Large Eddy Simulations(LES) in conjunction with the Flamelet Progress Variable(FPV) approach have been performed to investigate the flame and large-scale flow structures in the bluff-body stabilized non-premixed flame...Large Eddy Simulations(LES) in conjunction with the Flamelet Progress Variable(FPV) approach have been performed to investigate the flame and large-scale flow structures in the bluff-body stabilized non-premixed flames, HM1 and HM3. The validity of the numerical methods is first verified by comparing the predicted velocity and composition fields with experimental measurements. Then the evolution of the flame and large-scale flow structures is analyzed when the flames approach blow-off. The analysis of instantaneous and statistical data indicates that there exists a shift of the control mechanism in the recirculation zone in the two flames. In the recirculation zone, HM1 flame is mainly controlled by the mixing effect and ignition mainly occurs in the outer shear layer. In HM3 flame, both the chemical reactions and mixing are important in the recirculation zone. The Proper Orthogonal Decomposition(POD) results show that the fluctuations in the outer shear layer are more intense in HM1, while the flow structures are more obvious in the outer vortex structure in HM3, due to the different control mechanism in the recirculation zone.It further shows that the flow structures in HM1 spread larger in the intense mixing zone due to higher temperature and less extinction.展开更多
针对火焰检测过程中存在小目标难以检测的问题,提出了一种改进的YOLOv8n模型。首先,在双分支跨阶段局部特征融合(cross stage partial 2 with feature fusion,C2f)模块中加入动态蛇形卷积,有助于提取多尺度特征、增强特征表示。接着,将G...针对火焰检测过程中存在小目标难以检测的问题,提出了一种改进的YOLOv8n模型。首先,在双分支跨阶段局部特征融合(cross stage partial 2 with feature fusion,C2f)模块中加入动态蛇形卷积,有助于提取多尺度特征、增强特征表示。接着,将GhostnetV2引入到颈部网络中,不仅减少了模型的参数量,还提升了整体的检测精度和速度。然后,添加微小目标检测头以便更好地进行多尺度小目标的检测,基于局部和全局的挤压激励(squeeze and excitation,SE)注意力机制确保每一层的特征都得到充分优化,特别是小目标的细微特征。最后,基于最小点距离的交并比损失函数提高算法的收敛速度和定位精度。实验结果显示,改进YOLOv8n模型的P、R、FPS、mAP@0.5和mAP@0.5∶0.95指标平均值比YOLOv8n模型分别提高了3.34%、3.62%、14帧/s、3.01%和3.41%,表明模型拥有较好的小目标火焰检测能力。研究结果可为预防火灾等安全事故提供理论依据和决策支撑。展开更多
The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The resul...The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.展开更多
基金supported by the National Natural Science Foundation of China(Nos.91441202 and 51476087)
文摘Large Eddy Simulations(LES) in conjunction with the Flamelet Progress Variable(FPV) approach have been performed to investigate the flame and large-scale flow structures in the bluff-body stabilized non-premixed flames, HM1 and HM3. The validity of the numerical methods is first verified by comparing the predicted velocity and composition fields with experimental measurements. Then the evolution of the flame and large-scale flow structures is analyzed when the flames approach blow-off. The analysis of instantaneous and statistical data indicates that there exists a shift of the control mechanism in the recirculation zone in the two flames. In the recirculation zone, HM1 flame is mainly controlled by the mixing effect and ignition mainly occurs in the outer shear layer. In HM3 flame, both the chemical reactions and mixing are important in the recirculation zone. The Proper Orthogonal Decomposition(POD) results show that the fluctuations in the outer shear layer are more intense in HM1, while the flow structures are more obvious in the outer vortex structure in HM3, due to the different control mechanism in the recirculation zone.It further shows that the flow structures in HM1 spread larger in the intense mixing zone due to higher temperature and less extinction.
文摘针对火焰检测过程中存在小目标难以检测的问题,提出了一种改进的YOLOv8n模型。首先,在双分支跨阶段局部特征融合(cross stage partial 2 with feature fusion,C2f)模块中加入动态蛇形卷积,有助于提取多尺度特征、增强特征表示。接着,将GhostnetV2引入到颈部网络中,不仅减少了模型的参数量,还提升了整体的检测精度和速度。然后,添加微小目标检测头以便更好地进行多尺度小目标的检测,基于局部和全局的挤压激励(squeeze and excitation,SE)注意力机制确保每一层的特征都得到充分优化,特别是小目标的细微特征。最后,基于最小点距离的交并比损失函数提高算法的收敛速度和定位精度。实验结果显示,改进YOLOv8n模型的P、R、FPS、mAP@0.5和mAP@0.5∶0.95指标平均值比YOLOv8n模型分别提高了3.34%、3.62%、14帧/s、3.01%和3.41%,表明模型拥有较好的小目标火焰检测能力。研究结果可为预防火灾等安全事故提供理论依据和决策支撑。
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11732003 and 11521062)the National Key Research and Development Program of China(Grant No.2017YFC0804700)
文摘The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.
