Experimental study was explored with the ultrasonic Doppler velocimeter on the shell side fluid flow characteristics in helical baffled heat exchanger made from acrylic glass. Emphasis was on the axial velocity distribution characteristics. Results indicate that the axial velocity is uniformly distributed in the circumferential region of the shell side. Benefiting from the leakage flow in the triangular region

the corresponding axial velocity in the central region is rapidly increased. With the increase of mass flow rate

the variation magnitude of axial velocity in the central region is much larger than that in the circumferential region. An obvious fluctuation of axial velocity on the shell side of the helical baffled heat exchanger is also an important phenomenon. The mean velocity in the central region increases with the mass flow rate

but this feature is absent when the corresponding turbulent flow is dominant and backmixing flow occurs.