It wasn’t much until I had the first droplets on my screen (as pixels of course). Thanks to prof. Schrijer, we were able to quickly find out the right camera settings for this setup. It was about a PCO Sensicam with a Nikon ED 180mm and 68mm extension tubes. Because the lenses were rather far from the image sensor, the f stop was set to 4.
The software used to acquire the images was Davis 8.1.5 and the pressure upsetream the injector was recorded by Labview 15.
When looking at the droplets, it is important to freeze the jet as much as possible in order to reduce the blur. One could say that the shutter of the above mentioned camera would be enough, but in fact, is not that fast. In my case a light source was a better option. It can be triggered extremely fast with a perfect synchronization with the camera. The main advantage is that the pulse duration is much shorter than 1μs i.e. the exposure of the camera.
Now looking at the light position with respect to the camera, it is important to understand the goal of this measurements. We want to look at the droplet size and this means that they must be imaged in a high contrast homogeneous way. If the light source is placed behind the camera, the drops will be seen as bright spots only in the middle creating a non-homogeneous area since they are in fact, small spheres. This problem is solved using shadowgraphy, which means that the light source is placed facing the camera. In this way, the camera will see light everywhere except from the regions where droplets/liquid is between the light and the camera.
The difference between lighting from behind/above the camera and shadowgraphy can be seen in the images below.
Full cone – light from behind the camera and above the jet
Small side region – light facing the camera (shadowgraphy)
And the full setup:
In case you were wondering what happened with the injector, I switched to a pintle as I could not achieve atomization with the plain orifice with only 10 bar. The pintle injector was simply made by centering a flat screw inside a diverging fitting.
Yesterday I finished the first part of the setup i.e. the feed system: from the air supply to the water tank to the injector. Here are some pictures:
The air supply in the Aerodynamics lab is 10 bar. The gas passes a manual valve, a check valve, a relief valve set just above 10 bar and a regulator with a range of 1-10 bar. This is what you can see on the main wood panel. On its other side, the water tank is pressurized by the regulated air and the liquid goes towards the injector. It first passes through a pressure transducer and a solenoid valve connected to the electronic box. This consists of a small voltage divider and a data acquisition board (NI 6211). The pressure is displayed and the valve is controlled by the control panel made in LabView.
After the safety check is passed, next week, Prof. Ferry Schrijer will help me with the main part of this project i.e. measuring the size of the droplets. Since the jet velocity is 30 m/s and the size of the droplets is expected to be around 50 μm, the taken pictures must be completely frozen in order to collect relevant data. This cannot be done by the shutter of the high speed camera as it is too slow, but instead, a light source will be used for a shorter period (in the order of ns). More about the equipment will follow in a later post.
Last year, around October, I have been admitted to the Honours Programme by TU Delft’s faculty of Aerospace Engineering. This programme consists of extracurricular courses (such as Personal Leadership, Cybersecurity, Design Thinking, etc.) and a research project worth of 13 ECTS. The latter has been a challenge for me to choose because I was told that I can take any domain in our faculty to dig into. After several months of visiting different professors, I made my choice of working in Space Engineering with Prof. Barry Zandbergen, who is now my supervisor for the current research.
So what am I doing specifically? I can’t just say I am a space engineer who plays with rockets in his spare time (ok, maybe sometimes). The goal of this research is to characterize the droplets of a plain orifice rocket injector. Here is my literature study.
So why is this so important? There is an optimum when it comes to the size/volume of a droplet at a certain distance from the injector. If the droplet is too small, it will evaporate quickly, before penetrating into the combustion chamber. If the droplet is too big, the engine overall will not reach its maximum efficiency.
In order to achieve the objective, experiments are to be held as the behavior of the atomization process is non-linear and models are very hard to predict. Therefore in my experiments, water will be emerged through a simple injector and pictures will be taken at different upstream pressures. I will keep you posted with the experimental setup and the final results.