Paul Medwell | The University of Adelaide (original) (raw)
Papers by Paul Medwell
54th AIAA Aerospace Sciences Meeting, 2016
Moderate or Intense Low Oxygen Dilution (MILD) combustion of pulverised coal in a laboratory scal... more Moderate or Intense Low Oxygen Dilution (MILD) combustion of pulverised coal in a laboratory scale recuperative furnace is investigated in this study. High volatile Kingston brown coal and low volatile Bowen basin black coal with particle size in the range of 38-180 m were injected into the furnace using either CO 2 or N 2 as a carrier gas. A water cooled sampling probe was used to conduct in-furnace gas sampling. Measurements of in-furnace gas concentration of CO and NO, exhaust gas emissions, and in-furnace temperatures are presented. A CFD model was developed for the CO 2 carried Kingston brown coal case and results are compared with experimental data. Ash content analysis showed that black coal was not burnt effectively, which is thought to be due to the relatively short furnace residence times.
A numerical study of pulverised coal combustion under Moderate or Intense Low oxygen Dilution (MI... more A numerical study of pulverised coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions was carried out in a parallel jet self-recuperative MILD combustion furnace. The three-dimensional model solved the Reynolds-Averaged Navier-Stokes equations and used the Eddy Dissipation Concept (EDC) model to describe the turbulence-chemistry interaction. The main aim of this work is to investigate the MILD combustion characteristics of pulverised Kingston brown coal using N2 as a carrier gas in the furnace. The modelling results are validated using our experimental measurements. The characteristics of thermal-NO, prompt-NO, fuel-NO, and NO reburning mechanisms and the influence of heterogeneous reactions on char burnout rate for the MILD combustion of pulverised coal are presented and discussed. The Results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.7% to the total NO emissions, whilst the fuel-...
22nd AIAA Computational Fluid Dynamics Conference, 2015
ABSTRACT Combustion in next-generation aero-engines may occur in conditions in, or approaching, m... more ABSTRACT Combustion in next-generation aero-engines may occur in conditions in, or approaching, moderate or intense low oxygen dilution (MILD) combustion. Under these conditions, fresh fuel is injected into a hot, low oxygen environment. Autoignition delays for ethylene, with detailed chemical kinetics, in three different oxidants are systematically analysed in a simulated perfectly-stirred batch reactor (batch PSR), with initial conditions based on the composition and adiabatic mixing temperature of the separate fuel and oxidant streams. The analysis of the same non-premixed streams in a transient, one-dimensional, opposed laminar diffusion flame shows ignition initiating at the same equivalence ratio for a diluted ethylene fuel with a high temperature air oxidant, albeit over-predicting ignition delay. Further analysis through means of a two-dimensional simulation shows good agreement between the batch PSR analysis and the location of the flame base. These analysis are subsequently applied to a MILD flame and a flame bordering the MILD and autoignitive lifted-flame regimes. Whilst the batch PSR analysis is able to predict the ignition equivalence ratio of the transitional flame, agreement with lift-off height is superior using a measure of 10K increase above oxidant conditions. This temperature metric is also applied to a MILD flame to show good agreement between a 10K temperature increase and visible chemiluminescence, however occurring in richer conditions than predicted by the batch PSR. The onset of thermal runaway in a batch PSR shows good predictive value for ignition in hot, vitiated environments, such as those anticipated in next generation aero-engines, despite the significant differences in flame structure between ignition in the lifted and MILD combustion regimes.
Applied Energy, 2015
ABSTRACT This paper presents an experimental study on moderate or intense low oxygen dilution (MI... more ABSTRACT This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverse-flow MILD combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and n-heptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with n-heptane being the most unstable due to its fast ignition under all high-pressure conditions studied. Measured CO emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low CO and NOx emission becomes narrower under elevated pressure as NOx emissions emitted from all fuels increased with pressure. The enhanced NOx formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher NOx emissions as pressure increases. The NOx emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The NOx emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing NOx emissions under these conditions, especially at elevated pressures.
JOURNAL OF SOUND AND VIBRATION
This paper presents the underlying theory, associated mathematical modelling and analysis of a sp... more This paper presents the underlying theory, associated mathematical modelling and analysis of a sponge-layer damping technique, termed the Time-Reversal-Sponge-Layer (TRSL), that significantly improves the performance of aeroacoustic Time-Reversal (TR). The TR technique requires the use of multiple Line Arrays (LAs) in a Time-Reversal Mirror (TRM) to accurately predict the source location and its characteristics. However, it is shown that when using multiple LAs, the interference between the opposite propagating fluxes near the LA boundaries results in the formation of spurious local maxima regions throughout the computational domain, thereby reducing the capacity of TR to resolve acoustic sources. The novel TRSL technique proposed in this work minimises this unwanted interference by damping the flux normally incident on a LA boundary and is implemented using the Pseudo-Characteristic Formulation (PCF) of the two-dimensional Linearised Euler Equations (LEE). The performance of TRSL is assessed by simulating a number of test cases such as an idealised time-harmonic monopole, dipole and lateral quadrupole sources as well as multiple (two) dipole sources of different strengths located in a nonuniform mean shear flow. The use of TRSL suppresses the formation of spurious maxima and significantly improves the source map, thereby demonstrating the effectiveness of this damping technique. The performance of TRSL is compared with two other methods: a TR superposition technique and Conventional Beamforming (CB). The TR superposition technique prevents the flux interference problem near the LA boundaries by
superposing the instantaneous time-reversed acoustic pressure fields computed from individual LAs. The source map obtained using the superposition technique was found to be identical to that obtained using the TRSL damping technique, however, the computational cost was much higher. A comparison with CB indicated that although CB accurately predicts the aeroacoustic source location, the relative magnitudes of the side-lobes in the CB source map(s) are substantially higher in comparison to the corresponding TR source map(s). It was also shown that TR with use of the TRSL is better suited for resolving multiple coherent sources in a domain than the CB method, especially when one source is much weaker than other sources in the domain.
Recent advances in diagnostic methods are providing new capacity for detailed measurement of turb... more Recent advances in diagnostic methods are providing new capacity for detailed measurement of turbulent, reacting flows in which heat transfer is dominant. Radiation typically becomes dominant in flames containing soot and/or with sufficient physical size, so is important in many flames of practical significance. The presence of particles, including soot, increases the coupling between the turbulence, chemistry and radiative heat transfer processes. Particles also increase the difficulties of laser-based measurements by increasing the interferences to the signal and the attenuation of the beam. The paper reviews recent advances in techniques to measure temperature, mixture fraction, soot volume fraction, velocity, particle number density and the scattered, absorbed and transmitted components of radiation propagation through particle laden systems.
Laser, 2005
The spatial distribution of OH, formaldehyde (H 2 CO) and temperature imaged by laser diagnostic ... more The spatial distribution of OH, formaldehyde (H 2 CO) and temperature imaged by laser diagnostic techniques are presented. The measurements are of nonpremixed jet flames in a hot and highly diluted co-flow. These conditions emulate those of Moderate and Intense Low Oxygen Dilution (MILD) combustion. This paper presents some results on the effect of O 2 co-flow level and jet Reynolds number on the structure of the flames for various fuels (each diluted by hydrogen 1:1 vol/vol). A reduction of O 2 levels is shown to lead to a substantial suppression of OH and a drop in peak temperatures. For strained flames, increased dilution of O 2 also leads to reduction of H 2 CO. Reynolds number effects on the radial profiles of the flame species considered are shown to be minor compared to O 2 levels. The drop of the peak temperature in a low O 2 co-flow leads to a reduction of thermal gradients and hence a laminarisation of these flames. Strain rate does have a significant effect on the relative levels of H 2 CO however, with levels increasing markedly with higher Reynolds number.
Experiments in Fluids, 1997
The link between fluctuations of a passive scalar and its dissipation is an important problem for... more The link between fluctuations of a passive scalar and its dissipation is an important problem for various aspects of turbulent flow modelling both with and without chemical reaction. This paper first reports experimental methods for simultaneous measurements of the three derivatives involved in temperature dissipation using cold wire probes in a boundary layer over a weakly heated flat plate. Particular
Applied Spectroscopy, 2010
This work investigates the first demonstration of nonlinear regime twoline atomic fluorescence (N... more This work investigates the first demonstration of nonlinear regime twoline atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is ;6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.
This work analyses the accuracy of numerical Time-Reversal (TR) simulations implemented using two... more This work analyses the accuracy of numerical Time-Reversal (TR) simulations implemented using two different Time-Reversal Mirror (TRM) configurations for localising and characterising a stationary acoustic dipole source in a mean flow. The forward time evolution of the acoustic fields is simulated by means of the numerical solution of the inhomogeneous 2-D Linearised Euler Equations (LEE) with uniform subsonic mean flow. Only the acoustic pressure is recorded with two line arrays (LAs) of boundary nodes in a TRM corresponding to the top and bottom boundaries. The time-reversed acoustic pressure history is used as input data for simulating two numerical TR experiments; (a) one line array (LA) in a TRM corresponding to the top boundary and (b) two LAs in a TRM corresponding to the top and bottom boundaries. The Root-Mean-Square (RMS) of the time-reversed acoustic pressure field obtained by the first experiment indicates only one spatial maxima region (focal spot), therefore incorrectly suggests that the source is a monopole, whereas the second experiment correctly reveals the source to be a dipole. The local acoustic pressure history at two source locations is shown to be coherent with relative phase exactly equal to radian, thereby confirming the dipole source nature. This demonstrates that two LAs in a TRM located on either sides of the mean flow are required to take into consideration, the complete phase information and thereby accurately characterise a dipole.
This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-... more This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-resolution of aeroacoustic Time-Reversal (TR). A PTRSL is implemented on a square domain centered at the predicted source location and is based on damping the radial components of the incoming and outgoing fluxes propagating toward and away from the source, respectively. A PTRSL is shown to overcome the conventional half-wavelength diffraction-limit; its implementation significantly reduces the focal spot size to one-fifth of a wavelength for a monopole source. Furthermore, PTRSL reduces the focal spots of a dipole source to three-tenths of a wavelength, as compared to three-fifths without its implementation.
The 2-D Linearized Euler equations (LEE) with a superimposed uniform mean flow subject to anechoi... more The 2-D Linearized Euler equations (LEE) with a superimposed uniform mean flow subject to anechoic boundary conditions are numerically solved using the Pseudo-Characteristic Formulation (PCF) and the compact upwind biased Finite Difference (FD) schemes (developed in a companion paper). The third-order total variation diminishing (TVD) Runge-Kutta scheme is used for time integration. The anechoic boundary conditions in a 2-D computational domain are modeled by setting the incoming fluxes (in the PCF) to zero at the boundaries of the rectangular (square) domain and also by simultaneously using the sponge layer to damp the incoming fluxes. This ensures a reasonable suppression of the contaminant reflected waves from the boundaries. An initial monopole condition of the acoustic pressure field modeled by a Gaussian pulse is considered as a test case which initiates an acoustic wave that propagates and expands in time. The numerical simulations are carried for sufficient time duration to ensure that the wave completely propagates out of the computational domain, wherein only the acoustic pressure at the boundaries is stored after each step of timeintegration. The results are presented as forward time-evolution of acoustic pressure field at different time instants. These stored acoustic pressure time-histories are reversed in time and used as input data during the implementation of Time-Reversal (TR) simulations of the 2-D LEE to compute the backward time-evolution of the acoustic field variables, whereby the same numerical solver is used. The acoustic pressure field predicted by the TR procedure at final time-instant of simulation is found to be in excellent agreement with the initial conditions considered during the forward problem. This suggests the potential use of proposed numerical scheme (based on the PCF) with only the time-reversed acoustic pressure history as the input inflow boundary conditions during TR simulations, in localizing an acoustic source in a 2-D domain.
ABSTRACT Moderate or Intense Low oxygen Dilution (MILD) combustion is a particular combustion reg... more ABSTRACT Moderate or Intense Low oxygen Dilution (MILD) combustion is a particular combustion regime which offers improved thermal efficiency and a reduction of nitrogen oxide (NOx) pollutants and soot. In this paper computational fluid dynamics (CFD) is employed to model turbulent jet flames issuing into a hot and diluted coflow stream, with a view to develop fundamental level understanding of the MILD combustion regime. Reynolds averaged Navier-Stokes (RANS) simulations are coupled with the Eddy Dissipation Concept (EDC) turbulence-chemistry model and the computational results are compared with experimental measurements. For the turbulent ethylene-hydrogen (C2H4/H2) fuel jet, a modified k-epsilon turbulence model, the standard Reynolds Stress Model (RSM), Shear Stress Transport (SST) model, k-omega models, and a modified RSM are considered. Results show that a mesh, constructed with 53610, primarily rectangular, elements with a characteristic length of 0.950mm, is sufficient to model the combustion processes in the MILD configuration of the JHC burner. The most accurate reacting flow field is predicted using the modified RSM, by adjustment of the factor C1epsilon to 1.6 from the default 1.44. The RSM is the most computationally expensive model, being an anisotropic extension of the standard k-epsilon model, however the increased computational cost is small in comparison to the cost of solving the detailed, finite-rate chemistry required for modelling MILD combustion. The modified RSM is therefore deemed to be superior in this application in comparison to the other turbulence models investigated.
54th AIAA Aerospace Sciences Meeting, 2016
Moderate or Intense Low Oxygen Dilution (MILD) combustion of pulverised coal in a laboratory scal... more Moderate or Intense Low Oxygen Dilution (MILD) combustion of pulverised coal in a laboratory scale recuperative furnace is investigated in this study. High volatile Kingston brown coal and low volatile Bowen basin black coal with particle size in the range of 38-180 m were injected into the furnace using either CO 2 or N 2 as a carrier gas. A water cooled sampling probe was used to conduct in-furnace gas sampling. Measurements of in-furnace gas concentration of CO and NO, exhaust gas emissions, and in-furnace temperatures are presented. A CFD model was developed for the CO 2 carried Kingston brown coal case and results are compared with experimental data. Ash content analysis showed that black coal was not burnt effectively, which is thought to be due to the relatively short furnace residence times.
A numerical study of pulverised coal combustion under Moderate or Intense Low oxygen Dilution (MI... more A numerical study of pulverised coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions was carried out in a parallel jet self-recuperative MILD combustion furnace. The three-dimensional model solved the Reynolds-Averaged Navier-Stokes equations and used the Eddy Dissipation Concept (EDC) model to describe the turbulence-chemistry interaction. The main aim of this work is to investigate the MILD combustion characteristics of pulverised Kingston brown coal using N2 as a carrier gas in the furnace. The modelling results are validated using our experimental measurements. The characteristics of thermal-NO, prompt-NO, fuel-NO, and NO reburning mechanisms and the influence of heterogeneous reactions on char burnout rate for the MILD combustion of pulverised coal are presented and discussed. The Results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.7% to the total NO emissions, whilst the fuel-...
22nd AIAA Computational Fluid Dynamics Conference, 2015
ABSTRACT Combustion in next-generation aero-engines may occur in conditions in, or approaching, m... more ABSTRACT Combustion in next-generation aero-engines may occur in conditions in, or approaching, moderate or intense low oxygen dilution (MILD) combustion. Under these conditions, fresh fuel is injected into a hot, low oxygen environment. Autoignition delays for ethylene, with detailed chemical kinetics, in three different oxidants are systematically analysed in a simulated perfectly-stirred batch reactor (batch PSR), with initial conditions based on the composition and adiabatic mixing temperature of the separate fuel and oxidant streams. The analysis of the same non-premixed streams in a transient, one-dimensional, opposed laminar diffusion flame shows ignition initiating at the same equivalence ratio for a diluted ethylene fuel with a high temperature air oxidant, albeit over-predicting ignition delay. Further analysis through means of a two-dimensional simulation shows good agreement between the batch PSR analysis and the location of the flame base. These analysis are subsequently applied to a MILD flame and a flame bordering the MILD and autoignitive lifted-flame regimes. Whilst the batch PSR analysis is able to predict the ignition equivalence ratio of the transitional flame, agreement with lift-off height is superior using a measure of 10K increase above oxidant conditions. This temperature metric is also applied to a MILD flame to show good agreement between a 10K temperature increase and visible chemiluminescence, however occurring in richer conditions than predicted by the batch PSR. The onset of thermal runaway in a batch PSR shows good predictive value for ignition in hot, vitiated environments, such as those anticipated in next generation aero-engines, despite the significant differences in flame structure between ignition in the lifted and MILD combustion regimes.
Applied Energy, 2015
ABSTRACT This paper presents an experimental study on moderate or intense low oxygen dilution (MI... more ABSTRACT This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverse-flow MILD combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and n-heptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with n-heptane being the most unstable due to its fast ignition under all high-pressure conditions studied. Measured CO emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low CO and NOx emission becomes narrower under elevated pressure as NOx emissions emitted from all fuels increased with pressure. The enhanced NOx formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher NOx emissions as pressure increases. The NOx emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The NOx emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing NOx emissions under these conditions, especially at elevated pressures.
JOURNAL OF SOUND AND VIBRATION
This paper presents the underlying theory, associated mathematical modelling and analysis of a sp... more This paper presents the underlying theory, associated mathematical modelling and analysis of a sponge-layer damping technique, termed the Time-Reversal-Sponge-Layer (TRSL), that significantly improves the performance of aeroacoustic Time-Reversal (TR). The TR technique requires the use of multiple Line Arrays (LAs) in a Time-Reversal Mirror (TRM) to accurately predict the source location and its characteristics. However, it is shown that when using multiple LAs, the interference between the opposite propagating fluxes near the LA boundaries results in the formation of spurious local maxima regions throughout the computational domain, thereby reducing the capacity of TR to resolve acoustic sources. The novel TRSL technique proposed in this work minimises this unwanted interference by damping the flux normally incident on a LA boundary and is implemented using the Pseudo-Characteristic Formulation (PCF) of the two-dimensional Linearised Euler Equations (LEE). The performance of TRSL is assessed by simulating a number of test cases such as an idealised time-harmonic monopole, dipole and lateral quadrupole sources as well as multiple (two) dipole sources of different strengths located in a nonuniform mean shear flow. The use of TRSL suppresses the formation of spurious maxima and significantly improves the source map, thereby demonstrating the effectiveness of this damping technique. The performance of TRSL is compared with two other methods: a TR superposition technique and Conventional Beamforming (CB). The TR superposition technique prevents the flux interference problem near the LA boundaries by
superposing the instantaneous time-reversed acoustic pressure fields computed from individual LAs. The source map obtained using the superposition technique was found to be identical to that obtained using the TRSL damping technique, however, the computational cost was much higher. A comparison with CB indicated that although CB accurately predicts the aeroacoustic source location, the relative magnitudes of the side-lobes in the CB source map(s) are substantially higher in comparison to the corresponding TR source map(s). It was also shown that TR with use of the TRSL is better suited for resolving multiple coherent sources in a domain than the CB method, especially when one source is much weaker than other sources in the domain.
Recent advances in diagnostic methods are providing new capacity for detailed measurement of turb... more Recent advances in diagnostic methods are providing new capacity for detailed measurement of turbulent, reacting flows in which heat transfer is dominant. Radiation typically becomes dominant in flames containing soot and/or with sufficient physical size, so is important in many flames of practical significance. The presence of particles, including soot, increases the coupling between the turbulence, chemistry and radiative heat transfer processes. Particles also increase the difficulties of laser-based measurements by increasing the interferences to the signal and the attenuation of the beam. The paper reviews recent advances in techniques to measure temperature, mixture fraction, soot volume fraction, velocity, particle number density and the scattered, absorbed and transmitted components of radiation propagation through particle laden systems.
Laser, 2005
The spatial distribution of OH, formaldehyde (H 2 CO) and temperature imaged by laser diagnostic ... more The spatial distribution of OH, formaldehyde (H 2 CO) and temperature imaged by laser diagnostic techniques are presented. The measurements are of nonpremixed jet flames in a hot and highly diluted co-flow. These conditions emulate those of Moderate and Intense Low Oxygen Dilution (MILD) combustion. This paper presents some results on the effect of O 2 co-flow level and jet Reynolds number on the structure of the flames for various fuels (each diluted by hydrogen 1:1 vol/vol). A reduction of O 2 levels is shown to lead to a substantial suppression of OH and a drop in peak temperatures. For strained flames, increased dilution of O 2 also leads to reduction of H 2 CO. Reynolds number effects on the radial profiles of the flame species considered are shown to be minor compared to O 2 levels. The drop of the peak temperature in a low O 2 co-flow leads to a reduction of thermal gradients and hence a laminarisation of these flames. Strain rate does have a significant effect on the relative levels of H 2 CO however, with levels increasing markedly with higher Reynolds number.
Experiments in Fluids, 1997
The link between fluctuations of a passive scalar and its dissipation is an important problem for... more The link between fluctuations of a passive scalar and its dissipation is an important problem for various aspects of turbulent flow modelling both with and without chemical reaction. This paper first reports experimental methods for simultaneous measurements of the three derivatives involved in temperature dissipation using cold wire probes in a boundary layer over a weakly heated flat plate. Particular
Applied Spectroscopy, 2010
This work investigates the first demonstration of nonlinear regime twoline atomic fluorescence (N... more This work investigates the first demonstration of nonlinear regime twoline atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is ;6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.
This work analyses the accuracy of numerical Time-Reversal (TR) simulations implemented using two... more This work analyses the accuracy of numerical Time-Reversal (TR) simulations implemented using two different Time-Reversal Mirror (TRM) configurations for localising and characterising a stationary acoustic dipole source in a mean flow. The forward time evolution of the acoustic fields is simulated by means of the numerical solution of the inhomogeneous 2-D Linearised Euler Equations (LEE) with uniform subsonic mean flow. Only the acoustic pressure is recorded with two line arrays (LAs) of boundary nodes in a TRM corresponding to the top and bottom boundaries. The time-reversed acoustic pressure history is used as input data for simulating two numerical TR experiments; (a) one line array (LA) in a TRM corresponding to the top boundary and (b) two LAs in a TRM corresponding to the top and bottom boundaries. The Root-Mean-Square (RMS) of the time-reversed acoustic pressure field obtained by the first experiment indicates only one spatial maxima region (focal spot), therefore incorrectly suggests that the source is a monopole, whereas the second experiment correctly reveals the source to be a dipole. The local acoustic pressure history at two source locations is shown to be coherent with relative phase exactly equal to radian, thereby confirming the dipole source nature. This demonstrates that two LAs in a TRM located on either sides of the mean flow are required to take into consideration, the complete phase information and thereby accurately characterise a dipole.
This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-... more This letter presents the Point-Time-Reversal-Sponge-Layer (PTRSL) technique to enhance the focal-resolution of aeroacoustic Time-Reversal (TR). A PTRSL is implemented on a square domain centered at the predicted source location and is based on damping the radial components of the incoming and outgoing fluxes propagating toward and away from the source, respectively. A PTRSL is shown to overcome the conventional half-wavelength diffraction-limit; its implementation significantly reduces the focal spot size to one-fifth of a wavelength for a monopole source. Furthermore, PTRSL reduces the focal spots of a dipole source to three-tenths of a wavelength, as compared to three-fifths without its implementation.
The 2-D Linearized Euler equations (LEE) with a superimposed uniform mean flow subject to anechoi... more The 2-D Linearized Euler equations (LEE) with a superimposed uniform mean flow subject to anechoic boundary conditions are numerically solved using the Pseudo-Characteristic Formulation (PCF) and the compact upwind biased Finite Difference (FD) schemes (developed in a companion paper). The third-order total variation diminishing (TVD) Runge-Kutta scheme is used for time integration. The anechoic boundary conditions in a 2-D computational domain are modeled by setting the incoming fluxes (in the PCF) to zero at the boundaries of the rectangular (square) domain and also by simultaneously using the sponge layer to damp the incoming fluxes. This ensures a reasonable suppression of the contaminant reflected waves from the boundaries. An initial monopole condition of the acoustic pressure field modeled by a Gaussian pulse is considered as a test case which initiates an acoustic wave that propagates and expands in time. The numerical simulations are carried for sufficient time duration to ensure that the wave completely propagates out of the computational domain, wherein only the acoustic pressure at the boundaries is stored after each step of timeintegration. The results are presented as forward time-evolution of acoustic pressure field at different time instants. These stored acoustic pressure time-histories are reversed in time and used as input data during the implementation of Time-Reversal (TR) simulations of the 2-D LEE to compute the backward time-evolution of the acoustic field variables, whereby the same numerical solver is used. The acoustic pressure field predicted by the TR procedure at final time-instant of simulation is found to be in excellent agreement with the initial conditions considered during the forward problem. This suggests the potential use of proposed numerical scheme (based on the PCF) with only the time-reversed acoustic pressure history as the input inflow boundary conditions during TR simulations, in localizing an acoustic source in a 2-D domain.
ABSTRACT Moderate or Intense Low oxygen Dilution (MILD) combustion is a particular combustion reg... more ABSTRACT Moderate or Intense Low oxygen Dilution (MILD) combustion is a particular combustion regime which offers improved thermal efficiency and a reduction of nitrogen oxide (NOx) pollutants and soot. In this paper computational fluid dynamics (CFD) is employed to model turbulent jet flames issuing into a hot and diluted coflow stream, with a view to develop fundamental level understanding of the MILD combustion regime. Reynolds averaged Navier-Stokes (RANS) simulations are coupled with the Eddy Dissipation Concept (EDC) turbulence-chemistry model and the computational results are compared with experimental measurements. For the turbulent ethylene-hydrogen (C2H4/H2) fuel jet, a modified k-epsilon turbulence model, the standard Reynolds Stress Model (RSM), Shear Stress Transport (SST) model, k-omega models, and a modified RSM are considered. Results show that a mesh, constructed with 53610, primarily rectangular, elements with a characteristic length of 0.950mm, is sufficient to model the combustion processes in the MILD configuration of the JHC burner. The most accurate reacting flow field is predicted using the modified RSM, by adjustment of the factor C1epsilon to 1.6 from the default 1.44. The RSM is the most computationally expensive model, being an anisotropic extension of the standard k-epsilon model, however the increased computational cost is small in comparison to the cost of solving the detailed, finite-rate chemistry required for modelling MILD combustion. The modified RSM is therefore deemed to be superior in this application in comparison to the other turbulence models investigated.