Master's thesis Thorsten Driemeier

 

Experimental investigation of the flow and entrainment behavior of a turbulent round jet in a channel geometry with a single-sided opening

Flow channel and contour plot of the velocity distribution in the entrainment zone Copyright: EBC Flow channel and contour plot of the velocity distribution in the entrainment zone

The influence of a spatial confined ambience on the flow field and the entrainment behavior of a free jet is investigated. For this purpose an experimental setup was designed where the free jet issues from a round nozzle into a flow channel with an opening to one side. This opening has a variable length Lind. Through this opening the free jet is able to entrain fluid from an ambience. To measure the flow field inside the flow channel the Particle Image Velocimetry method (PIV) is used. The measurements of the flow field are carried out at Reynolds numbers of Re = 10000 and Re = 6000 for four different opening ratios between Lind/D = 4...25 to a downstream distance of x/D = 25 to the nozzle. The entrainment ratio is determined by measuring the volume flow via a differential pressure method. The exit conditions of the free jet are investigated and compared with the exit conditions of a smooth contraction nozzle. The quarter circle nozzle shows an enhanced jet contraction and a thicker shear layer than the smooth contraction nozzle. The jet development and the expansion of the jet are analyzed quantitatively with the contour lines of the time averaged mean velocities and the instantaneous pictures of the flow field. It is shown that the flow behaves differently for different opening ratios. The distance to the nozzle where the streamlines of the jet reattach to the floor of the channel is identified. This reattachment length depends on the Reynolds number and the opening ratio. The position of the jets maximum mean velocity in flow direction is analyzed and the length of the potential core is determined. The decay of the maximum mean velocity is compared with the theoretical development of a free jet. The mean velocity at the opening is calculated from the velocity distribution at the opening. With this mean velocity the entrainment ratio is determined. In addition the entrainment ratio is measured via volume flow measurements in dependence of different characteristic ratios. The results are compared with the calculations of the PIV. At first the entrainment ratio increases linearly with the opening ratio. At a length of the opening of Lind/H = 1.5 the entrainment ratio tends towards a constant value. The entrainment ratio is proportional to the ratio of channel height and nozzle diameter H/D. By changing the position of the nozzle in the channel the entrainment ratio varied. A maximum value was found for an offset ratio of h/D = 6.67. If the aspect ratio of the channel is decreased the entrainment ratio decreases as well. By using an elliptic nozzle with an aspect ratio of S = 2 the entrainment ratio was increased. In the considered area of the Reynolds number between Re = 2000...20000 it is observed that the entrainment ratio decreases monotonously for increasing Reynolds numbers.