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Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.
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Analysis of the Closures of Sub-grid Scale Variance of Reaction Progress Variable for Turbulent Bunsen Burner Flames at Different Pressure Levels
Flow Turbulence and Combustion, 2020Co-Authors: Felix Benjamin Keil, Nilanjan Chakraborty, Markus KleinAbstract:The statistical behaviour and modelling of the sub-grid variance of reaction progress variable have been analysed based on a priori analysis of direct numerical simulation (DNS) data of turbulent premixed Bunsen Burner flames at different pressure levels. An algebraic expression for sub-grid variance, which can be derived based on a presumed bi-modal sub-grid distribution of reaction progress variable with impulses at unburned reactants and fully burned products, has been found to be inadequate for the purpose of prediction of sub-grid variance even for the flames in the wrinkled flamelets/corrugated flamelets regime. Moreover, an algebraic model, which is often used for modelling sub-grid variance of passive scalars, has been found to significantly overpredict the sub-grid variance of reaction progress variable for all the cases considered here. The modelling of the unclosed terms of the sub-grid variance transport equation has been analysed in detail. Suitable model expressions have been identified for the sub-grid flux of variance, reaction rate contribution and scalar dissipation rate based on a priori analysis of DNS data. It has been found that the alternation of pressure does not have any significant impact on the closures of sub-grid flux of variance but a model parameter for the Favre-filtered scalar dissipation rate needs to be modified to account for the variation of pressure.
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On the validity of Damköhler's first hypothesis in turbulent Bunsen Burner flames: A computational analysis
Proceedings of the Combustion Institute, 2019Co-Authors: Nilanjan Chakraborty, Dana Alwazzan, Markus Klein, R. Stewart CantAbstract:Abstract The validity of Damkohler's first hypothesis, which relates the turbulent flame speed to turbulent flame surface area under the condition where the integral length scale of turbulence is greater than the flame thickness, has been assessed using three-dimensional Direct Numerical Simulations (DNS) of turbulent premixed Bunsen Burner flames over a range of values of Reynolds number, pressure and turbulence intensity. It has been found for the Bunsen configuration that the proportionality between volume-integrated burning rate and the overall flame surface area is not strictly maintained according to Damkohler's first hypothesis. The discrepancy is found to originate physically from the local stretch rate dependence of displacement speed, and this helps to explain differences observed previously between flames with and without mean curvature. Approximating the local density-weighted flame propagation speed with the unstrained laminar burning velocity is shown to be inaccurate, and can have a significant influence on the prediction of the overall burning rate for flames with non-zero mean curvature. Using a two-dimensional projection of the actual scalar gradient for flame area evaluation is shown to exacerbate the loss of proportionality between volume-integrated burning rate and the overall flame surface area. The current analysis identifies the conditions under which Damkohler's hypothesis remains valid and the necessary correction for non-zero mean flame curvature. Further, it has been demonstrated that surface-weighted stretch effects on displacement speed need to be accounted for in order to ensure the validity of Damkohler's hypothesis under all circumstances. Finally, it has been found that the volume-integrated density-weighted scalar dissipation rate remains proportional to the overall burning rate for all flames considered here irrespective of the value of Reynolds number, pressure and turbulence intensity. However, this proportionality is lost when the scalar dissipation rate is evaluated using the two-dimensional projection of the actual scalar gradient.
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A Direct Numerical Simulation analysis of pressure variation in turbulent premixed Bunsen Burner flames-part 2: Surface Density Function transport statistics
Computers & Fluids, 2018Co-Authors: Markus Klein, Dana Alwazzan, Nilanjan ChakrabortyAbstract:Abstract The effects of pressure variation on the transport statistics of the magnitude of the reaction progress gradient (i.e. Surface Density Function (SDF)) have been investigated based on three-dimensional simple chemistry Direct Numerical Simulations (DNS) of Bunsen Burner flames representing the flamelet regime of combustion. The large length scale separation between the nozzle diameter and flame thickness for high pressure flames makes the Darrieus–Landau (DL) instability highly likely, which in turn affects the curvature stretch. It has been found that the effective normal strain rate remains insensitive to the pressure variation for the parameter range considered here, which makes the flamelet thickness in turbulent flames comparable to the laminar flame thickness. The influences of the DL instability on the positive mean tangential strain rate counter the effects of instability on the negative mean curvature stretch and thus the effective tangential strain rate (or net flame stretch rate) remains mostly unaffected by the pressure variation within the strict flamelet regime (i.e. wrinkled flamelets and corrugated flamelets regimes) of combustion. The similarities in the SDF and the effective strain rate statistics for different values of pressure suggest that the models for the Flame Surface Density and Scalar Dissipation Rate, which were originally proposed and validated for atmospheric combustion, might remain valid also for elevated pressures.
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A Direct Numerical Simulation analysis of pressure variation in turbulent premixed Bunsen Burner flames-Part 1: Scalar gradient and strain rate statistics
Computers & Fluids, 2018Co-Authors: Markus Klein, Dana Alwazzan, Nilanjan ChakrabortyAbstract:Abstract Three-dimensional simple chemistry Direct numerical simulations (DNS) of Bunsen Burner flames have been carried out for different pressure values. A number of cases have been considered for the same set of values of mean and root-mean-square inlet velocities normalised by the laminar burning velocity and the integral length scale normalised by the nozzle diameter. The modifications of laminar burning velocity and flame thickness with pressure lead to an increase in both flow and turbulent Reynolds numbers with increasing pressure. This also gives rise to changes in Damkohler number and Karlovitz numbers for these flames and thus they occupy different locations on the regime diagram. For this reason, two additional cases at the lowest pressure have been simulated to match the turbulent Reynolds number of the highest-pressure case by changing the normalised root-mean-square velocity in one case, whereas the integral length scale is modified in the other case. It has been found that pressure and turbulent Reynolds number variations do not have significant influences on the mean behaviours of the magnitude of the reaction progress gradient (i.e. Surface Density Function) and fluid-dynamic normal strain rate. However, the length scale separation between the nozzle diameter and flame thickness increases with increasing pressure, which makes the occurrence of the Darrieus–Landau (DL) instability highly likely for the flames at elevated pressures. The presence of the DL instability affects the flame curvature statistics, which in turn influence the mean behaviours of the dilatation rate and fluid-dynamic tangential strain rate.
Markus Klein - One of the best experts on this subject based on the ideXlab platform.
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Analysis of the Closures of Sub-grid Scale Variance of Reaction Progress Variable for Turbulent Bunsen Burner Flames at Different Pressure Levels
Flow Turbulence and Combustion, 2020Co-Authors: Felix Benjamin Keil, Nilanjan Chakraborty, Markus KleinAbstract:The statistical behaviour and modelling of the sub-grid variance of reaction progress variable have been analysed based on a priori analysis of direct numerical simulation (DNS) data of turbulent premixed Bunsen Burner flames at different pressure levels. An algebraic expression for sub-grid variance, which can be derived based on a presumed bi-modal sub-grid distribution of reaction progress variable with impulses at unburned reactants and fully burned products, has been found to be inadequate for the purpose of prediction of sub-grid variance even for the flames in the wrinkled flamelets/corrugated flamelets regime. Moreover, an algebraic model, which is often used for modelling sub-grid variance of passive scalars, has been found to significantly overpredict the sub-grid variance of reaction progress variable for all the cases considered here. The modelling of the unclosed terms of the sub-grid variance transport equation has been analysed in detail. Suitable model expressions have been identified for the sub-grid flux of variance, reaction rate contribution and scalar dissipation rate based on a priori analysis of DNS data. It has been found that the alternation of pressure does not have any significant impact on the closures of sub-grid flux of variance but a model parameter for the Favre-filtered scalar dissipation rate needs to be modified to account for the variation of pressure.
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On the validity of Damköhler's first hypothesis in turbulent Bunsen Burner flames: A computational analysis
Proceedings of the Combustion Institute, 2019Co-Authors: Nilanjan Chakraborty, Dana Alwazzan, Markus Klein, R. Stewart CantAbstract:Abstract The validity of Damkohler's first hypothesis, which relates the turbulent flame speed to turbulent flame surface area under the condition where the integral length scale of turbulence is greater than the flame thickness, has been assessed using three-dimensional Direct Numerical Simulations (DNS) of turbulent premixed Bunsen Burner flames over a range of values of Reynolds number, pressure and turbulence intensity. It has been found for the Bunsen configuration that the proportionality between volume-integrated burning rate and the overall flame surface area is not strictly maintained according to Damkohler's first hypothesis. The discrepancy is found to originate physically from the local stretch rate dependence of displacement speed, and this helps to explain differences observed previously between flames with and without mean curvature. Approximating the local density-weighted flame propagation speed with the unstrained laminar burning velocity is shown to be inaccurate, and can have a significant influence on the prediction of the overall burning rate for flames with non-zero mean curvature. Using a two-dimensional projection of the actual scalar gradient for flame area evaluation is shown to exacerbate the loss of proportionality between volume-integrated burning rate and the overall flame surface area. The current analysis identifies the conditions under which Damkohler's hypothesis remains valid and the necessary correction for non-zero mean flame curvature. Further, it has been demonstrated that surface-weighted stretch effects on displacement speed need to be accounted for in order to ensure the validity of Damkohler's hypothesis under all circumstances. Finally, it has been found that the volume-integrated density-weighted scalar dissipation rate remains proportional to the overall burning rate for all flames considered here irrespective of the value of Reynolds number, pressure and turbulence intensity. However, this proportionality is lost when the scalar dissipation rate is evaluated using the two-dimensional projection of the actual scalar gradient.
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A Direct Numerical Simulation analysis of pressure variation in turbulent premixed Bunsen Burner flames-part 2: Surface Density Function transport statistics
Computers & Fluids, 2018Co-Authors: Markus Klein, Dana Alwazzan, Nilanjan ChakrabortyAbstract:Abstract The effects of pressure variation on the transport statistics of the magnitude of the reaction progress gradient (i.e. Surface Density Function (SDF)) have been investigated based on three-dimensional simple chemistry Direct Numerical Simulations (DNS) of Bunsen Burner flames representing the flamelet regime of combustion. The large length scale separation between the nozzle diameter and flame thickness for high pressure flames makes the Darrieus–Landau (DL) instability highly likely, which in turn affects the curvature stretch. It has been found that the effective normal strain rate remains insensitive to the pressure variation for the parameter range considered here, which makes the flamelet thickness in turbulent flames comparable to the laminar flame thickness. The influences of the DL instability on the positive mean tangential strain rate counter the effects of instability on the negative mean curvature stretch and thus the effective tangential strain rate (or net flame stretch rate) remains mostly unaffected by the pressure variation within the strict flamelet regime (i.e. wrinkled flamelets and corrugated flamelets regimes) of combustion. The similarities in the SDF and the effective strain rate statistics for different values of pressure suggest that the models for the Flame Surface Density and Scalar Dissipation Rate, which were originally proposed and validated for atmospheric combustion, might remain valid also for elevated pressures.
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A Direct Numerical Simulation analysis of pressure variation in turbulent premixed Bunsen Burner flames-Part 1: Scalar gradient and strain rate statistics
Computers & Fluids, 2018Co-Authors: Markus Klein, Dana Alwazzan, Nilanjan ChakrabortyAbstract:Abstract Three-dimensional simple chemistry Direct numerical simulations (DNS) of Bunsen Burner flames have been carried out for different pressure values. A number of cases have been considered for the same set of values of mean and root-mean-square inlet velocities normalised by the laminar burning velocity and the integral length scale normalised by the nozzle diameter. The modifications of laminar burning velocity and flame thickness with pressure lead to an increase in both flow and turbulent Reynolds numbers with increasing pressure. This also gives rise to changes in Damkohler number and Karlovitz numbers for these flames and thus they occupy different locations on the regime diagram. For this reason, two additional cases at the lowest pressure have been simulated to match the turbulent Reynolds number of the highest-pressure case by changing the normalised root-mean-square velocity in one case, whereas the integral length scale is modified in the other case. It has been found that pressure and turbulent Reynolds number variations do not have significant influences on the mean behaviours of the magnitude of the reaction progress gradient (i.e. Surface Density Function) and fluid-dynamic normal strain rate. However, the length scale separation between the nozzle diameter and flame thickness increases with increasing pressure, which makes the occurrence of the Darrieus–Landau (DL) instability highly likely for the flames at elevated pressures. The presence of the DL instability affects the flame curvature statistics, which in turn influence the mean behaviours of the dilatation rate and fluid-dynamic tangential strain rate.
R. Stewart Cant - One of the best experts on this subject based on the ideXlab platform.
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On the validity of Damköhler's first hypothesis in turbulent Bunsen Burner flames: A computational analysis
Proceedings of the Combustion Institute, 2019Co-Authors: Nilanjan Chakraborty, Dana Alwazzan, Markus Klein, R. Stewart CantAbstract:Abstract The validity of Damkohler's first hypothesis, which relates the turbulent flame speed to turbulent flame surface area under the condition where the integral length scale of turbulence is greater than the flame thickness, has been assessed using three-dimensional Direct Numerical Simulations (DNS) of turbulent premixed Bunsen Burner flames over a range of values of Reynolds number, pressure and turbulence intensity. It has been found for the Bunsen configuration that the proportionality between volume-integrated burning rate and the overall flame surface area is not strictly maintained according to Damkohler's first hypothesis. The discrepancy is found to originate physically from the local stretch rate dependence of displacement speed, and this helps to explain differences observed previously between flames with and without mean curvature. Approximating the local density-weighted flame propagation speed with the unstrained laminar burning velocity is shown to be inaccurate, and can have a significant influence on the prediction of the overall burning rate for flames with non-zero mean curvature. Using a two-dimensional projection of the actual scalar gradient for flame area evaluation is shown to exacerbate the loss of proportionality between volume-integrated burning rate and the overall flame surface area. The current analysis identifies the conditions under which Damkohler's hypothesis remains valid and the necessary correction for non-zero mean flame curvature. Further, it has been demonstrated that surface-weighted stretch effects on displacement speed need to be accounted for in order to ensure the validity of Damkohler's hypothesis under all circumstances. Finally, it has been found that the volume-integrated density-weighted scalar dissipation rate remains proportional to the overall burning rate for all flames considered here irrespective of the value of Reynolds number, pressure and turbulence intensity. However, this proportionality is lost when the scalar dissipation rate is evaluated using the two-dimensional projection of the actual scalar gradient.
Robert J. Santoro - One of the best experts on this subject based on the ideXlab platform.
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Characterization of syngas laminar flames using the Bunsen Burner configuration
International Journal of Hydrogen Energy, 2011Co-Authors: Nicolas Bouvet, Christian Chauveau, Iskender Gökalp, Robert J. SantoroAbstract:Laminar flame speeds of syngas mixtures (H2/CO/Air) have been studied using the Bunsen flame configuration with both straight and nozzle Burners. The flame surface area and flame cone angle methodologies, respectively based on the OH* chemiluminescence and Schlieren imaging techniques, have been performed to extract flame speeds for a wide range of equivalence ratios (0.3
Robert W. Bilger - One of the best experts on this subject based on the ideXlab platform.
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Characterization of Turbulence Generated by Perforated Plugs of Different Hole Sizes Placed Upstream of a Bunsen Burner
2006Co-Authors: Yung-cheng Chen, Robert W. BilgerAbstract:The digital particle imaging velocimetry (DPIV) technique i s applied to measure simultaneously flow velocity and turbulence integral length scale on a Bunsen Burner of 36 mm in diameter. Three perforated plugs of different hole diameters at 2, 4, and 6 mm are used as the turbulence generator placed at 45 mm upstream of the Burner exit. The plug opening is controlled to be approximately the same at 55% of the plug surface area. Mean and rms velocities, and the integral length scales are reported at 40 mm downstream of the Burner exit Similar turbulence properties are found for the 4-mm and 6-mm plugs with the decay of centreline rms velocity following the power-law relationship expected for conventional gridgenerated turbulence. There is, however, evidence indicating jet coalescence from the holes of the 2-mm plug. The integral length scales do not show clear dependency on the hole diameters of the plugs. This is likely due to low grid Reynolds number as well as intense turbulence, much higher than generated from the plugs, at the shear layer. The current design of turbulence generator requires further improvement for it t o be suitable for use in studying low-turbulence premixed flames.
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Turbulence measurements on a Bunsen Burner inserted with perforated plugs of different hole sizes using DPIV
Experimental Thermal and Fluid Science, 2003Co-Authors: Yung-cheng Chen, Robert W. BilgerAbstract:Abstract The digital particle imaging velocimetry (DPIV) technique is applied to measure simultaneously flow velocity and the turbulence integral length scale on a Bunsen Burner of 36 mm in diameter. A perforated plug is inserted at 45 mm upstream of the Burner exit for turbulence adjustment. The plug consists of a hexagonal array of circular holes at a diameter of 2, 4, or 6 mm. The opening of these three plugs is controlled to be approximately the same at 55 % of the plug surface area. Radial profiles of the mean and rms velocities, and the longitudinal and transverse integral length scales are reported at 40 mm downstream of the Burner exit. The turbulence properties are found similar for the 4- and 6-mm plugs with the decay of the centreline rms velocity following a power–law relationship expected for conventional grid-generated turbulence. Unstable reattachment of the flow is likely to occur in the holes of the relatively thick 2-mm plug and may interact with the downstream flow development. The integral length scales do not show a clear dependency on the hole diameter of the plugs. This is likely due to low grid Reynolds numbers as well as effects of intense shear-layer turbulence, much higher than generated by the plugs. Further improvement on the current plug design as a turbulence regulator is suggested for future use in studying low-turbulence premixed flames.
