Bachelor's thesis Martin Cardaun

Experimental investigation of the properties of a radial turbulent free jet in transverse flow in a channel geometry

As part of this work the flow and entrainment behaviour of a free jet in a channel geometry with
variable diffusion angle is investigated. In an existing test bench the entrainment ratio is measured
using the zero-pressure-method. By implementing an automatic volume flow control for the
secondary air it is possible to artificially impose a pressure loss, whereby the influence of various
heat exchanger geometries can be simulated. The flow field in the channel geometry is measured
using stereoscopic Particle Image Velocimetry (2D3C-PIV). The test bench consists of a round nozzle
through which air is flowing into the channel. The channel has an opening in the ceiling wall of
a variable length Lind . Through this opening the induced air flows into the channel. It is first examined
how the opening ratio Lind /D at a Reynolds number of Re 10000 and a height ratio of
H/D 15 affects the entrainment ratio I . The entrainment ratio is the ratio of the volume flows of
the secondary air to the primary air. A monotonous increase with the opening ratio is observed. By
varying the bottom angle in a range between Lind /D 10...35 highest values of I are found for
0. The angle at which the maximum occurs increases with the opening ratio. At much greater
angles the entrainment ratio decreases significantly. The optical analysis of the flow field shows
characteristic flow structures in the channel. The jet is deflected to the bottom wall.With increasing
bottom angle a decrease of the reattachment point is observed. Further, the influence of a pressure
loss in the induction zone is observed for an area of Lind /D 4...20 and Re 3000...10000. It shows
that a higher Reynolds number increases the entrainment ratio at a certain pressure loss. Increasing
the opening ratio also improves entrainment. The optical analysis shows that higher velocities are
conserved further downstream the channel due to less mixing. The jet is not distracted to the bottom
wall because of the lower momentum of the induced fluid from above. The conditions of the
induced flow are investigated. It is observed that a smaller curvature of the flow at the edge of the
top opening has an effect on the entrainment for opening ratio values of Lind /D<17