Abstract
INVESTIGATING
LAMINAR FLOW
IN ROCKET NOZZLES
Researching the effect of laminar flow of air by changing the length of the throat of the nozzle on the thrust of the air exiting the nozzle.
About
The following abstract is about my investigation regarding the effects of laminar flow in rocket engine nozzles. I conducted this investigation with the advice of a supervisor as part of the IB Extended Essay. The investigation lasted approximately 6 months, beginning at the end of my junior year and finishing at start of my senior year. This included planning, preliminary research, designing the investigation, building the setup, collecting the results, and documenting my findings.
Introduction
Aerospace has always been at the leading edge of engineering and innovation. Aerospace activity has proved to be tremendously beneficial beyond its own aims, with various industrial, commercial and military applications. Dr. G.V.R. Rao deduced the optimal bell nozzle design in 1955, which is now the most commonly used rocket nozzle shape. It is designed to mitigate oblique shocks and maximize efficiency and performance. However, the throat properties of a rocket nozzle could be further examined and tested to further improve efficiency of these vehicles. This investigation aims to deduce the extent of the effect laminar flow has on the thrust exiting the nozzle by changing the length of the throat of a nozzle (modeled after those in rockets).
Method
This investigation uses a small-scale setup to explore the effects of laminar flow. The setup includes two plastic bottles cut at the top to emulate the shape of rocket nozzles, while the throats are PVC pipes cut to the variable sizes: 4cm, 8cm, 12cm, 16cm, 20cm, 24cm, and 28cm. The source of air flow was a desk fan, which maintained a consistent flow rate. The setup was built on top of jack stands to allow for easy change in height to match the height of the desk fan being used. An anemometer was used to record the exit velocity of the air. The fan is turned on when the setup is ready and the readings of the anemometer are recorded. Each trial lasted a total of 30 seconds. The process is then repeated for all the throat lengths with 5 trials for each length. Finally, the data was collated, averaged and then used to calculate the thrust at the end of the nozzle.
Results
The outcome of this investigation was that thrust from the exit nozzle increased with increasing nozzle throat lengths until 17cm, and begins decreasing after, showing a quadratic relationship between throat length and thrust exiting the nozzle. The data also showed the linear increase in pressure loss with increasing throat length. The quadratic relationship is due to the effects of laminar flow, where after a certain length, the effects of friction on the fluid with the boundary become greater than the effect of the increased velocity due to the fluid being choked for a longer duration with larger throat lengths.
Conclusion
This investigation uses a small-scale setup to explore the effects of laminar flow. The setup includes two plastic bottles cut at the top to emulate the shape of rocket nozzles, while the throats are PVC pipes cut to the variable sizes: 4cm, 8cm, 12cm, 16cm, 20cm, 24cm, and 28cm. The source of air flow was a desk fan, which maintained a consistent flow rate. The setup was built on top of jack stands to allow for easy change in height to match the height of the desk fan being used. An anemometer was used to record the exit velocity of the air. The fan is turned on when the setup is ready and the readings of the anemometer are recorded. Each trial lasted a total of 30 seconds. The process is then repeated for all the throat lengths with 5 trials for each length. Finally, the data was collated, averaged and then used to calculate the thrust at the end of the nozzle.