The systematic method for constructing Lewis representations is a method for representing chemical bonds between atoms in a molecule. It uses symbols to represent the valence electrons of the atoms involved in the bon...The systematic method for constructing Lewis representations is a method for representing chemical bonds between atoms in a molecule. It uses symbols to represent the valence electrons of the atoms involved in the bond. Using a number of rules in a defined order, it is often better suited to complicated cases than the Lewis representation of atoms. This method allows us to determine the formal charge and oxidation number of each atom in the edifice more efficiently than other methods.展开更多
In this work, the effect of various effective dimensionless numbers and moisture contents on initiation of instability in combustion of moisty organic dust is calculated. To have reliable model, effect of thermal radi...In this work, the effect of various effective dimensionless numbers and moisture contents on initiation of instability in combustion of moisty organic dust is calculated. To have reliable model, effect of thermal radiation is taken into account. One- dimensional flame structure is divided into three zones： preheat zone, reaction zone and post-flame zone. To investigate pulsating characteristics of flame, governing equations are rewritten in dimensionless space-time （（, r/, ~） coordinates. By solving these newly achieved governing equations and combining them, which is completely discussed in body of article, a new expression is obtained. By solving this equation, it is possible to predict initiation of instability in organic dust flame. According to the obtained results by increasing Lewis number, threshold of instability happens sooner. On the other hand, pulsating is postponed by increasing Damk6hler number, pyrolysis temperature or moisture content. Also, by considering thermal radiation effect, burning velocity predicted by our model is closer to experimental results.展开更多
The structure and stability of non-adiabatic reverse smolder waves are investigated numerically. First, the 1D steady-state responses of reverse smolder waves in the presence of convective heat losses are studied with...The structure and stability of non-adiabatic reverse smolder waves are investigated numerically. First, the 1D steady-state responses of reverse smolder waves in the presence of convective heat losses are studied with the rate of incoming air flow as the control parameter. Based on the 1D steady solutions, the linear stability and the Lewis number effects on the stability are examined by a numerical normal mode analysis. Finally, the dynamical evolution processes of unstable reverse smolder waves are studied by direct numerical simulations. It is shown that, in comparison with the adiabatic case, the presence of heat losses leads to a backward shift of the extinction limit. For varying Lewis numbers, the extinction limit shifts forward with the increase of the Lewis number while the smolder temperature remains unchanged. Furthermore, results of a linear stability analysis show that the maximum growth rate decreases with the increasing Lewis number, implying that increasing the Lewis number tends to weaken the thermal-diffusive instability of non-adiabatic reverse smolder waves. Direct numerical simulation results show that, on the fuel-rich branch, the unstable plane reverse smolder wave gradually develops to a regular steady fingering pattern, whereas on the fuel-lean branch, similar to the adiabatic case, vigorous fragmentation instability occurs, and is accompanied by a substantial local temperature rise, which may be sufficiently high to trigger the transition to flaming combustion.展开更多
Asymptotic analysis is conducted for outwardly propagating spherical flames with large activation energy. The spherical flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Analytical sol...Asymptotic analysis is conducted for outwardly propagating spherical flames with large activation energy. The spherical flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Analytical solutions are separately obtained in these three zones and then asymp- totically matched. In the asymptotic analysis, we derive a correlation describing the spherical flame temperature and propagation speed changing with the flame radius. This cor- relation is compared with previous results derived in the limit of infinite value of activation energy. Based on this correla- tion, the properties of spherical flame propagation are inves- tigated and the effects of Lewis number on spherical flame propagation speed and extinction stretch rate are assessed. Moreover, the accuracy and performance of different mod- els used in the spherical flame method are examined. It is found that in order to get accurate laminar flame speed and Markstein length, non-linear models should be used.展开更多
The present pagination reports both Brownian diffusion and thermophoresis aspects subject to magneto hydrodynamic Williamson fluid model.Assuming the flow is unsteady and blood is treated as Williamson fluid over a we...The present pagination reports both Brownian diffusion and thermophoresis aspects subject to magneto hydrodynamic Williamson fluid model.Assuming the flow is unsteady and blood is treated as Williamson fluid over a wedge with radiation.The governing equations are transformed into ordinary differential equations by using similarity variables.The analytical solutions of the transformed governing equations are obtained by using the RK 4th order method along with shooting technique solver.The effects of various physical parameters such as Hartmann number,local Weissenberg number,radiation parameter,unsteadiness parameter,Prandtl number,Lewis number,Brownian diffusion,thermophoresis,wedge angle parameter,moving wedge parameter,on velocity,temperature,concentration,skin friction,heat transfer rate and mass transfer rate have been discussed in detail.The velocity and temperature profile deprives for larger We and an opposite trend is observed for concentration.The radiation parameter is propositional to temperature and a counter behaviour is observed for Pr.展开更多
The flame displacement speed is one of the major characteristics in turbulent premixed flames. The flame displacement speed is experimentally obtained from the displacement normal to the flame surface, while it is num...The flame displacement speed is one of the major characteristics in turbulent premixed flames. The flame displacement speed is experimentally obtained from the displacement normal to the flame surface, while it is numerically evaluated by the transport equation of the flame surface. The flame displacement speeds obtained both experimentally and numerically cannot be compared directly because their definitions are different. In this study, two kinds of experimental flame displacement speeds—involving the mean inflow velocity and the local flow velocity—were simulated using the DNS data with the different Lewis numbers, and were compared with the numerical flame displacement speed. The simulated experimental flame displacement speed involving the mean inflow velocity had no correlation with the numerical flame displacement speed, while the simulated displacement speed involving the local flow velocity had a clear correlation with the numerical displacement speed in the cases of higher Lewis number than unity. The correlation coefficient of the simulated displacement speed involving the local flow velocity with the numerical displacement speed had a maximum value on the isosurface of the reaction progress variable with the maximum temperature gradient where the dilation effect of the flame is strongest.展开更多
Based on premixed flame, the theoretical model of transport properties with temperature variation was established inside a preheated zone. Lewis number of the deficient-to- stoichiometric hydrocarbon/air mixture has b...Based on premixed flame, the theoretical model of transport properties with temperature variation was established inside a preheated zone. Lewis number of the deficient-to- stoichiometric hydrocarbon/air mixture has been theoretically predicted over a wide range of preheated temperature. These predictions are compared with the experimental data on disport properties that exist in the literature. The response of the burning velocity to flame stretch can be parameterized by the laminar flame speed and Markstein length. Therefore, if the laminar flame speed and Markstein number could be accurately simulated by using an analytic expression of characterized temperature, equivalence ratio, and Lewis number, the results are applicable to the prediction of methane, acetylene, ethylene, ethane, and propane flames. Expanding previous studies on the extinction of premixed flames under the influence of stretch and incomplete reaction, the results were further classified and rescaled. Finally, it could be inferred that parameter Pq, the rescaled extinction Karlovitz number could be used to explain the degree or flame quench.展开更多
A three-dimensional Direct numerical simulation(DNS)with complex chemistry was employed to examine the statistical behavior of turbulent kinetic energy(TKE)and enstrophy transport equations in hydrogen(Lewis number(Le...A three-dimensional Direct numerical simulation(DNS)with complex chemistry was employed to examine the statistical behavior of turbulent kinetic energy(TKE)and enstrophy transport equations in hydrogen(Lewis number(Le)≈0.4)and dodecane(Le≈4.2)flames.The Karlovitz(Ka)numbers ranged from 4 to 150,involving both the thin and broken reaction zones.Budget analyses of TKE and enstrophy transport equations are performed,and scaling terms in the literature are re-examined.Similar to thin reaction zone flames,viscous dissipation term appears to be the most important term in the TKE balance,while viscous dissipation and vortex-stretching terms are the dominant terms in the enstrophy transport equation at high Ka number.The velocity-pressure gradient and the mean velocity dilatation in the TKE transport equation and the dilatation term in enstrophy budget are found to be affected by the Le.Modified scaling estimations for those terms affected by Le are proposed in this work to account for the Le effects spanning different combustion regimes.This work confirmed that Kolmogorov’s first hypothesis is not valid for low Ka number flames investigated in this study,where the vortex stretching and viscous dissipation terms cannot be scaled with local dissipation and viscosity.At sufficiently high Ka number flames,the vorticity can be scaled with the Kolmogorov time scale,and the mean enstrophy value approaches homogeneous,isotropic,non-reacting turbulence flow,but lower Le fuels require much higher Ka number to achieve that.展开更多
In the present investigation,an analytical analysis has been carried out to study theinfluence of chemical reaction on MHD flow of a nanofluid in an expanding or contractingporous pipe in the presence of heat sourcels...In the present investigation,an analytical analysis has been carried out to study theinfluence of chemical reaction on MHD flow of a nanofluid in an expanding or contractingporous pipe in the presence of heat sourcelsink.The pipe wall expands or contracts uniformlyat a time dependent rate.Similarity transfomations have been invoked to reduce the governingflow equations into coupled nonlinear ordinary differential equations.An analytical approach,namely the homotopy analysis method(HAM)is employed to obtain the analytical solutions ofthe ordinary differential equations.The convergence of the obtained series solutions isanalyzed.The effects of various physical parameters such as wall expansion ratio,Brownianmotion parameter,thermophoresis parameter,Lewis number,chemical reaction parameter andheat sourcelsink parameter on flow variables have been discussed.Further,for the case ofhydrodynamic viscous fluid,we find a good agreement between the HAM solutions andsolutions already reported in the literature.展开更多
文摘The systematic method for constructing Lewis representations is a method for representing chemical bonds between atoms in a molecule. It uses symbols to represent the valence electrons of the atoms involved in the bond. Using a number of rules in a defined order, it is often better suited to complicated cases than the Lewis representation of atoms. This method allows us to determine the formal charge and oxidation number of each atom in the edifice more efficiently than other methods.
文摘In this work, the effect of various effective dimensionless numbers and moisture contents on initiation of instability in combustion of moisty organic dust is calculated. To have reliable model, effect of thermal radiation is taken into account. One- dimensional flame structure is divided into three zones： preheat zone, reaction zone and post-flame zone. To investigate pulsating characteristics of flame, governing equations are rewritten in dimensionless space-time （（, r/, ~） coordinates. By solving these newly achieved governing equations and combining them, which is completely discussed in body of article, a new expression is obtained. By solving this equation, it is possible to predict initiation of instability in organic dust flame. According to the obtained results by increasing Lewis number, threshold of instability happens sooner. On the other hand, pulsating is postponed by increasing Damk6hler number, pyrolysis temperature or moisture content. Also, by considering thermal radiation effect, burning velocity predicted by our model is closer to experimental results.
基金Project supported by the Shanghai Rising Star Program (No. 09QA1402300)the Scientific Research Innovation Program of Shanghai Education Commission
文摘The structure and stability of non-adiabatic reverse smolder waves are investigated numerically. First, the 1D steady-state responses of reverse smolder waves in the presence of convective heat losses are studied with the rate of incoming air flow as the control parameter. Based on the 1D steady solutions, the linear stability and the Lewis number effects on the stability are examined by a numerical normal mode analysis. Finally, the dynamical evolution processes of unstable reverse smolder waves are studied by direct numerical simulations. It is shown that, in comparison with the adiabatic case, the presence of heat losses leads to a backward shift of the extinction limit. For varying Lewis numbers, the extinction limit shifts forward with the increase of the Lewis number while the smolder temperature remains unchanged. Furthermore, results of a linear stability analysis show that the maximum growth rate decreases with the increasing Lewis number, implying that increasing the Lewis number tends to weaken the thermal-diffusive instability of non-adiabatic reverse smolder waves. Direct numerical simulation results show that, on the fuel-rich branch, the unstable plane reverse smolder wave gradually develops to a regular steady fingering pattern, whereas on the fuel-lean branch, similar to the adiabatic case, vigorous fragmentation instability occurs, and is accompanied by a substantial local temperature rise, which may be sufficiently high to trigger the transition to flaming combustion.
基金supported by the National Natural Science Foundation of China (50976003,51136005)Doctoral Fund of Ministry of Education of China (20100001120003)
文摘Asymptotic analysis is conducted for outwardly propagating spherical flames with large activation energy. The spherical flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Analytical solutions are separately obtained in these three zones and then asymp- totically matched. In the asymptotic analysis, we derive a correlation describing the spherical flame temperature and propagation speed changing with the flame radius. This cor- relation is compared with previous results derived in the limit of infinite value of activation energy. Based on this correla- tion, the properties of spherical flame propagation are inves- tigated and the effects of Lewis number on spherical flame propagation speed and extinction stretch rate are assessed. Moreover, the accuracy and performance of different mod- els used in the spherical flame method are examined. It is found that in order to get accurate laminar flame speed and Markstein length, non-linear models should be used.
文摘The present pagination reports both Brownian diffusion and thermophoresis aspects subject to magneto hydrodynamic Williamson fluid model.Assuming the flow is unsteady and blood is treated as Williamson fluid over a wedge with radiation.The governing equations are transformed into ordinary differential equations by using similarity variables.The analytical solutions of the transformed governing equations are obtained by using the RK 4th order method along with shooting technique solver.The effects of various physical parameters such as Hartmann number,local Weissenberg number,radiation parameter,unsteadiness parameter,Prandtl number,Lewis number,Brownian diffusion,thermophoresis,wedge angle parameter,moving wedge parameter,on velocity,temperature,concentration,skin friction,heat transfer rate and mass transfer rate have been discussed in detail.The velocity and temperature profile deprives for larger We and an opposite trend is observed for concentration.The radiation parameter is propositional to temperature and a counter behaviour is observed for Pr.
文摘The flame displacement speed is one of the major characteristics in turbulent premixed flames. The flame displacement speed is experimentally obtained from the displacement normal to the flame surface, while it is numerically evaluated by the transport equation of the flame surface. The flame displacement speeds obtained both experimentally and numerically cannot be compared directly because their definitions are different. In this study, two kinds of experimental flame displacement speeds—involving the mean inflow velocity and the local flow velocity—were simulated using the DNS data with the different Lewis numbers, and were compared with the numerical flame displacement speed. The simulated experimental flame displacement speed involving the mean inflow velocity had no correlation with the numerical flame displacement speed, while the simulated displacement speed involving the local flow velocity had a clear correlation with the numerical displacement speed in the cases of higher Lewis number than unity. The correlation coefficient of the simulated displacement speed involving the local flow velocity with the numerical displacement speed had a maximum value on the isosurface of the reaction progress variable with the maximum temperature gradient where the dilation effect of the flame is strongest.
文摘Based on premixed flame, the theoretical model of transport properties with temperature variation was established inside a preheated zone. Lewis number of the deficient-to- stoichiometric hydrocarbon/air mixture has been theoretically predicted over a wide range of preheated temperature. These predictions are compared with the experimental data on disport properties that exist in the literature. The response of the burning velocity to flame stretch can be parameterized by the laminar flame speed and Markstein length. Therefore, if the laminar flame speed and Markstein number could be accurately simulated by using an analytic expression of characterized temperature, equivalence ratio, and Lewis number, the results are applicable to the prediction of methane, acetylene, ethylene, ethane, and propane flames. Expanding previous studies on the extinction of premixed flames under the influence of stretch and incomplete reaction, the results were further classified and rescaled. Finally, it could be inferred that parameter Pq, the rescaled extinction Karlovitz number could be used to explain the degree or flame quench.
基金supported by the National Natural Science Foundation of China(Grant Nos.91752201 and 11672123)Shenzhen Science and Technology Program(Grant Nos.JCYJ20170412151759222,JCYJ20180302173952945,and KQTD20180411143441009)+1 种基金Department of Science and Technology of Guangdong Province(Grant No.2019B21203001)Project No.LCH-2019011 under the Joint Program of Shenzhen Clean Energy Research Institute and SUSTech through contract CERI-KY-2019-003.
文摘A three-dimensional Direct numerical simulation(DNS)with complex chemistry was employed to examine the statistical behavior of turbulent kinetic energy(TKE)and enstrophy transport equations in hydrogen(Lewis number(Le)≈0.4)and dodecane(Le≈4.2)flames.The Karlovitz(Ka)numbers ranged from 4 to 150,involving both the thin and broken reaction zones.Budget analyses of TKE and enstrophy transport equations are performed,and scaling terms in the literature are re-examined.Similar to thin reaction zone flames,viscous dissipation term appears to be the most important term in the TKE balance,while viscous dissipation and vortex-stretching terms are the dominant terms in the enstrophy transport equation at high Ka number.The velocity-pressure gradient and the mean velocity dilatation in the TKE transport equation and the dilatation term in enstrophy budget are found to be affected by the Le.Modified scaling estimations for those terms affected by Le are proposed in this work to account for the Le effects spanning different combustion regimes.This work confirmed that Kolmogorov’s first hypothesis is not valid for low Ka number flames investigated in this study,where the vortex stretching and viscous dissipation terms cannot be scaled with local dissipation and viscosity.At sufficiently high Ka number flames,the vorticity can be scaled with the Kolmogorov time scale,and the mean enstrophy value approaches homogeneous,isotropic,non-reacting turbulence flow,but lower Le fuels require much higher Ka number to achieve that.
基金The authors gratefully acknowledge National Board forHigher Mathematics,India for sanctioning a major researchproject under the Grant no.2/48(19)/2012/NBHM(R.P.)/R&D II/9137.The authors thank all the three anonymousreviewers for their constructive comments and suggestions.
文摘In the present investigation,an analytical analysis has been carried out to study theinfluence of chemical reaction on MHD flow of a nanofluid in an expanding or contractingporous pipe in the presence of heat sourcelsink.The pipe wall expands or contracts uniformlyat a time dependent rate.Similarity transfomations have been invoked to reduce the governingflow equations into coupled nonlinear ordinary differential equations.An analytical approach,namely the homotopy analysis method(HAM)is employed to obtain the analytical solutions ofthe ordinary differential equations.The convergence of the obtained series solutions isanalyzed.The effects of various physical parameters such as wall expansion ratio,Brownianmotion parameter,thermophoresis parameter,Lewis number,chemical reaction parameter andheat sourcelsink parameter on flow variables have been discussed.Further,for the case ofhydrodynamic viscous fluid,we find a good agreement between the HAM solutions andsolutions already reported in the literature.
