Simulating Exhaust Plumes of a Rocket Engine to Observe Inefficiencies in ANSYS Fluent
Anomalies occur within rockets quite frequently. A lot of the time, these anomalies are fixed shortly after testing due to the fact that they are relatively small roadblocks. Rocket engines need to be fine-tuned so that in actual flights with cargo (passengers or other payloads) there are no setbacks.
SpaceX has had a past with rockets blowing up when landing back on the launch pad due to various problems. These problems have been overcome by the company allowing for the Falcon to be declared as a very safe rocket. As we see the new innovations the company is coming up with, there have been failures with testing in the past.
During a Merline engine test in 2017 in Mcgregor, Texas, the engine exploded. This had damaged the engine severely and surrounding equipment although there were no reported injuries to people working in the area.
After further investigation into what happened, it was figured out that the engine exploded during a LOX (liquid oxygen) drop. A LOX drop is a test that indicates if there’s a leak within the engine. During this test, the engine was somehow ignited without any supporting parameters which caused the engine to explode.
Errors like these have the potential to slow down the development of larger rockets for more impactful projects such as advanced rocket propulsion systems in the future.
A Potential Solution
To prevent something that occurred with the Merlin engine or with other anomalies such as exhaust flow formations and temperature variations, computational fluid dynamics (CFD) is an effective solution.
CFD allows the user to try inputting various parameters in a risk-free environment to see the effects visually, Through the inclusion of various contours, the colour contrast alongside easy-to-read labels help provide great insight into the efficacy of the model. CFD is constantly being used in the aerospace field to help prevent anomalies from occurring. The results would help propulsion engineers to optimize the design of the rocket to meet the needs of the mission.
Brief Overview of Exhaust Flow and Shock Diamonds
To understand the results of the CFD that I simulated in ANSYS, it’s important to analyze how a nozzle works and the exhaust flow produced by the nozzle. Nozzles work by taking the hot gas that is produced by the combustion engine of a rocket and then eject that gas at fast speeds in 1 direction.
Before the hot gas (mixture of oxidizer and propellants) enters the nozzle, it is flowing in multiple directions. Nozzles are designed in a way to make the flow more directional to help propel the rockets in the opposite direction (newton’s 3rd law). To improve efficiencies within the flow of rockets, you can increase the speed of the exhaust flow (also known as supersonic flow).
To do this, engineers have created something known as a converging-diverging nozzle. This type of nozzle has the throat of the nozzle in a compressed state where the amount of gas has to accelerate at higher speeds.
When the nozzle walls expand, the pressure and temperature of the exhaust is actually lowered while the velocity of the flow increases. This occurrence makes it so that the pressure of the exhaust is the same as the pressure of the air surrounding the nozzle. This allows the rocket to have an exhaust flow that is optimally expanded.
If the specific nozzle design is relatively small when the atmospheric pressure is a certain condition, then the nozzle will become under expanded. If the atmospheric pressure is too high, then the nozzle will become over-expanded.
Vacuum engines are designed to accommodate higher pressures, so many non-industry workers have thought about using vacuum engines for the whole duration of the flight.
The problem with this is that the nozzle is bigger than the flow being that air will push against the flow and cause damage to the overall nozzle.
This is known as flow separation.
Over expanded nozzles create a pattern in their exhaust flow known as shock diamonds. Shock diamonds are generated due to the fact that the exhaust pressure is lower than the ambient air which forms these patterns. When testing rockets on the ground, this pattern is highly visible.
The SpaceX Starhopper test also showed shock diamonds in the exhaust flow.
ANSYS Fluent
Ansys Fluent is the CFD software I used to help simulate the exhaust flow of a rocket engine. There are various computational fluid dynamic softwares although ANSYS was the optimal one for this project.
ANSYS is a very visual software with a user-friendly interface that is easy to understand. It took me around 3 hours to get familiar with most of the features.
Fluent is an application inside of the ANSYS workspace which is used specifically for fluid dynamics. Fluent is also widely used in the space technology industry. Using this software would familiarize me with softwares being used in the actual industry. The whole process was outlined in 5 steps; Geometry, Meshing, Setup, Solution, and Results.
Analyzing the Model at a Deeper Level
To fully understand the model, I’ll be breaking down the explanation into the 5 steps so that each part of the model is discussed.
Step 1 : The Geometry
The image shows the 2D geometry of the rocket nozzle that I created. The bottom left part is the converging-diverging nozzle and the rest is known as the shock domain or the area surrounding the nozzle.
The geometry was simply a sketch made with the design features available in Fluent. I then selected the overall geometry and clicked the generate button to fill the surface area.
I then used the face split method to the overall geometry into multiple faces to improve the meshing (the next step). The geometry was difficult at times due to errors faced with the sketches although it was a pretty simple process. I used the XY plane as it gave a frontal view of the geometry.
Step 2 : Meshing of the Nozzle
Meshing is an extremely important stage of the overall CFD process. Meshing in Ansys is used to help make the process of running the calculation faster and provide more accurate results.
Originally I had given a mesh to the geometry that was made up of triangles and the bottom portion of the geometry was inaccurate. Due to this flaw in the quality of the mesh, I got a floating-point error in the calculation process.
To fix this error, I refined the mesh and instead used quadrilaterals over triangles. Another feature I used in the meshing was the number of divisions feature. This feature helped provide mesh sizing to make the mesh more accurate. I had to select the desired surfaces and then add the number of divisions as per my geometry.
The next part of the meshing phase was to create named selections. Named selections help the model recognize each portion of the geometry when running the calculation.
The named selections were :
- Pressure Inlet
- Pressure Outlet
- Nozzle Wall
- Far-field
The pressure inlet is where the flow would start to form. The pressure outlet is basically the opposite side or when the flow would end. The nozzle walls are the walls that make up the whole shape of the nozzle. Naming the far-field is simply showing where the shock domain ends.
Step 3/4 : Setting up the Calculation & the Solution
I combined steps 3 and 4 as they are very interlinking, the setup is essentially choosing how you want the calculation to run on Fluent. I had selected parallel processing which allows for faster processing and selected 4 solver processes (this allows for faster processing on a local unit). After this step came the actual solution. First of all, I clicked density-based and kept the rest the same.
After this, I selected the energy equation to be on for this model alongside changing the viscous model to k-epsilon as it best suits a turbulent flow like this.
Then I changed a few of the parameters for the air as the fluid. The air is included due to the effect it has on the simulation.
The next step was to add our boundary conditions, this required the gauge pressure of the inlet and outlet alongside the temperature in kelvin. Then I simply went to initialization and computed from the pressure inlet (the starting point) for the solution portion to be completed.
For running the calculation, I identified my time steps and then had 5000 iterations; the recommended value for this type of model. The image shows half of the flow that formed with the nozzle.
Step 5 : The Results
The image is a fully developed flow of the simulation. I had made the geometry as half the nozzle for the flow to develop quicker and for complicity's sake. In the results tab, I simply mirrored the effect and added the same contour effect as the other half. You can see the full nozzle shape.
The numbers on the side with the contours are the mach numbers. You can see shock diamonds forming, meaning that this is an over-expanded flow. The mach number also falls at the end of the flow, meaning the speed of the flow needs to be increased. To do this, you could probably compress the throat further to reach the maximum speed (the speed of sound).
In Ansys, you could also analyze other features such as temperature etc. Using these observations regarding inefficiencies, propulsion engineers would be able to refine engines to suit the need of the rocket.
Conclusion
CFD empowers the space technology industry to try and innovate the standard chemical propulsion systems into more advanced methods of propulsion. CFD will allow engineers to test out the engines of antimatter and nuclear rockets that will help pave the path for interstellar travel.
TL ; DR
- Anomalies occur in rocket engines which slows down the development of more innovative rockets.
- Computational fluid dynamics or CFD helps engineers detect inefficiencies and allows them to try different parameters/variations of the engine in a risk-free environment.
- Nozzles work by taking in the hot gas produced by the combustion chamber and directing it to flow in 1 direction.
- To help improve the efficiencies of rockets, engineers had created a converging-diverging nozzle. This nozzle had a throat that was compressed which allowed the same amount of gas to flow although just in a faster way.
- Think of this as putting your thumb on a garden hose, the same amount of water has to flow, just in a smaller space.
- When the pressure of the exhaust flowing out of the nozzle is the same as the ambient air, the nozzle is optimally expanded.
- When the air pressure is lower than the pressure of the exhaust, the nozzle is under expanded.
- When the air pressure is higher than the exhaust pressure, then the nozzle is over-expanded. When this occurs, shock diamonds form in the exhaust. These shock diamonds are actually a sign of inefficiency.
- The simulation I conducted was split into 5 parts. The geometry was essentially creating the nozzle for the simulation.
- Meshing helps in making a more accurate result while improving the speed of the calculation.
- I had input a few parameters for the setup and then moved on to the solution where I changed some of the viscous models and boundary conditions for the nozzle.
- The result was the image shown in the article that showed the fully developed exhaust flow. Overall, there were a few inefficiencies with the mach number and the shock diamonds of the results I got.
- CFD would help overcome these errors by trying different nozzle designs and parameters. CFD can also enable the faster development of advanced methods of rocket propulsion (antimatter, nuclear, etc).
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