| With the increasing demand of economy performances of marine low-speed diesel engines from the shipbuilding industry,a centrifugal compressor with a high flow rate and a high-pressure ratio has been used in turbocharges of marine low-speed diesel engines.However,a high-pressure ratio induces complex internal flows in the compressor,which increases flow induced noise.Noise regulations for marine low-speed diesel engines are stricter than ever,the current IMO standard requires no more than 105 d B(A)for the noise in the main engine room,while the noise limit of marine diesel engine turbocharge with a pressure ratio greater than 3.5 in China is still 120 d B(A).Noise sources of a marine low-speed diesel engine mainly include combustion noise,mechanical noise and aerodynamic noise.In addition,aerodynamic noise is dominant at medium and high frequencies.Moreover,a centrifugal compressor is the main contributor to aerodynamic noise of a diesel engine.Therefore,reducing the aerodynamic noise of a centrifugal compressor can effectively reduce the diesel engine noise as well.However,the marine turbocharger operates in a wide range of conditions,noise characteristics vary accordingly.Moreover,it is very difficult to decompose complex noise sources,and the noise mechanism still needs to be further studied.The aerodynamic noise of centrifugal compressors in turbochargers has become a hot topic for Chinese and foreign scholars in the past few decades.Unfortunately,the majority of the current research is limited to automotive turbochargers.There is much less research on marine diesel engine turbochargers.This paper experimentally and numerically studies the aerodynamic noise of a centrifugal compressor in a marine low-speed diesel engine turbocharger.,The accuracy and applicability of hybrid computational aeroacoustics(CAA)are analyzed,flow characteristics and the noise mechanism of a centrifugal compressor are discussed,which can be used for aerodynamic noise control and low-noise compressor design in the future.The main contributions of this research are as follows:(1)A series of experiments were conducted,including a radiation noise test of a marine low-speed diesel engine,a radiation noise test of a marine diesel engine turbocharger and an aerodynamic noise test of a centrifugal compressor.The aerodynamic noise characteristics of a centrifugal compressor and the contribution of compressor aerodynamic noise to diesel engine noise are analyzed based on the experimental results.The aerodynamic noise of a centrifugal compressor is mainly composed of tonal noise,buzz-saw noise and broadband noise.Specifically,tonal noise is a dominant component,the buzz-saw noise peaks at high operating speeds and also has a high sound pressure level(SPL).The artificial neural network(ANN)model constructed by the experimental results of limited operating conditions can be used for noise analysis at any operating conditions within the test operating range.In addition,the noise map of a centrifugal compressor can also be obtained using the ANN model.The ANN model can accurately predict aerodynamic noise spectra at interested operating condition,the discrepancies between the predicted overall SPL using ANN model and experimental measurements are less than 3.2d B.However,the ANN model has some shortcomings in predicting details of the linear spectra due to the limited training data.(2)The hybrid CAA approach based on computational fluid dynamics(CFD)and acoustic boundary element method(BEM)in the prediction of aerodynamic noise is validated.The impacts of the numerical setup of unsteady CFD simulations on the prediction of compressor flow and aerodynamic noise are discussed.It is found that the whole flow passage is able to take into account distinctions and interactions between different blades;therefore,it can accurately predict the compressor aerodynamic noise.The turbulent intensity is enhanced when the blade surface has a large roughness which increases the broadband noise.It is recommended that Ra should be less than 12.1μm in real applications,which means that the blade surface roughness cannot be ignored in CFD simulations for a noise study.A large time step would result in unreliable numerical results,the time step 2° can get accurately predicted results while reducing the computational efforts.It is also found that a large total simulation time with a reasonable time step is helpful to improve predicted noise spectra.The average SPL discrepancy of aerodynamic noise between numerical prediction and experimental measurement is only 0.5d B.The numerical prediction is performed using the selected numerical model and time parameters.(3)The unsteady flow and aerodynamic noise characteristics of the compressor at the design condition,the near surge condition of a design speed and the upper limit condition are studied using the hybrid CAA method based on CFD and BEM,in which the compressor operates with ideal intake.The relationships between unsteady flow features and aerodynamic noise characteristics of the compressor are analyzed,and the fluid mechanism of different aerodynamic noise components is discussed.When the compressor operates with ideal intake without an intake muffler,the tonal noise at the design condition is induced by the blade aerodynamic force.This unsteady loading is influenced by the interaction between the rotating impeller blades and the intake flow,and the interaction between the rotating impeller blades and the diffuser.The buzz-saw noise results from the shock waves at the leading edge of the impeller blades.The interaction between the impeller and the volute tongue has a certain influence on the tonal noise and the buzz-saw noise.The stall phenomenon appears at the near surge condition of the design speed and the upper limit condition,which significantly increases the broadband noise,the SPL of broadband noise increases over 20 d B,but the tonal noise is still a dominant noise component.Meanwhile,the SPL of broadband noise is higher than buzz-saw noise when stall occurs in a compressor,which reduces the influence of buzz-saw noise peaks.(4)The hybrid CAA approach based on computational fluid dynamics(CFD)and acoustic finite element method(FEM)in the prediction of aerodynamic noise is validated.The aerodynamic noise of a compressor operating with in-cabin intake can be effectively reduced by an intake muffler.However,the muffler induces complex internal flows inside the compressor,and the structure of the intake muffler has a negative impact on the compressor aerodynamic performance.It is found that the hybrid CAA method can also be used in predicting the insertion loss of the intake muffler,and the discrepancy is less than 3d B.The unsteady flow and aerodynamic noise characteristics of the compressor with a straight inlet pipe at different operating conditions are analyzed.The results show that the straight pipe has little influence on the flow and aerodynamic noise of the compressor at the design condition and the near surge condition of design speed.The aerodynamic noise characteristics and flow mechanism of the compressor with straight intake pipe are nearly the same as the compressor with an ideal intake.The influence of variable cross-section pipes on the unsteady flow and aerodynamic noise of the compressor is analyzed.It is found that a large intake section area improves the compressor aerodynamic performance,but the size contraction of the cross-section causes flow deterioration in the compressor,thus,reduces the aerodynamic performance.The compressor aerodynamic noise characteristics in the taper pipe with gradual changed cross-section area are similar to those of the straight pipe.The nozzle pipe induces a severe stall in the compressor.As a result,the radiation noise in the nozzle pipe is increased,and the contribution of broadband noise to the overall SPL is also increased.The aerodynamic noise source is enhanced in the compressor with the contraction pipe,but the radiated sound wave is reflected at the section with the minimal area of the contraction pipe,which leads lower aerodynamic noise in the pipe. |
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