Chapter 3 – Silencer Designs
This chapter introduces the silencer designs tested during the course of this research. The chapter is separated into two main sections: silencers tested on the ZDZ 40-RE 40 and 3W-150iB2 engines, respectively. This separation is made because the ZDZ 40-RE engine has a single exhaust port, while the 3W-150iB2 engine has two exhaust ports. Some of the silencers tested were commercially available designs. The purpose of testing commercially available silencers was to provide a baseline so comparisons could be made to refractory foam silencers. However, most of the silencers were fabricated specifically to solve the acoustic problem presented by the UAV. The silencer descriptions include their internal volumes, weights, exit-to-inlet area ratios, and fabrication materials. The volume occupied by these silencers constitutes a key parameter in whether or not they can be effective in this UAV vehicle. Weight is also a large factor in designing a UAV silencer. Silencer exit-to-inlet area ratios provide a method of comparing the constriction imposed on the exhaust gas flow due to exit and inlet geometries. It is also an important parameter in noise attenuation. The materials used to fabricate a silencer have a large effect on the weight and effectiveness of the design. Standard materials such as aluminum and steel were used in many of the tested silencer designs, but more advanced materials were also explored. Pictures for each silencer and drawings showing the external and internal dimensions will be presented. The inside dimensions for the commercial designs are not known
Silencers designed for the single cylinder engine
Seven different silencers were tested on the ZDZ 40-RE 2-stroke single cylinder engine. Two of these were commercially available silencers and the remaining five silencers take advantage of refractory foam. Out of the five refractory foam silencers, one was previously tested for use on small passenger planes , and two were modifications of silencers previously tested on small passenger planes. The remaining two silencers were designed and fabricated in-house based on knowledge gained from testing previous designs
Double exhaust exit:
The double exhaust exit silencer is commercially available as the ZDZ 40 RV-L and it was specifically designed for this engine . Figure 3.1 shows the outer dimensions of the silencer, and a picture of the double exhaust exit design can be seen in Figure 3.2. It is made of aluminum and its inner volume is 6.9 in3 . It weighs 186.6g. The double exhaust exit has an exit-to-inlet area ratio of 0.79, which causes a small constriction in the exhaust gas flow.
Black single exit:
The black single exit is a silencer commercially available as the SMQ-M . Figure 3.3 is a drawing showing its outer dimensions, and a picture of this silencer is shown in Figure 3.4. The exit-to-inlet area ratio is 0.39, which makes this the most constrictive silencer tested. The inner volume of the black single exit silencer is 15.5 in3 , and its weight is 218.3g.
The airplane silencer incorporates SiC refractory foam material and it was previously tested as a small passenger plane silencer . Therefore, it is larger than many of the acoustic solutions explored in this research, but knowledge of its acoustic performance will still be valuable. This design is very similar to the common dissipative design, i.e. an inner perforated tube and a mineral wool/steel wool filled chamber. The airplane silencer improves on this basic dissipative design by maximizing contact with the absorptive material. This improvement is accomplished by replacing the perforated pipe and mineral wool/steel wool combination with the refractory foam cylinder. By relying on the inherent stiffness of refractory foam, the perforated pipe can be removed. The design of the airplane silencer is shown in Figure 3.5. It incorporates a 1” thick cylindrical refractory foam absorbing liner housed inside a welded steel shell. The volume of the airplane silencer is 165.2 in3 , and its weight is 1969.0g. This silencer was originally designed for a larger engine, so it has an exit-to-inlet area ratio of 9.82, which is the largest of the silencers tested. The exhaust port area of the single cylinder engine is only 0.5in 2 , so an adapter had to be made to match up to the 2.5” diameter silencer inlet used for the larger airplane exhaust it was originally designed for. According to Selamet , removing the inner perforate tube can greatly increases the effectiveness of a dissipative silencer. The stiffness of refractory foam provides a significant advantage over similar designs that require a perforated tube to keep the absorbent in place. A picture of the airplane silencer design can be seen in Figure 3.6
Airplane silencer w/ plug:
Since the airplane silencer was originally designed for a much larger engine, improvement in noise attenuation can be achieved by reducing the silencers exit-to-inlet area ratio. This is achieved by reducing the exit area, which causes an increase in acoustic reflections inside the silencer. This increase in reflections will provide more contact time with the absorbent. To achieve this in the airplane silencer, a two layer perforated plug of refractory foam material and pine was added to the exit pipe of the airplane silencer as shown in Figure 3.7. The plug in the airplane silencer w/ plug design was perforated with 3 quarter inch holes, thus reducing the exit-to-inlet area ratio to 0.30. The added plug increased the weight to 2020.0g.
The chevron liner silencer implements a more complicated flow pattern than the constant cross-section seen in the airplane silencer. It was named for the basic chevron shape of its refractory foam liner. A drawing of the chevron liner can be seen in Figure 3.8. It’s 178.3 in3 volume makes it the largest silencer tested. This refractory foam liner was also designed for silencing small passenger airplane noise in the same project that the airplane silencer was used . The shell is made of aluminum sheet metal wrapped around machined aluminum end caps. A picture of the chevron liner silencer is shown in Figure 3.9. The purpose of this flow path was to break up pressure pulses by eliminating line-of-sight flow, and to provide a decrease in flow velocity by incorporating a small expansion in the middle of the liner. The exit-to-inlet area ratio for the chevron liner is 1.57, and its weight is 2356.0g. This means the exit area is larger than the entrance area, which allows for a more restrictive internal refractory foam flow path without greatly increasing the backpressure. This design is a good example of the kind of imaginative flow paths that can be formed using refractory foam.
This design again takes advantage of the complex shapes that can be machined out of refractory foam. The bend flow silencer uses circular 1” thick cross sections of refractory foam glued together in alternating orientations. The internal dimensions that result from these cross sections can be seen in Figure 3.10. This method increases the effective length of the silencer by “bending” the path of least resistance through three half-inch diameter holes in each of the five walls. It is important to note that the path of least resistance is not the only path available to the flow. The porosity of the refractory foam causes absorption of exhaust noise as it flows though the walls. This design technique maximizes the exhaust flow contact with the refractory foam. A picture of the bend flow design can be seen in figure 3.11. For its 165.7 in3 internal volume, this design provides more refractory foam area than the simple airplane silencer. The bend flow silencer has the same 1.57 exit-to-inlet area ratio as the chevron liner because they use the same aluminum end caps. The weight of this silencer is 2238.7g.
Bend flow half:
This design uses the same concept as the previous bend flow silencer, but at 77.8 in3 , the bend flow half incorporates less than half the volume of the bend flow design. The dimensions of the bend flow half are shown in Figure 3.12, and a picture of this design can be seen in Figure 3.13. The bend flow half was designed to take what was learned from the bend flow silencer, and apply it to develop a more compact version. The exit-to-inlet area ratio of the bend flow half is the same as the bend flow silencer because it shares the same shell design as the two previous silencers. The reduced length of this design brought the weight to 1942.4g. This weight reduction is minimal because the aluminum end caps used on the last 3 designs account for 1510g. The bend flow half is a very promising silencer design for the UAV application, because it utilizes a small and lightweight package that still provides adequate flow contact with the absorptive material
Chapter 1 Introduction
1.1 Engine Noise Sources
1.2 Review of Conventional Silencers
1.3 Research Objective and Approach
1.4 Thesis Organization
Chapter 2 Experimental Setup & Data Processing
2.1 Preliminary speaker tests
2.2 Engine performance tests
2.3 Data post processing
Chapter 3 Silencer Designs
3.1 Silencers designed for the single cylinder engine
3.2 Silencers designed for the twin-cylinder engine
Chapter 4 Preliminary Speaker Test Results
4.1 Acoustic contribution of inlet and exit pipe geometry
4.2 Speaker input test results
Chapter 5 Engine Test Results
5.1 Preliminary engine tests
5.2 Engine test results for single cylinder engine designs
5.3 Engine test results for twin cylinder engine designs
Chapter 6 Conclusions
6.1 Refractory foam conclusions
6.2 Recommendations for future research
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