# Green Shipping--Multifunctional Marine Scrubbers for Emission Control: Silencing Effect
## 3 Results and Discussion
### Effects of scrubber Design on Transmission Loss  
The basic design of the scrubber (i.e., characteristics of the perforated plates and scrubber diameter and height) should comply with the requirements of the chemical processing of exhaust gases and cannot be modified to improve the acoustic performance of the component. As such, the influence of modifications on the TL that do not influence the chemical process, such as the inlet/outlet pipes diameter and length, were investigated using the assessed FEM model without tube discontinuity. Moreover, the influence on the acoustic performance of the insertion of elements as perforated pipes or filler was studied.  
The first modification that was performed was the increase and the reduction of the inlet pipe diameter by 20 mm in respect to the original one (35 mm) while keeping the other dimensions constant. The results reported in Figure 15a clearly show that reducing the inlet diameter leads to an increment of the TL along the whole range of considered frequencies, whereas the opposite occurs upon increasing the inlet diameter.  
Figure 15b reports that the results for the outlet diameter change by 20 mm, while the other dimensions were maintained as the originals, reducing the diameter, allowing the TL amplitudes to increase but decreasing the TL minima. The increase in the diameter also leads to a worsening of the TL in this case.  
Considering the effects obtained by alternately varying the inlet and outlet diameters, it was decided the influence of the simultaneous reduction of the diameters by 20 mm should be evaluated.  
Considering the effects obtained by alternately varying the diameters of the inlet and outlet pipes, the influence of the simultaneous reduction of the diameters of both the inlet and outlet pipes by 20 mm was evaluated. Figure 15c shows a clear increment of the TL due to this change. This result is consistent with the literature [22]: increasing the expansion ratio between the cross-sectional area of the expansion chamber (body of the component) and the inlet/outlet pipe, the TL amplitudes increase. In the presented study, the expansion ratio of the original model-scale scrubber and the modified one with inlet and outlet pipe diameters of 15 mm were, 5.9 and 32.0, respectively.  
The lengths of the inlet and outlet pipes were also increased and decreased by 50 mm without any appreciable effects on the TL (data not reported for brevity), which is consistent with the plane wave propagations in the tube [37].  
Finally, the insertion of perforated pipes and filler into the model-scale scrubber, as illustrated in Figure 16, was investigated. Two perforated pipes with the following characteristics were considered: a pipe with a diameter of 17 mm, a thickness of 1.5 mm, holes featuring a diameter of 4 mm, and holes spacing of 8 mm. The filler had a flow resistivity of 3000 Pa\(\cdot\)s/m\({}^{2}\), which is typical of a rigid metal foam, and was 30 mm thick. Moreover, the water at the bottom of the scrubber was removed, as a water drain can be added to the scrubber in order to increment the TL and without influencing the chemical properties.  
A perusal of Figure 17 shows that the insertion of the perforated pipes not only increases the TL amplitudes but also changes the fundamental frequencies of the system; this aspect has to be taken into account when considering coupling with the engine, as the transparent frequencies must not match with the exciting ones. The insertion of the filler clearly improves the TL and changes its features due to the viscous dissipation generated by the passage of the sound waves through its porous structure. The simultaneous addition of filler and perforated pipes further increased the TL, coupling the dissipation effects of the porous structure and the holes.  
Figure 15: Influence of the pipe diameters on the model-scale scrubber TL: (**a**) inlet pipe, (**b**) outlet pipe, (**c**) combined inlet/outlet pipes.