Dynamics Of Flame Spread And Extinction Over Electrical Wires:Effect Of Convection

Fire poses a significant threat to human society,with electrical wire fire identified as a primary cause during the electrification of modern society.Convection boundary conditions,such as reduced-buoyant flow in space exploration missions and external airflow,influence practical wire fire scenarios.However,current fire safety standards for flammability evaluation of electrical wires do not account for these effects.Controlling mechanisms of heat and mass transfer,as well as chemical reaction kinetics,are significantly impacted by convection effects in combustion systems,ultimately modifying the flammability of solids.Thus,optimizing existing fire safety standards has become an essential practical issue.Due to the surface curvature and heat conduction through the metal core,the fire behaviors of electrical wires are more complex than those of homogeneous materials.However,the effect of convection on flame spread and extinction over electrical wires remains poorly understood.The classical fire dynamics theory and the methodologies used to simulate space gravity,have limitations.To address this gap,this study investigates the flame spread and extinction over electrical wires under reduced buoyant convection(depressurized atmosphere),natural convection,and mixed convection.The effects of surface curvature and heat conduction through the metal core are also explored to improve flammability evaluation of electrical wires under different convection boundary conditions and enhance fire safety during human electrification.Comparisons with experimental results and low-gravity conditions reveal errors in ground-based simulations of wire fire under low-speed convection,prompting refinement of pressure-gravity scaling relations to facilitate precise and efficient use of ground-based facilities for simulating low-speed convection.Major findings in this study provide crucial insights into improving fire safety and evaluating flammability under various convection boundary conditions,including:(1)Flame spread behaviors over electrical wires under natural convection,and its controlling heat transfer mechanism.The variation of the upward-and downward flame spread rates(FSR)with the change of wire configuration is thoroughly investigated.By further concerning the surface curvature effect,the dependence of the flame length on heat release rate per unit perimeter can be well explained by scale analysis.It turns out that the growth of flame length can be categorized in to two regions,one is controlled by laminar molecular diffusion followed the other controlled by turbulent entrainment.The evolution of FSR with wire configuration is well interpreted by a theoretical model,which describes the heat recirculation through flame-insulation-core thoroughly based on thermodynamics theories,and attempts to consider the effects of temperature gap between core and insulation at pyrolysis front,burning fraction and melting heat.The validity of this new model is well confirmed by measured flame spread rate.(2)Reduced-buoyancy aided flame spread over electrical wires in depressurized atmosphere.The most noteworthy finding is that the upward FSR increases with reducing ambient pressure monotonically,which shows an opposite trend to that of flat materials.It indicates that the classical similarity law between low-pressure and lowgravity based on Grashof number is no longer applicable for upward flame spread over wires.Based on the diffusive combustion theory,the time-and length scales involved in heat/mass transfer,hydrodynamics,and chemical reaction kinetics are modelled by incorporating the effects of surface curvature and finite chemical reaction rates.A group of criteria for simulating wire fires under partial gravity by reducing ambient pressure are proposed.Based on these criterion,the experimental data collected in partial gravity are qualtatively reproduced in depressurized atmosphere,which further verifies the validity of the theoretical analysis.(3)Flame spread and extinction over electrical wires under mixed convection induced by transverse flow.With the increase of transverse flow velocity,the FSR over horizontal wire shows a "wavy" non-monotonic pattern.A theoretical model is developed to interpret the transition in the controlling heat transfer mechanism with the transverse flow.Based on the boundary layer theory,a mixed flow velocity coupling the transverse flow velocity and the buoyancy-induced flow velocity is proposed.This mixed flow velocity establishes a correlation between transverse flow velocity and gravity acceleration.Results indicate that the "blow off" effect alone does not dictate the flame extinction over wires.Based on the diffusive combustion theory,a modified Damk?hler number coupling the solid-phase heat conduction is developed as a new flame extinction criterion.The analysis takes the pyrolysis zone as the control body,so as to avoid the difficulties in modelling with complicated effect such as phase change and dripping in preheat zone.