Numerical Study on the Influence of Branch Angle and Branch Slope on Smoke Transportation Characteristics for Branched Tunnel Fires Under Natural Ventilation

In order to study the change of influence of branch angle and branch slope on smoke transportation characteristics of branched tunnel fire under natural ventilation, the numerical simulation of branched tunnel with different slopes (0, 6 and 9%) and angles (90°, 45° and 15°) was carried out. Mass Transfer-Volumetric Heating Source (MT-VHS) model was used. The results show that when the branch slope is in the range of 0–9%, the higher the slope is, the more hot smoke will enter the branch; And the reduction of pressure leads to the smoke acceleration at the branch. The branch angle will also affect the hot smoke entering the branch, but there is no obvious monotonic feature. The influence of different branch angles and slopes on the longitudinal temperature decay of branched tunnel under natural ventilation is analyzed. The flame morphology variation is discussed and it is found that the flame tilts towards the sidewall. The flame tilt is then explained by comparing the surface force of the fire source.