Martinez, Matthew Aaron (2018-08). Passively Actuated Vibration to Decrease Membrane Fouling Rate for Centrifugal Reverse Osmosis. Master's Thesis.
A novel method of shaking a CRO membrane was designed. This method, called the Plug and Pipe method, was simulated to determine how well it would work in a full CRO system. This was done by a series of simulations. First, the geometry was meshed and a CFX simulation was performed. This would be used to find the water velocity at discrete points along the radial direction of the orifice. These discrete velocities were then put into a curve fitting program, MATLAB's cftool which provided a velocity distribution across the orifice. This velocity distribution was then integrated and to find the flow requirements and water jet force, Fvjet. Fvjet was found to depend in large part on the orifice size, while other variations, such as Pipe length and diameter, had little to no effect. Fvjet varied from 27.13 N to 94.64 N, based on orifice size. This water jet force was then included in a dynamic model, which was created to describe the nonlinear equation of motion of the shaker. This dynamic model had two parts. The first was the script into which the characteristics of the system were put, called osc_model, while the second was the function which held the nonlinear forcing functions for the water jet and wall interactions. The simulated shaker was designed based on a test size membrane of 28 kg. It was determined that the shaker would be capable of providing the proper dynamic characteristics based on previous work, which were a 1mm amplitude of vibration at 20 Hz. Another model was created to show the amount of mass that could be shaken by a full shaker based on the orifice size and wall stiffness. The orifice diameter was varied from 0.16 in to 0.5 in, while the wall stiffness varied from 10^5N=m to 10^8N=m. It was found that, as stiffness approached 10^8 N=m, the amount of mass that could be shaken began to plateau, meaning that this was the value at which the wall began to act rigidly for this system. The flow requirements were provided for each orifice size as well, and it was determined that any orifice diameter above 0.25 in would have excessive flow requirements and would limit a CRO rig's ability to purify water. An experimental rig was built based on the specifications set out in the simulations as well. A high pressure gear pump was used to bring the water up to osmotic pressure, about 800 psi. This high pressure water went into the shaker. It was intended that once the shaker showed vibration, adjustments would be made to achieve the 1 mm and 20 Hz vibration set by previous work. Upon final installation, however, there were misalignment problems between the shaft and the linear bearings, which prevented the shaker from vibrating at all.