Theoretical analysis on plane fire plume in a longitudinally ventilated tunnel

Abstract This paper presents a theoretical analysis on the longitudinal tunnel ventilation in fire emergency based on a plane fire plume model. The objective of the current research is to understand the physical mechanism leading to the decoupling of critical velocity and HRR in the case of large tunnel fire. The solution of the analysis has established a critical Fr curve that changes continuously from what can be approximated by Thomas’ cubic root correlation to a constant limited by Archimedes’ principle. The turning point between the two ventilation regimes is the unit relative tunnel height to the height of flame. The current analysis has revealed that in order to predict the behaviour of critical velocity correctly, all three controlling factors, namely fire, ventilation and tunnel height must be taken into account. Both traditional ceiling plume theory and Thomas’ correlation are inadequate in analysing tunnel fire ventilation. The former does not include forced ventilation and the later has left tunnel height out. The current Fr curve has provided the theoretical maximum critical velocity that can serve as guidance for fan sizing in tunnel ventilation design. Comparison between the prediction from the current theory and three independent sets of experimental data has shown excellent agreement.