Application of computational fluid dynamics in tunnel ventilation system design and operation under fire scenario

The approach to tunnel ventilation system design has been a controversy for engineers for many years. The present methodologies and criteria used for quantifying the capacity of emergency tunnel ventilation in tunnel fire scenarios are mostly based on a simple empirical method or one-dimensional fluid dynamics network model. Due to the simple formulation of these methodologies, detailed dynamic flow behaviour of hot smoke from a tunnel fire at any location, which might cause the onset of backlayering, could not be predicted. To overcome this concern, a basic analysis of the complex phenomenon between ambient and hot smoke laden air in the proximity of the fire is required. The Computational Fluid Dynamics (CFD) technique, which has been well-established and applied in the design of building ventilation systems, enables the prediction of fundamental field variables of pressure, velocities, temperature and smoke concentration at any location within the computational domain of the system being simulated. Due to the unique environment inside the tunnel, validation of simulation results is one of the most important issues in applying the CFD technique as a design tool for quantifying the capacity of emergency tunnel ventilation system. This paper describes the CFD simulations of different fire scenarios in a road tunnel. Simulation results are compared with the results obtained from a series of full-scale fire tests conducted in an abandoned road tunnel in the USA, known as the Memorial Tunnel Fire Ventilation Test Program. Backlayering and critical velocity will be addressed and comparison of results obtained from the CFD technique and the traditional design method using empirical equations will be made. Emphasis will be on the employ of CFD technique as a design tool for tunnel ventilation system. Keywords Computational fluid dynamics, critical velocity, tunnel fire, k-ε turbulence model, longitudinal tunnel ventilation system