A new study from the Health and Safety Laboratory (HSL) provides guidelines, recommendations and best practices for the practical application of computational fluid dynamics to the modeling of smoke movement in enclosed spaces. HSL modeled three realistic fire safety engineering cases: a subway station, an accommodation module on an offshore platform, and a high-rise building under construction. Aspects of the modeling process — including the computational grid, the discretization scheme and the turbulence model — were varied for each scenario, and the resulting predictions of smoke transport were compared.

All of the different approaches provided realistic results, indicating that they can be used as the basis for an engineered approach to fire safety. However, a comparison of quantitative data, such as the temperature of the hot layer, showed that key output results can vary greatly depending on the modeling approach used. HSL has published guidelines that we believe will help both CFD practitioners and regulators identify the most appropriate CFD modeling approach for the scenario under investigation and for the purpose of the simulation. Scenario examples include evaluating the time available for evacuation before smoke becomes too dense or checking the effectiveness of a ventilation design in clearing the smoke from exit routes.

The ANSYS CFX code was used for all cases for several reasons. CFD work employing ANSYS CFX played a crucial role in identifying the cause of the rapid spread of fire during London’s Kings Cross subway station fire investigation in 1987, which was led by HSL. As a consequence, HSL decided to develop an in-house expertise in CFD. Health and Safety Laboratory opted for ANSYS CFX, which remains its main code used to investigate an increasing range of health and safety issues. ANSYS CFX was employed for the latest smoke movement study since it is a commercial CFD package with a wide range of physical and numerical sub-models suited for fire safety engineering. In addition, ANSYS CFX has been widely used for smoke movement applications. Furthermore, HSL has had considerable experience using ANSYS CFX.

A four-level underground station on the Jubilee Line Extension of the London Underground was used as one representative fire scenario. It was assumed that the main source of the fire was a passenger’s suitcase that contained clothes of different fabrics. The ANSYS CFX results for the underground station show that in the five-minute period before forced ventilation is initiated, smoke is transported throughout most of the ticket hall and appears to extend to the main exit routes. Following the start-up of forced ventilation, smoke is cleared from the paid side of the ticket hall and the area past the ticket barrier by being convected toward the exits and into the dome. A series of small-scale experiments was undertaken to provide data for the evaluation of CFD modeling of smoke movement. Overall, the CFD simulations captured many of the observed gross flow conditions. In some cases, details of the temperature field also were well predicted. However, the simulations were found to be very sensitive to the wall heat transfer boundary condition.

HSL has published a report targeted for regulators to help them assess CFD predictions of smoke movement in complex enclosed spaces [pdf]. Another document aimed at CFD users is under preparation. It will be published later this year by NAFEMS under the title “How to Undertake a Smoke Movement Analysis in Complex Enclosed Spaces Using CFD.” HSL is grateful to ANSYS for assistance during the course of the project in implementing the CFD models in ANSYS CFX and in running some of the simulations on their computers. The guidelines produced should help to increase the safety of future structures designed and evaluated using the recommendations.

ANSYS CFX prediction of smoke propagation to the upper floor and into the atrium of an eighteen-storey office building under construction.

CFD prediction of smoke propagation in the corridor of an offshore accommodation module.

Smoke visualisation in a laboratory-scale Plexiglas model representing an inclined corridor.

Temperatures predicted by ANSYS CFX in the laboratory-scale model representing an inclined corridor.