Analysis of upward- and downward flame spread over vertical installed polyethylene-insulated electrical wires

Abstract

Upward- (UFS) and downward (DFS) flame spread along a vertical installed electrical wire has been examined experimentally to gain better understandings of the heat transfer mechanism and burning behavior over electrical wires. UFS is considered as a much more hazardous fire scenario than DFS, however, UFS over electrical wires was very limited addressed. Fundamentally, the controlling heat transfer boundary over the preheating zone, hence the flame spread behaviors are quite different between DFS and UFS. This problem is more complicated for an electrical wire, for which the metal conduction is involved additionally, playing an important role in the heat transfer process. Thirteen types of polyethylene-insulated electrical wires with Cu- and NiCr-core were used to investigate the effect of metal core thermal conduction and wire configuration for both UFS and DFS. During the flame spread process, the pyrolysis length of UFS was enlarged, while only portion of virgin material of DFS were burnt as the result of distinct dripping. By further concerning the curvature effect, the dependence of the flame length on heat release rate per unit width (i.e. perimeter for cylinder) can be well explained by non-dimensional analysis. It turns out that the growth of flame length will be restrained as radius decreases significantly, and can be categorized in to two regions, one is controlled by laminar molecular diffusion followed the other controlled by turbulent entrainment. The variation of flame spread rate (FSR) with wire configuration and metal core thermal conduction are well interpreted by a theoretical model, which describes the heat recirculation through flame-insulation-core comprehensively 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 FSR. An important finding is that a highly conductive core will trigger an acceleration of FSR over the wire, primarily because heat flux in gas phase is more sensitive to the change of core thermal conductivity than heat conduction through core. Even though the heat diffusion length increases at the same time, however, the heat flux in gas phase grows faster than the corresponding heat loss through core, thereby compensate the corresponding heat loss. This work could facilitate understanding of heat transfer mechanism and burning behavior not only on the electrical wires, but also on the composite material with high thermal-conductive media.