The major issues of Micro-Thermo-Photo-Voltaic(MTPV) micro-combustors are flame instabilities, which narrow the operational range, and non-uniform wall temperature, which lowers the overall efficiency. The purpose of ...The major issues of Micro-Thermo-Photo-Voltaic(MTPV) micro-combustors are flame instabilities, which narrow the operational range, and non-uniform wall temperature, which lowers the overall efficiency. The purpose of the present research is to propose a novel micro-combustor with combined baffle and cavity configuration to address these issues. For this aim, a numerical modeling approach is validated and used. The performance of the improved combustor is compared with another recent baffle-bluff configuration. It is shown that the novel design improves the average wall temperature by 36.4 K and mitigates its standard deviation by 13.4 K. Moreover, using the optimal baffle thickness, these enhancements can be augmented by 4% raise of average wall temperature, 62%increase of temperature uniformity, and 20% reduction in overall emission. The baffle length of 0.6 times the combustor length and thickness of 0.0625 times the baffle spacing result in the optimal operation due to the flame lift-off in the upstream direction. According to the sensitivity analysis, the most effective geometrical parameters are the baffle length and thickness. It is expected that using this novel micro-combustor with optimized design parameters improves the overall efficiency of MTPV systems.展开更多
In this work, we propose and examine an ammonia-hydrogen fueled micro-combustor with a single-channel inlet and double-channel outlet (SIDO). The combustion characteristics and nitrogen oxide emission of ammonia/hydro...In this work, we propose and examine an ammonia-hydrogen fueled micro-combustor with a single-channel inlet and double-channel outlet (SIDO). The combustion characteristics and nitrogen oxide emission of ammonia/hydrogen-oxygen premixed combustion are explored. Comparison is then made between the conventional and the proposed SIDO combustors. It is found that our proposed new design could lead to an increase of the outer wall temperature and reduced nitrogen oxide emission. The performances of different hydrogen blended ratios (Φ_(b)), inlet velocities (V_(in)), and equivalence ratios (Φ) are evaluated. It is found that increasing Φ_(b) reduces the maximum flame temperature and the pressure loss, enabling the flame to move upstream. When Φ_(b) is set to 25 %, the convective heat transfer performance reaches its optimal level. The wall temperature and its uniformity can be improved by increasing V_(in). However, it is accompanied by increased NO emissions at the outlet. Increasing Φ can significantly reduce nitrogen oxide emission, and such a reduction effect is much more remarkable at a lower Φ_(b). Examining the exergy efficiency is shown to be greatly improved by increasing Φ_(b). Increasing Φ could reduce the combustion efficiency of hydrogen in the mixed fuel and have almost no effect on ammonia. This study demonstrates the feasibility of improving thermal performance and reducing emissions by varying its structure for thermophotovoltaic applications.展开更多
This paper presents a design for a novel,palm sized,high-aspect-ratio engine.To simplify fabrication and keep the device unobtrusive,a design incorporating a two-cycle engine having a flat,rectangular piston with spri...This paper presents a design for a novel,palm sized,high-aspect-ratio engine.To simplify fabrication and keep the device unobtrusive,a design incorporating a two-cycle engine having a flat,rectangular piston with spring return was selected.An experimental engine based on this concept producing a significant amount of useful work was developed.Although results were encouraging,this geometry raised many issues to be investigated and resolved,including the effects of piston sealing,scavenging,heat losses,and combustion efficiency.Due to the complex interplay between these effects,experimental investigation was time consuming and simple models were found to be inadequate.Therefore,a more complex theoretical model accounting for these effects was developed and used to evaluate the sensitivity of engine performance to each of these parameters.The predictions of this model were used to develop recommendations for improving the experimentally developed engine.展开更多
“Flame-street”is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel.This experimental phenomenon was experimentally and n...“Flame-street”is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel.This experimental phenomenon was experimentally and numerically investigated with the focus on the steady-state,thermo-chemical flame structures in previous literature.In the present paper,the dynamic formation process of a methane-oxygen diffusion flame-street structure was simulated with a reacting flow solver developed based on the open-source framework OpenFOAM.By imposing a certain amount of ignition-energy near the channel outlet,a reaction-kernel was formed and bifurcated.Subsequently,three separate flames were consecutively generated from this kernel and propagated within the channel.The whole process was completed within 15 ms and all the discrete flames were eventually in a steady-state.Interestingly,different propagation features were observed for the three flame segments:The leading flame experienced a flame shape/type change from a tribrachial structure in its fastpropagating phase to a long,trailing diffusion tail after being anchored to the inlet.The successive flame had a much lower propagation speed,keeping its two wing-like(fuel-lean premixed and fuel-rich premixed)structure while moving toward its stabilization location,which was approximately in the middle of the channel.The last flame,after the ignition source was turned-off,was immediately convected a bit downstream,and eventually featured a similar two-branch-like structure as the second one.Moreover,chemical insights for the premixed and diffusion branches of the leading flame were also provided with the change of significance of some key elementary reactions focused on,in order to attain a detailed profiling of the flame-type transition.This paper is a first-ever one discussing the transient formation of flame-streets in literature and is believed to be useful for obtaining a comprehensive understanding of this unique flame characteristics from a dynamic point of view.展开更多
A versatile numerical tool based on the open-source framework Open FOAM has been developed in this paper for modeling time-accurate,low-Mach number reacting flows,with a particular interest in small-scale flames.This ...A versatile numerical tool based on the open-source framework Open FOAM has been developed in this paper for modeling time-accurate,low-Mach number reacting flows,with a particular interest in small-scale flames.This tool consists of a gas-phase Navier-Stokes solver and a solid-wall heat conduction solver which can be implemented alone,or used together in a coupled means to reveal the small-scale combustion’s characteristics of significantly enhanced flame-wall thermal coupling.Validation works has proved that the tool is capable of reproducing experimental flames at various scales(from conventional to small scales),including well-recognized micro-flame features in literature such as three modes of premixed flame dynamics(weak flames,flames with repetitive extinction and ignition,and stable flames).Then,an experimentally-already-found but rarely-simulated unique phenomenon of diffusion flame street is successfully reproduced with well-captured flame structures.Moreover,the conjugate heat transfer model with the specific formulation of solid-wall heat conduction enables an attempt to simulate a novel,thermally-orthotropic combustor with its axial thermal conductivities superior to the transverse ones.Finally,computational performance of the developed Open FOAM solver is compared to that of the previously-used compressible flow solver Eilmer.The Open FOAM solver is found to show better wave-damping abilities for overcoming acoustic wave effects at the initial stage of simulations,and is much more efficient in terms of the computational cost.展开更多
文摘The major issues of Micro-Thermo-Photo-Voltaic(MTPV) micro-combustors are flame instabilities, which narrow the operational range, and non-uniform wall temperature, which lowers the overall efficiency. The purpose of the present research is to propose a novel micro-combustor with combined baffle and cavity configuration to address these issues. For this aim, a numerical modeling approach is validated and used. The performance of the improved combustor is compared with another recent baffle-bluff configuration. It is shown that the novel design improves the average wall temperature by 36.4 K and mitigates its standard deviation by 13.4 K. Moreover, using the optimal baffle thickness, these enhancements can be augmented by 4% raise of average wall temperature, 62%increase of temperature uniformity, and 20% reduction in overall emission. The baffle length of 0.6 times the combustor length and thickness of 0.0625 times the baffle spacing result in the optimal operation due to the flame lift-off in the upstream direction. According to the sensitivity analysis, the most effective geometrical parameters are the baffle length and thickness. It is expected that using this novel micro-combustor with optimized design parameters improves the overall efficiency of MTPV systems.
基金support provided by the University of Canterbury,New Zealand(grant no.CPS20-03-002,grant no.452DISDZ)National Research Foundation Singapore(grant no.NRF2016 NRF-NSFC001-102)+1 种基金Hui Rong would like to thank College of Engineering,University of Canterbury and China Scholarship Council(grant no.202208250030)for providing his PhD studentship and scholarshipDS would like to thank for the financial support by the Science Center for Gas Turbine Project(P2022-C-II-003-001).
文摘In this work, we propose and examine an ammonia-hydrogen fueled micro-combustor with a single-channel inlet and double-channel outlet (SIDO). The combustion characteristics and nitrogen oxide emission of ammonia/hydrogen-oxygen premixed combustion are explored. Comparison is then made between the conventional and the proposed SIDO combustors. It is found that our proposed new design could lead to an increase of the outer wall temperature and reduced nitrogen oxide emission. The performances of different hydrogen blended ratios (Φ_(b)), inlet velocities (V_(in)), and equivalence ratios (Φ) are evaluated. It is found that increasing Φ_(b) reduces the maximum flame temperature and the pressure loss, enabling the flame to move upstream. When Φ_(b) is set to 25 %, the convective heat transfer performance reaches its optimal level. The wall temperature and its uniformity can be improved by increasing V_(in). However, it is accompanied by increased NO emissions at the outlet. Increasing Φ can significantly reduce nitrogen oxide emission, and such a reduction effect is much more remarkable at a lower Φ_(b). Examining the exergy efficiency is shown to be greatly improved by increasing Φ_(b). Increasing Φ could reduce the combustion efficiency of hydrogen in the mixed fuel and have almost no effect on ammonia. This study demonstrates the feasibility of improving thermal performance and reducing emissions by varying its structure for thermophotovoltaic applications.
文摘This paper presents a design for a novel,palm sized,high-aspect-ratio engine.To simplify fabrication and keep the device unobtrusive,a design incorporating a two-cycle engine having a flat,rectangular piston with spring return was selected.An experimental engine based on this concept producing a significant amount of useful work was developed.Although results were encouraging,this geometry raised many issues to be investigated and resolved,including the effects of piston sealing,scavenging,heat losses,and combustion efficiency.Due to the complex interplay between these effects,experimental investigation was time consuming and simple models were found to be inadequate.Therefore,a more complex theoretical model accounting for these effects was developed and used to evaluate the sensitivity of engine performance to each of these parameters.The predictions of this model were used to develop recommendations for improving the experimentally developed engine.
基金funded by the National Natural Science Foundation of China(Grant No.51806158)the Fundamental Research Funds for the Central Universities(WUT:2019IVB029).
文摘“Flame-street”is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel.This experimental phenomenon was experimentally and numerically investigated with the focus on the steady-state,thermo-chemical flame structures in previous literature.In the present paper,the dynamic formation process of a methane-oxygen diffusion flame-street structure was simulated with a reacting flow solver developed based on the open-source framework OpenFOAM.By imposing a certain amount of ignition-energy near the channel outlet,a reaction-kernel was formed and bifurcated.Subsequently,three separate flames were consecutively generated from this kernel and propagated within the channel.The whole process was completed within 15 ms and all the discrete flames were eventually in a steady-state.Interestingly,different propagation features were observed for the three flame segments:The leading flame experienced a flame shape/type change from a tribrachial structure in its fastpropagating phase to a long,trailing diffusion tail after being anchored to the inlet.The successive flame had a much lower propagation speed,keeping its two wing-like(fuel-lean premixed and fuel-rich premixed)structure while moving toward its stabilization location,which was approximately in the middle of the channel.The last flame,after the ignition source was turned-off,was immediately convected a bit downstream,and eventually featured a similar two-branch-like structure as the second one.Moreover,chemical insights for the premixed and diffusion branches of the leading flame were also provided with the change of significance of some key elementary reactions focused on,in order to attain a detailed profiling of the flame-type transition.This paper is a first-ever one discussing the transient formation of flame-streets in literature and is believed to be useful for obtaining a comprehensive understanding of this unique flame characteristics from a dynamic point of view.
基金funded by the National Natural Science Foundation of China(Grant Number 51806158)the Fundamental Research Funds for the Central Universities(WUT:2018IVA055,WUT:2019IVB029)。
文摘A versatile numerical tool based on the open-source framework Open FOAM has been developed in this paper for modeling time-accurate,low-Mach number reacting flows,with a particular interest in small-scale flames.This tool consists of a gas-phase Navier-Stokes solver and a solid-wall heat conduction solver which can be implemented alone,or used together in a coupled means to reveal the small-scale combustion’s characteristics of significantly enhanced flame-wall thermal coupling.Validation works has proved that the tool is capable of reproducing experimental flames at various scales(from conventional to small scales),including well-recognized micro-flame features in literature such as three modes of premixed flame dynamics(weak flames,flames with repetitive extinction and ignition,and stable flames).Then,an experimentally-already-found but rarely-simulated unique phenomenon of diffusion flame street is successfully reproduced with well-captured flame structures.Moreover,the conjugate heat transfer model with the specific formulation of solid-wall heat conduction enables an attempt to simulate a novel,thermally-orthotropic combustor with its axial thermal conductivities superior to the transverse ones.Finally,computational performance of the developed Open FOAM solver is compared to that of the previously-used compressible flow solver Eilmer.The Open FOAM solver is found to show better wave-damping abilities for overcoming acoustic wave effects at the initial stage of simulations,and is much more efficient in terms of the computational cost.
