Acoustic and flow design of turbocharger resonators

Nowadays comfort is one of the most important aspects of automobile industry. This has a great effect on the usage of resonator in all kind of cars. Because of the environmental noise regulations of governments, the nose reduction has come to a critical point. Most resonators are used in turbocharger units of diesel engine equipped cars. The reason of the need for a resonator comes from the noise created by the compressor unit. The high pressure profiles created inside the compressor travel through both structure and air inside the pipes towards the engine. In order to decrease the noise one of the best strategies goes through eliminating the source. This is not possible because without the compressor the amount of air burned inside the engine every cycle would be inefficient. The next best way to decrease the noise is to get as close as possible to the source and silence it with a resonator. In this study the steps to design a resonator which must be effective inside a broad frequency band for FIAT is described. The main restrictions are volume, acoustic performance and back pressure. Also the resonator must be able to maintain unharmed during work conditions. Design steps are described for each restriction. One of biggest problems in designing a resonator is that the acoustic performance and the pressure loss are effecting themselves. A better acoustic performance has a negative effect on the flow performance witch results in higher pressure loss of the system. So there must be an iterative solution in order to satisfy both restrictions. Beginning with the structural design, the geometry was chosen to be a pipe with a circumferential resonator volume around. This gives a better vibrational resistance. The resonator was chosen to be produced in two parts welded together. This secures that the production errors are minimized. After the structural analysis with the given boundary conditions from FIAT the structural design was finished. The next step was to get a high as possible acoustic performance. In order to get a large frequency band performance the resonator was divided into 3 acoustic chambers. Every chamber having a different natural frequency made sure that the acoustic performance was meeting the criteria given. The finite element analysis tool ACTRAN was used to calculate the TL values of each design. Firstly, the design was modified in order to be able to have a broad frequency band of performance. The acoustic channels between the chambers and the main pipe were enlarged for lower frequency and also one of the channel was made thinner in order to perform on the large frequencies. After the first couple of analysis, it was seen that the minimum frequency to have a 20 dB TL is 1625 Hz. The upper frequency limit due to the design criteria was optained as 3000 Hz. In order to be sure that the design is aerodynamically satisfying, the first mechanical design was made as straight as possible. With the CFD software Fluent the pressure loss value was calculated. Also the Mach number distribution was investigated in order to have no problematic areas where whistling may occur. After the pressure loss value was checked to be under 1 kPa, prototypes were produced and experimentally tested for their acoustic performance. The experimental acoustic tests were made with impedance tubes according to ASTM E2611-09 standard. The tests were made in a silenced room and und room conditions. Because of production problems many of the resonators show frequency change in the peak values. The prototypes were cut in half and investigated for further mechanical design changes. The correlation between room temperature and working temperature were made with the ACTRAN software, and as a conclusion the frequency of the prototypes were inside the specified frequency band. This study showed that CFD and FE Acoustic analysis are very good tools to design resonators for the industry. By following the steps in this study, it is possible to predict the working performance resonators.