Computational Study Of Acoustic Response Features For Non-premixed Flames By Direct Numerical Simulation

The rise of thermo-acoustic study provides fresh ideas for the detection and diagno-sis of fire-field.In addition to some well-known research focuses,such as combustion heat and products,the acoustical signal generated by flame can also carry fire-related informations.The multidisciplinary coupling of combustion and acoustics can offer a more comprehensive perspective for fire research.Due to the complexity of coupling,however,traditional experimental approaches may face difficulties in identifying the detailed thermo-acoustic process.By contrast,direct numerical simulation delivers fur-ther insight into this special problem since all relevant flow physics should be resolved.Non-premixed combustion is a phenomenon that occurs frequently in fire,however,for all kinds of combustion noise,the sound generated by non-premixed flame remains a challenge because of the limitation given by earlier thermo-acoustical theory.As such,this paper was motivated to seek insights into the sound problem of non-premixed flame based on numerical investigations.The main works include:(1)Constructing a high-precision platform based on di?rect numerical simulation to investigate the sound production of non-premixed flame.Especial attention will be paid to the simultaneous predictions of both the fields of sound and fire;(2)Examining the shear layer effect on the noise process of non-premixed flame,and the essential generation and evolution mechanisms of acoustical source will be provided;(3)Decoupling the effects of various flow instabilities on the noise process in non-premixed flames,and analyzing the variation of sound source structure when the domain is dominated by different instability effects;(4)Based on the theory of low-order model,an acoustic transfer function will be formulated to distinguish the acoustical re-sponse features of different flow instabilities at different flame positions.To predict the sound field simultaneously with flame simulation,Lilley's third-order sound equation is utilized in this paper.By introducing some low-order quantities,the high-order equation can be reduced to a first-order one,where the source term con-structed by flow dilatations is defined as acoustical source.In this way,the source term can be calculated by flow details,making it possible to collect fire and sound data syn-chronously.The numerical solution of Lilley's equation will be validated by comparing the predicted sound spectrum with the DNS results.On the ground that the generation of unstable shear flow is self-sustained,the ap-proach that varies the Prandtl number can be utilized to control the generation ability of unstable flow structure at different flame positions,which allows to explore the de-tailed effect of shear flow on combustion noise.The results showed that the vortical structures in shear layers are responsible for the initial formation of sound wave.The vortices in the vicinity of flame base dominate the low-frequency noise,while those in the downstream flame region is prone to bring influence on the high-frequency noise.Through the comparison of sound source structure and vorticity transport budgets,it can be found that the occurrence of acoustical source in non-premixed flame is attributed to combustion-induced buoyancy.Moreover,the source spread behavior is dominated by baroclinic torque.Further analyses show that buoyancy offers a positive influence on the development of acoustic source,while the volumetric expansion and flame defor-mation are both negative factors.The vortices in non-premixed flame are divided into those outside and inside the flame surface,dominated by buoyant and jet preferred instabilities,respectively.By varying the dimensionless parameters,the decouple of these two types of instability is achieved.In the case of buoyancy-dominated flames,simulations showed that buoy-ancy is positively correlated with both the noise level and peak frequency.When the buoyant effect is less than unit(Fr=1.0),only the low-frequency noise will be affected and when the effect is greater than unity,buoyancy has a further positive impact on those in the high-frequency range.Simulations without buoyancy showed that there is a step-like decrease of jet preferred instability along the streamwise direction,indicating that jet instability can only affect the sound source homed around the flame base.The weaker jet instability is not able to provide a clear acoustic response,but the stronger one can behave as the dominant pattern of noise process.Finally,the local acoustic response features of non-premixed flames under the re-sultant influence of buoyant and jet preferred instabilities are studied with the use of acoustic transfer function.The time-domain analyses show that there is a competition between the two instability effects,which depends on the locally relative intensity.Re-sults of the gain response show that the noise process induced by weak jet instability will be suppressed as buoyancy increases,however,the stronger jet instability can be easily amplified by flame into a clear signal with fixed sound pressure level and peak frequency.Results of the phase response illustrate that the level of buoyant instability is in positive correlation with the acoustic response rate of non-premixed flames.