Coupled hybrid modelling within the Fire Dynamics Simulator: transient transport and mass storage

Non-prescriptive design within fire engineering is becoming more prevalent as buildings get taller and more complex. This necessitates the increased use of reliable deterministic predictions, typically computational fluid dynamics-based models. Driven by time constraints, modellers are required to limit the domain to reduce wall time. This ignores the two-way coupling of a fire and a total building system which could embody risk to life. One way to address this risk is the use of coupled hybrid modelling to expand the domain, explicitly quantifying risk-related quantities in the far field whilst maintaining practicable wall times. Fire Dynamics Simulator (FDS) version 5.5 opened the door to coupled hybrid modelling within FDS and introduced the HVAC network submodel. Until FDS version 6.5.3, the submodel did not account for transient transport or mass storage. In this work, a new transient transport and mass storage subroutine has been introduced into HVAC which is available in FDS version 6.5.3 onwards. The relevant conservation equations and numerical solution are described. Successful verification cases are presented for various arrangements to test the implementation of the solution scheme. To demonstrate the benefits of the new method, a fire engineering test case is presented. The test case illustrates the potential risks contained within the pre-existing coupled hybrid modelling method. These risks include unrealistic predictions of hot layer height and head height temperatures and visibility. The test case demonstrates that the new coupled hybrid modelling method address these shortcomings and could form part of a most robust fire safety engineering solution. Based on experimental benchmarking exercises, recommended model correction coefficients are put forward. The new model implementation can be used by designers to quantitatively examine the fire hazard embodied within the two-way coupling of a fire and a total building system.