Discipline: Technology and Engineering
Subcategory: Civil/Mechanical/Manufacturing Engineering
Nialah Wilson - Howard University
The presence of gas bubbles in otherwise liquid-filled mechanisms can lead to unusual behavior when the system is vibrated. This is important for any system where gas-liquid systems are subject to vibration, for example in liquid-filled rocket fuel tanks, since the gas can form bubbles and create a multiphase mixture. The resonance of the system changes when gas is present. In order to understand and model this behavior, experiments were conducted with simple spring-mass-damper systems with pistons enclosed in silicon oil. A spring held the piston up against gravity. When a gas region was placed above the liquid, bubbles formed under certain vibration conditions, which changed the system dynamics and could cause the piston
to move downward and compress the spring. However, these experiments proved very difficult to repeat since bubble sizes and locations could not be controlled from test to test. It was therefore decided to replace the gas with three different bellows types which were selected to have compressibility similar to the bubbles so that experiments could be run multiple times to gather sufficient data to gain statistical significance. A Labworks ET-140 Electrodynamic Shaker was used to determine the resonance of the bellows, from which the bellows effective spring constant and damping could be calculated. A weight was glued to the bellows top before it was mounted on the shaker. The shaker oscillated over a controlled frequency and acceleration range. The motion of the bellows was captured by an Allied Vision Technologies camera so that the resonant frequency could be determined for each bellows. Spring constants ranged from 3555 to 11348 N/m for the three bellows types. A Texture Tester was also used to verify the data determined from the vibration tests. A force was applied to the bellows, and its movement was monitored. Hooke’s Law was then used to calculate the spring constant k. These tests proved the bellows of the same size had similar damping and spring constants. Once fully characterized, bellows were placed at the top and bottom of the oil filled piston apparatus and the system was subjected to vibration over a range of accelerations and frequencies. These tests, which are still ongoing, show that the size of the gas region controls the system dynamics. Therefore, any liquid-filled dynamic system that may be subject to vibration during operation must be carefully designed to prevent the presence of gas which can change its behavior.
Funder Acknowledgement(s): Sandia National Laboratories
Faculty Advisor: Timothy O'Hern, email@example.com