An experiment setup was built for investigating single-phase adiabatic flow characteristics of internal helical-rib roughness. The friction characteristics for 15% (by volume) water-ethylene glycol mixture flow in three internal finned tubes (two types of new developed internal helical-fins) and a smooth tube were obtained. The parameters of test tubes are nominal inside diameters (22 mm and 16 mm)

numbers of fins (60 and 38)

helix angles (45° and 60°) and fin height to inside diameter ratios (0.022 and 0.053). The lengths of the internal helical-rib tubes in test section were 2643 mm and 2945 mm

respectively. The Prandtl number varied from 13.9 to 23.2 and the Reynolds number ranged from 4000 to 33000. The smooth-tube results were compared to the Filonenko equation with satisfactory agreement. The experimental results of different length and same type multi-start internal helically-finned tubes also showed that the entrance effect on the friction factor in the multi-start internal helically-finned tube increased with the increase of Re. Especially for Re>20000

the criterion that entrance effect can be ignored when lo/di>60 (lo—length in test section

di—inside diameter) was not suitable for the multi-start internal helically-finned tube. All the three helically-finned tubes results indicated that there was a critical Re

Recr

a function of geometric variables of internal helically fins. The friction factor achieved the maximum when Re=Recr and for 4000<Re<Recr. The earlier friction factor correlations could not be in good agreement with experimental data. Much efforts should be done to enlarge the application range of the earlier empirical formula. Also

for 4000<Re<Recr

the variation tendency of friction factors in multi-start internal helically-finned tubes were greatly different from those in equivalent roughness tubes used in the Nikuradse experiment. The difference between resistance mechanisms of internal helically-finned tube and artificial equivalent roughness tube was analyzed and deserved more attention.