Twisted tape vortex generators in duct flow: Flow field and heat and mass transfer

The flow field in a rectangular duct behind a row of twisted tape vortex generators is investigated by means of laser-Doppler anemometry (LDA) and a mass transfer measuring method. The experimental apparatus consists of an open flow wind tunnel with a test section of three rectangular ducts. A row of twisted tape vortex generators is installed in each duct, which produces a regular field of longitudinal vortices. The flow velocity field behind the vortex generators is measured by LDA, and then the amplitude of the cross-flow velocity distribution Δu/2 of the main flow velocity in the center of the duct is determined. The amplitude of this velocity distribution describes the downstream damping of vortices. The local mass transfer is measured in the same duct with a photometric method, and the local mass transfer and the cross-flow amplitude of the mass transfer distribution corresponding to the amplitude of the velocity distribution is determined. The results show that the damping of the vortices can be investigated either by LDA measurements of the velocity field or by measuring the mass transfer distribution with the mass transfer measuring method. The analogy between the decay of the velocity and mass transfer amplitude with this vortex generator is evident but must still be proved for other types of vortex generators and duct geometries. The knowledge of the vortex damping is of particular importance for optimization of heat and mass transfer equipment. Then the distance to the next row of vortex generators can easily be optimized, leading to optimal improvement in heat and mass transfer along the whole surface of the duct. In laminar duct flow the existing range of the vortices downstream of the vortex generators gets smaller with the Reynolds number. The largest amplification of the vortices here takes place at Re /2˜ 2500.