Thread injection is a promising method for different minimally invasive medical applications. This paper documents an experimental study dealing with an axially moving thread in annular pipe flow. Mass flow and axial force on the thread are measured for a 0.46 mm diameter thread in pipes with diameters between 0.55 and 1.35 mm. The experiments with thread velocities of up to 1.5 ms−1 confirm the findings of theoretical studies that for clinical requirements the radius ratio between thread and pipe is crucial for the adjustments of mass ow and force on the thread.In both regimes, laminar and turbulent flow, the thread shows a characteristic oscillatory behaviour without touching the pipe wall. Resonance-like oscillations indicate circular thread motions around the pipe centre. The oscillating eccentricities may arise from longitudinal inhomogeneities of the thread shape and flow disturbances which cause non-stationary lateral momenta. According to established findings on annular flow with eccentric cylinders we assume that the mass flow, which is high compared to the concentric model, is caused by the temporary thread eccentricities. These findings should be considered in clinical applications to avoid possible thread blockage due to resonance-like vibrations.

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