Smoke buildup and light scattering in a cylindrical cavity above a uniform flow

In this study, we use computational fluid dynamics (CFD) and aerosol dynamics modeling to investigate the buildup of smoke and light scattering in a cylindrical cavity geometry, considered to be an idealized representation of a photoelectric smoke detector. CFD coupled with the quadrature method of moments (QMOM) is used for simulation of aerosol dynamics. The Rayleigh-Debye-Gans/polydisperse fractal aggregate (RDGPFA) theory is used for calculation of smoke extinction and angular light scattering. It is seen that the flow external to the cavity sets up a recirculating flow pattern within the cavity and that the flow processes determine the spatial distribution of smoke. Aerosol extinction and scattering calculations are performed to examine the time varying profiles of the intensity along a simulated LED light beam and the scattered intensity at different angles. The variation of the detector activation time with inlet velocity and smoke volume fraction is obtained from a calculation of the angular light scattering. The results are compared with calculations using an empirically determined detector response function and with a simpler model that assumes a uniform distribution of smoke inside the cavity. Results indicate that although the distribution of smoke inside the cavity is not uniformly mixed, the simple first-order mixing model with appropriately chosen parameters is valid for predicting detector activation time.