The foregoing discussion is applicable for the plain fin geometry. Test results for the highly interrupted surface like louver or semi-dimple VG surfaces, however, show a different heat transfer performance in response to the effect of fin pitch. Basically, both louver and semi-dimple VG geometry reveals that the heat transfer coefficients for Fp= 1.6 mm is normally higher than those of Fp= 2.0 mm throughout the test range. Yet the difference in heat transfer coefficient between louver and semi-dimple VG is very small when the frontal velocity is less than 1 m s1 and this phenomenon prevails for both fin pitches. However, the difference in heat transfer coefficient amid louver and semi-dimple VG increases considerably over 20% for Fp= 1.6 mm when the velocity is as high as 5 m s 1 . However, the difference is reduced to about 10% when