Diffusers, ducts that slow down the flow through them, are very common in the Aeronautical industry. Slowing down the flow, diffusers also increase the pressure of the fluid. Since high pressure flow is very useful, diffusers which can increase effectively the pressure of a fluid are desirable. The increase in the fluid's pressure is proportional to the increase in the diffuser's opening angle. However, at great opening angles (with respect to the diffuser's centerline), the flow separates from the walls, or stalls. When the flow stalls, its pressure rise ceases to be proportional to greater opening angles. Therefore, the opening angle of a diffuser is set by the angle when the flow stalls. Research shows that certain disturbances in the diffuser's flow can eliminate the adverse effect that stall has in the pressure rise of the fluid.1 These disturbances can be controlled so that the diffuser can be opened more while still increasing the flow's pressure rise. This wider diffuser can also be shorter and therefore lighter. Thus, a controlled diffuser can be designed which will save money to industry.
A diffuser will be designed and built with a closed loop control which senses stall and creates the necessary disturbances to maintain a pressure rise in the flow. The effect of the control system on the performance of the diffuser will be measured as described in the 'Approach' section. The results will be compared to the research referenced and recommendations will be made for further work and future diffuser designs.
The diffuser to be designed and built will be 2D subsonic. It will be attached to the 1 foot by 1 foot wind tunnel in MIT Building 17A. The wind tunnel will generate flow at different velocities through the diffuser. For simplicity, only one of the symmetrical halves of the designed diffuser will be built. That is to say, the built diffuser will have a straight wall and an angled wall. Stall will only occur in the angled wall, so less sensors will be needed. The angled wall will be able to rotate so that tests can be conducted at a variety of opening angles. Hot wires will be placed on this wall at a variety of positions and depths, where the device will measure the flow's velocity. These readings will be used by the control system to sense when stall occurs. A signal will be fed back to an actuator that will create the disturbances in the flow. The actuator will be either jets that will inject air into the flow, oscillating flaps or sound speakers. Pitot tubes will also be placed on the diffuser's angled wall at a variety of locations. These will measure the pressure of the flow. The pressure rise in the flow will be used to quantify the performance of the diffuser.
1. Oh, David, Active Control of Airflow Instabilities in 2D Diffusers. 16.622 Final Report, MIT, 1990.