The effects of separation distance and environmental parameters (such as ambient pressure and oxygen concentration) on the upward flame spread over two parallel paper sheets were investigated in this work, which would be conducive to enriching the fire dynamic theories and providing a reference for the risk evaluation in building and space fires. It was found that, with the increase of separation distance, the flame spread regimes can be roughly divided into four classes: the flame quenching regime (without inner flame), where the interaction of two parallel papers is mainly dominated by the solid surface radiation; the complete fusion regime, limited by the oxygen diffusion; the partial fusion regime, dominated by the convection; the separated flames regime, dominated by the gas-phase radiation. The flames inside the narrow space are fused under the flame-induced horizontal flow, and extinguished for the oxygen starvation, which reflects the interaction of two parallel samples. Interestingly, although the flames inside and outside the parallel sheets often have different flame lengths and burning intensities, they spread at almost the same rate, which is ascribed to the overlapping of the pyrolysis region on both sides of the thermal thin paper. A critical separation distance was found, where the flame spread rate is largest due to the stoichiometric mixture fraction. The orange-blue layered flames were analyzed, which is related to the boundary layer of the flow field and the mixture of fuel and fresh air. The oxygen concentration has nearly equivalent impacts on the flame-induced buoyancy and horizontal flows, and thus a constant flame fusion angle is maintained. Nevertheless, the enhanced ambient pressure mainly promotes the buoyancy and reduces the flame fusion angle. The flames inside the narrow space are apt to be extinguished under lower ambient pressure with weak buoyancy similar to the reduced gravity and microgravity environments. The transformation of the dominating regimes were observed at a moderate separation distance: the flame quenching regime at pressures below 18 kPa, the complete fusion regime at pressures between 18 similar to 29 kPa, and the partial fusion regime at pressures over 29 kPa.
