Abstract
This paper presents an empirical convective heat transfer model aimed at improving the accuracy of large-scale fire spread predictions, where coarse near-wall (cm scale) grid is typically applied. The convective heat transfer model is formulated using the sub-grid thermal diffusivity and includes correction terms for temperature gradient and boundary layer mass transfer (blowing effect). The model parameters are optimized with wall-resolved simulations of 1.5 mm grid resolution using propylene fires. The convective heat transfer model is examined with different grid sizes (3 to 36 mm) and fuel flow rates (5 to 20 g/m2/s). The results show grid-independent heat flux predictions up to 36 mm grid resolution. Without the wall model, the convective heat flux is significantly underpredicted. The model is further evaluated in an intermediate-scale parallel-panel fire spread scenario (0.6 m wide and 2.4 m high) with two grid resolutions (12.7 and 25.4 mm). The modeled chemical heat release rates show good agreement with the measurements for both cases. Without the wall model, the predicted fire growth rate is grid dependent and slower compared to the experiment.
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