The Experts below are selected from a list of 9114 Experts worldwide ranked by ideXlab platform
Tatsuya Hasegawa - One of the best experts on this subject based on the ideXlab platform.
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a direct numerical simulation study of the influence of flame generated vorticity on reaction zone surface area in weakly turbulent premixed combustion
Physics of Fluids, 2019Co-Authors: Andrei Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., σ = 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large m...
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letter does flame generated vorticity increase turbulent burning velocity
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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Combustion-induced local shear layers within premixed Flamelets in weakly turbulent flows
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:3D direct numerical simulation data obtained from statistically stationary, planar, weakly turbulent, premixed flames, which are characterized by two different density ratios (7.53 and 2.50) and are associated with the flamelet combustion regime, are analyzed to investigate differences between velocity and pressure variations (i) in Flamelets in a weakly turbulent flow and (ii) in the counterpart laminar flame. Results show that while the thermo-chemical structure of the Flamelets is weakly affected by turbulence under the studied conditions, the local velocity, vorticity, and pressure fields within the Flamelets differ significantly from the velocity, vorticity, and pressure fields, respectively, within the laminar flame. In particular, local shear layers appear within Flamelets in the turbulent flow because acceleration of a reacting mixture by the local pressure gradient is inversely proportional to the mixture density and, hence, depends on the mixture state. The shear layers are characterized by large velocity gradients (both the tangential gradient of the normal velocity with respect to the flamelet surface and the normal gradient of the tangential velocity), whose magnitudes may be comparable with the magnitude of the velocity gradient across the laminar flame. In flamelet zones characterized by a relatively large magnitude of the locally normal gradient of the tangential velocity, the local vorticity magnitude is also large and such zones contribute substantially to the overall generation of vorticity due to baroclinic torque. These results cast doubts on the validity of a simple common modeling approach that consists in directly invoking expressions derived for the laminar flames in order to describe the influence of combustion-induced thermal expansion on weakly turbulent velocity and pressure fields.
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Does flame-generated vorticity increase turbulent burning velocity?
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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a balance equation for the mean rate of product creation in premixed turbulent flames
Proceedings of the Combustion Institute, 2017Co-Authors: Vladimir Sabelnikov, Nilanjan Chakraborty, Shinnosuke Nishiki, Andrei Lipatnikov, Tatsuya HasegawaAbstract:Transport equations for reaction rate W and its Favre-averaged value W are derived from first principle in the case of premixed turbulent combustion. The assumptions made for derivation hold for unity Lewis number premixed flames at least in the flamelet regime of turbulent burning. Analysis of the latter equation shows that it involves two dominant terms, but the difference between them vanishes if reaction zones retain the structure of the zone in the unperturbed laminar flame. However, in such a case, turbulent burning velocity cannot grow with time during interaction of an initially laminar flame with a turbulent flow. Therefore, the analysis indicates a vital role played by local perturbations of reaction zone structure in premixed turbulent combustion. The dominance of these two terms and the important role played by the difference between them are confirmed by analyzing three DNS databases associated with both the corrugated Flamelets and thin reaction zones regimes of premixed turbulent burning. Moreover, the DNS data show that perturbations of local displacement speed due to perturbations of local flamelet structure are also of paramount importance for modeling transport of flame surface density even in weakly turbulent flows. Finally, by simulating curved and/or strained laminar premixed flames and integrating the transport equation for W across the flames, the integral is shown to depend linearly on the stretch rate even in highly perturbed flames, with results obtained from variously stretched flames being close to each other. Based on this finding, the difference between the two dominant terms in the transport equation for the mean rate W is hypothesized to depend linearly on the stretch rate conditioned to the reaction zone. Application of this hypothesis to the DNS data associated with the corrugated Flamelets combustion regime yields encouraging results, thus, confirming a crucial role played by local perturbations of reaction zone structure even in weakly turbulent flames.
Jianren Fan - One of the best experts on this subject based on the ideXlab platform.
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A generalized flamelet tabulation method for partially premixed combustion
Combustion and Flame, 2018Co-Authors: Xu Wen, Xue-song Bai, Kun Luo, Haiou Wang, Yujuan Luo, Jianren FanAbstract:Abstract A flamelet tabulation method for partially premixed flames is proposed, in which partially premixed Flamelets are incorporated as the archetypal flamelet elements. This method considers triple flame structures with both the partial premixing of fuel in the oxidizer side and the partial premixing of oxidizer in the fuel side, by replacing the pure-air and pure-fuel in the counterflow diffusion flame with a range of fuel-lean and -rich mixtures, respectively. The thermo-chemical quantities in the partially premixed flamelet are stored in a four-dimensional flamelet library as a function of the mixture fraction Z, describing the mixing process, the reaction progress variable YPV, describing the progress of reactions, and the trajectory variables YF and YO, characterizing the partial premixings of fuel and oxidizer, respectively. The performance of the proposed partially premixed flamelet tabulation (PPFT) method is evaluated through both a priori and a posteriori tests on laminar tribrachial flames with different mixture fraction gradients. The PPFT results are compared with those from a premixed flamelet tabulation (PFT) method and a diffusion flamelet tabulation (DFT) method. It is found that the combustion-mode-sensitive species such as CO and H2 can be accurately predicted by the PPFT method for both the low and high mixture fraction gradient flame cases, which cannot be well predicted by the PFT and DFT methods.
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An a priori study of different tabulation methods for turbulent pulverised coal combustion
Combustion Theory and Modelling, 2018Co-Authors: Yujuan Luo, Xu Wen, Kun Luo, Haiou Wang, Hanhui Jin, Jianren FanAbstract:In many practical pulverised coal combustion systems, different oxidiser streams exist, e.g. the primary- and secondary-air streams in the power plant boilers, which makes the modelling of these systems challenging. In this work, three tabulation methods for modelling pulverised coal combustion are evaluated through an a priori study. Pulverised coal flames stabilised in a three-dimensional turbulent counterflow, consisting of different oxidiser streams, are simulated with detailed chemistry first. Then, the thermo-chemical quantities calculated with different tabulation methods are compared to those from detailed chemistry solutions. The comparison shows that the conventional two-stream flamelet model with a fixed oxidiser temperature cannot predict the flame temperature correctly. The conventional two-stream flamelet model is then modified to set the oxidiser temperature equal to the fuel temperature, both of which are varied in the Flamelets. By this means, the variations of oxidiser temperature can be...
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Numerical investigation of coal flamelet characteristics in a laminar counterflow with detailed chemistry
Fuel, 2017Co-Authors: Xu Wen, Kun Luo, Hanhui Jin, Jianren FanAbstract:Abstract The characteristics of Flamelets in coal flames stabilized in a laminar counterflow are numerically investigated with detailed chemistry. The behaviors of coal flamelet are compared with those of the gas flamelet. In addition, the effects of strain rate, coal particle mass flow rate and coal particle size on the behaviors of coal flamelet are studied in both the physical space and the mixture fraction space. The results show that the characteristics of coal flamelet are significantly different from those of the gas flamelet. On the one hand, unlike the monotonicity of the gaseous mixture fraction, the coal particle mixture fraction is non-monotonic due to the interphase mass transfer. On the other hand, due to the non-monotonicity of coal particle mixture fraction, the thermo-chemical quantities in the coal flame cannot be uniquely identified by a single variable of mixture fraction as those in the gas flame. It is also found that the strain rate, coal particle mass flow rate and coal particle size have significantly effects on the behaviors of coal flamelet.
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LES of pulverized coal combustion with a multi-regime flamelet model
Fuel, 2017Co-Authors: Xu Wen, Kun Luo, Hanhui Jin, Yujian Luo, Jianren FanAbstract:Abstract It is well known that premixed and non-premixed Flamelets coexist in the combustion system, especially for multiphase combustion (Luo et al., 2011; Bai et al., 2016). In this work, a multi-regime combustion model for partially premixed multiphase combustion (2PMC) is developed in the framework of large eddy simulation (LES). In this model, the multi-regime combustion mode is decomposed into two pure combustion regimes (i.e. premixed and non-premixed) through combustion regime indicator. Different combustion models are chosen for different combustion regimes. For example, for premixed combustion regime, the flamelet generated manifold (FGM) tabulation method in combination with the artificially thickened flame (ATF) approach is used. For non-premixed combustion regime, on the other hand, the thermo-chemical quantities will be extracted from the non-premixed chemtable generated by the flamelet/progress variable (FPV) approach. The proposed multi-regime flamelet model is then extended to adapt to the pulverized coal combustion (PCC) and applied to a laboratory-scale pulverized coal jet flame. The simulation results show that the percentage of the volume-weighted premixed combustion regime of this studied flame is up to 18%. Quantitative comparisons between the experimental data and the numerical results with the present multi-regime flamelet model, the FPV model, and the Eddy Break Up (EBU) model shows that the proposed multi-regime flamelet model has several advantages. It performs better than the FPV model in the regions where premixed combustion mode prevails and also much better than the EBU model in species concentration prediction.
Vladimir Sabelnikov - One of the best experts on this subject based on the ideXlab platform.
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a direct numerical simulation study of the influence of flame generated vorticity on reaction zone surface area in weakly turbulent premixed combustion
Physics of Fluids, 2019Co-Authors: Andrei Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., σ = 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large m...
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letter does flame generated vorticity increase turbulent burning velocity
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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Combustion-induced local shear layers within premixed Flamelets in weakly turbulent flows
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:3D direct numerical simulation data obtained from statistically stationary, planar, weakly turbulent, premixed flames, which are characterized by two different density ratios (7.53 and 2.50) and are associated with the flamelet combustion regime, are analyzed to investigate differences between velocity and pressure variations (i) in Flamelets in a weakly turbulent flow and (ii) in the counterpart laminar flame. Results show that while the thermo-chemical structure of the Flamelets is weakly affected by turbulence under the studied conditions, the local velocity, vorticity, and pressure fields within the Flamelets differ significantly from the velocity, vorticity, and pressure fields, respectively, within the laminar flame. In particular, local shear layers appear within Flamelets in the turbulent flow because acceleration of a reacting mixture by the local pressure gradient is inversely proportional to the mixture density and, hence, depends on the mixture state. The shear layers are characterized by large velocity gradients (both the tangential gradient of the normal velocity with respect to the flamelet surface and the normal gradient of the tangential velocity), whose magnitudes may be comparable with the magnitude of the velocity gradient across the laminar flame. In flamelet zones characterized by a relatively large magnitude of the locally normal gradient of the tangential velocity, the local vorticity magnitude is also large and such zones contribute substantially to the overall generation of vorticity due to baroclinic torque. These results cast doubts on the validity of a simple common modeling approach that consists in directly invoking expressions derived for the laminar flames in order to describe the influence of combustion-induced thermal expansion on weakly turbulent velocity and pressure fields.
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Does flame-generated vorticity increase turbulent burning velocity?
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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a balance equation for the mean rate of product creation in premixed turbulent flames
Proceedings of the Combustion Institute, 2017Co-Authors: Vladimir Sabelnikov, Nilanjan Chakraborty, Shinnosuke Nishiki, Andrei Lipatnikov, Tatsuya HasegawaAbstract:Transport equations for reaction rate W and its Favre-averaged value W are derived from first principle in the case of premixed turbulent combustion. The assumptions made for derivation hold for unity Lewis number premixed flames at least in the flamelet regime of turbulent burning. Analysis of the latter equation shows that it involves two dominant terms, but the difference between them vanishes if reaction zones retain the structure of the zone in the unperturbed laminar flame. However, in such a case, turbulent burning velocity cannot grow with time during interaction of an initially laminar flame with a turbulent flow. Therefore, the analysis indicates a vital role played by local perturbations of reaction zone structure in premixed turbulent combustion. The dominance of these two terms and the important role played by the difference between them are confirmed by analyzing three DNS databases associated with both the corrugated Flamelets and thin reaction zones regimes of premixed turbulent burning. Moreover, the DNS data show that perturbations of local displacement speed due to perturbations of local flamelet structure are also of paramount importance for modeling transport of flame surface density even in weakly turbulent flows. Finally, by simulating curved and/or strained laminar premixed flames and integrating the transport equation for W across the flames, the integral is shown to depend linearly on the stretch rate even in highly perturbed flames, with results obtained from variously stretched flames being close to each other. Based on this finding, the difference between the two dominant terms in the transport equation for the mean rate W is hypothesized to depend linearly on the stretch rate conditioned to the reaction zone. Application of this hypothesis to the DNS data associated with the corrugated Flamelets combustion regime yields encouraging results, thus, confirming a crucial role played by local perturbations of reaction zone structure even in weakly turbulent flames.
Xu Wen - One of the best experts on this subject based on the ideXlab platform.
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Multi-dimensional and transient effects on flamelet modeling for turbulent pulverized coal combustion
Fuel, 2019Co-Authors: Xu Wen, Oliver T. Stein, G.l. Tufano, Andreas Kronenburg, Arne Scholtissek, Christian HasseAbstract:Abstract The transport along mixture fraction iso-surfaces is generally denoted as multi-dimensional effect, which is not considered in the classical one-dimensional (1D) flamelet model. This work investigates multi-dimensional and transient effects on flamelet modeling for turbulent pulverized coal combustion. To this end, instantaneous Flamelets are extracted from a fully resolved 3D DNS database generated for a turbulent pulverized coal flame with a 3 3 array of particles. At first, the importance of transport along the iso-mixture fraction surface is analyzed based on the flamelet regime diagram. By this means, the flamelet regime of the local combustion is clarified. Then, the relevance of transient effects is investigated using budget analyses for transient terms in both physical space and composition space. Particularly, the significance of the Eulerian and Lagrangian transient terms is quantified. It is found that compared to the other transient processes, the transient change in the inner flame structure, i.e., the Lagrangian transient process, is dominant. Interestingly, the budget analyses show that the Lagrangian transient term can be partially recovered by the steady flamelet model. The findings obtained from the flamelet regime diagram and the budget analyses are consistent with the flamelet solutions where the multi-dimensional and transient terms are included in/excluded from the flamelet equations. The generalized flamelet equations including both multi-dimensional and transient terms can effectively reproduce the instantaneous Flamelets in the DNS. Finally, a priori analyses are conducted to evaluate the multi-dimensional and transient effects on the final performance of the flamelet model. Overall, the a priori results are consistent with findings obtained from the budget analyses.
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A generalized flamelet tabulation method for partially premixed combustion
Combustion and Flame, 2018Co-Authors: Xu Wen, Xue-song Bai, Kun Luo, Haiou Wang, Yujuan Luo, Jianren FanAbstract:Abstract A flamelet tabulation method for partially premixed flames is proposed, in which partially premixed Flamelets are incorporated as the archetypal flamelet elements. This method considers triple flame structures with both the partial premixing of fuel in the oxidizer side and the partial premixing of oxidizer in the fuel side, by replacing the pure-air and pure-fuel in the counterflow diffusion flame with a range of fuel-lean and -rich mixtures, respectively. The thermo-chemical quantities in the partially premixed flamelet are stored in a four-dimensional flamelet library as a function of the mixture fraction Z, describing the mixing process, the reaction progress variable YPV, describing the progress of reactions, and the trajectory variables YF and YO, characterizing the partial premixings of fuel and oxidizer, respectively. The performance of the proposed partially premixed flamelet tabulation (PPFT) method is evaluated through both a priori and a posteriori tests on laminar tribrachial flames with different mixture fraction gradients. The PPFT results are compared with those from a premixed flamelet tabulation (PFT) method and a diffusion flamelet tabulation (DFT) method. It is found that the combustion-mode-sensitive species such as CO and H2 can be accurately predicted by the PPFT method for both the low and high mixture fraction gradient flame cases, which cannot be well predicted by the PFT and DFT methods.
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An a priori study of different tabulation methods for turbulent pulverised coal combustion
Combustion Theory and Modelling, 2018Co-Authors: Yujuan Luo, Xu Wen, Kun Luo, Haiou Wang, Hanhui Jin, Jianren FanAbstract:In many practical pulverised coal combustion systems, different oxidiser streams exist, e.g. the primary- and secondary-air streams in the power plant boilers, which makes the modelling of these systems challenging. In this work, three tabulation methods for modelling pulverised coal combustion are evaluated through an a priori study. Pulverised coal flames stabilised in a three-dimensional turbulent counterflow, consisting of different oxidiser streams, are simulated with detailed chemistry first. Then, the thermo-chemical quantities calculated with different tabulation methods are compared to those from detailed chemistry solutions. The comparison shows that the conventional two-stream flamelet model with a fixed oxidiser temperature cannot predict the flame temperature correctly. The conventional two-stream flamelet model is then modified to set the oxidiser temperature equal to the fuel temperature, both of which are varied in the Flamelets. By this means, the variations of oxidiser temperature can be...
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Numerical investigation of coal flamelet characteristics in a laminar counterflow with detailed chemistry
Fuel, 2017Co-Authors: Xu Wen, Kun Luo, Hanhui Jin, Jianren FanAbstract:Abstract The characteristics of Flamelets in coal flames stabilized in a laminar counterflow are numerically investigated with detailed chemistry. The behaviors of coal flamelet are compared with those of the gas flamelet. In addition, the effects of strain rate, coal particle mass flow rate and coal particle size on the behaviors of coal flamelet are studied in both the physical space and the mixture fraction space. The results show that the characteristics of coal flamelet are significantly different from those of the gas flamelet. On the one hand, unlike the monotonicity of the gaseous mixture fraction, the coal particle mixture fraction is non-monotonic due to the interphase mass transfer. On the other hand, due to the non-monotonicity of coal particle mixture fraction, the thermo-chemical quantities in the coal flame cannot be uniquely identified by a single variable of mixture fraction as those in the gas flame. It is also found that the strain rate, coal particle mass flow rate and coal particle size have significantly effects on the behaviors of coal flamelet.
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LES of pulverized coal combustion with a multi-regime flamelet model
Fuel, 2017Co-Authors: Xu Wen, Kun Luo, Hanhui Jin, Yujian Luo, Jianren FanAbstract:Abstract It is well known that premixed and non-premixed Flamelets coexist in the combustion system, especially for multiphase combustion (Luo et al., 2011; Bai et al., 2016). In this work, a multi-regime combustion model for partially premixed multiphase combustion (2PMC) is developed in the framework of large eddy simulation (LES). In this model, the multi-regime combustion mode is decomposed into two pure combustion regimes (i.e. premixed and non-premixed) through combustion regime indicator. Different combustion models are chosen for different combustion regimes. For example, for premixed combustion regime, the flamelet generated manifold (FGM) tabulation method in combination with the artificially thickened flame (ATF) approach is used. For non-premixed combustion regime, on the other hand, the thermo-chemical quantities will be extracted from the non-premixed chemtable generated by the flamelet/progress variable (FPV) approach. The proposed multi-regime flamelet model is then extended to adapt to the pulverized coal combustion (PCC) and applied to a laboratory-scale pulverized coal jet flame. The simulation results show that the percentage of the volume-weighted premixed combustion regime of this studied flame is up to 18%. Quantitative comparisons between the experimental data and the numerical results with the present multi-regime flamelet model, the FPV model, and the Eddy Break Up (EBU) model shows that the proposed multi-regime flamelet model has several advantages. It performs better than the FPV model in the regions where premixed combustion mode prevails and also much better than the EBU model in species concentration prediction.
Shinnosuke Nishiki - One of the best experts on this subject based on the ideXlab platform.
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a direct numerical simulation study of the influence of flame generated vorticity on reaction zone surface area in weakly turbulent premixed combustion
Physics of Fluids, 2019Co-Authors: Andrei Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of the baroclinic torque term in the enstrophy transport equation, with the effect being more pronounced at larger values of the mean combustion progress variable. If the density ratio is low, e.g., σ = 2.5, the baroclinic torque weakly affects the vorticity field within the mean flame brush and the aforementioned effect is not pronounced.Direct numerical simulation data obtained from two statistically stationary, one-dimensional, planar, weakly turbulent, premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the surface area of the reaction zone. The two flames are associated with the flamelet combustion regime and are characterized by two significantly different density ratios σ = 7.53 and 2.5, with all other things being roughly equal. The obtained results indicate that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling of the reaction surface, reduce its area, and, hence, decrease the burning rate. Thus, these results call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of σ = 7.53, the local stretch rate, which quantifies the local rate of increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large m...
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letter does flame generated vorticity increase turbulent burning velocity
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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Combustion-induced local shear layers within premixed Flamelets in weakly turbulent flows
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:3D direct numerical simulation data obtained from statistically stationary, planar, weakly turbulent, premixed flames, which are characterized by two different density ratios (7.53 and 2.50) and are associated with the flamelet combustion regime, are analyzed to investigate differences between velocity and pressure variations (i) in Flamelets in a weakly turbulent flow and (ii) in the counterpart laminar flame. Results show that while the thermo-chemical structure of the Flamelets is weakly affected by turbulence under the studied conditions, the local velocity, vorticity, and pressure fields within the Flamelets differ significantly from the velocity, vorticity, and pressure fields, respectively, within the laminar flame. In particular, local shear layers appear within Flamelets in the turbulent flow because acceleration of a reacting mixture by the local pressure gradient is inversely proportional to the mixture density and, hence, depends on the mixture state. The shear layers are characterized by large velocity gradients (both the tangential gradient of the normal velocity with respect to the flamelet surface and the normal gradient of the tangential velocity), whose magnitudes may be comparable with the magnitude of the velocity gradient across the laminar flame. In flamelet zones characterized by a relatively large magnitude of the locally normal gradient of the tangential velocity, the local vorticity magnitude is also large and such zones contribute substantially to the overall generation of vorticity due to baroclinic torque. These results cast doubts on the validity of a simple common modeling approach that consists in directly invoking expressions derived for the laminar flames in order to describe the influence of combustion-induced thermal expansion on weakly turbulent velocity and pressure fields.
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Does flame-generated vorticity increase turbulent burning velocity?
Physics of Fluids, 2018Co-Authors: Andrei N. Lipatnikov, Vladimir Sabelnikov, Shinnosuke Nishiki, Tatsuya HasegawaAbstract:Direct numerical simulation data obtained from a statistically stationary, 1D, planar, weakly turbulent, premixed flame, which is associated with the flamelet combustion regime, are analyzed in order to show that generation of vorticity due to baroclinic torque within Flamelets can impede wrinkling the reaction surface, reduce its area, and decrease the burning rate. These data call for revisiting the widely accepted concept of combustion acceleration due to flame-generated turbulence.
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a balance equation for the mean rate of product creation in premixed turbulent flames
Proceedings of the Combustion Institute, 2017Co-Authors: Vladimir Sabelnikov, Nilanjan Chakraborty, Shinnosuke Nishiki, Andrei Lipatnikov, Tatsuya HasegawaAbstract:Transport equations for reaction rate W and its Favre-averaged value W are derived from first principle in the case of premixed turbulent combustion. The assumptions made for derivation hold for unity Lewis number premixed flames at least in the flamelet regime of turbulent burning. Analysis of the latter equation shows that it involves two dominant terms, but the difference between them vanishes if reaction zones retain the structure of the zone in the unperturbed laminar flame. However, in such a case, turbulent burning velocity cannot grow with time during interaction of an initially laminar flame with a turbulent flow. Therefore, the analysis indicates a vital role played by local perturbations of reaction zone structure in premixed turbulent combustion. The dominance of these two terms and the important role played by the difference between them are confirmed by analyzing three DNS databases associated with both the corrugated Flamelets and thin reaction zones regimes of premixed turbulent burning. Moreover, the DNS data show that perturbations of local displacement speed due to perturbations of local flamelet structure are also of paramount importance for modeling transport of flame surface density even in weakly turbulent flows. Finally, by simulating curved and/or strained laminar premixed flames and integrating the transport equation for W across the flames, the integral is shown to depend linearly on the stretch rate even in highly perturbed flames, with results obtained from variously stretched flames being close to each other. Based on this finding, the difference between the two dominant terms in the transport equation for the mean rate W is hypothesized to depend linearly on the stretch rate conditioned to the reaction zone. Application of this hypothesis to the DNS data associated with the corrugated Flamelets combustion regime yields encouraging results, thus, confirming a crucial role played by local perturbations of reaction zone structure even in weakly turbulent flames.