Numerical study of a fire-driven flow in a narrow cavity.

Air cavities and gaps between material layers are common in construction systems, e.g. ventilated façades. Air cavity may provide a pathway for smoke and flame spread in case of fire. Performing physical testing to investigate different systems and fire scenarios is resource demanding. Fire Dynamics Simulator (FDS version 6.7.0) was used to simulate fire driven flow between two parallel vertical walls. Flame heights, thermal impact to the interior wall surface and upward flow velocities were predicted with FDS and compared with experimental results. The fire source was a propane burner with 8 × 391 mm2 gas outlet area. Heat release rates were 6.6 kW and 12.4 kW and the distance between the parallel walls was 40 mm. Two different convective heat transfer coefficient sub grid scale models available in FDS were investigated. In this study the cavity width to mesh cell size ratio was equal or above 10, resulting in good predictions of flame heights, upward flow velocities and wall temperatures. 2 mm grid resulted in 25% lower HRR in locations near the burner gas inlet, compared to 4 mm grid, indicating the importance of well resolved gas outlet boundary. • Cavity width divided by mesh cell size should be greater than 10 in FDS simulations. • Predicted flame heights and flow velocities corresponded well with experiments. • Refining the grid in FDS resulted in lower cumulative HRR in the flame region. • FDS models for heat transfer at surfaces must be revised for transitional flows. [ABSTRACT FROM AUTHOR]