3.9.1 Assembly and alignment
According to the previous chapter, Schlieren and shadowgraph visualization techniques have been widely used for decades to visualize the gradient of the refractive index in different applications such as high-speed gas flow behaviour using convergent-divergent nozzles. The fundamental of the Schlieren and shadowgraph techniques have been mentioned in the previous chapter. Both Schlieren and shadowgraph techniques were employed in this study to visualize the turbulent flow interactions between the two gas jets. However, according to experimental results, it was found that the Schlieren method gave better results than the shadowgraph method.
The experiment investigation will therefore be conducted using Schlieren imaging [3].
The Schlieren and shadowgraph setups must be aligned correctly. Although, this may take some time, in order to obtain good results, it is important to align the setup properly [21]. The Schlieren setup needs to be sensitive enough to capture deflection of just a few arcseconds of angle. Since, large changes in a gas density corresponds to the small changes in a refractive index, it is crucial to ensure that the mirrors are well aligned to visualize these small deflections.
The schlieren setup utilized in this study can be seen in figure 3.6. Each component can be seen in the figure. Two off-axis parabolic mirrors are the main components of this technique. These mirrors have an off-axis angle of 30° and a focal point of 326.7 mm. Schlieren setup also includes a blue colourpoint light source, as well as a high-speed camera, two 670mm long plates and a knife-edge, which is normally a lazar bale. The knife edge is placed at the focal point of
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the second mirror. The percentage of the light blocked due to the knife-edge is called the cut-off percentage and it could be vary depending on the experiment objective [20]. The sensitivity of the Schlieren setup can be defined as the percentage of the knife-edge on the light source.
For instance, 100% cutoff describes the situation, where the light source is being blocked completely by the knife-edge. In the Schlieren imaging, only a very slight shadowgraph effect can be seen for 10% or 20% cutoff. Conversely, the Schlieren setup with high cutoff produces over-ranged imaged. As a consequence of over-ranging image, local flow details will be lost.
Therefore, for the Schlieren imaging it is necessary to choose an optimal cutoff percentage in order to visualize the smallest turbulent structure and meanwhile avoid over-ranging image.
The optimal percentage would be, for instance in the 30-60% cutoff range [34].
Figure 3.6 A top view of schlieren setup showing all the components and their positions in the arrangement and the light paths
3.9.2 Experimental procedure
First, two convergent-divergent nozzles were machined. To make the comparison easier, the dimensions of the nozzles were kept similar to what has been modelled in the simulation. A special glue was then used to ensure the nozzles facing each other are stable at 20°. The nozzles were then connected to a set of connections that contained regulators and a 1-meter metal hose to connect them to the air gas of 200-bar pressure that was fixed in its position using a chain.
Figure 3.7 shows the shadowgraph imaging arrangement used in this study. In order to get
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Schlieren images, a knife edge was added to the setup. In addition, figure 3.8 shows the nozzles with their connections.
1.5 m
Figure 3.7 Shadowgraph imaging setup and convergent-divergent nozzles connected to the pressurized gas cylinder
0.5 m
Figure 3.8 Zoomed view of nozzles and their connections
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When all the connections in the setup are connected, the next step is to make sure the components are all located in the right places and no leaks are coming from the gas cylinder or any of the other connections. After that, the video started recording with the camera control software (MotionBLITZ). The gases were released simultaneously.
It should be noted that both shadowgraph and Schlieren imaging techniques were employed with different cut-off percentages and cut-off directions (vertical and horizontal) in this study.
However, the results of the experiments show that Schlieren imaging with horizontal cut-off gave a better result than the others. Finally, when all trials were completed, the gas cylinder was carefully sealed and the remaining gas in the system was vented.
3.9.3 Schlieren image post-processing
A group of images with the highest level of quality has been selected from each trial. In order to prepare these images for comparison with the results of the CFD simulation, these images were transferred to the ImageJ software to modify the brightness and contrast. Moreover, a series of schlieren images have been post processed using image processing in MATLAB in order to get more information from the experiment.
3.9.4 Safety consideration and rules
The experiment was conducted in the basement lab under the supervision of project supervisors.
Synthetic compressed air was used to prevent asphyxia in this study. Furthermore, compressed air was used just for taking pictures to prevent further asphyxiation. Due to the high Mach number of gas jets from CD nozzles, earplugs are used to protect against any potential sound hazards. Additionally, eye protection was necessary to prevent damage from the blue light source used in the experiment.
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