The flow characteristics of deionized water in stainless steel microtubes were experimentally investigated in this study.The influences of relative surface roughness and inner diameter of tube on pressure drop were discussed.The inner diameters of microtubes were 543.3

654.4 and 737.2 μm

with the relative surface roughness 0.31%

0.16% and 0.1%

respectively.In experiments

long tube and short tube were used and compared to eliminate the effect of inlet and outlet.The relationships between pressure drop and Re with different diameters

between friction factor f and Re

and between Poiseuille number and Re were obtained.The results show that the variation of tube diameter affects the pressure gradient.With the decrease of tube diameter

the pressure gradient increases quickly.Large relative surface roughness(ε=0.31%

d=543.3 μm)results in higher friction factor and earlier transition point(Rec≈1500)

while smaller relative surface roughness(ε=0.16%

d=650 μm)has little effect on the laminar flow characteristics.