Effect of oil immersion on the horizontal flame spread over transformer insulating paperboard with different sizes

Transformer oil and insulating paperboard are widely used in oil-immersed transformers and may be ignited in the event of device failure. The present study aims to investigate the effect of oil immersion on the horizontal flame spread over the transformer insulating paperboard (TIP). A series of experiments were conducted on the samples of TIP and oil-immersed transformer insulating paperboard (OTIP) with 6 different thicknesses and 4 different widths. The flame spread rate and flame height increase with sample width under different conditions. In addition, the flame spread rate decreases with increasing sample thickness but the flame height is insensitive to the sample thickness. Oil immersion increases the flame height and the length of the burning region in the steady stage. Compared to TIP, the radiative heat feedback is enhanced but the energy required to ignite the sample is decreased by 40 % for the flame spread over OTIP, resulting in an increased flame spread rate. Convection from the ceiling flame underneath the sample is the dominant heat mode, accounting for more than 68 % of the total heat flux. A model for predicting the flame spread rate of OTIP and TIP with different thicknesses and widths was proposed.