Final Design

What is it?

The final design of our noise suppression consists of a full length diffuser. This design is based off of the V^8 law which states that the noise produced by a flow is proportional to the velocity to the eighth power, meaning that a slight reduction in exhaust velocity produces a large decrease in noise. The stainless steel diffuser is able to withstand the high temperatures of the exhaust, and the 24" length provides the longest distance for decreasing the bulk exhaust velocity while still adhering to the physical size constraints. The diffuser has a 2.48" inlet diameter connected to the turbine exit and a 4.5" exit diameter, resulting in the largest diffusing effects (or maximum noise output reduction) allowable due to space constraints.

How does it work?

Applying a simplified volume balance equation (which ignores compressibility effects) shows a significant reduction in exhaust velocity. According to this model, Density*Area*Velocity at the inlet equals that at the outlet. Rearranging the equation and assuming constant density yields Vout = 0.3*Vin.

This analysis is perhaps a little oversimplified. The compressibility of air can be ignored for small mach numbers (less than 0.3M) but this engine operates in the realm of ~0.5M. Additionally, the V^8 law works off of the maximum velocity of the flow; the flow has a relatively uniform velocity at the inlet of the diffuser, while the velocity of the flow spikes at the exit. These effects are extremely hard to model, especially for turbulent flow, but the idea behind the simplified model is accurate. As the exit area increases, the noise level drops!

What else was considered?

This Final Design as well as the Runner Up Design (see bottom of Multimedia page) were tested side by side. The Runner Up Design incorporated several tactics used in the jet engine industry, such as using chevrons to help mix the exhaust with the ambient air and a secondary, lower velocity flow which separates the main exhaust flow from the ambient. The Runner Up Design also incorporated a slight diffuser. The maximum noise reduction in the jet engine industry is around 2.7 dB, and the Runner Up Design was on par with this figure, while the diffuser was able to decrease the noise output by an average of 8 dB and a maximum (directly behind the turbine) of over 15 dB!

How well does it work?

Although the 8 dB decrease experienced does not seem substantial when compared to the 120 dB source, it is actually a significant achievement. As a sense of magnitude of this result, a 3 dB decrease in noise is the same as saying that the noise output contains half of the power. Therefore, a 9 dB decrease has 1/8 the power!

Sound intensity (Power/Area) is a function proportional to 1/Distance^2. In comparison to the noise decrease experienced, the listener would need to be 2.5 times closer to hear the same level of noise. The Final Design does an excellent job of decreasing the noise of the Jet Engine.