Studies Of Submarine Engine Exhaust Silencers And Thermal Infrared Signature Reduction

The 3-D numerical methods are used to predict the acoustic attenuation performance and flow resistance characteristics of submarine engine exhaust reactive silencers. In addition, simulations of temperature drop of exhaust gas by the use of dry type cooling silencer and water injection in tail pipe through spray nozzle are also performed. In this thesis, the fundamental theory of acoustic finite element method and brief introduction to the finite volume method, as well as the related software SYSNOISE and FLUENT are introduced. The transmission loss is predicted by finite element method using SYSNOISE, while pressure drop is calculated by finite volume method using FLUENT. Temperature drop simulations of both dry type cooling silencers and water injection in tail pipe are carried out by using heat transfer module and spray model in discrete phase modeling of FLUENT software.During surfacing and snorkeling, a submarine has to compromise its inherent stealth, as Exhaust Noise of diesel engines and the associated IR/Thermal Signatures of hot exhaust gases increases the vulnerability towards detection by sonars and IR detectors of hostile ASW aircrafts and vessels. Submarine diesel engine exhaust noise is controlled through the use of reactive silencers. The objective of the present work has been to simulate the acoustic and aerodynamic performance of selected submarine reactive silencers and to devise a method to minimize the IR/Thermal signatures of engine exhaust. Cooling of exhaust gases will result in a decrease of temperature of tail pipe or exhaust mast and a shift in the wavelength, thus masking the associated IR signatures.Numerical results of transmission loss of simple expansion chamber with offset extended inlet/outlet and expansion chamber with offset perforated extended inlet/outlet tubes showed that repeating dome behavior characteristics of one dimensional propagation do not extend beyond the first asymmetric (1,0) mode. Also predicted pressure drop of these two silencers are almost the same. Double chamber reactive silencers with double inter-connecting tubes have higher pressure drop and provide higher acoustic attenuation than the single chamber silencer at most frequencies after its first pass frequency. Using guiding annulus, the pressure drop may be decreased effectively, with negligible effect on acoustic attenuation performance. The influence of increasing the value of clearance 'L_C' on the acoustic performance of reversing flow silencer is negligible at low frequencies and limited at higher frequencies. For reversing flow silencer, pressure drop varies inversely with the clearance space between the shell and guide plates.Temperature drop numerical simulation results of cooling silencers show that increasing the exhaust inlet velocity adversely affect the cooling process, resulting in a lower overall temperature drop. Temperature drop of exhaust gas passing through cooling silencer depends upon the cooling silencer dimensions i.e. larger the heat transfer surface area, greater will be reduction in the exhaust gas temperature. Water injection in the exhaust gas through spray nozzle has remarkable results and results in a greater reduction in exhaust gas temperature as compared to dry type cooling silencer.