Score-StoveTM a clean-burning cooking stove that also generates electricity was tested using a pressurized kerosene burner. The Score-Stove works on the principle of thermo-acoustics to gen- erate small-scale electric...Score-StoveTM a clean-burning cooking stove that also generates electricity was tested using a pressurized kerosene burner. The Score-Stove works on the principle of thermo-acoustics to gen- erate small-scale electricity. The device having hot-end, cold-end and regenerator acts in a way similar to a stirling cycle generating acoustic power, which is then converted to electricity using a linear actuator. It can supply small power for applications such as LED lighting, mobile phone charging and radios particularly in rural areas without grid electricity as well as improving house- hold air pollution. After assessing the needs of the rural communities through a survey, tea-stalls and small restaurants owners were identified as clients with the most potential of using the stove in Bangladesh. Bangladesh University of Engineering and Technology ((BUET) modified a Score- Stove to use both wood and a pressurized kerosene burner of a design that is widely used for cooking in rural areas of Bangladesh. The design was adapted to meet performance needs such as: heating rate, cooking efficiency, energy distribution, electric power generation, exhaust emissions and time taken to boil water using standardized water boiling tests. Performance was also compared with conventional (non-electrically generating) stoves that use a pressurized kerosene burn- er. The Score-Stove performance was then evaluated while increasing the pressure of the sealed working fluid (air in this case) from atmospheric to about 1.4 bar. The pressurization was found to almost double the power generation. An arrangement for utilizing cooling water waste heat was also devised in order to improve the thermal performance of the stove by 18%. Technical deficiencies are documented and recommendations for improvements and future research in order to obtain wider end-user acceptance are made.展开更多
Thermoacoustic imaging with current injection(TAI-CI) is a novel imaging technology that couples with electromagnetic and acoustic research, which combines the advantages of high contrast of the electrical impedance t...Thermoacoustic imaging with current injection(TAI-CI) is a novel imaging technology that couples with electromagnetic and acoustic research, which combines the advantages of high contrast of the electrical impedance tomography and the high spatial resolution of sonography, and therefore has the potential for early diagnosis. To verify the feasibility of TAI-CI for complex bone-containing biological tissues, the principle of TAI-CI and the coupling characteristics of fluid and solid were analyzed. Meanwhile, thermoacoustic(TA) effects for fluid model and fluid–solid coupling model were analyzed by numerical simulations. Moreover, we conducted experiments on animal cartilage, hard bone and biological soft tissue phantom with low conductivity(0.5 S/m). By injecting a current into the phantom, the thermoacoustic signal was detected by the ultrasonic transducer with a center frequency of 1 MHz, thereby the B-scan image of the objects was obtained. The B-scan image of the cartilage experiment accurately reflects the distribution of cartilage and gel, and the hard bone has a certain attenuation effect on the acoustic signal. However, compared with the ultrasonic imaging, the thermoacoustic signal is only attenuated during the outward propagation. Even in this case, a clear image can still be obtained and the images can reflect the change of the conductivity of the gel. This study confirmed the feasibility of TAI-CI for the imaging of biological tissue under the presence of cartilage and the bone. The novel TAI-CI method provides further evidence that it can be used in the diagnosis of human diseases.展开更多
Pulse decomposition has been proven to be efficient to analyze complicated signals and it is introduced into the photo-acoustic and thermo-acoustic tomography to eliminate reconstruction distortions caused by negative...Pulse decomposition has been proven to be efficient to analyze complicated signals and it is introduced into the photo-acoustic and thermo-acoustic tomography to eliminate reconstruction distortions caused by negative lobes.During image reconstruction,negative lobes bring errors in the estimation of acoustic pulse amplitude,which is closely related to the distribution of absorption coefficient.The negative lobe error degrades imaging quality seriously in limited-view conditions because it cannot be offset so well as in full-view conditions.Therefore,a pulse decomposition formula is provided with detailed deduction to eliminate the negative lobe error and is incorporated into the popular delay-and-sum method for better reconstructing the image without additional complicated computation.Numerical experiments show that the pulse decomposition improves the image quality obviously in the limited-view conditions,such as separating adjacent absorbers,discovering a small absorber despite disturbance from a big absorber nearby,etc.展开更多
Direct and inverse scattering problems connected with the wave equation in non-homogeneous bounded domains constitute challenging actual subjects for both mathematicians and engineers. Among them one can mention, for ...Direct and inverse scattering problems connected with the wave equation in non-homogeneous bounded domains constitute challenging actual subjects for both mathematicians and engineers. Among them one can mention, for example, inverse source problems in seismology, nondestructive archeological probing, mine prospecting, inverse initial-value problems in acoustic tomography, etc. In spite of its crucial importance, almost all of the available rigorous investigations concern the case of unbounded simple domains such as layered planar or cylindrical or spherical structures. The main reason for the lack of the works related to non-homogeneous bounded structures is the extreme complexity of the explicit expressions of the Green’s functions. The aim of the present work consists in discovering some universal properties of the Green’s functions in question, which reduce enormously the difficulties arising in various applications. The universality mentioned here means that the properties are not depend on the geometrical and physical properties of the configuration. To this end one considers first the case when the domain is partially-homogeneous. Then the results are generalized to the most general case. To show the importance of the universal properties in question, they are applied to an inverse initial-value problem connected with photo-acoustic tomography.展开更多
Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swi...Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swirl interaction patterns include equal swirl intensity interaction and strong-weak swirl interaction.The thermo-acoustic instability characteristics indicate that increasing the central nozzle swirl intensity transforms the interaction pattern from equal swirl intensity interaction to strong-weak swirl interaction,which can significantly weaken the thermo-acoustic coupling effect under low equivalence ratio conditions,and substantially reduce the dynamic pressure amplitude during unstable combustion.The instantaneous flame structures show that the multi-swirl flames exhibit chaotic oscillations under low equivalence ratio conditions.With equivalence ratios greater than 0.71,a clear flame interaction boundary appears,and the flames can exhibit periodic oscillations in a regular structure.However,different interaction patterns result in the completely different phase oscillations in the central and outer flames.The time-averaged flame structures also indicate that strong-weak swirl interaction leads to an increase in the flame angle and a decrease in the flame length for both the central and outer flames,and the variations in the flame angle and length have great impacts on the thermo-acoustic instability mode.The fuel-staging combustion characteristics demonstrate that the instability combustion conditions with a dominant frequency of 100 Hz are greatly broadened by the strong-weak swirl interaction pattern,and the overlapping operating conditions between this mode and other modes are greatly increased.This implies that it is more flexible to adjust the thermo-acoustic unstable mode,which is conducive to the passive suppression of thermo-acoustic instability.展开更多
Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasoun...Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation.展开更多
Dual-volume Helmholtz dampers with two resonant frequencies are proposed to simultaneously attenuate longitudinal and azimuthal thermo-acoustic instabilities in annular combustors. Thermo-acoustic instabilities in a s...Dual-volume Helmholtz dampers with two resonant frequencies are proposed to simultaneously attenuate longitudinal and azimuthal thermo-acoustic instabilities in annular combustors. Thermo-acoustic instabilities in a swirled annular combustor equipped with dual-volume dampers are numerically investigated by the Helmholtz method, combined with a measured flame transfer function and the established damper impedance model. Furthermore, the influences of the damper number and circumferential configurations on oscillation attenuations and mode structures are explored. The established dual-volume damper model is well validated by the impedance tube tests. Numerical results indicate velocity fluctuation levels of the longitudinal and azimuthal modes decline after installing Helmholtz dampers, whereas those of the azimuthal modes further decrease by around 16% after using four retuned dual-volume dampers. The eigenfrequencies of the first longitudinal and azimuthal modes decrease and increase after installing dampers, respectively. After installing dual-volume dampers, the difference between the pressure fluctuation in the plenum and combustion chamber is reduced, and pressure waveforms of the azimuthal modes along the circumferential direction shifts. The pressure distribution of azimuthal modes becomes more uniform after using more dual-volume dampers. The specific absorption frequency band for azimuthal modes introduced by the dual-volume damper may lead to decreased oscillations and mode evolutions. The maximal absorbing ability can be approached by installing dampers with the same angle between adjacent dampers. When dampers are unevenly distributed, the symmetry between two azimuthal modes is broken and standing modes will emerge.展开更多
Effects of liquid fuel composition variations on characteristics of self-excited thermo-acoustic instabilities in a lean premixed,pre-vaporized gas turbine model combustor were experimentally studied.Test fuels includ...Effects of liquid fuel composition variations on characteristics of self-excited thermo-acoustic instabilities in a lean premixed,pre-vaporized gas turbine model combustor were experimentally studied.Test fuels included practical RP-3 jet fuel and its blending with iso-octane and n-dodecane,which were branched and linear alkanes respectively.Under the test conditions,dynamic pressure measurements indicated that the dominant instability frequency was highest for RP-3 flame,while RP-3/ndodecane flame exhibited the strongest instability strength.A further analysis showed that the instability frequency correlated well with the profiles of adiabatic flame temperature,and the strength of the instability highly depended on the ignition delay times of the fuels.Measurements of the flame structure and flow field with OH*chemiluminescence (CL) imaging and twodimensional particle image velocimetry (PIV) techniques indicated that changes in the fuel composition did not alter the unstable modes and general sequences of flame-flow structure oscillations.Further power spectra and proper orthogonal decomposition(POD) analysis suggested that axial oscillations along with precessing vortex core (PVC) induced helical motion predominated periodic flame structure and flow field oscillations.展开更多
Compared with the traditional engines, the thermo-acoustic engines are relatively new and can act as the linear compressors for refrigerators. Many institutes have shown great interest in this kind of machine for its ...Compared with the traditional engines, the thermo-acoustic engines are relatively new and can act as the linear compressors for refrigerators. Many institutes have shown great interest in this kind of machine for its absence of moving mechanical part. In this paper, the influence of the dimensions of the main parts of the small- scale Stifling thermo-acoustic engine was numerically simulated using a computer code called DeltaEC. The resonator and the resonator cavity were found to be the most convenient and effective in improving the perfor- mance of the engine. Based on the numerical simulation, a small-scale Stifling thermo-acoustic engine were constructed and experimentally investigated. Currently, with a resonator length of only 1 m, the working frequency of the engine was decreased to 90 Hz and the onset temperature difference was decreased to 198.2 K.展开更多
Unlike electric vehicles and electric aircrafts,hydrocarbon-fuelled(fossil)engine systems are much noisier.By conducting one-step chemical reaction-thermodynamics-acoustics coupling studies and experimental measuremen...Unlike electric vehicles and electric aircrafts,hydrocarbon-fuelled(fossil)engine systems are much noisier.By conducting one-step chemical reaction-thermodynamics-acoustics coupling studies and experimental measurements,we explore the universal physics of how hydrocarbon-fuelled combustion is a noise maker.We also explain that how combustion-sustained noise at a particular frequencyωis intrinsically selected.These frequencies correspond to the acoustic resonance nature of the combustor.We find that a reacting gas in which the rate of chemical reacting increases with temperature is intrinsically and naturally unstable with respect to acoustic wave motion,since its modal growth rateαis greater than 0.Acoustic disturbances tend to exponentially i.e.exp(αt)increase first and then are limited by nonlinear effects and finally grow into limit cycle oscillations.The growth rateαis found to increase first and then decrease with the gradient of the heat release rate with respect to the temperature change,i.e.heat capacity.The maximum(α/ω)_(max)depends on the specific heat ratioγ,which is related to the speed of sound.The unstable nature could be changed by introducing some acoustic dissipative/damping mechanisms,such as the boundary layer viscous drag and boundary losses.It is shown that such losses could lead to increased critical heat capacity,below which stable combustors can be designed.Finally,the acoustical energy consisting of both potential and kinetic energy is found to grow exponentially faster by 100%than the acoustic disturbance amplitude.展开更多
Open-loop control of self-excited flame pulsating oscillations and thermo-acoustic instability is considered in this work.The performance of the control strategy is numerically evaluated in a 2 D Rij ke-type combustor...Open-loop control of self-excited flame pulsating oscillations and thermo-acoustic instability is considered in this work.The performance of the control strategy is numerically evaluated in a 2 D Rij ke-type combustor with a perfo rated pipe implemented.It is found that approximately 38 dB sound pressure level(SPL)reduction can be achieved by actively tuning the cooling flow through the perforated pipe.Furthermore,the vorticity-induced damping performance is contributing to the breaking up of flame-acoustics coupling.However,the shedding of vortices is not uniformly distributed along the perforated pipe.To apply the control strategy in practice and to validate the findings,experimental studies are performed on a customerdesigned Rij ke-type combustor with a perfo rated liner implemented.To mimic practical engines,a cooling flow generated by a centrifugal pump is provided to pass through the perforated pipe.Properly tuning the cooling flow rate is found to lead to the unstable combustor being successfully stabilized.SPL is red uced by approximately 35 dB at ω1/2π≈245 Hz,and harmonic thermoacoustic modes are completely attenuating.Further study is conducted by suddenly re moving the perforated pipe section.The combustion system is found to be associated with not only classical thermo-acoustic limit cycle oscillations with a dominant mode at 2.45 × 102 Hz,but also beating oscillations at 1.4 × 1 00 Hz.It is revealed that increasing acoustic losses by implementing the perforated pipe is another critical mechanism contributing to attenuating flame pulsating instability.The present work opens up an applicable means to attenuate both selfexcited high-frequency thermoacoustic and low-frequency flame pulsating oscillations.展开更多
文摘Score-StoveTM a clean-burning cooking stove that also generates electricity was tested using a pressurized kerosene burner. The Score-Stove works on the principle of thermo-acoustics to gen- erate small-scale electricity. The device having hot-end, cold-end and regenerator acts in a way similar to a stirling cycle generating acoustic power, which is then converted to electricity using a linear actuator. It can supply small power for applications such as LED lighting, mobile phone charging and radios particularly in rural areas without grid electricity as well as improving house- hold air pollution. After assessing the needs of the rural communities through a survey, tea-stalls and small restaurants owners were identified as clients with the most potential of using the stove in Bangladesh. Bangladesh University of Engineering and Technology ((BUET) modified a Score- Stove to use both wood and a pressurized kerosene burner of a design that is widely used for cooking in rural areas of Bangladesh. The design was adapted to meet performance needs such as: heating rate, cooking efficiency, energy distribution, electric power generation, exhaust emissions and time taken to boil water using standardized water boiling tests. Performance was also compared with conventional (non-electrically generating) stoves that use a pressurized kerosene burn- er. The Score-Stove performance was then evaluated while increasing the pressure of the sealed working fluid (air in this case) from atmospheric to about 1.4 bar. The pressurization was found to almost double the power generation. An arrangement for utilizing cooling water waste heat was also devised in order to improve the thermal performance of the stove by 18%. Technical deficiencies are documented and recommendations for improvements and future research in order to obtain wider end-user acceptance are made.
基金Project supported by the National Natural Science Foundation of China(Grant No.51477161)the National Key Research and Development Program of China(Grant No.2018YFC0115200)the Fund from the Chinese Academy of Sciences(Grant No.YZ201507)
文摘Thermoacoustic imaging with current injection(TAI-CI) is a novel imaging technology that couples with electromagnetic and acoustic research, which combines the advantages of high contrast of the electrical impedance tomography and the high spatial resolution of sonography, and therefore has the potential for early diagnosis. To verify the feasibility of TAI-CI for complex bone-containing biological tissues, the principle of TAI-CI and the coupling characteristics of fluid and solid were analyzed. Meanwhile, thermoacoustic(TA) effects for fluid model and fluid–solid coupling model were analyzed by numerical simulations. Moreover, we conducted experiments on animal cartilage, hard bone and biological soft tissue phantom with low conductivity(0.5 S/m). By injecting a current into the phantom, the thermoacoustic signal was detected by the ultrasonic transducer with a center frequency of 1 MHz, thereby the B-scan image of the objects was obtained. The B-scan image of the cartilage experiment accurately reflects the distribution of cartilage and gel, and the hard bone has a certain attenuation effect on the acoustic signal. However, compared with the ultrasonic imaging, the thermoacoustic signal is only attenuated during the outward propagation. Even in this case, a clear image can still be obtained and the images can reflect the change of the conductivity of the gel. This study confirmed the feasibility of TAI-CI for the imaging of biological tissue under the presence of cartilage and the bone. The novel TAI-CI method provides further evidence that it can be used in the diagnosis of human diseases.
基金supported by the National Basic Research Program of China(Grant No.2012CB921504)the National Natural Science Foundation of China(Grant Nos.11274167,11274171,61201450,61201495,and 61302175)the Chongqing Science and Technology Commission of China(Grant Nos.2012jjA40058 and 2012jjA40006)
文摘Pulse decomposition has been proven to be efficient to analyze complicated signals and it is introduced into the photo-acoustic and thermo-acoustic tomography to eliminate reconstruction distortions caused by negative lobes.During image reconstruction,negative lobes bring errors in the estimation of acoustic pulse amplitude,which is closely related to the distribution of absorption coefficient.The negative lobe error degrades imaging quality seriously in limited-view conditions because it cannot be offset so well as in full-view conditions.Therefore,a pulse decomposition formula is provided with detailed deduction to eliminate the negative lobe error and is incorporated into the popular delay-and-sum method for better reconstructing the image without additional complicated computation.Numerical experiments show that the pulse decomposition improves the image quality obviously in the limited-view conditions,such as separating adjacent absorbers,discovering a small absorber despite disturbance from a big absorber nearby,etc.
文摘Direct and inverse scattering problems connected with the wave equation in non-homogeneous bounded domains constitute challenging actual subjects for both mathematicians and engineers. Among them one can mention, for example, inverse source problems in seismology, nondestructive archeological probing, mine prospecting, inverse initial-value problems in acoustic tomography, etc. In spite of its crucial importance, almost all of the available rigorous investigations concern the case of unbounded simple domains such as layered planar or cylindrical or spherical structures. The main reason for the lack of the works related to non-homogeneous bounded structures is the extreme complexity of the explicit expressions of the Green’s functions. The aim of the present work consists in discovering some universal properties of the Green’s functions in question, which reduce enormously the difficulties arising in various applications. The universality mentioned here means that the properties are not depend on the geometrical and physical properties of the configuration. To this end one considers first the case when the domain is partially-homogeneous. Then the results are generalized to the most general case. To show the importance of the universal properties in question, they are applied to an inverse initial-value problem connected with photo-acoustic tomography.
基金the National Science and Technology Major Project(HT-J2019-II-0018-0039)of China for financial support。
文摘Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swirl interaction patterns include equal swirl intensity interaction and strong-weak swirl interaction.The thermo-acoustic instability characteristics indicate that increasing the central nozzle swirl intensity transforms the interaction pattern from equal swirl intensity interaction to strong-weak swirl interaction,which can significantly weaken the thermo-acoustic coupling effect under low equivalence ratio conditions,and substantially reduce the dynamic pressure amplitude during unstable combustion.The instantaneous flame structures show that the multi-swirl flames exhibit chaotic oscillations under low equivalence ratio conditions.With equivalence ratios greater than 0.71,a clear flame interaction boundary appears,and the flames can exhibit periodic oscillations in a regular structure.However,different interaction patterns result in the completely different phase oscillations in the central and outer flames.The time-averaged flame structures also indicate that strong-weak swirl interaction leads to an increase in the flame angle and a decrease in the flame length for both the central and outer flames,and the variations in the flame angle and length have great impacts on the thermo-acoustic instability mode.The fuel-staging combustion characteristics demonstrate that the instability combustion conditions with a dominant frequency of 100 Hz are greatly broadened by the strong-weak swirl interaction pattern,and the overlapping operating conditions between this mode and other modes are greatly increased.This implies that it is more flexible to adjust the thermo-acoustic unstable mode,which is conducive to the passive suppression of thermo-acoustic instability.
基金Supported by the National Natural Science Foundation of China(Grant No.60771042)the National Hi-Tech Research and Development Program("863"Project)(Grant No.2007AA12Z159)+2 种基金111Project(Grant No.B07046)SiChuan Excellent Youth Foun-dation(Grant No.08ZQ026-039)Program for New Century Excellent Talents in University of China and Program for Changjiang Scholars
文摘Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation.
基金funded by the National Science and Technology Major Project(J2019-Ⅲ-0020-0064)。
文摘Dual-volume Helmholtz dampers with two resonant frequencies are proposed to simultaneously attenuate longitudinal and azimuthal thermo-acoustic instabilities in annular combustors. Thermo-acoustic instabilities in a swirled annular combustor equipped with dual-volume dampers are numerically investigated by the Helmholtz method, combined with a measured flame transfer function and the established damper impedance model. Furthermore, the influences of the damper number and circumferential configurations on oscillation attenuations and mode structures are explored. The established dual-volume damper model is well validated by the impedance tube tests. Numerical results indicate velocity fluctuation levels of the longitudinal and azimuthal modes decline after installing Helmholtz dampers, whereas those of the azimuthal modes further decrease by around 16% after using four retuned dual-volume dampers. The eigenfrequencies of the first longitudinal and azimuthal modes decrease and increase after installing dampers, respectively. After installing dual-volume dampers, the difference between the pressure fluctuation in the plenum and combustion chamber is reduced, and pressure waveforms of the azimuthal modes along the circumferential direction shifts. The pressure distribution of azimuthal modes becomes more uniform after using more dual-volume dampers. The specific absorption frequency band for azimuthal modes introduced by the dual-volume damper may lead to decreased oscillations and mode evolutions. The maximal absorbing ability can be approached by installing dampers with the same angle between adjacent dampers. When dampers are unevenly distributed, the symmetry between two azimuthal modes is broken and standing modes will emerge.
基金the National Natural Science Foundation of China(Grant Nos.91641202 and 501100001809)the Program of Shanghai Subject Chief Scientist(Grant No.19XD1401800)。
文摘Effects of liquid fuel composition variations on characteristics of self-excited thermo-acoustic instabilities in a lean premixed,pre-vaporized gas turbine model combustor were experimentally studied.Test fuels included practical RP-3 jet fuel and its blending with iso-octane and n-dodecane,which were branched and linear alkanes respectively.Under the test conditions,dynamic pressure measurements indicated that the dominant instability frequency was highest for RP-3 flame,while RP-3/ndodecane flame exhibited the strongest instability strength.A further analysis showed that the instability frequency correlated well with the profiles of adiabatic flame temperature,and the strength of the instability highly depended on the ignition delay times of the fuels.Measurements of the flame structure and flow field with OH*chemiluminescence (CL) imaging and twodimensional particle image velocimetry (PIV) techniques indicated that changes in the fuel composition did not alter the unstable modes and general sequences of flame-flow structure oscillations.Further power spectra and proper orthogonal decomposition(POD) analysis suggested that axial oscillations along with precessing vortex core (PVC) induced helical motion predominated periodic flame structure and flow field oscillations.
文摘Compared with the traditional engines, the thermo-acoustic engines are relatively new and can act as the linear compressors for refrigerators. Many institutes have shown great interest in this kind of machine for its absence of moving mechanical part. In this paper, the influence of the dimensions of the main parts of the small- scale Stifling thermo-acoustic engine was numerically simulated using a computer code called DeltaEC. The resonator and the resonator cavity were found to be the most convenient and effective in improving the perfor- mance of the engine. Based on the numerical simulation, a small-scale Stifling thermo-acoustic engine were constructed and experimentally investigated. Currently, with a resonator length of only 1 m, the working frequency of the engine was decreased to 90 Hz and the onset temperature difference was decreased to 198.2 K.
基金the financial support from the University of Canterbury,New Zealand(Grant No.452STUPDZ)Singapore National Research Foundation(Grant No.NRF2016 NRF-NSFC001-102)National Natural Science Foundation of China(11661141020)。
文摘Unlike electric vehicles and electric aircrafts,hydrocarbon-fuelled(fossil)engine systems are much noisier.By conducting one-step chemical reaction-thermodynamics-acoustics coupling studies and experimental measurements,we explore the universal physics of how hydrocarbon-fuelled combustion is a noise maker.We also explain that how combustion-sustained noise at a particular frequencyωis intrinsically selected.These frequencies correspond to the acoustic resonance nature of the combustor.We find that a reacting gas in which the rate of chemical reacting increases with temperature is intrinsically and naturally unstable with respect to acoustic wave motion,since its modal growth rateαis greater than 0.Acoustic disturbances tend to exponentially i.e.exp(αt)increase first and then are limited by nonlinear effects and finally grow into limit cycle oscillations.The growth rateαis found to increase first and then decrease with the gradient of the heat release rate with respect to the temperature change,i.e.heat capacity.The maximum(α/ω)_(max)depends on the specific heat ratioγ,which is related to the speed of sound.The unstable nature could be changed by introducing some acoustic dissipative/damping mechanisms,such as the boundary layer viscous drag and boundary losses.It is shown that such losses could lead to increased critical heat capacity,below which stable combustors can be designed.Finally,the acoustical energy consisting of both potential and kinetic energy is found to grow exponentially faster by 100%than the acoustic disturbance amplitude.
基金supported by the University of Canterbury, New Zealand with Grant No. 452STUPDZNational Research Foundation, Prime Minister’s Office, Singapore, with Grant No. NRF2016NRF-NSFC001-102National Natural Science Foundation of China (11661141020)
文摘Open-loop control of self-excited flame pulsating oscillations and thermo-acoustic instability is considered in this work.The performance of the control strategy is numerically evaluated in a 2 D Rij ke-type combustor with a perfo rated pipe implemented.It is found that approximately 38 dB sound pressure level(SPL)reduction can be achieved by actively tuning the cooling flow through the perforated pipe.Furthermore,the vorticity-induced damping performance is contributing to the breaking up of flame-acoustics coupling.However,the shedding of vortices is not uniformly distributed along the perforated pipe.To apply the control strategy in practice and to validate the findings,experimental studies are performed on a customerdesigned Rij ke-type combustor with a perfo rated liner implemented.To mimic practical engines,a cooling flow generated by a centrifugal pump is provided to pass through the perforated pipe.Properly tuning the cooling flow rate is found to lead to the unstable combustor being successfully stabilized.SPL is red uced by approximately 35 dB at ω1/2π≈245 Hz,and harmonic thermoacoustic modes are completely attenuating.Further study is conducted by suddenly re moving the perforated pipe section.The combustion system is found to be associated with not only classical thermo-acoustic limit cycle oscillations with a dominant mode at 2.45 × 102 Hz,but also beating oscillations at 1.4 × 1 00 Hz.It is revealed that increasing acoustic losses by implementing the perforated pipe is another critical mechanism contributing to attenuating flame pulsating instability.The present work opens up an applicable means to attenuate both selfexcited high-frequency thermoacoustic and low-frequency flame pulsating oscillations.
