A series of evaporation and condensation experiments have been performed on horizontal microfin tubes with refrigerants R134a and R410A. Mass flux varied from 75 kg/m 2 -s to 700 kglm 2 -s and average test section quality varied from 5 percent to 80 percent. Zero degree (axial grooving) and 18 degree helix micro fin tube configurations have been examined in both condensation and evaporation. Four tubes were examined (7.3mm and 8.9mm diameters with axial and 18 degree helix angle microfins). Evaporation and adiabatic conditions in microfin tubes generally show similar void fraction trends found in smooth tubes. Condensation void fraction results tend to show a reduction in void fraction relative to smooth tube results, possibly indicating liquid filling of groove regions.
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