Thread-annular flow in vertical pipes

Thread injection is a promising method for different minimally; invasive me dical applications. This paper documents an experimental study dealing with an axially moving thread in annular pipe flow. Mass flow and axial force o n the thread are measured for a 0.46 mm diameter thread in pipes with diame ters between 0.55 and 1.35 mm. The experiments with thread velocities of up to 1.5 m s(-1) confirm the findings of theoretical studies that for clinic al requirements the radius ratio between thread and pipe is crucial for the adjustments of mass flow and force on the thread. In both regimes, laminar and turbulent now, the thread shows a characterist ic oscillatory behaviour without touching the pipe wall. Resonance-like osc illations indicate circular thread motions around the pipe centre. The osci llating eccentricities may arise from longitudinal inhomogeneities of the t hread shape and flow disturbances which cause non-stationary lateral moment a. According to established findings on annular flow with eccentric cylinde rs we assume that the mass flow, which is high compared to the concentric m odel, is caused by the temporary thread eccentricities. These findings shou ld be considered in clinical applications to avoid possible thread blockage due to resonance-like vibrations.