Study On The Flow And Acoustic Characteristics And The Noise Reduction Methods Of Submerged Exhaust

Submerged exhaust, which is defined as the process of gas exhausting into liquid environment, is usually accompanied by complex flow phenomena and strong noise radiation. In industry fields, many processes such as bubbling, aeration, jet mixing etc. can involve in the submerged exhaust. In these areas, the use of acoustic signal processing has gradually shown great advantages in obtaining flow field information, and has more and more become one of the hottest topics for scientific researches and engineering application. In the underwater equipment field, the engine exhaust also refers to this submerged exhaust process. The noise generated by this process will not only cause heavy interference to vehicle-mounted Sonar and communication systems, but also make the underwater equipment more exposed to noise radiation and more easily detected by non-friendly Sonars, not mention to polluting the underwater environment. Thus how to control the noise generated by submerged exhaust process has been an attractive but challenging subject these days. No matter how to utilize or suppress the submerged exhaust noise, it is highly necessary and urgently demanding to have a deep insight and systematic analysis on the flow and acoustic characteristics of this process.This paper presents the study on the flow and acoustic characteristics, as well as the noise reduction methods for submerged exhaust progress. The flow behavior, main noise sources and noise mechanisms of the submerged exhaust are comprehensively investigated through detailed analysis of the experimental data getting from a carefully designed experiment system. Based on these analyses, a lobed nozzle is appropriately designed so as to control the flow near the nozzle exit and the noise emitting from it. Moreover, several schemas of targeted noise reduction are put forward and validated by various experiments. The main content includes the following aspects:(1) First of all, this paper provides a series of designs for the experiment system and the test schema which are suitable for the study of the flow and acoustic characteristics of submerged exhaust. And it proposes the methods of image sampling and processing, as well as the schema of the signal-video synchronized acquisition with high precision.(2) The flow behavior of submerged exhaust under the conditions of large orifice, high gas flow rates and wide gas velocity are experimentally studied. The periodicity and randomness of the flow are analyzed. The similarity of flow behavior under different flow patterns is investigated and summarized as alternating necking and expansion. The regularity of expansion time and necking positions are also statistically analyzed.(3) The acoustic characteristics of submerged exhaust are studied by means of various experiments. Then it reveals the noise contributions of a variety of flow phenomena. The main noise sources of different frequency bands, especially of the low frequency sounds which dominate the sound spectrum, are found. A noise generation mechanism is found out that loud low frequency sounds are produced by the vibrations of large size bubbles which are excited by the necking. Moreover, the effects of operating conditions on the noise of different frequency bands are also exploited.(4) A flow and noise control schema by improving the nozzle structure is put forward. According to this schema, a lobed nozzle is designed. The effects of the lobed nozzle on the flow behavior, noise and pressure fluctuations related to the submerged exhaust are investigated by using numerical simulations and experimental methods. The results show that the lobed nozzle can improve the continuity of the flow, and can attenuate the pressure fluctuation in the upstream pipeline caused by the necking phenomenon, and can suppress the 10-1000 Hz noise as well. The overall sound pressure level of 10-4600 Hz can be reduced by 3.3-8.6 dB. Moreover, the noise reduction mechanism of the lobed nozzle is analyzed to some extension.(5) Owing to the well understanding of the noise mechanism, several targeted noise reduction schema are put forward, such as the multi-nozzle schema, perforated tube schema and water injection schema. The experiment results show that all these schemas can achieve good enough noise reduction effects. Finally, the suggestions for the design of submerged exhaust noise reduction are summarized.To sum up, the flow and acoustic characteristics of submerged exhaust are systematically studied in this paper. According to the studies of the noise sources and mechanisms, some feasible noise reduction methods are put forward. Some useful noise reduction mechanism is proposed. The contents and conclusions of the current dissertation can provide solid and concrete references for the noise reduction design of submerged exhaust in underwater equipment.