Research On The Coupling Mechanism Between 3D Flow Field And Flow-Induced Acoustic Field For Centrifugal Pumps Based On Powell Vortex Sound Theory

Acoustic performance is one of the evaluation criteria of comprehensive performance for centrifugal pumps,and it is also a vital evaluation criterion for centrifugal pumps used in ship,military and other fields.The flow-induced noise of centrifugal pumps deteriorates the working and living environment,and is also one of the most important issues that affect the operating reliability of centrifugal pumps.The flow-induced noise of centrifugal pumps is associated with the unsteady flow,so it is crucial to research the coupling mechanism between 3D flow field and flow-induced acoustic field.By analyzing the generation mechanism,distribution characteristics and propagation mechanism of acoustic perturbation in the fluid,the coupling mechanism between 3D flow and acoustic fields for centrifugal pumps can be revealed comprehensively.The results can lay a theoretical foundation on the flow-induced noise control technology research,provide guideline for the 3D optimization design of centrifugal pumps,and also guide the related research on the flow-induced noise of other turbomachinery.Vortex sound theory points out that the generation of acoustic perturbation is associated closely with the vortex,potential flow and so on in flow field,which is of great significance to research the coupling mechanism between flow and acoustic fields.Based on Powell vortex sound theory,this thesis focuses on the key problems,such as the generation mechanism of acoustic perturbation,the distribution characteristics of acoustic field,as well as the flow-induced noise propagation mechanism of centrifugal pumps.The purpose of this research is to establish a numerical computation model to research systematically the acoustic perturbation generation mechanism,distribution characteristics and propagation mechanism of flow-induced noise through experimental verification,and to present the coupling mechanism between 3D flow field and flow-induced acoustic field for centrifugal pumps.The main contents and results are as follows:(1)The numerical solution of Powell vortex sound equation is constructed and the corresponding User Define Function(UDF)is compiled.A 3D implicit numerical computation model coupling flow and acoustic fields is established,which combines the Powell vortex sound equation with computational fluid dynamics,and the time-domain acoustic source and frequency-domain acoustic source intensity(ASI)are introduced to characterize the acoustic source magnitude,which lays a theoretical foundation on exploring the generation mechanism of acoustic perturbation and the distribution characteristics of acoustic field.Besides,the flow-induced noise measurement platform is improved,and the noise in the centrifugal pump under various operating conditions is measured.In addition,the frequency response characteristics of noise,and the changing rules of noise with operating condition are analyzed,which verifies the accuracy of the numerical computation model,and also lays a foundation for the research on the noise propagation mechanism.(2)The unsteady fluctuation characteristics of fluid velocity and vorticity in the centrifugal pump are analyzed under various operating conditions according to the Powell vortex sound theory,which lays a foundation for the research on the acoustic perturbation generation mechanism and the acoustic field distribution characteristics.It is shown that the unsteady fluctuation characteristics of fluid velocity and vorticity in the pump are related to the position of blade relative to volute tongue.The blade motion process is accompanied by the phenomena such as the vortex generation,migration,mergence and rupture,etc.With increasing rotational speed or decreasing flow rate,the fluid flow in the pump tends to be disordered.(3)The temporal-spatial distribution characteristics and evolution rules of acoustic field in the centrifugal pump are researched based on the numerical computation model under various operating conditions.The generation mechanism of acoustic perturbation is revealed.It is proposed that the time-domain acoustic source fluctuation induced by unsteady fluid flow results in the generation of acoustic perturbation in the pump.The time-domain acoustic source obtained by velocity and vorticity is less than 0 basically,and the acoustic pressure perturbation is less than 0 accordingly.The minimum value region of the two parameters is distributed near the volute tongue.During the periodically rotational process of impeller,under the influence of unsteady flow characteristics of flow field,the minimum value region of time-domain acoustic source in the pump shows the evolution rules that concentrate at the volute tongue,then break,extend,migrate,and reaggretate near the tongue with the blade motion process,which causes the similar evolution rules of acoustic pressure perturbation(4)Introduce the acoustic source standard deviation(ASSTD)to characterize the time-domain acoustic source fluctuation degree.The ASSTD distribution characteristics of time-domain acoustic source in the centrifugal pump is discussed,the main acoustic source region and main characterized acoustic source in the pump are revealed.It is shown that the maximum value region of ASSTD includes the volute tongue and the region of the tongue extending slightly downstream along the rotational direction of impeller,which is the main acoustic source region in the pump.The source component induced by non-uniformity of fluid kinetic energy is the main characterized acoustic source,take the volute tongue as an example,the ASSTD of the characterized source accounts for about 95%of the overall AASTD.(5)The frequency-domain distribution characteristics of acoustic field in the centrifugal pump,including the acoustical intensity changing rules from impeller inlet to outlet in the impeller and the directivity distribution characteristics of acoustic field in the volute,are explored based on the numerical computation model under various operating conditions.It is shown that in the impeller passage,the total acoustic source intensity(TASI)on the curve far away from the tongue increases rapidly first,then increases slowly from inlet to outlet of impeller under various operating conditions,and the total acoustic pressure level(TAPL)shows the similar increasing trends.The two parameters increase by 4.5%and 6.9%from inlet to outlet of impeller under various operating conditions,respectively.The results indicate that the change in operating condition has little effect on the evolution process of blade-volute wall interaction intensity.While on the curve near the tongue in the impeller,a mutation of TASI appears near the impeller outlet,which results in the similar mutation of TAPL,and the two parameters increase by 6.6%and 7.6%from inlet to outlet of impeller under various operating conditions,respectively.Besides,the maximum mutation range of TASI and TAPL moves to the middle section of impeller with increasing rotational speed or decreasing flow rate.The results reflect that a mutation of blade-tongue interaction intensity exists near the impeller outlet,and the maximum mutation range moves to the middle section of impeller with the intensification of flow instability.In the volute passage,the blade motion leads to the difference of TAS1 at different locations in circumferential direction,the dipole distribution characteristics and the distribution characteristics that the maximum is located at the tongue of TASI are formed,which further results in the similar directivity distribution characteristics of TAPL.(6)The difference of the changing characteristics between the noise at pump inlet and outlet and the overall noise in the centrifugal pump with operating condition is compared.It is found that when the rotational speed is lower than the rated rotational speed,the noise at inlet and outlet under various rotational speeds with rated flow rate is 0.5%and 0.3%higher than that under various rotational speeds with the opening of the valve kept unchanged,respectively.While the overall noise under various rotational speeds with rated flow rate is 0.7%lower than that under various rotational speeds with the opening of the valve kept unchanged.Therefore,the comprehensive results of several representative monitoring points should be considered to evaluate the flow-induced noise in centrifugal pumps.(7)The changing mechanism of the temporal-spatial distribution characteristics and the frequency-domain distribution characteristics of acoustic field,as well as the noise at different locations and the overall noise in the centrifugal pump with blade outlet angle is explored based on the numerical computation model.It is revealed that the temporal-spatial distribution characteristics and the frequency-domain distribution characteristics of acoustic field change little with the change in the angle.Beside,the increase in the angle causes the interaction intensity of blade tail-volute wall and blade tail-tongue decreasing and increasing,respectively,which is characterized by the decrease and increase of the time-domain acoustic source fluctuation range and TASI at corresponding locations,respectively,and causes the TAPL changing accordingly.When the angle increases from 15°to 35°,the TASI and TAPL at impeller outlet far away from the tongue decrease by 1.2%,respectively,while those near the tongue increase respectively by 0.6%and 0.9%,and the two parameters at the volute tongue increase by 1.6%and 2.4%,respectively.Additionally,the overall time-domain acoustic source fluctuation degree and frequency-domain acoustic source intensity increase in the pump with increasing the angle,and causes the overall noise increasing gradually.When the angle increases from 150 to 35°,the overall frequency-domain acoustic source intensity and noise increase by 0.9%and 1.4%,respectively.(8)Introduce the time-domain-averaged synergy angle(?tave)to characterize the whole synergy degree between flow and acoustic fields,the corresponding UDF is compiled based on the flow and acoustic fields synergy analysis.A 3D numerical computation model used for investigating the flow-induced noise propagation mechanism is established.The comparison between the noise measurement results and the synergy degree results obtained by numerical simulation is conducted.It is shown that the ?tave in the impeller and volute is 85.9° and 91.20 under rated operating condition,respectively.With the growth of rotational speed or flow rate,the ?tave in the pump moves far away from 90°,meanwhile,the noise outside the pump increases gradually.The changing trends of noise and ?tave with operating condition are consistent basically.The results manifest that the flow-induced noise propagation characteristics in the pump can be well described by the change in the synergy degree between flow and acoustic fields,and the increase in the whole synergy degree would cause the flow-induced noise in the pump tending to propagate outside,i.e.,the synergy degree between flow and acoustic fields affects the noise outside the pump.In addition,the?tave in the pump is close to 900 gradually with increasing the blade outlet angle.When the blade outlet angle increases from 15° to 35°,the deviation of ?tave in the pump from 900 decreases from 6.5° to 4.7°,the whole synergy degree between flow and acoustic fields decreases,indicating that the increase in the blade outlet angle can weaken the tendency of flow-induced noise in the pump propagating outside.In summary,this thesis establishes a comprehensive numerical computation model to research the flow-induced noise.The generation mechanism of acoustic perturbation,the distribution characteristics of acoustic field and the noise propagation mechanism in the centrifugal pump are explored systematically,and the coupling mechanism between 3D flow and acoustic fields for centrifugal pumps is revealed,which is of great theoretical significance.Additionally,this thesis points out the clear location for the acoustic source control in centrifugal pumps,and puts forwards a more accurate evaluation criterion for flow-induced noise in centrifugal pumps and the flow-induced noise control method that takes the noise propagation characteristics into account,which has an important engineering application value.Meanwhile,the numerical computation model has a wide applicability,and can be used in the related research on flow-induced noise of other turbomachinery.