher evaporation coefficients than existing single-groove micro-fin designs. The new geometries include both single-helix and cross-grooved surfaces. The single-groove geometries have 74-80 internal fins, 0.35 mm fin height, and 30 degrees fin included angle. The cross-groove geometries are formed by applying a second set of grooves at the same helix angle, but opposite angular direction as the first set. Data are provided for varying second groove depths. Data are reported for evaporation at 2.2 degrees C in a 2.44 m long test section for 45-181 kg h(-1) mass Bow rate. The series 1 tests are for inlet and exit qualities of 0.20 and 0.80, respectively. The evaporation coefficient reaches a maximum at 20 degrees helix angle and then decreases for higher helix angle. The highest performance was provided by a cross-grooved tube having 20 degrees helix angle. Its evaporation coefficient is 23% higher than an existing 75 groove, single-helix tube. The pressure drop is 6% higher than in the 75 groove tube. Reduced performance occurs in the cross-grooved tube when the second groove depth exceeds 60% of the depth of the first set of grooves. The series 2 data stimulate complete evaporation with exit superheat in circuit lengths of 7.3, 9.8 and 12.2 m. These data clearly show that the evaporation coefficient attains a maximum as the average vapor quality in the 2.44 m test section approaches 90%. The vapor quality at which the maximum occurs decreases with increasing flow rate or heat flux.