ADVANCED MICRO-FIN TUBES FOR EVAPORATION

R-22 evaporation data are presented for new micro-fin geometries appli ed to the inner surface of 15.88 mm outside diameter tubes. The purpos e of the work was to develop internal geometries having higher evapora tion coefficients than existing single-groove micro-fin designs. The n ew geometries include both single-helix and cross-grooved surfaces. Th e single-groove geometries have 74-80 internal fins, 0.35 mm fin heigh t, and 30 degrees fin included angle. The cross-groove geometries are formed by applying a second set of grooves at the same helix angle, bu t opposite angular direction as the first set. Data are provided for v arying 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 r ate. 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 hi ghest performance was provided by a cross-grooved tube having 20 degre es helix angle. Its evaporation coefficient is 23% higher than an exis ting 75 groove, single-helix tube. The pressure drop is 6% higher than in the 75 groove tube. Reduced performance occurs in the cross-groove d tube when the second groove depth exceeds 60% of the depth of the fi rst set of grooves. The series 2 data stimulate complete evaporation w ith exit superheat in circuit lengths of 7.3, 9.8 and 12.2 m. These da ta clearly show that the evaporation coefficient attains a maximum as the average vapor quality in the 2.44 m test section approaches 90%. T he vapor quality at which the maximum occurs decreases with increasing flow rate or heat flux.